WO2022221662A1

WO2022221662A1 - Low-loss optical materials and processes

Info

Publication number: WO2022221662A1
Application number: PCT/US2022/025021
Authority: WO
Inventors: Sara AZARI; Austin Lane; Feyza Dundar Arisoy; Ankit Vora; Nihar Ranjan Mohanty; Vivek Gupta; Keren ZHANG
Original assignee: Meta Platforms Technologies, Llc
Priority date: 2021-04-15
Filing date: 2022-04-15
Publication date: 2022-10-20
Also published as: EP4323815A1

Abstract

An optical device includes a substrate, a first surface-relief grating including grooves and ridges formed on or in the substrate, a first overcoat layer in the grooves of the first surface-relief grating, and a first antireflective layer on the first overcoat layer. The ridges of the first surface-relief grating include high-refractive index, photoactive metal oxide nanoparticles and a material of the first overcoat layer in regions between the metal oxide nanoparticles, or the first overcoat layer includes the metal oxide nanoparticles and a material of the first antireflective layer in regions between the metal oxide nanoparticles. Methods of fabricating the optical device are also described.

Description

LOW-LOSS OPTICAL MATERIALS AND PROCESSES

FIELD OF THE DISCLOSURE

[0001] This disclosure relates generally to surface-relief gratings. More specifically, disclosed herein are techniques for forming low-loss, high-refractive index surface-relief gratings or overcoat layers of surface-relief gratings.

BACKGROUND

[0002] An artificial reality system, such as a head-mounted display (HMD) or heads-up display (HUD) system, generally includes a near-eye display (e.g., in the form of a headset or a pair of glasses) configured to present content to a user via an electronic or optic display within, for example, about 10-20 mm in front of the user’s eyes. The near-eye display may display virtual objects or combine images of real objects with virtual objects, as in virtual reality (VR), augmented reality (AR), or mixed reality (MR) applications. For example, in an AR system, a user may view both images of virtual objects (e.g., computer-generated images (CGIs)) and the surrounding environment by, for example, seeing through transparent display glasses or lenses (often referred to as optical see-through).

[0003] One example of an optical see-through AR system may use a waveguide-based optical display, where light of projected images may be coupled into a waveguide (e.g., a transparent substrate), propagate within the waveguide, and be coupled out of the waveguide at different locations. In some implementations, the light of the projected images may be coupled into or out of the waveguide using diffractive optical elements, such as surface-relief gratings or volume Bragg gratings. Light from the surrounding environment may pass through a see-through region of the waveguide and reach the user’s eyes as well.

SUMMARY

[0004] This disclosure relates generally to surface-relief gratings. More specifically, disclosed herein are techniques for forming low-loss, high-refractive index surface-relief gratings or overcoat layers of surface-relief gratings. Various inventive embodiments are described herein, including devices, systems, methods, processes, materials, compositions, and the like.

[0005] According to certain embodiments, an optical device may include a substrate, a first surface-relief grating including grooves and ridges formed on or in the substrate, a first overcoat layer in the grooves of the first surface-relief grating, and a first antireflective layer on the first overcoat layer. In some embodiments, the ridges of the first surface-relief grating may include metal oxide nanoparticles and a material of the first overcoat layer in regions between the metal oxide nanoparticles, where the metal oxide nanoparticles may be photoactive. In some embodiments, the first overcoat layer may include the metal oxide nanoparticles and a material of the first antireflective layer in regions between the metal oxide nanoparticles.

[0006] In some embodiments of the optical device, the metal oxide nanoparticles may include TiOx nanoparticles, NbOx nanoparticles, or a combination thereof. The ridges of the first surface-relief grating or the first overcoat layer may be characterized by a refractive index greater than 2.0 for visible light. A total optical absorption of the first surface-relief grating and the first overcoat layer may be lower than 0.1% for visible light. The material in the regions between the metal oxide nanoparticles in the ridges of the first surface-relief grating or the material between the metal oxide nanoparticles in the first overcoat layer may include acrylate, polystyrenics, epoxy, siloxane, AI2O3, S1O2, ZrC , Nb20s, T1O2, Ta20.y or a combination thereof. The ridges of the first surface-relief grating or the first overcoat layer may have a carbon atomic concentration less than 10%, less than 5%, less than 2%, or less than 1%, such as close to about 0%.

[0007] In some embodiments, the optical device may also include a second surface-relief grating including grooves and ridges formed on or in the substrate, a second overcoat layer in the grooves of the second surface-relief grating, and a second antireflective layer on the second overcoat layer. The second surface-relief grating and the first surface-relief grating may be on opposite sides of the substrate. In some embodiments, the ridges of the second surface-relief grating may include the metal oxide nanoparticles and a material of the second overcoat layer in regions between the metal oxide nanoparticles. In some embodiments, the second overcoat layer may include the metal oxide nanoparticles and a material of the second antireflective layer in regions between the metal oxide nanoparticles.

[0008] According to certain embodiments, a method may include depositing an overcoat layer on a surface-relief grating to at least partially fill grating grooves of the surface-relief grating, curing the overcoat layer by heat or electromagnetic radiation, performing a high- energy treatment on the overcoat layer to remove light-absorbing organics and form porous structures in the overcoat layer, and depositing a first layer of an organic material, an inorganic material, or a combination thereof on the overcoat layer to fill the porous structures in the overcoat layer. The overcoat layer may include a sol-gel material including metal oxide precursors, or a resin layer including metal oxide nanoparticles. In some embodiments, the method may include, before curing the overcoat layer, imprinting the overcoat layer using a planar imprint stamp to planarize a top surface of the overcoat layer. [0009] In some embodiments, the high-energy treatment may include rapid thermal annealing (RTA), laser spike annealing (LSA), or ultraviolet (UV) light irradiation. Depositing the first layer of the organic material, the inorganic material, or a combination thereof on the overcoat layer may include an inkjet coating process, a spin coating process, an atomic layer deposition (ALD) process, a sequential infiltration synthesis (SIS) process, a physical vapor deposition (PVD) process, an initiated chemical vapor deposition (iCVD) process, or a combination thereof. ALD precursors used in the ALD process include TiCh and EhO, tetrakis (dimethylamino) htanium(IV) (TDMAT) and O3, tetrakis (dimethylamido) zirconium (IV) (TDMAZ) / trimethylaluminum (TMA) and H2O, or a combination thereof. [0010] In some embodiments, the organic material may include acrylate, polystyrenics, epoxy, siloxane, or a combination thereof. The inorganic material may include, for example, AI2O3, SiCh, ZrCh, M>2q5, TiCh, Ta205, or a combination thereof. The sol-gel material may include organic solvent soluble Titania with organic ligands; and a solvent including propylene glycol methyl ether acetate (PGMEA), dipropylene glycol methyl ether (DPGME)/tripropylene glycol monomethyl ether (TPM), or a combination thereof. The resin layer may include an acrylate resin including photo radical generator or thermal radical generator; TiO_x or NbOx nanoparticles decorated with organic ligands; and a solvent including PGMEA, DPGME/TPM, or a combination thereof. The first layer of the organic material, the inorganic material, or a combination thereof may form an antireflective coating layer on the overcoat layer.

[0011] According to certain embodiments, a method may include depositing, on a substrate, a resin layer that includes metal oxide nanoparticles; imprinting a surface-relief grating in the resin layer; performing a high-energy treatment on the surface-relief grating to remove light-absorbing organics and form porous structures in the surface-relief grating; and depositing a layer of an organic material, an inorganic material, or a combination thereof on the surface-relief grating to fill grating grooves of the surface-relief grating and the porous structures in the surface-relief grating. The high-energy treatment may include rapid thermal annealing (RTA), laser spike annealing (LSA), or ultraviolet (UV) light irradiation. The resin layer may include an acrylate resin including photo radical generator or thermal radical generator; TiCh or NbCL nanoparticles decorated with organic ligands; and a solvent including propylene glycol methyl ether acetate (PGMEA), dipropylene glycol methyl ether (DPGME)/tripropylene glycol monomethyl ether (TPM), or a combination thereof. The layer of the organic material, the inorganic material, or a combination thereof may include acrylate, polystyrenics, epoxy, siloxane, AI2O3, SiCh, ZrCh, MnCh, TiCh, Ta2Ch, or a combination thereof.

[0012] This summary is neither intended to identify key or essential features of the claimed subject matter, nor is it intended to be used in isolation to determine the scope of the claimed subject matter. The subject matter should be understood by reference to appropriate portions of the entire specification of this disclosure, any or all drawings, and each claim. The foregoing, together with other features and examples, will be described in more detail below in the following specification, claims, and accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS [0013] Illustrative embodiments are described in detail below with reference to the following figures.

[0014] FIG. 1 is a simplified block diagram of an example of an artificial reality system environment including a near-eye display according to certain embodiments.

[0015] FIG. 2 is a perspective view of an example of a near-eye display in the form of a head-mounted display (HMD) device for implementing some of the examples disclosed herein.

[0016] FIG. 3 is a perspective view of an example of a near-eye display in the form of a pair of glasses for implementing some of the examples disclosed herein.

[0017] FIG. 4 illustrates an example of an optical see-through augmented reality system including a waveguide display according to certain embodiments.

[0018] FIG. 5 illustrates an example of an optical see-through augmented reality system including a waveguide display for exit pupil expansion according to certain embodiments. [0019] FIG. 6 illustrates examples of propagations of display light and external light in an example of a waveguide display.

[0020] FIG. 7 illustrates an example of a slanted grating coupler in a waveguide display according to certain embodiments.

[0021] FIG. 8A illustrates an example of a waveguide-based near-eye display where display light for all fields of view is substantially uniformly output from different regions of a waveguide display.

[0022] FIG. 8B illustrates an example of a waveguide-based near-eye display where display light may be coupled out of a waveguide display at different angles in different regions of the waveguide display according to certain embodiments.

[0023] FIG. 9A illustrates an example of a slanted surface-relief grating with variable etch depths according to certain embodiments.

[0024] FIG. 9B illustrates an example of a slanted surface-relief grating with variable etch depths and variable duty cycles according to certain embodiments.

[0025] FIGS. 10A-10D illustrate an example of a process for fabricating a soft stamp for nanoimprinting according to certain embodiments. FIG. 10A shows a master mold. FIG. 10B illustrates the master mold coated with a soft stamp material layer. FIG. IOC illustrates a lamination process for laminating a soft stamp foil onto the soft stamp material layer. FIG. 10D illustrates a delamination process, where the soft stamp including the soft stamp foil and the attached soft stamp material layer is detached from the master mold.

[0026] FIGS. 11 A-l ID illustrate an example of an imprinting process using a soft stamp according to certain embodiments. FIG. 11 A shows a waveguide coated with an imprint resin layer. FIG. 1 IB shows the lamination of the soft stamp onto the imprint resin layer. FIG. 11C shows the delamination of the soft stamp from the imprint resin layer. FIG. 1 ID shows an example of an imprinted slanted grating formed on the waveguide.

[0027] FIGS. 12A-12C illustrate an example of a method of forming a planarized overcoat layer on a surface-relief grating according to certain embodiments.

[0028] FIGS. 13A-13B illustrate an example of a method of forming a planarized overcoat layer on a surface-relief grating using inkjet printing and imprinting techniques according to certain embodiments.

[0029] FIG. 14A illustrates an example of the photocatalytic activity of TiCh nanoparticles. [0030] FIG. 14B illustrates an example of organic degradation caused by the photocatalytic activity of TiCh nanoparticles.

[0031] FIGS. 15A-15D illustrate an example of a process of fabricating a surface-relief grating that includes overcoat layers with organics and metal oxide nanoparticles.

[0032] FIG. 15E illustrates changes in the optical losses of examples of waveguide displays including surface-relief gratings having different metal oxide nanoparticles and organics during the fabrication processes.

[0033] FIGS. 16A-16E illustrate an example of a process of fabricating a surface-relief grating including a high-refractive index and low-loss overcoat layer according to certain embodiments.

[0034] FIG. 17A illustrates an example of a back-filled nanoparticle layers according to certain embodiments.

[0035] FIG. 17B illustrates an example of a surface-relief grating including a back-filled high-refractive index overcoat layer according to certain embodiments.

[0036] FIG. 18A illustrates an X-ray photoelectron spectroscopy (XPS) profile of an example of a structure including a substrate and a resin layer with organics removed by rapid thermal annealing (RTA) according to certain embodiments.

[0037] FIG. 18B illustrates an XPS profile of an example of a structure including a substrate, a back-filled nanoparticle layer, and a back-fill layer according to certain embodiments.

[0038] FIG. 18C illustrates an XPS profile of an example of a structure including a substrate, a back-filled nanoparticle layer, and an AI2O3 back-fill layer according to certain embodiments.

[0039] FIG. 18D illustrates correlations between refractive indices and densities of various layers in the examples of structures of FIGS. 18A-18C.

[0040] FIGS. 19A-19E illustrate an example of a process of fabricating a surface-relief grating including an overcoat layer having a high refractive index and a low loss according to certain embodiments.

[0041] FIGS. 20A-20E illustrate an example of a process of fabricating a surface-relief grating including grating ridges having a high refractive index and a low loss according to certain embodiments.

[0042] FIG. 21 includes a flowchart illustrating an example of a process of fabricating a surface-relief grating having a high refractive index contrast and a low loss according to certain embodiments.

[0043] FIG. 22 is a simplified block diagram of an example electronic system of an example near-eye display for implementing some of the examples disclosed herein.

[0044] The figures depict embodiments of the present disclosure for purposes of illustration only. One skilled in the art will readily recognize from the following description that alternative embodiments of the structures and methods illustrated may be employed without departing from the principles, or benefits touted, of this disclosure.

[0045] In the appended figures, similar components and/or features may have the same reference label. Further, various components of the same type may be distinguished by following the reference label by a dash and a second label that distinguishes among the similar components. If only the first reference label is used in the specification, the description is applicable to any one of the similar components having the same first reference label irrespective of the second reference label.

DETAILED DESCRIPTION

[0046] This disclosure relates generally to surface-relief gratings. More specifically, disclosed herein are techniques for forming low-loss, high-refractive index surface-relief gratings or overcoat layers of surface-relief gratings. Various inventive embodiments are described herein, including devices, systems, methods, processes, materials, compositions, and the like.

[0047] Surface-relief gratings may be used in some optical devices to manipulate behavior of light. For example, straight or slanted surface-relief gratings may be used in near-eye display systems to couple display light into or out of a waveguide. An overcoat layer having a refractive index different from the refractive index of the grating ridges of the surface-relief grating may be formed on the surface-relief gratings to fill the grating grooves and protect the straight or slanted structures. In general, it is desirable to have a large refractive index contrast between the grating ridges and the overcoat layer to improve, for example, the light coupling efficiency, the field-of-view range, and the wavelength range of the near-eye display system. In some surface-relief gratings, the grating ridges may be made using a high- refractive index nanoimprinting material (e.g., a resin with high-refractive index metal oxide nanoparticles), and an overcoat layer with a low refractive index may be used to achieve the large refractive index contrast. In some surface-relief gratings, the grating ridges may be made of a low-refractive index material (e.g., glass), and an overcoat layer with a high refractive index (e.g., about 2.0 or larger) may be used to achieve the large refractive index contrast. The high-refractive index material for the grating ridges or the overcoat layer may include a resin dispersed with high-refractive index nanoparticles, such as TiCh nanoparticles, NbOx nanoparticles, ZrCh nanoparticles, and the like, or may include a sol-gel material that includes a solution and a high-refractive index oxide precursor (e.g., Ti precursor) and has been partially or fully condensed into an extended network.

[0048] TiCh has a higher refractive index (e.g., greater than about 2.5 for visible light) than ZrCh (e.g., about 2.15 for visible light) and other dielectric materials (e.g., metal oxides such as NbOx, LaOx, TaOx, AhCh, etc.), and thus may be mixed with a resin material to form a high-refractive index resin (e.g., with a refractive index about 2.0 or higher). However, high- refractive index resin materials, such as organics mixed with TiCh particles, may have high absorption in visible wavelengths, and the absorption may increase during the fabrication and usage of the devices made of these high-refractive index resin materials. For example, due to the photo-activity of titanium oxide, TiCh nanoparticles exposed to ultraviolet (UV) light may interact with moisture and/or oxygen to generate radicals (e.g., hydroxyl radicals) that may oxidize and degrade organics (e.g., ligands and resin) in the high-refractive index resin materials, where the degraded organics may absorb visible light. These interactions may also be thermally induced, in addition to titanium oxide photoexcitation. Due to their high surface- volume ratios, TiCh nanoparticles may have high light absorption rates, which may increase the surface photo-induced carrier density that can lead to higher surface photoactivity and enhanced photocatalytic activity of TiCh nanoparticles. As a result, films including titanium oxide nanoparticles and organics may show some levels of light absorption in the visible spectrum. The absorption of the films may increase over time, as the films undergo further redox interactions with their environment, such as upon exposure to ultraviolet (UV) light and heat. Surface-relief gratings and/or overcoat layers made of the high-absorption films may lead to high loss and low throughput for waveguide displays.

[0049] According to certain embodiments, organics in an imprinted grating layer or an overcoat layer, such as organic ligands on metal oxide (e.g., TiCh or NbOx) nanoparticles that may have been degraded (oxidized) or may not have been degraded, may be removed and/or replaced with another material, such as an inorganic material (e.g., Si-containing material) or another organic material that may have a low optical loss and may not be degraded by the photocatalytic activity of the metal oxide nanoparticles. For example, the organic ligands on metal oxide nanoparticles in a grating layer or an overcoat layer may be removed (e.g., burned) by a high-energy treatment process, such as rapid thermal annealing (RTA) or laser spike annealing (LSA), to form porous structures in the grating layer or the overcoat layer.

An organic or inorganic material layer (e.g., an overcoat layer or an antireflective coating layer) may then be formed on the grating layer or the overcoat layer to fill the porous structures in the grating layer or the overcoat layer, thereby increasing the refractive index while reducing the absorption loss of the grating layer or the overcoat layer.

[0050] In one example, a method of forming a high-refractive index and low-loss overcoat layer on a surface-relief grating may include depositing a first overcoat layer including high- refractive index nanoparticles dispersed in a resin material on a first side of a surface-relief grating to at least partially fill grating grooves of the surface relief grating; optionally imprinting the first overcoat layer to form a planar top surface on the first overcoat layer; curing the first overcoat layer by heat or electromagnetic radiation; optionally depositing a second overcoat layer including the nanoparticles dispersed in the resin material on a second side of the surface-relief grating; optionally curing the second overcoat layer by heat or electromagnetic radiation; performing RTA or LSA on the first overcoat layer and the second overcoat layer to remove light-absorbing organic ligands on the nanoparticles and form porous structures in the first overcoat layer and the second overcoat layer; and depositing an organic and/or inorganic material layer (e.g., an antireflective coating layer) on each of the first overcoat layer and the second overcoat layer by, for example, an inkjet coating process, a spin coating process, an atomic layer deposition (ALD) process, a sequential infiltration synthesis (SIS) process, a physical vapor deposition (PVD) process, an initiated chemical vapor deposition (iCVD) process, and the like, to back fill the porous structures with the organic and/or inorganic material.

[0051] In another example, a method of forming a high-refractive index and low-loss surface-relief grating may include depositing, on a substrate, a resin layer that includes metal oxide nanoparticles; imprinting a surface-relief grating in the resin layer; performing a high- energy treatment on the surface-relief grating to remove light-absorbing organics and form porous structures in the surface-relief grating; and depositing a layer of an organic material, an inorganic material, or a combination thereof on the surface-relief grating using, for example, an inkjet coating process, a spin coating process, an ALD process, an SIS process, a PVD process, or an iCVD process, to fill the porous structures in the surface-relief grating. [0052] According to certain embodiments, the high-refractive index material for imprinting the surface-relief grating or the overcoat layer may include a nanoparticle-based formulation or a sol-gel-based formulation. The nanoparticle-based formulation, sol-gel-based formulation, or a mixture of the two types of formulation can be annealed using RTA or LSA to form porous structures in the imprinted surface-relief grating or the overcoat layer. The imprinted surface-relief grating or the overcoat layer with the porous structures may be back filled with an organic and/or inorganic material to achieve a high refractive index and ultra- low absorption. The organic and/or inorganic material that fills the porous structures can have low optical loss, and may include, for example, organic materials such as acrylate, polystyrenics, epoxy based organic materials; siloxane-based materials; inorganic materials such as AI2O3, S1O2, ZrC , M>2q5, TiC , Ta20s, and the like; or a combination of organic materials and inorganic materials describe above.

[0053] In some embodiments, the sol-gel-based formulation may include EMD MHM-63, Philips high-refractive index sol-gel formulation, or organic solvent soluble Titania with ligands in PGMEA solvent (for spin coating) or DPGME/TPM solvent (for inkjet coating). In some embodiments, the nanoparticle-based formulation may include acrylate resin (with a refractive index between about 1.55 and about 1.7) with photo radical generator or thermal radical generator; 75%-90% TiOx, ZrOx, NbOx nanoparticles with acrylate and non-functional decoration ligands; and propylene glycol methyl ether acetate (PGMEA) solvent (for spin coating), dipropylene glycol methyl ether (DPGME)/ tripropylene glycol monomethyl ether (TPM) solvent (for inkjet coating), or a solvent blend including one or more solvents that may or may not have boiling points greater than 200°C. In some embodiments, the ALD precursor may include TiCU and H2O, tetrakis (dimethyl amino) titanium(IV) (TDMAT) and O3, tetrakis (dimethylamido) zirconium (IV) (TDMAZ) / trimethylaluminum (TMA) and H2O, or the like.

[0054] Unless defined otherwise, all technical and scientific terms used herein have the same meaning as is commonly understood by one of ordinary skill in the art to which this disclosure belongs.

[0055] As used herein, the term “about” means that dimensions, sizes, formulations, parameters, shapes and other quantities and characteristics are not and need not be exact, but may be approximate and/or larger or smaller, as desired, reflecting tolerances, conversion factors, rounding off, measurement error and the like, and other factors known to those of skill in the art. In general, a dimension, size, formulation, parameter, shape or other quantity or characteristic is “about” or “approximate” whether or not expressly stated to be such. It is noted that embodiments of very different sizes, shapes and dimensions may employ the described arrangements.

[0056] In the following description, for the purposes of explanation, specific details are set forth in order to provide a thorough understanding of examples of the disclosure. However, it will be apparent that various examples may be practiced without these specific details. For example, devices, systems, structures, assemblies, methods, and other components may be shown as components in block diagram form in order not to obscure the examples in unnecessary detail. In other instances, well-known devices, processes, systems, structures, and techniques may be shown without necessary detail in order to avoid obscuring the examples. The figures and description are not intended to be restrictive. The terms and expressions that have been employed in this disclosure are used as terms of description and not of limitation, and there is no intention in the use of such terms and expressions of excluding any equivalents of the features shown and described or portions thereof. The word “example” is used herein to mean “serving as an example, instance, or illustration.” Any embodiment or design described herein as “example” is not necessarily to be construed as preferred or advantageous over other embodiments or designs.

[0057] FIG. 1 is a simplified block diagram of an example of an artificial reality system environment 100 including a near-eye display 120 in accordance with certain embodiments. Artificial reality system environment 100 shown in FIG. 1 may include near-eye display 120, an optional external imaging device 150, and an optional input/output interface 140, each of which may be coupled to an optional console 110. While FIG. 1 shows an example of artificial reality system environment 100 including one near-eye display 120, one external imaging device 150, and one input/output interface 140, any number of these components may be included in artificial reality system environment 100, or any of the components may be omitted. For example, there may be multiple near-eye displays 120 monitored by one or more external imaging devices 150 in communication with console 110. In some configurations, artificial reality system environment 100 may not include external imaging device 150, optional input/output interface 140, and optional console 110. In alternative configurations, different or additional components may be included in artificial reality system environment 100.

[0058] Near-eye display 120 may be a head-mounted display that presents content to a user. Examples of content presented by near-eye display 120 include one or more of images, videos, audio, or any combination thereof. In some embodiments, audio may be presented via an external device (e.g, speakers and/or headphones) that receives audio information from near-eye display 120, console 110, or both, and presents audio data based on the audio information. Near-eye display 120 may include one or more rigid bodies, which may be rigidly or non-rigidly coupled to each other. A rigid coupling between rigid bodies may cause the coupled rigid bodies to act as a single rigid entity. A non-rigid coupling between rigid bodies may allow the rigid bodies to move relative to each other. In various embodiments, near-eye display 120 may be implemented in any suitable form-factor, including a pair of glasses. Some embodiments of near-eye display 120 are further described below with respect to FIGS. 2 and 3. Additionally, in various embodiments, the functionality described herein may be used in a headset that combines images of an environment external to near-eye display 120 and artificial reality content (e.g., computer-generated images). Therefore, near eye display 120 may augment images of a physical, real-world environment external to near eye display 120 with generated content (e.g, images, video, sound, etc.) to present an augmented reality to a user.

[0059] In various embodiments, near-eye display 120 may include one or more of display electronics 122, display optics 124, and an eye-tracking unit 130. In some embodiments, near-eye display 120 may also include one or more locators 126, one or more position sensors 128, and an inertial measurement unit (IMU) 132. Near-eye display 120 may omit any of eye tracking unit 130, locators 126, position sensors 128, and IMU 132, or include additional elements in various embodiments. Additionally, in some embodiments, near-eye display 120 may include elements combining the function of various elements described in conjunction with FIG. 1.

[0060] Display electronics 122 may display or facilitate the display of images to the user according to data received from, for example, console 110. In various embodiments, display electronics 122 may include one or more display panels, such as a liquid crystal display (LCD), an organic light emitting diode (OLED) display, an inorganic light emitting diode (ILED) display, a micro light emitting diode (pLED) display, an active-matrix OLED display (AMOLED), a transparent OLED display (TOLED), or some other display. For example, in one implementation of near-eye display 120, display electronics 122 may include a front TOLED panel, a rear display panel, and an optical component (e.g., an attenuator, polarizer, or diffractive or spectral film) between the front and rear display panels. Display electronics 122 may include pixels to emit light of a predominant color such as red, green, blue, white, or yellow. In some implementations, display electronics 122 may display a three-dimensional (3D) image through stereoscopic effects produced by two-dimensional panels to create a subjective perception of image depth. For example, display electronics 122 may include a left display and a right display positioned in front of a user’s left eye and right eye, respectively. The left and right displays may present copies of an image shifted horizontally relative to each other to create a stereoscopic effect (i.e., a perception of image depth by a user viewing the image).

[0061] In certain embodiments, display optics 124 may display image content optically (e.g., using optical waveguides and couplers) or magnify image light received from display electronics 122, correct optical errors associated with the image light, and present the corrected image light to a user of near-eye display 120. In various embodiments, display optics 124 may include one or more optical elements, such as, for example, a substrate, optical waveguides, an aperture, a Fresnel lens, a convex lens, a concave lens, a filter, input/output couplers, or any other suitable optical elements that may affect image light emitted from display electronics 122. Display optics 124 may include a combination of different optical elements as well as mechanical couplings to maintain relative spacing and orientation of the optical elements in the combination. One or more optical elements in display optics 124 may have an optical coating, such as an antireflective coating, a reflective coating, a filtering coating, or a combination of different optical coatings.

[0062] Magnification of the image light by display optics 124 may allow display electronics 122 to be physically smaller, weigh less, and consume less power than larger displays. Additionally, magnification may increase a field of view of the displayed content. The amount of magnification of image light by display optics 124 may be changed by adjusting, adding, or removing optical elements from display optics 124. In some embodiments, display optics 124 may project displayed images to one or more image planes that may be further away from the user’s eyes than near-eye display 120.

[0063] Display optics 124 may also be designed to correct one or more types of optical errors, such as two-dimensional optical errors, three-dimensional optical errors, or any combination thereof. Two-dimensional errors may include optical aberrations that occur in two dimensions. Example types of two-dimensional errors may include barrel distortion, pincushion distortion, longitudinal chromatic aberration, and transverse chromatic aberration. Three-dimensional errors may include optical errors that occur in three dimensions. Example types of three-dimensional errors may include spherical aberration, comatic aberration, field curvature, and astigmatism.

[0064] Locators 126 may be objects located in specific positions on near-eye display 120 relative to one another and relative to a reference point on near-eye display 120. In some implementations, console 110 may identify locators 126 in images captured by external imaging device 150 to determine the artificial reality headset’s position, orientation, or both. A locator 126 may be a light-emitting diode (LED), a comer cube reflector, a reflective marker, a type of light source that contrasts with an environment in which near-eye display 120 operates, or any combination thereof. In embodiments where locators 126 are active components (e.g., LEDs or other types of light emitting devices), locators 126 may emit light in the visible band (e.g., about 380 nm to 750 nm), in the infrared (IR) band (e.g., about 750 nm to 1 mm), in the ultraviolet band (e.g, about 10 nm to about 380 nm), in another portion of the electromagnetic spectrum, or in any combination of portions of the electromagnetic spectrum.

[0065] External imaging device 150 may include one or more cameras, one or more video cameras, any other device capable of capturing images including one or more of locators 126, or any combination thereof. Additionally, external imaging device 150 may include one or more filters (e.g., to increase signal to noise ratio). External imaging device 150 may be configured to detect light emitted or reflected from locators 126 in a field of view of external imaging device 150. In embodiments where locators 126 include passive elements (e.g., retroreflectors), external imaging device 150 may include a light source that illuminates some or all of locators 126, which may retro-reflect the light to the light source in external imaging device 150. Slow calibration data may be communicated from external imaging device 150 to console 110, and external imaging device 150 may receive one or more calibration parameters from console 110 to adjust one or more imaging parameters (e.g., focal length, focus, frame rate, sensor temperature, shutter speed, aperture, etc.). [0066] Position sensors 128 may generate one or more measurement signals in response to motion of near-eye display 120. Examples of position sensors 128 may include accelerometers, gyroscopes, magnetometers, other motion-detecting or error-correcting sensors, or any combination thereof. For example, in some embodiments, position sensors 128 may include multiple accelerometers to measure translational motion (e.g., forward/back, up/down, or left/right) and multiple gyroscopes to measure rotational motion (e.g., pitch, yaw, or roll). In some embodiments, various position sensors may be oriented orthogonally to each other.

[0067] IMU 132 may be an electronic device that generates fast calibration data based on measurement signals received from one or more of position sensors 128. Position sensors 128 may be located external to IMU 132, internal to IMU 132, or any combination thereof. Based on the one or more measurement signals from one or more position sensors 128, IMU 132 may generate fast calibration data indicating an estimated position of near-eye display 120 relative to an initial position of near-eye display 120. For example, IMU 132 may integrate measurement signals received from accelerometers over time to estimate a velocity vector and integrate the velocity vector over time to determine an estimated position of a reference point on near-eye display 120. Alternatively, IMU 132 may provide the sampled measurement signals to console 110, which may determine the fast calibration data. While the reference point may generally be defined as a point in space, in various embodiments, the reference point may also be defined as a point within near-eye display 120 (e.g., a center of IMU 132).

[0068] Eye-tracking unit 130 may include one or more eye-tracking systems. Eye tracking may refer to determining an eye’s position, including orientation and location of the eye, relative to near-eye display 120. An eye-tracking system may include an imaging system to image one or more eyes and may optionally include a light emitter, which may generate light that is directed to an eye such that light reflected by the eye may be captured by the imaging system. For example, eye-tracking unit 130 may include a non-coherent or coherent light source (e.g., a laser diode) emitting light in the visible spectrum or infrared spectrum, and a camera capturing the light reflected by the user’s eye. As another example, eye-tracking unit 130 may capture reflected radio waves emitted by a miniature radar unit. Eye-tracking unit 130 may use low-power light emitters that emit light at frequencies and intensities that would not injure the eye or cause physical discomfort. Eye-tracking unit 130 may be arranged to increase contrast in images of an eye captured by eye-tracking unit 130 while reducing the overall power consumed by eye-tracking unit 130 (e.g., reducing power consumed by a light emitter and an imaging system included in eye-tracking unit 130). For example, in some implementations, eye-tracking unit 130 may consume less than 100 milliwatts of power. [0069] Near-eye display 120 may use the orientation of the eye to, e.g., determine an inter pupillary distance (IPD) of the user, determine gaze direction, introduce depth cues (e.g., blur image outside of the user’s main line of sight), collect heuristics on the user interaction in the VR media (e.g., time spent on any particular subject, object, or frame as a function of exposed stimuli), some other functions that are based in part on the orientation of at least one of the user’s eyes, or any combination thereof. Because the orientation may be determined for both eyes of the user, eye-tracking unit 130 may be able to determine where the user is looking. For example, determining a direction of a user’s gaze may include determining a point of convergence based on the determined orientations of the user’s left and right eyes. A point of convergence may be the point where the two foveal axes of the user’s eyes intersect. The direction of the user’s gaze may be the direction of a line passing through the point of convergence and the mid-point between the pupils of the user’s eyes.

[0070] Input/output interface 140 may be a device that allows a user to send action requests to console 110. An action request may be a request to perform a particular action. For example, an action request may be to start or to end an application or to perform a particular action within the application. Input/output interface 140 may include one or more input devices. Example input devices may include a keyboard, a mouse, a game controller, a glove, a button, a touch screen, or any other suitable device for receiving action requests and communicating the received action requests to console 110. An action request received by the input/output interface 140 may be communicated to console 110, which may perform an action corresponding to the requested action. In some embodiments, input/output interface 140 may provide haptic feedback to the user in accordance with instructions received from console 110. For example, input/output interface 140 may provide haptic feedback when an action request is received, or when console 110 has performed a requested action and communicates instructions to input/output interface 140. In some embodiments, external imaging device 150 may be used to track input/output interface 140, such as tracking the location or position of a controller (which may include, for example, an IR light source) or a hand of the user to determine the motion of the user. In some embodiments, near-eye display 120 may include one or more imaging devices to track input/output interface 140, such as tracking the location or position of a controller or a hand of the user to determine the motion of the user.

[0071] Console 110 may provide content to near-eye display 120 for presentation to the user in accordance with information received from one or more of external imaging device 150, near-eye display 120, and input/output interface 140. In the example shown in FIG. 1, console 110 may include an application store 112, a headset tracking module 114, an artificial reality engine 116, and an eye-tracking module 118. Some embodiments of console 110 may include different or additional modules than those described in conjunction with FIG. 1. Functions further described below may be distributed among components of console 110 in a different manner than is described here.

[0072] In some embodiments, console 110 may include a processor and a non-transitory computer-readable storage medium storing instructions executable by the processor. The processor may include multiple processing units executing instructions in parallel. The non- transitory computer-readable storage medium may be any memory, such as a hard disk drive, a removable memory, or a solid-state drive ( e.g ., flash memory or dynamic random access memory (DRAM)). In various embodiments, the modules of console 110 described in conjunction with FIG. 1 may be encoded as instructions in the non-transitory computer- readable storage medium that, when executed by the processor, cause the processor to perform the functions further described below.

[0073] Application store 112 may store one or more applications for execution by console 110. An application may include a group of instructions that, when executed by a processor, generates content for presentation to the user. Content generated by an application may be in response to inputs received from the user via movement of the user’s eyes or inputs received from the input/output interface 140. Examples of the applications may include gaming applications, conferencing applications, video playback application, or other suitable applications.

[0074] Headset tracking module 114 may track movements of near-eye display 120 using slow calibration information from external imaging device 150. For example, headset tracking module 114 may determine positions of a reference point of near-eye display 120 using observed locators from the slow calibration information and a model of near-eye display 120. Headset tracking module 114 may also determine positions of a reference point of near-eye display 120 using position information from the fast calibration information. Additionally, in some embodiments, headset tracking module 114 may use portions of the fast calibration information, the slow calibration information, or any combination thereof, to predict a future location of near-eye display 120. Headset tracking module 114 may provide the estimated or predicted future position of near-eye display 120 to artificial reality engine 116. [0075] Artificial reality engine 116 may execute applications within artificial reality system environment 100 and receive position information of near-eye display 120, acceleration information of near-eye display 120, velocity information of near-eye display 120, predicted future positions of near-eye display 120, or any combination thereof from headset tracking module 114. Artificial reality engine 116 may also receive estimated eye position and orientation information from eye-tracking module 118. Based on the received information, artificial reality engine 116 may determine content to provide to near-eye display 120 for presentation to the user. For example, if the received information indicates that the user has looked to the left, artificial reality engine 116 may generate content for near-eye display 120 that mirrors the user’s eye movement in a virtual environment. Additionally, artificial reality engine 116 may perform an action within an application executing on console 110 in response to an action request received from input/output interface 140, and provide feedback to the user indicating that the action has been performed. The feedback may be visual or audible feedback via near-eye display 120 or haptic feedback via input/output interface 140. [0076] Eye-tracking module 118 may receive eye-tracking data from eye-tracking unit 130 and determine the position of the user’s eye based on the eye tracking data. The position of the eye may include an eye’s orientation, location, or both relative to near-eye display 120 or any element thereof. Because the eye’s axes of rotation change as a function of the eye’s location in its socket, determining the eye’s location in its socket may allow eye-tracking module 118 to more accurately determine the eye’s orientation.

[0077] FIG. 2 is a perspective view of an example of a near-eye display in the form of an HMD device 200 for implementing some of the examples disclosed herein. HMD device 200 may be a part of, e.g., a VR system, an AR system, an MR system, or any combination thereof. HMD device 200 may include a body 220 and a head strap 230. FIG. 2 shows a bottom side 223, a front side 225, and a left side 227 of body 220 in the perspective view. Head strap 230 may have an adjustable or extendible length. There may be a sufficient space between body 220 and head strap 230 of HMD device 200 for allowing a user to mount HMD device 200 onto the user’s head. In various embodiments, HMD device 200 may include additional, fewer, or different components. For example, in some embodiments, HMD device 200 may include eyeglass temples and temple tips as shown in, for example, FIG. 3 below, rather than head strap 230.

[0078] HMD device 200 may present to a user media including virtual and/or augmented views of a physical, real-world environment with computer-generated elements. Examples of the media presented by HMD device 200 may include images (e.g., two-dimensional (2D) or three-dimensional (3D) images), videos (e.g., 2D or 3D videos), audio, or any combination thereof. The images and videos may be presented to each eye of the user by one or more display assemblies (not shown in FIG. 2) enclosed in body 220 of HMD device 200. In various embodiments, the one or more display assemblies may include a single electronic display panel or multiple electronic display panels (e.g., one display panel for each eye of the user). Examples of the electronic display panel(s) may include, for example, an LCD, an OLED display, an ILED display, a pLED display, an AMOLED, a TOLED, some other display, or any combination thereof. HMD device 200 may include two eye box regions. [0079] In some implementations, HMD device 200 may include various sensors (not shown), such as depth sensors, motion sensors, position sensors, and eye tracking sensors. Some of these sensors may use a structured light pattern for sensing. In some implementations, HMD device 200 may include an input/output interface for communicating with a console. In some implementations, HMD device 200 may include a virtual reality engine (not shown) that can execute applications within HMD device 200 and receive depth information, position information, acceleration information, velocity information, predicted future positions, or any combination thereof of HMD device 200 from the various sensors. In some implementations, the information received by the virtual reality engine may be used for producing a signal (e.g., display instructions) to the one or more display assemblies. In some implementations, HMD device 200 may include locators (not shown, such as locators 126) located in fixed positions on body 220 relative to one another and relative to a reference point. Each of the locators may emit light that is detectable by an external imaging device. [0080] FIG. 3 is a perspective view of an example of a near-eye display 300 in the form of a pair of glasses for implementing some of the examples disclosed herein. Near-eye display 300 may be a specific implementation of near-eye display 120 of FIG. 1, and may be configured to operate as a virtual reality display, an augmented reality display, and/or a mixed reality display. Near-eye display 300 may include a frame 305 and a display 310. Display 310 may be configured to present content to a user. In some embodiments, display 310 may include display electronics and/or display optics. For example, as described above with respect to near-eye display 120 of FIG. 1, display 310 may include an LCD display panel, an LED display panel, or an optical display panel (e.g., a waveguide display assembly).

[0081] Near-eye display 300 may further include various sensors 350a, 350b, 350c, 350d, and 350e on or within frame 305. In some embodiments, sensors 350a-350e may include one or more depth sensors, motion sensors, position sensors, inertial sensors, or ambient light sensors. In some embodiments, sensors 350a-350e may include one or more image sensors configured to generate image data representing different fields of views in different directions. In some embodiments, sensors 350a-350e may be used as input devices to control or influence the displayed content of near-eye display 300, and/or to provide an interactive VR/AR/MR experience to a user of near-eye display 300. In some embodiments, sensors 350a-350e may also be used for stereoscopic imaging.

[0082] In some embodiments, near-eye display 300 may further include one or more illuminators 330 to project light into the physical environment. The projected light may be associated with different frequency bands (e.g., visible light, infra-red light, ultra-violet light, etc.), and may serve various purposes. For example, illuminator(s) 330 may project light in a dark environment (or in an environment with low intensity of infra-red light, ultra-violet light, etc.) to assist sensors 350a-350e in capturing images of different objects within the dark environment. In some embodiments, illuminator(s) 330 may be used to project certain light patterns onto the objects within the environment. In some embodiments, illuminator(s) 330 may be used as locators, such as locators 126 described above with respect to FIG. 1.

[0083] In some embodiments, near-eye display 300 may also include a high-resolution camera 340. High-resolution camera 340 may capture images of the physical environment in the field of view. The captured images may be processed, for example, by a virtual reality engine (e.g., artificial reality engine 116 of FIG. 1) to add virtual objects to the captured images or modify physical objects in the captured images, and the processed images may be displayed to the user by display 310 for AR or MR applications.

[0084] FIG. 4 illustrates an example of an optical see-through augmented reality system 400 including a waveguide display according to certain embodiments. Augmented reality system 400 may include a projector 410 and a combiner 415. Projector 410 may include a light source or image source 412 and projector optics 414. In some embodiments, light source or image source 412 may include one or more micro-LED devices described above. In some embodiments, image source 412 may include a plurality of pixels that displays virtual objects, such as an LCD display panel or an LED display panel. In some embodiments, image source 412 may include a light source that generates coherent or partially coherent light. For example, image source 412 may include a laser diode, a vertical cavity surface emitting laser, an LED, and/or a micro-LED described above. In some embodiments, image source 412 may include a plurality of light sources (e.g., an array of micro-LEDs described above), each emitting a monochromatic image light corresponding to a primary color (e.g., red, green, or blue). In some embodiments, image source 412 may include three two-dimensional arrays of micro-LEDs, where each two-dimensional array of micro-LEDs may include micro-LEDs configured to emit light of a primary color (e.g., red, green, or blue). In some embodiments, image source 412 may include an optical pattern generator, such as a spatial light modulator. Projector optics 414 may include one or more optical components that can condition the light from image source 412, such as expanding, collimating, scanning, or projecting light from image source 412 to combiner 415. The one or more optical components may include, for example, one or more lenses, liquid lenses, mirrors, apertures, and/or gratings. For example, in some embodiments, image source 412 may include one or more one-dimensional arrays or elongated two-dimensional arrays of micro-LEDs, and projector optics 414 may include one or more one-dimensional scanners (e.g., micro-mirrors or prisms) configured to scan the one dimensional arrays or elongated two-dimensional arrays of micro-LEDs to generate image frames. In some embodiments, projector optics 414 may include a liquid lens (e.g., a liquid crystal lens) with a plurality of electrodes that allows scanning of the light from image source 412.

[0085] Combiner 415 may include an input coupler 430 for coupling light from projector 410 into a substrate 420 of combiner 415. Combiner 415 may transmit at least 50% of light in a first wavelength range and reflect at least 25% of light in a second wavelength range. For example, the first wavelength range may be visible light from about 400 nm to about 650 nm, and the second wavelength range may be in the infrared band, for example, from about 800 nm to about 1000 nm. Input coupler 430 may include a volume holographic grating, a diffractive optical element (DOE) (e.g., a surface-relief grating), a slanted surface of substrate 420, or a refractive coupler (e.g., a wedge or a prism). For example, input coupler 430 may include a reflective volume Bragg grating or a transmissive volume Bragg grating. Input coupler 430 may have a coupling efficiency of greater than 30%, 50%, 75%, 90%, or higher for visible light. Light coupled into substrate 420 may propagate within substrate 420 through, for example, total internal reflection (TIR). Substrate 420 may be in the form of a lens of a pair of eyeglasses. Substrate 420 may have a flat or a curved surface, and may include one or more types of dielectric materials, such as glass, quartz, plastic, polymer, poly(methyl methacrylate) (PMMA), crystal, or ceramic. A thickness of the substrate may range from, for example, less than about 1 mm to about 10 mm or more. Substrate 420 may be transparent to visible light.

[0086] Substrate 420 may include or may be coupled to a plurality of output couplers 440, each configured to extract at least a portion of the light guided by and propagating within substrate 420 from substrate 420, and direct extracted light 460 to an eyebox 495 where an eye 490 of the user of augmented reality system 400 may be located when augmented reality system 400 is in use. The plurality of output couplers 440 may replicate the exit pupil to increase the size of eyebox 495 such that the displayed image is visible in a larger area. As input coupler 430, output couplers 440 may include grating couplers (e.g., volume holographic gratings or surface-relief gratings), other diffraction optical elements, prisms, etc. For example, output couplers 440 may include reflective volume Bragg gratings or transmissive volume Bragg gratings. Output couplers 440 may have different coupling (e.g., diffraction) efficiencies at different locations. Substrate 420 may also allow light 450 from the environment in front of combiner 415 to pass through with little or no loss. Output couplers 440 may also allow light 450 to pass through with little loss. For example, in some implementations, output couplers 440 may have a very low diffraction efficiency for light 450 such that light 450 may be refracted or otherwise pass through output couplers 440 with little loss, and thus may have a higher intensity than extracted light 460. In some implementations, output couplers 440 may have a high diffraction efficiency for light 450 and may diffract light 450 in certain desired directions (i.e., diffraction angles) with little loss. As a result, the user may be able to view combined images of the environment in front of combiner 415 and images of virtual objects projected by projector 410.

[0087] In some embodiments, projector 410, input coupler 430, and output coupler 440 may be on any side of substrate 420. Input coupler 430 and output coupler 440 may be reflective gratings (also referred to as reflective gratings) or transmissive gratings (also referred to as transmissive gratings) to couple display light into or out of substrate 420.

[0088] FIG. 5 illustrates an example of an optical see-through augmented reality system 500 including a waveguide display for exit pupil expansion according to certain embodiments. Augmented reality system 500 may be similar to augmented reality system 500, and may include the waveguide display and a projector that may include a light source or image source 510 and projector optics 520. The waveguide display may include a substrate 530, an input coupler 540, and a plurality of output couplers 550 as described above with respect to augmented reality system 500. While FIG. 5 only shows the propagation of light from a single field of view, FIG. 5 shows the propagation of light from multiple fields of view.

[0089] FIG. 5 shows that the exit pupil is replicated by output couplers 550 to form an aggregated exit pupil or eyebox, where different regions in a field of view (e.g., different pixels on image source 510) may be associated with different respective propagation directions towards the eyebox, and light from a same field of view (e.g., a same pixel on image source 510) may have a same propagation direction for the different individual exit pupils. Thus, a single image of image source 510 may be formed by the user’s eye located anywhere in the eyebox, where light from different individual exit pupils and propagating in the same direction may be from a same pixel on image source 510 and may be focused onto a same location on the retina of the user’s eye. FIG. 5 shows that the image of the image source is visible by the user’s eye even if the user’s eye moves to different locations in the eyebox. [0090] FIG. 6 illustrates propagations of display light 640 and external light 630 in an example waveguide display 600 including a waveguide 610 and a grating coupler 620. Waveguide 610 may be a flat or curved transparent substrate with a refractive index m greater than the free space refractive index m (e.g., 1.0). Grating coupler 620 may be, for example, a Bragg grating or a surface-relief grating.

[0091] Display light 640 may be coupled into waveguide 610 by, for example, input coupler 430 of FIG. 4 or other couplers (e.g., a prism or slanted surface) described above. Display light 640 may propagate within waveguide 610 through, for example, total internal reflection. When display light 640 reaches grating coupler 620, display light 640 may be diffracted by grating coupler 620 into, for example, a 0^th order diffraction (i.e., reflection) light 642 and a -1st order diffraction light 644. The 0^th order diffraction may propagate within waveguide 610, and may be reflected by the bottom surface of waveguide 610 towards grating coupler 620 at a different location. The -1st order diffraction light 644 may be coupled (e.g, refracted) out of waveguide 610 towards the user’s eye, because a total internal reflection condition may not be met at the bottom surface of waveguide 610 due to the diffraction angle.

[0092] External light 630 may also be diffracted by grating coupler 620 into, for example, a 0^th order diffraction light 632 and a -1st order diffraction light 634. Both the 0^th order diffraction light 632 and the -1st order diffraction light 634 may be refracted out of waveguide 610 towards the user’s eye. Thus, grating coupler 620 may act as an input coupler for coupling external light 630 into waveguide 610, and may also act as an output coupler for coupling display light 640 out of waveguide 610. As such, grating coupler 620 may act as a combiner for combining external light 630 and display light 640. In general, the diffraction efficiency of grating coupler 620 (e.g., a surface-relief grating coupler) for external light 630 (i.e., transmissive diffraction) and the diffraction efficiency of grating coupler 620 for display light 640 (i.e., reflective diffraction) may be similar or comparable.

[0093] In order to diffract light at a desired direction towards the user’s eye and to achieve a desired diffraction efficiency for certain diffraction orders, grating coupler 620 may include a blazed or slanted grating, such as a slanted Bragg grating or surface-relief grating, where the grating ridges and grooves may be tilted relative to the surface normal of grating coupler 620 or waveguide 610.

[0094] FIG. 7 illustrates an example of a slanted grating 720 in a waveguide display 700 according to certain embodiments. Slanted grating 720 may be an example of input coupler 430, output couplers 440, or grating coupler 620. Waveguide display 700 may include slanted grating 720 on a waveguide 710, such as substrate 420 or waveguide 610. Slanted grating 720 may act as a grating coupler for couple light into or out of waveguide 710. In some embodiments, slanted grating 720 may include a one-dimensional periodic structure with a period p. For example, slanted grating 720 may include a plurality of ridges 722 and grooves 724 between ridges 722. Each period of slanted grating 720 may include a ridge 722 and a groove 724, which may be an air gap or a region filled with a material with a refractive index n 2. The ratio between the width d of a ridge 722 and the grating period p may be referred to as duty cycle. Slanted grating 720 may have a duty cycle ranging, for example, from about 10% to about 90% or greater. In some embodiments, the duty cycle may vary from period to period. In some embodiments, the period p of the slanted grating may vary from one area to another on slanted grating 720, or may vary from one period to another (i.e., chirped) on slanted grating 720.

[0095] Ridges 722 may be made of a material with a refractive index of n_gi, such as silicon containing materials (e.g., SiC , S13N4, SiC, SiO_xN_y, or amorphous silicon), organic materials (e.g, spin on carbon (SOC) or amorphous carbon layer (ACL) or diamond like carbon (DLC)), or inorganic metal oxide layers (e.g., TiOx, AlOx, TaOx, HfOx, etc.). Each ridge 722 may include a leading edge 730 with a slant angel a and a trailing edge 740 with a slant angle b. In some embodiments, leading edge 730 and trailing edge 740 of each ridge 722 may be parallel to each other. In other words, slant angle a is approximately equal to slant angle b. In some embodiments, slant angle a may be different from slant angle b. In some embodiments, slant angle a may be approximately equal to slant angle b. For example, the difference between slant angle a and slant angle b may be less than 20%, 10%, 6%, 1%, or less. In some embodiments, slant angle a and slant angle b may range from, for example, about 30° or less to about 70% or larger.

[0096] In some implementations, grooves 724 between the ridges 722 may be over-coated or filled with a material having a refractive index n_g2 higher or lower than the refractive index of the material of ridges 722. For example, in some embodiments, a high refractive index material, such as Hafinia, Titania, Tantalum oxide, Tungsten oxide, Zirconium oxide, Gallium sulfide, Gallium nitride, Gallium phosphide, silicon, and a high refractive index polymer, may be used to fill grooves 724. In some embodiments, a low refractive index material, such as silicon oxide, alumina, porous silica, or fluorinated low index monomer (or polymer), may be used to fill grooves 724. As a result, the difference between the refractive index of the ridges and the refractive index of the grooves may be greater than 0.1, 0.2, 0.3, 0.5, 1.0, or higher.

[0097] The user experience with an artificial reality system may depend on several optical characteristics of the artificial reality system, such as the field of view (FOV), image quality ( e.g ., resolution), size of the eye box of the system (to accommodate for eye and/or head movement), the distance of eye relief, optical bandwidth, and brightness of the displayed image. In general, the FOV and the eye box need to be as large as possible, the optical bandwidth needs to cover the visible band, and the brightness of the displayed image needs to be high enough (especially for optical see-through AR systems).

[0098] In a waveguide-based near-eye display, the output area of the display may be much larger than the size of the eyebox of the near-eye display system. The portion of light that may reach a user’s eyes may depend on the ratio between the size of the eyebox and the output area of the display, which, in some cases, may be less than 10% for a certain eye relief and field of view. In order to achieve a desired brightness of the displayed image perceived by user’s eyes, the display light from the projector or the light source may need to be increased significantly, which may increase the power consumption and cause some safety concerns.

[0099] FIG. 8A illustrates an example of a waveguide-based near-eye display where display light for all fields of view is substantially uniformly output from different regions of a waveguide display 810. The near-eye display may include a projector 820 and waveguide display 810. Projector 820 may be similar to projector 410 and may include a light source or image source similar to light source or image source 412 and projector optics similar to projector optics 414. Waveguide display 810 may include a waveguide (e.g., a substrate), one or more input couplers 812, and one or more output couplers 814. Input couplers 812 may be configured to couple display light from different fields of view (or viewing angles) into the waveguide, and output couplers 814 may be configured to couple display light out of the waveguide. The input and output couplers may include, for example, slanted surface-relief gratings or volume Bragg gratings. In the example shown in FIG. 8, output coupler 814 may have similar grating parameters across the full region of the output coupler other than parameters that may be varied to adjust the coupling efficiency for more uniform output light. Thus, the display light may be partially coupled out of the waveguide at different regions of waveguide display 810 in a similar manner as shown in FIG. 8 A, where display light from all fields of view of the near-eye display may be partially coupled out of the waveguide at any given region of waveguide display 810.

[0100] As also shown in FIG. 8A, the near-eye display system may have an eyebox at a certain eyebox position 890 and having a limited size and thus a limited field of view 830. As such, not all light coupled out of the waveguide in waveguide display 810 may reach the eyebox at eyebox position 890. For example, display light 832, 834, and 836 from waveguide display 810 may not reach the eyebox at eyebox position 890, and thus may not be received by the user’s eyes, which may result in significant loss of the optical power from projector 820.

[0101] In certain embodiments, an optical coupler (e.g., a slanted surface-relief grating) for a waveguide-based display may include a grating coupler that includes multiple regions (or multiple multiplexed grating), where different regions of the grating coupler may have different angular selectivity characteristics (e.g., constructive interference conditions) for the incident display light such that, at any region of the waveguide-based display, diffraction light that would not eventually reach user’s eyes may be suppressed (i.e., may not be diffracted by the grating coupler so as to be coupled into or out of the waveguide and thus may continue to propagate within the waveguide), while light that may eventually reach the user’s eyes may be diffracted by the grating coupler and be coupled into or out of the waveguide.

[0102] FIG. 8B illustrates an example of a waveguide-based near-eye display where display light may be coupled out of a waveguide display 840 at different angles in different regions of the waveguide display according to certain embodiments. Waveguide display 840 may include a waveguide (e.g., a substrate), one or more input couplers 842, and one or more output couplers 844. Input couplers 842 may be configured to couple display light from different fields of view (e.g., viewing angles) into the waveguide, and output couplers 844 may be configured to couple display light out of the waveguide. The input and output couplers may include, for example, slanted surface-relief gratings or other types of gratings or reflectors. The output couplers may have different grating parameters and thus different angular selectivity characteristics at different regions of the output couplers. Thus, at each region of the output couplers, only display light that would propagate in a certain angular range towards the eyebox at eyebox position 890 of the near-eye display may be coupled out of the waveguide, while other display light may not meet the angular selectivity condition at the region and thus may not be coupled out of the waveguide. In some embodiments, the input couplers may also have different grating parameters and thus different angular selectivity characteristics at different regions of the input couplers, and thus, at each region of an input coupler, only display light from a respective field of view may be coupled into the waveguide. As a result, most of the display light coupled into the waveguide and propagating in the waveguide can be efficiently sent to the eyebox, thus improving the power efficiency of the waveguide-based near-eye display system.

[0103] The refractive index modulation of a slanted surface-relief grating, and other parameters of the slanted surface-relief grating, such as the grating period, the slant angle, the duty cycle, the depth, and the like, may be configured to selectively diffract incident light within a certain incident angular range (e.g., FOV) and/or a certain wavelength band at certain diffraction directions (e.g., within an angular range shown by field of view 830). For example, when the refractive index modulation is large (e.g., > 0.2), a large angular bandwidth (e.g., > 10°) may be achieved at the output couplers to provide a sufficiently large eyebox for the waveguide-based near-eye display system.

[0104] FIG. 9A illustrates an example of a slanted grating 900 with variable etch depths according to certain embodiments. Slanted grating 900 may include a substrate 910 (e.g., a glass substrate) and a grating layer 920 (e.g., a dielectric or polymer layer) formed on substrate 910. A plurality of grating grooves 922 may be etched or otherwise formed (e.g., imprinted) in grating layer 920. Grating grooves 922 may have non-uniform depths, widths, and/or separations. As such, slanted grating 900 may have variable grating periods, depths, and/or duty cycles.

[0105] FIG. 9B illustrates an example of a slanted grating 905 with variable etch depths and duty cycles according to certain embodiments. In the example shown in FIG. 9B, slanted grating 905 may be etched in a dielectric layer 930, which may have a refractive index, for example, between about 1.46 and about 2.4. As illustrated, slanted grating 905 may have different etch depths and duty cycles at different regions. The grating period may also be different at the different regions. As such, different regions of slanted grating 905 may have different desire diffraction characteristics as described above with respect to, for example, FIG. 8B.

[0106] The surface-relief gratings with parameters and configurations (e.g., duty cycles, depths, or refractive index modulations) varying over the regions of the gratings described above and other surface-relief gratings (e.g., gratings used for eye-tracking) may be fabricated using many different nanofabrication techniques. The nanofabrication techniques generally include a patterning process and a post-patterning (e.g., over-coating) process. The patterning process may be used to form slanted ridges or grooves of the slanted grating. There may be many different nanofabrication techniques for forming the slanted ridges. For example, in some implementations, the slanted grating may be fabricated using lithography techniques including slanted etching. In some implementations, the slanted grating may be fabricated using nanoimprint lithography (NIL) molding techniques, where a master mold including slanted structures may be fabricated using, for example, slanted etching techniques, and may then be used to mold slanted gratings or different generations of soft stamps for nanoimprinting. The post-patterning process may be used to over-coat the slanted ridges and/or to fill the gaps between the slanted ridges with a material having a different refractive index than the slanted ridges. The post-patterning process (e.g., overcoating and planarization) may be independent from the patterning process. Thus, a same post-patterning process may be used on slanted gratings fabricated using any patterning technique.

[0107] In a NIL molding process, a substrate (e.g., a waveguide) may be coated with a NIL resin layer. The NIL resin layer may include, for example, a butyl-acrylate-based resin doped with a sol-gel precursor (e.g., titanium butoxide), a monomer containing a reactive functional group for subsequent infusion processes (such as acrylic acid), and/or high refractive index nanoparticles (e.g., titanium oxide, zirconium oxide, hafnium oxide, tungsten oxide, zinc tellurium, or gallium phosphide). In some embodiments, the NIL resin layer may include polydimethylsiloxane (PDMS) or another silicone elastomer or silicon-based organic polymer. The NIL resin layer may be deposited on the substrate by, for example, spin coating, lamination, or inkjet printing. A NIL mold with a nanostructure formed thereon may be pressed against the NIL resin layer and the substrate for molding a nanostructure in the NIL resin layer. The NIL resin layer may be cured subsequently (e.g., crosslinked) using heat and/or ultraviolet (UV) light. After the curing, the NIL mold may be detached from the NIL resin layer and the substrate. After the NIL mold is detached from the NIL resin layer and the substrate, a nanostructure (e.g., a slanted grating) that is complementary to the nanostructure in the NIL mold may be formed in the NIL resin layer on the substrate.

[0108] In some embodiments, a master NIL mold (e.g., a hard mold including a rigid material, such as Si, SiCh, S13N4, or a metal) may be fabricated first using, for example, slanted etching, micromachining, or 3-D printing. A soft stamp may be fabricated using the master NIL mold, and the soft stamp may then be used as the working stamp to fabricate the slanted grating or may be used to fabricate a next generation soft stamp. In such a process, the slanted grating structure in the master NIL mold may be similar to the slanted grating of the grating coupler for the waveguide display, and the slanted grating structure on the soft stamp may be complementary to the slanted grating structure in the master NIL mold and the slanted grating of the grating coupler for the waveguide display. Compared with a hard stamp or hard mold, a soft stamp may offer more flexibility during the molding and demolding processes.

[0109] FIGS. 10A-10D illustrate an example of a process 1000 for fabricating a soft stamp for nanoimprinting according to certain embodiments. FIG. 10A shows a master mold 1010 (e.g., a hard mold or hard stamp). Master mold 1010 may include a rigid material, such as a semiconductor substrate (e.g., Si or GaAs), an oxide (e.g., SiCh, S13N4, TiOx, AlOx, TaOx, or HfOx), or a metal plate. Master mold 1010 may be fabricated using , for example, a slanted etching process using reactive ion beams or chemically assisted reactive ion beams, a micromachining process, or a 3-D printing process. As shown in FIG. 10A, master mold 1010 may include a slanted grating 1020 that may in turn include a plurality of slanted ridges 1022 with gaps 1024 (referred to as grooves) between slanted ridges 1022.

[0110] FIG. 10B illustrates master mold 1010 coated with a soft stamp material layer 1030. Soft stamp material layer 1030 may include, for example, a resin material or a curable polymer material. In some embodiments, soft stamp material layer 1030 may include polydimethylsiloxane (PDMS) or another silicone elastomer, or a silicon-based organic polymer. In some embodiments, soft stamp material layer 1030 may include ethylene tetrafluoroethylene (ETFE), perfluoropoly ether (PFPE), or other fluorinated polymer or acrylic polymer materials. In some embodiments, soft stamp material layer 1030 may be coated on master mold 1010 by, for example, spin-coating or inkjet printing.

[0111] FIG. IOC illustrates a lamination process for laminating a soft stamp foil 1040 onto soft stamp material layer 1030. A roller 1050 may be used to press soft stamp foil 1040 against soft stamp material layer 1030. The lamination process may also be a planarization process to make the thickness of soft stamp material layer 1030 substantially uniform. After the lamination process, soft stamp foil 1040 may be tightly or securely attached to soft stamp material layer 1030.

[0112] FIG. 10D illustrates a delamination process, where a soft stamp including soft stamp foil 1040 and attached soft stamp material layer 1030 is detached from master mold 1010. Soft stamp material layer 1030 may include a slanted grating structure that is complementary to the slanted grating structure on master mold 1010. Because the flexibility of soft stamp foil 1040 and attached soft stamp material layer 1030, the delamination process may be relatively easy compared with a demolding process using a hard stamp or mold. In some embodiments, a roller (e.g., roller 1050) may be used in the delamination process to ensure a constant or controlled delamination speed. In some embodiments, roller 1050 may not be used during the delamination. In some implementations, an anti-sticking layer may be formed on master mold 1010 before soft stamp material layer 1030 is coated on master mold 1010. The anti-sticking layer may facilitate the delamination process. After the delamination of the soft stamp from master mold 1010, the soft stamp may be used to mold the slanted grating on a waveguide of a waveguide display.

[0113] FIGS. 11A-11D illustrate an example of an imprinting process 1100 using a soft stamp according to certain embodiments. FIG. 11A shows a waveguide 1110 coated with an imprint resin layer 1120. Imprint resin layer 1120 may include, for example, a butyl-acrylate based resin doped with a sol-gel precursor (e.g., titanium butoxide), a monomer containing a reactive functional group for subsequent infusion processes (such as acrylic acid), and/or high refractive index nanoparticles (e.g., TiC , NbOx, ZrCh, GaP, HfCh, WO3, GaAs, etc.). In some embodiments, imprint resin layer 1120 may include polydimethylsiloxane (PDMS) or another silicone elastomer, or silicon-based organic polymer. In some embodiments, imprint resin layer 1120 may include ethylene tetrafluoroethylene (ETFE), perfluoropoly ether (PFPE), or other fluorinated polymer or acrylic polymer materials. Imprint resin layer 1120 may be deposited on waveguide 1110 by, for example, spin-coating, lamination, or inkjet printing. A soft stamp 1130 including slanted ridges 1132 attached to a soft stamp foil 1140 may be used for the imprint.

[0114] FIG. 11B shows the lamination of soft stamp 1130 onto imprint resin layer 1120. Soft stamp 1130 may be pressed against imprint resin layer 1120 and waveguide 1110 using a roller 1150, such that slanted ridges 1132 may be pressed into imprint resin layer 1120. Imprint resin layer 1120 may be cured subsequently. For example, imprint resin layer 1120 may be crosslinked using heat and/or ultraviolet (UV) light.

[0115] FIG. llC shows the delamination of soft stamp 1130 from imprint resin layer 1120. The delamination may be performed by lifting soft stamp foil 1140 to detach slanted ridges 1132 of soft stamp 1130 from imprint resin layer 1120. Imprint resin layer 1120 may now include a slanted grating 1122, which may be used as the grating coupler or may be over coated to form the grating coupler for the waveguide display. As described above, because of the flexibility of soft stamp 1130, the delamination process may be relatively easy compared with a demolding process using a hard stamp or mold. In some embodiments, a roller (e.g., roller 1150) may be used in the delamination process to ensure a constant or controlled delamination speed. In some embodiments, roller 1150 may not be used during the delamination.

[0116] FIG. 11D shows an example imprinted slanted grating 1122 formed on waveguide 1110 using soft stamp 1130. As described above, slanted grating 1122 may include ridges and grooves between the ridges and thus may be over-coated with a material having a refractive index different from imprint resin layer 1120 to fill the grooves and form the grating coupler for the waveguide display.

[0117] The period of the slanted grating may vary from one area to another on slanted grating 1122, or may vary from one period to another (i.e., chirped) on slanted grating 1122. Slanted grating 1122 may have a duty cycle ranging, for example, from about 11% to about 100% or greater. In some embodiments, the duty cycle may vary from period to period. In some embodiments, the depth of the grooves or height of the ridges of slanted grating 1122 may be greater than about 50 nm, about 110 nm, about 200 nm, about 300 nm, about 400 nm, or higher. The slant angles of the leading edges of the ridges of slanted grating 1122 and the slant angles of the trailing edges of the ridges of slanted grating 1122 may be greater than about 30°, about 45°, about 60°, or higher. In some embodiments, the leading edge and training edge of each ridge of slanted grating 1122 may be parallel to each other. In some embodiments, the difference between the slant angle of the leading edge of a ridge of slanted grating 1122 and the slant angle of the trailing edge of the ridge of slanted grating 1122 may be less than about 20%, about 11%, about 5%, about 1%, or less.

[0118] Different generations of NIL stamps may be made and used as the working stamp to mold the slanted gratings. For example, in some embodiments, a master mold (which may be referred to as a generation 0 mold) may be fabricated (e.g., etched) in, for example, a semiconductor substrate, a quartz, or a metal plate. The master mold may be a hard stamp and may be used as the working stamp to mold the slanted grating directly, which may be referred to as hard stamp NIL or hard NIL. In such case, the slanted structure on the mold may be complimentary to the desired slanted structure of the slanted grating used as the grating coupler on a waveguide display.

[0119] In order to protect the master NIL mold, the master NIL mold may be fabricated first, and a hybrid stamp (which may be referred to as generation 1 mold or stamp) may then be fabricated using the master NIL mold. The hybrid stamp may be used as the working stamp for nanoimprinting. The hybrid stamp may include a hard stamp, a soft stamp, or a hard-soft stamp. Nanoimprinting using a soft stamp may be referred to as soft stamp NIL or soft NIL. In some embodiments, the hybrid mold may include a plastic backplane with soft or hard patterned polymer (e.g., having a Young’s modulus about 1 GPa). In some embodiments, the hybrid mold may include a glass backplane with soft or hard patterned polymer (e.g., having a Young’s modulus about 1 GPa). In some embodiments, the hybrid mold may include a glass/plastic laminated backplane with soft or hard patterned polymer. In some embodiments, a generation 2 hybrid mold may be made from the generation 1 mold, and may then be used as the working stamp for the nanoimprinting. In some embodiments, generation 3 hybrid molds, generation 4 hybrid molds, and the like, may be made and used as the working stamp. NIL molding may significantly reduce the cost of making the slanted surface-relief structures because the molding process may be much faster than the etching process and no expensive reactive ion etching equipment may be needed.

[0120] FIGS. 12A-12C illustrate an example of a method of forming a planarized overcoat layer on a surface-relief grating using an imprint process according to certain embodiments. FIG. 12A shows that an overcoat material as described above may be dispensed on a surface- relief grating 1220 on a substrate 1210 to form an overcoat layer 1230. The overcoat material may be dispensed by, for example, by spin-coating or inkjet printing. The amount of overcoat material dispensed on surface-relief grating 1220 may be determined based on the dimensions of the grating grooves and the desired thickness of the overburden (e.g., the portion of the overcoat layer on top of the grating ridges). For example, as described above, it may be desirable that the overburden thickness is less than about 20 nm. After dispensing the overcoat material, the top surface of overcoat layer 1230 may not be flat as shown in the illustrated example. For example, the height of the top surface at the grating ridge regions may be greater than the height of the top surface at the grating groove regions.

[0121] In some embodiments, the overcoat layer may optionally be baked to remove the solvent from the overcoat layer. Air trapping (e.g., air bubbles) may occur in the grating grooves when spin-coating techniques are employed. The solvent in the overcoat material may have not completely evaporated. The solvent and/or trapped air may result in a varying refractive index of the overcoat layer and an efficiency loss. The problems may be exacerbated when the grating grooves are relatively deep (e.g., 120 nm, 200 nm, 300 nm, or greater), when the grating grooves are relatively narrow (e.g., gratings with large duty cycles), and/or when the slant angles of the grating ridges are relatively large, such as greater than about 30°, 45°, 50°, 70°, or larger. Baking may help to remove the solvent and the trapped air such that the overcoat material in the overcoat layer may be more uniform or homogeneous.

[0122] FIG. 12B shows that a planar imprint stamp 1240 (e.g., a soft stamp or mold described above) may be laminated or otherwise applied on overcoat layer 1230 in an imprint process. As described above, in some embodiments, planar imprint stamp 1240 may include polydimethylsiloxane (PDMS) or another silicone elastomer or silicon-based organic polymer. In some embodiments, planar imprint stamp 1240 may include ethylene tetrafluoroethylene (ETFE), perfluoropoly ether (PFPE), or other fluorinated polymer or acrylic polymer materials. The bottom surface of planar imprint stamp 1240 that contacts overcoat layer 1230 may have a low surface roughness. Planar imprint stamp 1240 may be pressed against overcoat layer 1230, for example, using a roller as described above with respect to, for example, FIG. 12B. Thus, some portions of the overcoat materials on top of the grating ridges may be forced to the regions on top of the grating grooves, and the flat bottom surface of planar imprint stamp 1240 may be transferred to the top surface of overcoat layer 1230.

[0123] The overcoat layer may then be cured (e.g., using UV light) to crosslink and fix the base resin in the overcoat material as described above. After the curing and the crosslinking of the overcoat material, the planar imprint stamp may be delaminated or otherwise detached from the overcoat layer and the surface-relief grating, as described above with respect to, for example, FIG. 12C.

[0124] FIG. 12C shows that, after the delamination of planar imprint stamp 1240, the top surface of overcoat layer 1230 can be flat and smooth. In addition, the overburden can be very thin, such as less than about 20 nm.

[0125] FIGS. 13A-13B illustrate an example of a method of forming a planarized overcoat layer on a surface-relief grating 1300 having variable grating parameters according to certain embodiments. FIG. 13A shows surface-relief grating 1300 including grating ridges 1320 on a substrate 1310 (or a grating material layer or waveguide), where the duty cycle and slant angle of surface-relief grating 1300 may vary from region to region. Inkjet printing or 3-D printing techniques may be used to dispense different amounts of overcoat material 1330 in different regions of surface-relief grating 1300. For example, a larger number of small drops of overcoat material 1330 may be dispensed in regions where the dimensions of the grating grooves are larger, and a smaller number of small drops of overcoat material 1330 may be deposited in regions where the dimensions of the grating grooves are smaller. The amount of overcoat material dispensed may be more precisely controlled by dispensing small drops of overcoat material 1330.

[0126] FIG. 13B shows that a planar imprint stamp 1340 may be laminated or otherwise applied on overcoat material 1330, for example, after an optional baking process to remove solvent and/or trapped air. Planar imprint stamp 1340 may be pressed against overcoat material 1330, for example, using a roller as described above with respect to, for example, FIG. IOC. Thus, some portions of the overcoat material 1330 on top of grating ridges 1320 may be forced to the regions on top of the grating grooves, and the flat bottom surface of planar imprint stamp 1340 may be transferred to the top surface of the overcoat layer formed by overcoat material 1330. Overcoat material 1330 may then be cured (e.g., using UV light) to crosslink and fix the base resin as described above. After the curing and the crosslinking of overcoat material 1330, planar imprint stamp 1340 may be delaminated or otherwise detached from the overcoat material layer and the surface-relief grating, as described above with respect to, for example, FIG. IOC. In this way, the top surface of the overcoat layer may be flat. Because the amount of overcoat material dispensed can be more precisely controlled, the thickness of the overburden can be more precisely controlled, and can be very thin, such as less than about 20 nm, less than about 10 nm, or less.

[0127] In general, it is desirable to have a large refractive index contrast between the grating ridges and the overcoat layer to improve, for example, the light coupling efficiency, the field-of-view range, and the wavelength range of the near-eye display system. In some surface-relief gratings, the grating ridges may be formed in a high-refractive index material layer (e.g., including a resin with high-refractive index nanoparticles) using nanoimprinting techniques described above with respect to, for example, FIGS. 10A-11D, and an overcoat layer with a low refractive index may be used to achieve the large refractive index contrast.

In some surface-relief gratings, the grating ridges may be made of a low-refractive index material (e.g., glass), and an overcoat layer with a high refractive index (e.g., about 2.0 or larger) may be used to achieve the large refractive index contrast. The high-refractive index material for the grating ridges or the overcoat layer may include a resin dispersed with high- refractive index nanoparticles, such as TiCh nanoparticles, ZrCh nanoparticles, NbOx nanoparticles, and the like, or may include a sol-gel material that includes a solution and a high-refractive index oxide precursor (e.g., Ti precursor) and has been partially or fully condensed into an extended network.

[0128] TiCh has a higher refractive index (e.g., greater than 2.5 for visible light) than ZrCh (e.g., about 2.15 for visible light) and some other materials (e.g., metal oxides such as NbOx, LaOx, TaOx, AI2O3, etc.), and thus may be mixed with a resin material to form a high refractive index resin (e.g., with a refractive index about 2.0 or higher) for nanoimprinting or overcoating. For example, a 200-nm-thick resin layer including 90 wt. % of ZrCh nanoparticles may have a refractive index about 1.78 for 520-nm light, while a 200-nm-thick resin layer including 90 wt. % of TiCh nanoparticles may have a refractive index about 2.05 for 520-nm light. However, due to its bandgap energy (e.g., about 3.0 to 3.2 eV), TiCh may absorb UV light (e.g., with a wavelength l < 388 nm) to excite electrons (e ) to the conduction band and leave holes (h⁺) in the valence band. The electrons and holes may migrate to the surface of the nanoparticles to reduce and/or oxidize reactants adjacent to the nanoparticles.

[0129] FIG. 14A illustrates an example of the photocatalytic activity of TiCh nanoparticles. As illustrated, upon exposure to UV light, electrons in TiCh particles may be excited to the conduction band, to generate free electrons and holes. Some free electrons and holes may recombine within a short time period to release energy in the form of heat or photons. Some electrons and holes may migrate to the surfaces of the TiCh nanoparticles. The photo- produced holes may generate hydroxyl radicals ·OH by the oxidation of OH and/or H2O molecules that may be absorbed onto the surfaces of TiCh nanoparticles. The photo-produced electrons may facilitate the reduction of O2 molecules absorbed onto the surfaces of TiCh nanoparticles to form superoxide radical anions ^«0^ . These photo-generated hydroxyl radicals and superoxide radicals may in turn oxidize and degrade adjacent organic and/or inorganic materials. These reduction and oxidation reactions shown in FIG. 14A may be used in photocatalytic hydrogen production and photocatalytic water/air purification, respectively. Other metal oxides, such as NbOx, which may have a refractive index between about 2.3 and about 2.35, may also be photoactive.

[0130] FIG. 14B illustrates an example of organic degradation caused by the photocatalytic activity of TiCh nanoparticles 1410. As described above, due to the photoactivity of titanium oxide, TiCh nanoparticles 1410 may interact with moisture (H2O) and/or oxygen (O2) to generate radicals (e.g., hydroxyl radicals and superoxide radical anions). The hydroxyl radicals may oxidize and degrade organics 1420 (e.g., ligands and resin) adjacent to TiCh nanoparticles 1410. These interaction may be triggered via titanium oxide photoexcitation and/or may be thermally induced. TiCh nanoparticles 1410, due to their high surface-volume ratios, may have high light absorption rate, which may increase the surface photo-induced carrier density that can lead to higher surface photoactivity and enhanced photocatalytic activity of TiCh nanoparticles. As a result, more organics 1420 may be oxidized and degraded to absorb visible light. Therefore, a film including titanium oxide nanoparticles 1410 and organics 1420 may show some levels of light absorption in the visible spectrum even though TiCh nanoparticles 1410 are transparent to visible light. The absorption of visible light by the film including titanium oxide nanoparticles and organics may increase over time, as the film undergoes further redox interactions with the environment, such as upon further exposure to UV and heat. Therefore, surface-relief gratings or overcoat layers made (e.g., imprinted) using films including titanium oxide nanoparticles and organics may lead to high absorption and low throughput for waveguide displays.

[0131] FIGS. 15A-15D illustrate an example of a process of fabricating a surface-relief grating that includes overcoat layers with metal oxide nanoparticles and organics. FIG. 15A illustrates an example of a substrate 1510 including one or more surface-relief gratings 1512 formed in or on substrate 1510. In some embodiments, surface-relief gratings 1512 may be fabricated by etching substrate 1510 using wet or dry etching techniques, such as ion beam etching. In some embodiments, surface-relief gratings 1512 may be fabricated by depositing a nanoimprint resin layer and imprinting the nanoimprint resin layer using a mold or a soft stamp as described above. In some embodiments, the nanoimprint resin layer may include high-refractive index metal oxide nanoparticles (e.g., TiC nanoparticles, NbOx nanoparticles, and/or ZrCh nanoparticles) dispersed in a resin. The resin may include, for example, acrylate resin with photo radical generator or thermal radical generator. The refractive index of the acrylate resin may be between about 1.55 and about 1.7. In one example, the resin may include difunctional high-refractive index monomers, such as Ml from TOKYO OHKA KOGYO CO., LTD. (TOK).

[0132] FIG. 15B illustrates an example of a first overcoat layer 1520 formed on a surface- relief grating 1512 on one side of substrate 1510. In some embodiments, first overcoat layer 1520 may be formed on surface-relief grating 1512 using techniques disclosed above with respect to, for example, FIGS. 12A-13B. In some embodiments, first overcoat layer 1520 may be a nanoimprint resin layer that includes high-refractive index metal oxide nanoparticles (e.g., TiCh nanoparticles, NbOx nanoparticles, and/or ZrCh nanoparticles) dispersed in a resin. The resin may include, for example, acrylate resin with photo radical generator or thermal radical generator. The refractive index of the acrylate resin may be between about 1.55 and about 1.7. In one example, the resin may include difunctional high- refractive index monomers, such as Ml from TOK. First overcoat layer 1520 may be deposited on surface-relief grating 1512, baked at an elevated temperature (e.g., about 90 °C), and cured using UV light 1505. In some embodiments, before the curing using UV light 1505, the nanoimprint resin layer may be imprinted using a flat stamp as described above with respect to, for example, FIGS. 12A-13B. Curing first overcoat layer 1520 using UV light may degrade the organics in first overcoat layer 1520 due to the photocatalytic activity of the metal oxide (e.g., TiCh or NbOx) nanoparticles, as described above with respect to, for example, FIGS. 14A and 14B.

[0133] FIG. 15C shows that a second overcoat layer 1530 may be formed on a surface- relief grating 1512 on a second side of substrate 1510. Second overcoat layer 1530 may be formed on surface-relief grating 1512 using techniques disclosed above with respect to, for example, FIGS. 12A-13B. In some embodiments, second overcoat layer 1530 may be a nanoimprint resin layer that includes high-refractive index metal oxide nanoparticles dispersed in a resin as described above with respect to first overcoat layer 1520, where second overcoat layer 1530 may be deposited on surface-relief grating 1512, baked at an elevated temperature (e.g., about 90 °C), and then cured using UV light 1505. In some embodiments, before the curing using UV light 1505, the nanoimprint resin layer may be imprinted using a flat stamp, as described above with respect to, for example, FIGS. 12A-13B and 15B. Curing second overcoat layer 1530 using UV light 1505 may degrade the organics in second overcoat layer 1530 due to the photocatalytic activity of the metal oxide (e.g., TiCh or NbOx) nanoparticles. Curing second overcoat layer 1530 using UV light 1505 may further expose first overcoat layer 1520 to UV light 1505, and thus may further degrade the organics in first overcoat layer 1520 due to the photocatalytic activity of the metal oxide (e.g., TiCh or NbOx) nanoparticles exposed to UV light.

[0134] FIG. 15D shows that antireflective coating layers 1540 may be coated on first overcoat layer 1520 and second overcoat layer 1530 to form a waveguide display 1500. Antireflective coating layers 1540 may be used to reduce Fresnel reflection at the interfaces between waveguide display 1500 and air. In some embodiments, antireflective coating layers 1540 may be baked and cured using UV light 1505. Curing antireflective coating layers 1540 using UV light 1505 may further expose first overcoat layer 1520 and second overcoat layer 1530 to UV light 1505, and thus may further degrade the organics in first overcoat layer 1520 and second overcoat layer 1530 due to the photocatalytic activity of the metal oxide (e.g., TiCh or NbOx) nanoparticles.

[0135] FIG. 15E illustrates changes in the optical losses of examples of waveguide displays including surface-relief gratings having different metal oxide nanoparticles and organics during the fabrication processes. As described above, the metal oxide nanoparticles may be photocatalysts that can cause photoactivities upon exposure to light of certain wavelengths. For example, the bandgap energy of TiCh may be about 3.0 eV to about 3.2 eV, and may absorb UV light (e.g., with a wavelength l < 388 nm) to generate free electron-hole pairs that may in turn generate radicals that can react with and degrade adjacent organic or inorganic materials to reduce the transmissivity of the waveguide display. In contrast, the bandgap energy of ZrCh may be about 5.0 eV, much higher than that of TiCh, and thus may only absorb deep UV light. Therefore, a resin layer including ZrCh nanoparticles may have a lower level of degradation of the organic materials after UV curing and thermal processing, and thus may have a lower optical loss for visible light.

[0136] In one example, the material used to form first overcoat layer 1520 and second overcoat layer 1530 may be a mixture including Ml from TOK, ZrCh nanoparticles, and a solvent, where the ZrCh nanoparticles may be about 90 wt. % of the mixture, and the refractive index of first overcoat layer 1520 and second overcoat layer 1530 may be about 1.78. A bar 1552 in FIG. 15E shows the optical loss of an example of the structure shown in FIG. 15B after first overcoat layer 1520 is baked at 90 °C and before first overcoat layer 1520 is cured using UV light, where first overcoat layer 1520 may include ZrCh nanoparticles and the thickness of first overcoat layer 1520 may be about 140 nm. A bar 1554 shows the optical loss of the example of the structure shown in FIG. 15B after first overcoat layer 1520 is cured using UV light. A bar 1556 shows the optical loss of an example of the structure shown in FIG. 15C after second overcoat layer 1530 is baked at 90 °C and is cured using UV light, where second overcoat layer 1530 may include ZrCh nanoparticles and the thickness of second overcoat layer 1530 may be about 140 nm. A bar 1558 shows the optical loss of waveguide display 1500 shown in FIG. 15D after antireflective coating layers 1540 are baked at 90 °C and are cured using UV light. Thus, the optical losses of the overcoat layers including organics and include ZrCh nanoparticles may increase slightly during the fabrication processes.

[0137] In another example, the material used to form first overcoat layer 1520 and second overcoat layer 1530 may be a mixture including Ml from TOK, TiCh nanoparticles, and a solvent, where the TiCh nanoparticles may be about 90 wt. % of the mixture, and the refractive index of first overcoat layer 1520 and second overcoat layer 1530 may be about 2.05. A bar 1562 in FIG. 15E shows the optical loss of an example of the structure shown in FIG. 15B after first overcoat layer 1520 is baked at 90 °C and before first overcoat layer 1520 is cured using UV light, where first overcoat layer 1520 may include TiCh nanoparticles and the thickness of first overcoat layer 1520 may be about 140 nm. A bar 1564 shows the optical loss of the example of the structure shown in FIG. 15B after first overcoat layer 1520 is cured using UV light. A bar 1566 shows the optical loss of an example of the structure shown in FIG. 15C after second overcoat layer 1530 is baked at 90 °C and is cured using UV light, where second overcoat layer 1530 may include TiCh nanoparticles and the thickness of second overcoat layer 1530 may be about 140 nm. A bar 1568 shows the optical loss of an example of waveguide display 1500 shown in FIG. 15D after antireflective coating layers 1540 are baked at 90 °C and are cured using UV light. FIG. 15E shows that the optical loss of first overcoat layer 1520 and second overcoat layer 1530 may increase significantly during the fabrication processes, and can be very high when TiCh nanoparticles are used in the overcoat layers. Thus, it can be difficult to achieve both a high refractive index and a low optical loss for an overcoat layer.

[0138] According to certain embodiments, organics in an imprinted grating layer or an overcoat layer, such as organic ligands on metal oxide (e.g., TiCh or NbCh) nanoparticles that may have been degraded (oxidized) or may not have been degraded, may be removed and/or replaced with another material, such as an inorganic material (e.g., Si-containing material) or another organic material that may have a low optical loss and may not be degraded by the photocatalytic activity of the metal oxide nanoparticles. For example, the organic ligands on metal oxide nanoparticles in a grating layer or an overcoat layer may be removed (e.g., burned) by a high-energy treatment process, such as rapid thermal annealing (RTA) or laser spike annealing (LSA), to form porous structures in the grating layer or the overcoat layer.

An organic or inorganic material layer (e.g., an overcoat layer or an antireflective coating layer) may then be formed on the grating layer or the overcoat layer to fill the porous structures in the grating layer or the overcoat layer, thereby increasing the refractive index while reducing the absorption loss of the grating layer or the overcoat layer. For example, the imprinted surface-relief grating or the overcoat layer of the surface-relief grating may be characterized by a first refractive index greater than 2.0 for visible light and an absorption lower than 0.05% for visible light.

[0139] According to certain embodiments, a method may include depositing a nanoimprint material layer (including nanoparticles dispersed in a resin material) on a substrate, imprinting a surface-relief grating in the nanoimprint material layer using a stamp, curing the nanoimprint material layer by heat or electromagnetic radiation, delaminating the stamp, performing an RTA process or an LSA process on the imprinted surface-relief grating to remove light-absorbing organics and form porous structures in the imprinted surface-relief grating, and depositing an overcoat layer that includes an organic material, an inorganic material, or a combination thereof on the imprinted surface-relief grating to fill the porous structures in the imprinted surface-relief grating and increase the refractive index of the grating ridges of the imprinted surface-relief grating.

[0140] According to certain embodiments, a method may include depositing an overcoat layer including a sol-gel material or nanoparticles dispersed in a resin material on a surface- relief grating to at least partially fill grating grooves of the surface-relief grating, optionally imprinting the overcoat layer using a planar imprint stamp, curing the overcoat layer by heat or electromagnetic radiation, performing an RTA process or an LSA process on the overcoat layer to remove light-absorbing organics and form porous structures in the overcoat layer, and depositing an antireflective coating layer that may include an organic material, an inorganic material, or a combination thereof on the overcoat layer to fill the porous structures in the overcoat layer and increase the refractive index of the overcoat layer.

[0141] In some embodiments, the nanoimprint material layer or the overcoat layer may include nanoparticle-based formulation or sol-gel-based formulation. In some embodiments, the nanoparticle-based formulation may include acrylate resin (with a refractive index between about 1.55 and about 1.7) with photo radical generator or thermal radical generator, 75%-90% TiOx, ZrOx, and/or NbOx nanoparticles with acrylate and non-functional decoration ligands, and a solvent. The solvent may include, for example, propylene glycol methyl ether acetate (PGMEA) (for spin coating) or dipropylene glycol methyl ether (DPGME)/ tripropylene glycol monomethyl ether (TPM) for inkjet coating. In some embodiments, the solvent may include a solvent blend including one or more solvents that may or may not have boiling points greater than 200°C. In some embodiments, the sol-gel-based formulation may include EMD MHM-63, Philips high refractive index sol-gel formulation, or organic solvent- soluble Titania with ligands in a solvent, such as PGMEA or DPGME/TPM.

[0142] In some embodiments, the organic and/or inorganic material that back-fills the porous structures in the imprinted surface-relief grating or the overcoat layer may have low optical loss, and may include, for example, organic materials such as acrylate, polystyrenics, epoxy based organic materials, siloxane based materials, inorganic materials such as AI2O3, S1O2, ZrCh, M>2q5, TiCh, Ta20s, and the like, or a combination of organic materials and inorganic materials describe above. In some aspects, depositing the first layer of the organic material, the inorganic material, or a combination thereof on the imprinted surface-relief grating or the overcoat layer may include an inkjet coating process, a spin coating process, an atomic layer deposition (ALD) process, a physical vapor deposition (PVD) process, an initiated chemical vapor deposition (iCVD) process, or a combination thereof. In some embodiments, the ALD process may use ALD precursors such as TiCU and H2O, tetrakis (dimethylamino) titanium(IV) (TDMAT) and O3, tetrakis (dimethylamido) zirconium (IV) (TDMAZ) / trimethylaluminum (TMA) and H2O, or the like.

[0143] FIGS. 16A-16E illustrate an example of a process of fabricating a surface-relief grating including a high-refractive index and low-loss overcoat layer according to certain embodiments. FIG. 16A illustrates an example of a substrate 1610 including one or more surface-relief gratings 1612 formed in or on substrate 1610. Surface-relief gratings 1612 may be fabricated by etching substrate 1610 using wet or dry etching techniques, such as ion beam etching, or may be fabricated by depositing a nanoimprint resin layer and imprinting the nanoimprint resin layer using a mold or a soft stamp as described above.

[0144] FIG. 16B illustrates an example of a first overcoat layer 1620 formed on a surface- relief grating 1612 on one side of substrate 1610. In some embodiments, first overcoat layer 1620 may be a nanoimprint resin layer that includes high-refractive index metal oxide nanoparticles (e.g., T1O2 nanoparticles, NbOx nanoparticles, and/or ZrCh nanoparticles) dispersed in a resin and a solvent. The resin may include, for example, acrylate resin with photo radical generator or thermal radical generator. The refractive index of the acrylate resin may be between about 1.55 and about 1.7. In one example, the resin may include difunctional high-refractive index monomers, such as Ml from TOK. In some embodiments, the nanoparticles may be about 75-90 wt. % of the nanoimprint resin layer, and may include TiOx, ZrOx, and/or NbOx nanoparticles with acrylate and non-functional decoration ligands. The solvent may include, for example, PGMEA (for spin coating) or DPGME/TPM (for inkjet coating). In some embodiments, the solvent may include a solvent blend with one or more solvents that may or may not have boiling points greater than 200°C. In some embodiments, first overcoat layer 1620 may include a sol-gel material that includes a solution and a high-refractive index oxide precursor (e.g., Ti precursor) and has been partially or fully condensed into an extended network. In some embodiments, the sol-gel material may include EMD MHM-63, Philips high refractive index sol-gel formulation, or organic solvent-soluble Titania with ligands in a solvent, such as PGMEA or DPGME/TPM.

[0145] In some embodiments, first overcoat layer 1620 may be formed on the surface-relief grating 1612 using techniques disclosed above, for example, with respect to FIGS. 12A-13B. In one example, first overcoat layer 1620 may be deposited on surface-relief grating 1612, baked at an elevated temperature (e.g., about 90 °C), and cured using UV light 1605. In some embodiments, before the curing using UV light 1605, the nanoimprint resin layer may be imprinted using a flat stamp as described above to form first overcoat layer 1620 with a flat top surface. Curing first overcoat layer 1620 using UV light may degrade the organics (e.g., ligands on the nanoparticles) in first overcoat layer 1620 due to the photocatalytic activity of the metal oxide (e.g., TiCh or NbOx) nanoparticles described above with respect to, for example, FIGS. 14A and 14B.

[0146] FIG. 16C shows that a second overcoat layer 1630 may be formed on a surface- relief grating 1612 on a second side of substrate 1610. In some embodiments, second overcoat layer 1630 may be formed on surface-relief gratings 1612 using materials and techniques disclosed above, for example, with respect to FIGS. 12A-13B and 16B. In some embodiments, second overcoat layer 1630 may be deposited on surface-relief grating 1612, baked at an elevated temperature (e.g., about 90 °C), and then cured using UV light 1605. In some embodiments, before the curing using UV light 1605, the nanoimprint resin layer may be imprinted using a flat stamp to form a flat top surface on second overcoat layer 1630. Curing second overcoat layer 1630 using UV light may degrade the organics in second overcoat layer 1630 due to the photocatalytic activity of the metal oxide (e.g., TiCh or NbOx) nanoparticles. Curing second overcoat layer 1630 may further expose first overcoat layer 1620 to UV light 1605, and thus may further degrade the organics in first overcoat layer 1620 due to the photocatalytic activity of the metal oxide nanoparticles.

[0147] FIG. 16D shows that organic materials in the first overcoat layer 1620 and second overcoat layer 1630, including organics (e.g., organic ligands on nanoparticles) degraded during the fabrication processes, may be removed (e.g., burned) by a high-energy treatment process, such as rapid thermal annealing (RTA), laser spike annealing (LSA), or UV irradiation, to form porous structures 1622 and 1632 that may include metal oxide nanoparticles and pores between the metal oxide nanoparticles. For example, rapid thermal annealing processes may rapidly heat samples to high temperatures to oxidize the organics on a timescale of a few seconds to about 30 minutes. In one example, the samples may be heated to or cooled down from a temperature between about 200 °C and about 1300 °C, with a ramp rate about 20-200 °C/sec, combined with gas ambient control. The short treatment may activate catalytic reactions to quickly remove the organic materials. The rapid heating rate can be achieved by, for example, high intensity lamps (e.g. near-infrared light sources, such as tungsten-halogen lamps) controlled by pyrometer and thermocouples that measure the sample temperature. Cooling may also be controlled to prevent dislocations and sample breakage. LSA processes may use a narrow laser beam to heat the sample from a low temperature to a peak annealing temperature to bum the organic materials. After the removal of the organic materials, porous structures 1622 and 1632 may have lower refractive indices than first overcoat layer 1620 and second overcoat layer 1630.

[0148] FIG. 16E shows that antireflective coating layers 1640 may be coated on first overcoat layer 1620 and second overcoat layer 1630 to form a waveguide display. Antireflective coating layers 1640 may be used to reduce Fresnel reflection at the interfaces between the waveguide display and air. In some embodiments, antireflective coating layers 1640 may be baked and cured using UV light 1605. Antireflective coating layers 1640 may include organic and/or inorganic materials, and may fill the pores in porous structures 1622 and 1632 to form filled overcoat layer 1624 and filled overcoat layer 1634 that may have a higher refractive index (due to the back fill) and a lower absorption loss (due to the removal of the degraded organics and other organics that may be degraded upon exposure to UV light or heat). The organic and/or inorganic materials that fill the pores in porous structures 1622 and 1632 may have low optical loss, and may include, for example, organic materials such as acrylate, polystyrenics, epoxy based organic materials, siloxane based materials, inorganic materials such as AI2O3, S1O2, ZrCh, Nb20s, T1O2, Ta20s, and the like, or a combination of organic materials and inorganic materials describe above. In some aspects, depositing antireflective coating layer 1640 may include an inkjet coating process, a spin coating process, ALD process, a PVD process, an iCVD process, or a combination thereof. In some embodiments, the ALD process may use ALD precursors, such as TiCL and H2O, TDMAT and O3, TDMAZ/TMA and H2O, or the like.

[0149] FIG. 17A illustrates an example of a device 1700 including a back-filled nanoparticle layer 1720 according to certain embodiments. In the illustrated example, device 1700 may include a substrate 1710, back-filled nanoparticle layer 1720, and an antireflective coating layer 1730. Back-filled nanoparticle layer 1720 may be formed by, for example, depositing a resin layer including T1O2 nanoparticles as described above, curing the resin layer, treating the resin layer using RTA (e.g., at about 600 °C) to remove organic materials and form nanoporous structures, and depositing antireflective coating layer 1730 on the nanoporous structures such that materials (e.g., a siloxane based material) in antireflective coating layer 1730 may fill the nanoporous structures in the resin layer to form back-filled nanoparticle layer 1720 that may have a high refractive index and a low optical absorption. In the illustrated example, antireflective coating layer 1730 may have a thickness about 100 nm and a refractive index about 1.5, and back-filled nanoparticle layer 1720 may have a thickness about 150 nm and a refractive index about 2.02. The optical loss of device 1700 for visible light is measured to be about 0.13%.

[0150] FIG. 17B illustrates an example of a surface-relief grating 1705 including a back filled high-refractive index overcoat layer 1760 according to certain embodiments. Surface- relief grating 1705 may include a substrate 1740, a grating layer 1750, overcoat layer 1760, and an antireflective coating layer 1770. Grating layer 1750 may include surface-relief structures formed therein. Overcoat layer 1760 may fill grooves in grating layer 1750, and may be formed by processes described above with respect to, for example, FIGS. 16A-16E.

In the illustrated example, antireflective coating layer 1770 may have a thickness about 50 nm and may include AI2O3 with a refractive index about 1.6. Antireflective coating layer 1770 may be deposited using, for example, ALD or SIS processes described above. The deposited AI2O3 may fill nanoporous structures in overcoat layer 1760. The back-filled overcoat layer 1760 may have a thickness about 180 nm and a refractive index between about 2.07 and about 2.11. The optical loss of surface-relief grating 1705 for visible light is measured to be about 0.06%.

[0151] FIG. 18A illustrates an X-ray photoelectron spectroscopy (XPS) profile of an example of a structure (e.g., the structure shown in FIG. 16D) including a substrate and an overcoat layer with organics removed by RTA at about 600 °C according to certain embodiments. In the illustrated example, the overcoat layer has a thickness about 135 nm, and is made using a resin material that includes organics and T1O2 nanoparticles as described above. Curves 1810, 1820, 1830, and 1840 in FIG. 18A show the atomic concentrations of carbon, oxygen, silicon, and titanium, respectively, in the substrate and the overcoat layer after the RTA. As shown by curve 1810, in the overcoat layer, after the RTA, the atomic concentration of carbon is close to zero, the atomic concentration of titanium is about 33%, and the atomic concentration of oxygen is about 66%. Therefore, after the RTA, the carbon- based resin is removed from the overcoat layer, such that the remaining overcoat layer may include nanoporous structures formed by T1O2 nanoparticles and pores between the T1O2 nanoparticles. The remaining overcoat layer may have a refractive index about 1.87 and a density about 2.67 g/cm³ as measured using x-ray reflectivity.

[0152] FIG. 18B illustrates an XPS profile of an example of a structure (e.g., device 1700 or the structure shown in FIG. 16E) including a substrate, an overcoat layer, and a back-fill layer according to certain embodiments. In the illustrated example, the overcoat layer has a thickness about 135 nm, and may be made using a resin material including T1O2 nanoparticles dispersed in a carbon-based resin. The carbon-based resin may be removed from the overcoat layer using the high-energy treatment processes described above (e.g.,

RTA at about 600 °C), such that the overcoat layer may include nanoporous structures formed by TiCh nanoparticles and pores between the TiCh nanoparticles. The back-fill layer may be an antireflective coating layer, and may include, for example, a thin film including a siloxane based material and having a thickness about 100 nm. The siloxane based material may have a refractive index about 1.5 and a density about 1.37 g/cm³. The siloxane based material may fill the pores between the TiCh nanoparticles in the nanoporous structures. Curves 1812, 1822, 1832, 1842, and 1850 in FIG. 18B show the atomic concentrations of carbon, oxygen, silicon, titanium, and aluminum, respectively, in the substrate, the overcoat layer, and the back-fill layer. As shown by curve 1812, the atomic concentration of carbon is greater than about 60% in the back-fill layer, and about 5% or less in the overcoat layer. As shown by curve 1822, the atomic concentration of oxygen is about 20% in the back-fill layer, and about 60% in the overcoat layer. As shown by curve 1832, the atomic concentration of silicon is about 15% in the back-fill layer, and about 5% in the overcoat layer. As shown by curve 1842, the atomic concentration of titanium is 0% in the back-fill layer, and about 30% in the overcoat layer. FIG. 18B shows that the atomic concentrations of silicon, carbon, and titanium in the back-filled overcoat layer may be uniform in the thickness (depth) direction. The back-filled overcoat layer may have a refractive index about 2.02 and a density about 2.99 g/cm³.

[0153] FIG. 18C illustrates an XPS profile of an example of a structure including a substrate, an overcoat layer, and an AI2O3 layer formed on the overcoat layer according to certain embodiments. In the illustrated example, the overcoat layer has a thickness about 175 nm, and may be made using a resin material including TiCh nanoparticles dispersed in a carbon-based resin. The carbon-based resin may be removed from the overcoat layer using the high-energy treatment processes described above (e.g., RTA at about 600 °C), such that the overcoat layer may include nanoporous structures formed by TiCh nanoparticles and pores between the TiCh nanoparticles. The AhCh layer may be an antireflective coating layer, and may have a thickness about 50 nm. The AI2O3 layer may have a refractive index about 1.6 and a density about 2.66 g/cm³. The AI2O3 layer may be deposited on the nanoporous structures by, for example, atomic layer deposition. The aluminum atoms in the AI2O3 layer may fill the pores between the TiCh nanoparticles in the nanoporous structures. Curves 1814, 1824, 1834, 1844, and 1852 in FIG. 18C show the atomic concentrations of carbon, oxygen, silicon, titanium, and aluminum, respectively, in the substrate, the overcoat layer, and the AI2O3 layer. As shown by curve 1814, the atomic concentration of carbon is 0% in the AI2O3 layer, and about 0% in the overcoat layer. As shown by curve 1824, the atomic concentration of oxygen is about 65% in the AhCrilayer. and greater than about 66% in the overcoat layer. Curve 1834 shows that the atomic concentration of silicon is about 0% in the back-fill layer and the overcoat layer. As shown by curve 1844, the atomic concentration of titanium is about 0% in the AI2O3 layer, and about 25% in the overcoat layer. As shown by curve 1852, the atomic concentration of aluminum is about 35% in the AI2O3 layer, and about 10% in the overcoat layer. FIG. 18C shows that the atomic concentrations of aluminum and titanium in the back-filled overcoat layer may be uniform in the thickness (depth) direction. The back filled overcoat layer may have a refractive index about 2.07 and a density about 3.32 g/cm³. [0154] FIG. 18D illustrates correlations between the refractive indices and the densities of various layers in the examples of structures of FIGS. 18A-18C. A data point 1860 in FIG.

18D shows the refractive index (1.87) and the density (2.67 g/cm³) of the overcoat layer of FIG. 18 A. A data point 1862 in FIG. 18D shows the refractive index (2.02) and the density (2.99 g/cm³) of the overcoat layer of FIG. 18B that is back-filled with a siloxane-based material, which has a refractive index about 1.5 and a density about 2.99 g/cm³ as represented by a data point 1864. A data point 1866 in FIG. 18D shows the refractive index (2.07) and the density (3.32g/cm³) of the overcoat layer of FIG. 18C that is back-filled with an AI2O3 layer, which has a refractive index about 1.6 and a density about 2.66 g/cm³ as represented by a data point 1868.

[0155] FIGS. 19A-19E illustrate an example of a process of fabricating a surface-relief grating including an overcoat layer having a high refractive index and a low loss according to certain embodiments. FIG. 19A shows that an overcoat material may be dispensed on a surface-relief grating 1910 on a substrate 1905 to form an overcoat layer 1920 for surface- relief grating 1910. As described above, the overcoat material may include a resin (e.g., acrylate resin with photo radical generator or thermal radical generator), nanoparticles (e.g., T1O2 nanoparticles and/or NbOx nanoparticles), and a solvent (e.g., PGMEA or DPGME/TPM). In some embodiments, the overcoat material may include a sol-gel material that includes a solution and a high-refractive index oxide precursor (e.g., Ti precursor) and has been partially or fully condensed into an extended network. For example, the sol-gel material may include EMD MHM-63, Philips high refractive index sol-gel formulation, or organic solvent-soluble Titania with ligands in a solvent (e.g., PGMEA or DPGME/TPM). In one example, the overcoat material may have a refractive index about 1.95. The overcoat material may be dispensed by, for example, spin coating or inkjet printing. The amount of the overcoat material dispensed on surface-relief grating 1910 may be determined based on the dimensions of the grating grooves and the desired thickness of the overburden (e.g., the portion of the overcoat layer on top of the grating ridges). After dispensing the overcoat material, the top surface of overcoat layer 1920 may not be flat as shown in the illustrated example. For example, the height of the top surface at the grating ridge regions may be greater than the height of the top surface at the grating groove regions. In some embodiments, overcoat layer 1920 may be baked to remove the solvent from the overcoat layer. Baking may help to remove the solvent and the trapped air such that the overcoat material in the overcoat layer may be more uniform or homogeneous.

[0156] FIG. 19B shows that a planar imprint stamp 1930 (e.g., a soft stamp or mold described above) may be laminated or otherwise applied on overcoat layer 1920 in an imprint process. As described above, in some embodiments, planar imprint stamp 1930 may include PDMS or another silicone elastomer or silicon-based organic polymer. In some embodiments, planar imprint stamp 1930 may include ETFE, PFPE, or other fluorinated polymer or acrylic polymer materials. The bottom surface of planar imprint stamp 1930 that contacts overcoat layer 1920 may have a low surface roughness. Planar imprint stamp 1930 may be pressed against overcoat layer 1920, for example, using a roller as described above with respect to, for example, FIG. 1 IB. Thus, some portions of the overcoat materials on top of the grating ridges may be forced to the regions on top of the grating grooves, and the flat bottom surface of planar imprint stamp 1930 may be transferred to the top surface of overcoat layer 1920. [0157] FIG. 19C shows that overcoat layer 1920 may then be cured (e.g., using UV light) to crosslink and fix the base resin in the overcoat material as described above. After the curing and the crosslinking of the overcoat material, planar imprint stamp 1930 may be delaminated or otherwise detached from overcoat layer 1920 and surface-relief grating 1910, as described above with respect to, for example, FIG. 11C. After the delamination of planar imprint stamp 1930, the top surface of overcoat layer 1920 can be flat and smooth. In addition, the overburden can be thin, such as less than about 20 nm.

[0158] FIG. 19D shows that a high-energy treatment process (e.g., RTA, LSA, or UV irradiation) may be performed on overcoat layer 1920 to remove the organic materials (e.g., organic ligands that decorate the TiCh nanoparticles and/or NbOx nanoparticles,) in overcoat layer 1920. After the high-energy treatment, overcoat layer 1920 may become a porous layer 1922 that may have a lower refractive index due to the removal of the organic materials. For example, overcoat layer 1920 may have a refractive index about 1.95, while porous layer 1922 may have a refractive index about 1.88 or lower. [0159] FIG. 19E shows that an antireflective coating (ARC) layer 1940 may be deposited on porous layer 1922. As described above, ARC layer 1940 may include organic and/or inorganic materials and may back-fill porous layer 1922 to form a back-filled overcoat layer 1924 that may have a higher refractive index (due to the back fill of the porous structures) and a low absorption loss (due to the removal of the degraded organics and other organics that can be degraded). For example, porous layer 1922 may have a refractive index about 1.88, while back-filled overcoat layer 1924 may have a refractive index about 2.03 or higher. The organic and/or inorganic materials that back-fill porous layer 1922 may have a low optical loss, and may include, for example, organic materials such as acrylate, polystyrenics, epoxy based organic materials, siloxane based materials, inorganic materials such as AI2O3, S1O2, ZrC , Nb205, T1O2, Ta20s, and the like, or a combination of organic materials and inorganic materials describe above. Depositing ARC layer 1940 may include an inkjet coating process, a spin coating process, an ALD process, a PVD process, an CVD process, or a combination thereof. In some embodiments, the ALD process may use ALD precursors, such as TiCL and H2O, TDMAT and O3, TDMAZ/TMA and H2O, or the like.

[0160] FIGS. 20A-20E illustrate an example of a process of fabricating a surface-relief grating including grating ridges having a high refractive index and a low loss according to certain embodiments. FIG. 20A shows that a resin material 2010 may be dispensed on a substrate 2005. As described above, resin material 2010 may include a resin (e.g., acrylate resin with photo radical generator or thermal radical generator), nanoparticles (e.g., T1O2 nanoparticles and/or NbOx nanoparticles), and a solvent (e.g., PGMEA or DPGME/TPM). In one example, resin material 2010 may have a refractive index about 1.95. Resin material 2010 may be dispensed by, for example, spin coating or inkjet coating. The amount of resin material 2010 dispensed on substrate 2005 may be determined based on, for example, the depth of the grating grooves and the duty cycle of the grating. In some embodiments, resin material 2010 may be baked to remove the solvent and the trapped air such that the resin material may be more uniform or homogeneous.

[0161] FIG. 20B shows that an imprint stamp 2020 (e.g., a soft stamp) may be laminated or otherwise applied on resin material 2010 in an imprint process. As described above, in some embodiments, imprint stamp 2020 may include PDMS or another silicone elastomer or silicon-based organic polymer. In some embodiments, imprint stamp 2020 may include ETFE, PFPE, or other fluorinated polymer or acrylic polymer materials. The surfaces of imprint stamp 2020 that contacts resin material 2010 may have surface-relief structures. Imprint stamp 2020 may be pressed against resin material 2010, for example, using a roller as described above with respect to, for example, FIG. 1 IB. Thus, the surface-relief structures in imprint stamp 2020 may be transferred to resin material 2010 such that surface-relief structures complementary to the surface-relief structures in imprint stamp 2020 may be formed in resin material 2010.

[0162] FIG. 20C shows that resin material 2010 may be cured (e.g., using UV light) to crosslink and fix the base resin in the resin material as described above. After the curing and the crosslinking of resin material 2010, imprint stamp 2020 may be delaminated or otherwise detached from resin material 2010, thereby forming surface-relief grating 2012 as described above with respect to, for example, FIG. 11C. Even though the example of surface-relief grating 2012 shown in FIG. 20C includes straight grating ridges, in some embodiments, surface-relief grating 2012 may include slanted grating ridges. The grating period, duty cycle, and/or depth of surface-relief grating 2012 may be constant or may be variable across the grating.

[0163] FIG. 20D shows that a high-energy treatment process (e.g., RTA, LSA, or UV irradiation) may be performed on surface-relief grating 2012 to remove the organic materials (e.g., organic ligands that decorate TiC nanoparticles and/or NbOx nanoparticles) in surface- relief grating 2012. After the high-energy treatment, surface-relief grating 2012 may become porous and thus may have a reduced refractive index due to the removal of the organic materials. For example, surface-relief grating 2012 may have a refractive index about 1.95 before the high-energy treatment process, and may have a refractive index about 1.87 after the high-energy treatment process.

[0164] FIG. 20E shows that an overcoat layer 2030 may be deposited on surface-relief grating 2012. As described above, overcoat layer 2030 may include organic and/or inorganic materials and may back-fill the porous surface-relief grating 2012 to form a back-filled surface-relief grating 2012 that may have a higher refractive index (due to the back fill of the porous structures) and a low absorption loss (due to the removal of the degraded organics and other organics that can be degraded). For example, surface-relief grating 2012 may have a refractive index about 1.87 after the high-energy treatment process, and may have a refractive index about 2.07 after overcoat layer 2030 is deposited on surface-relief grating 2012 to fill pores in surface-relief grating 2012. The organic and/or inorganic materials that back-fill the pores in surface-relief grating 2012 may have a low optical loss, and may include, for example, organic materials such as acrylate, polystyrenics, epoxy based organic materials, siloxane based materials, inorganic materials such as AI2O3, S1O2, ZrC , Nb20s, T1O2, TarOy and the like, or a combination of organic materials and inorganic materials describe above. Depositing overcoat layer 2030 may include, for example, an inkjet-coating process or a spin coating process.

[0165] FIG. 21 includes a flowchart 2100 illustrating an example of a process of fabricating a surface-relief grating that has a high refractive index contrast and a low loss according to certain embodiments. It is noted that the specific operations illustrated in FIG.

21 provide a particular process of fabricating a surface-relief grating. Other sequences of operations may be performed according to alternative embodiments. Moreover, the individual operations illustrated in FIG. 21 may include multiple sub-steps that may be performed in various sequences as appropriate to the individual operation. Furthermore, additional operations may be added or some operations may not be performed depending on the particular applications. One of ordinary skill in the art would recognize many variations, modifications, and alternatives.

[0166] Operations in block 2110 may include depositing a resin layer (or a sol-gel layer) that includes an organic material and metal oxide nanoparticles dispersed in the organic material. The resin layer may include, for example, an acrylate resin including photo radical generator or thermal radical generator, 75%-90% metal oxide (e.g., TiOx, ZrOx, and/or NbOx) nanoparticles decorated with organic ligands, and a solvent including PGMEA, DPGME/TPM, or a combination. In some embodiments, the resin layer may have a refractive index about 1.95 or higher. The resin layer may be deposited on a substrate or a surface-relief grating using spin coating, inkjet coating, or spray coating. In some embodiments, the resin layer may be baked to remove the solvent. Baking may help to remove the solvent and the trapped air such that the material in the resin layer may be more uniform or homogeneous. In some embodiments, a sol-gel layer, rather than a resin layer, may be deposited on a surface- relief grating to form an overcoat layer. The sol-gel material may include a solution and a high-refractive index oxide precursor (e.g., Ti precursor), and may have been at least partially condensed into an extended network. For example, the sol-gel material may include organic solvent soluble Titania with organic ligands, and a solvent including PGMEA,

DPGME/TPM, or a combination thereof. In some embodiments, the sol-gel material may include EMD MHM-63 or Philips high-refractive index sol-gel formulation.

[0167] Optional operations in block 2115 may include imprinting the resin layer by, for example, laminating a planar imprint stamp or an imprint stamp with structures complementary to the structures of a surface-relief grating on the resin layer. As described above, the planar imprint stamp may include PDMS or another silicone elastomer or silicon- based organic polymer. In some embodiments, the planar imprint stamp may include ETFE, PFPE, or other fluorinated polymer or acrylic polymer materials. The planar imprint stamp may be used to form an overcoat layer with a flat top surface. The imprint stamp with structures complementary to the structures of a surface-relief grating may be used to form the surface-relief grating in the resin layer.

[0168] Operations in block 2120 may include curing the resin layer (e.g., using UV light) to crosslink and fix the base resin in the resin layer. The crosslinked base resin may hold the metal oxide nanoparticles in place. After the curing, the imprint stamp (if used) may be delaminated from the resin layer to leave a flat top surface (for an overcoat layer) on the resin layer or a surface-relief grating in the resin layer.

[0169] Operations in block 2130 may include performing a high-energy treatment process (e.g., RTA, LSA, or UV irradiation) on the cured resin layer to remove the organic materials (e.g., organic ligands that decorate TiCh nanoparticles and/or NbOx nanoparticles), including the organic materials that have been degraded during the fabrication processes (e.g., UV curing). After the high-energy treatment, the resin layer may become a porous layer that may include metal oxide nanoparticles and pores between the metal oxide nanoparticles. For example, as shown in FIG. 18A, the porous layer may mainly include Ti and oxygen atoms. The porous layer may have a lower refractive index due to the removal of the organic materials. For example, the resin layer before the high-energy treatment may have a refractive index about 1.95, while the porous layer may have a refractive index about 1.88. [0170] Operations in block 2140 may include depositing a back-fill material that has a low optical loss on the porous layer to fill the pores in the porous layer. Depositing the back-fill material may include an inkjet coating process, a spin coating process, an ALD process, an SIS process, a PVD process, an iCVD process, or a combination thereof. The ALD precursors used in the ALD process may include, for example, TiCU and FLO, TDMAT and O3, TDMAZ/TMA and FLO, or a combination thereof. The back-fill material may include of an organic material, an inorganic material, or a combination thereof. The organic material may include, for example, acrylate, polystyrenics, epoxy, siloxane, or a combination thereof. The inorganic material may include, for example, AI2O3, S1O2, ZrCh, M>2q5, TiCh, Ta20s, or a combination thereof. The porous layer with the pores filled with the back-fill material may have a higher refractive index, such as greater than about 2.0. Thus, a low-loss, high- refractive index overcoat layer or surface-relief grating may be formed. The back-fill material may form an ARC layer on a high-refractive index overcoat layer that is on a low-refractive index surface-relief grating, or may form a low-refractive index overcoat layer on a high- refractive index surface-relief grating. [0171] Embodiments of the invention may include or be implemented in conjunction with an artificial reality system. Artificial reality is a form of reality that has been adjusted in some manner before presentation to a user, which may include, for example, a virtual reality (VR), an augmented reality (AR), a mixed reality (MR), a hybrid reality, or some combination and/or derivatives thereof. Artificial reality content may include completely generated content or generated content combined with captured (e.g., real-world) content. The artificial reality content may include video, audio, haptic feedback, or some combination thereof, and any of which may be presented in a single channel or in multiple channels (such as stereo video that produces a three-dimensional effect to the viewer). Additionally, in some embodiments, artificial reality may also be associated with applications, products, accessories, services, or some combination thereof, that are used to, for example, create content in an artificial reality and/or are otherwise used in (e.g., perform activities in) an artificial reality. The artificial reality system that provides the artificial reality content may be implemented on various platforms, including a head-mounted display (HMD) connected to a host computer system, a standalone HMD, a mobile device or computing system, or any other hardware platform capable of providing artificial reality content to one or more viewers. [0172] FIG. 22 is a simplified block diagram of an example electronic system 2200 of an example near-eye display (e.g., HMD device) for implementing some of the examples disclosed herein. Electronic system 2200 may be used as the electronic system of an HMD device or other near-eye displays described above. In this example, electronic system 2200 may include one or more processor(s) 2210 and a memory 2220. Processor(s) 2210 may be configured to execute instructions for performing operations at a number of components, and can be, for example, a general-purpose processor or microprocessor suitable for implementation within a portable electronic device. Processor(s) 2210 may be communicatively coupled with a plurality of components within electronic system 2200. To realize this communicative coupling, processor(s) 2210 may communicate with the other illustrated components across a bus 2240. Bus 2240 may be any subsystem adapted to transfer data within electronic system 2200. Bus 2240 may include a plurality of computer buses and additional circuitry to transfer data.

[0173] Memory 2220 may be coupled to processor(s) 2210. In some embodiments, memory 2220 may offer both short-term and long-term storage and may be divided into several units. Memory 2220 may be volatile, such as static random access memory (SRAM) and/or dynamic random access memory (DRAM) and/or non-volatile, such as read-only memory (ROM), flash memory, and the like. Furthermore, memory 2220 may include removable storage devices, such as secure digital (SD) cards. Memory 2220 may provide storage of computer-readable instructions, data structures, program modules, and other data for electronic system 2200. In some embodiments, memory 2220 may be distributed into different hardware modules. A set of instructions and/or code might be stored on memory 2220. The instructions might take the form of executable code that may be executable by electronic system 2200, and/or might take the form of source and/or installable code, which, upon compilation and/or installation on electronic system 2200 (e.g., using any of a variety of generally available compilers, installation programs, compression/decompression utilities, etc.), may take the form of executable code.

[0174] In some embodiments, memory 2220 may store a plurality of application modules 2222 through 2224, which may include any number of applications. Examples of applications may include gaming applications, conferencing applications, video playback applications, or other suitable applications. The applications may include a depth sensing function or eye tracking function. Application modules 2222-2224 may include particular instructions to be executed by processor(s) 2210. In some embodiments, certain applications or parts of application modules 2222-2224 may be executable by other hardware modules 2280. In certain embodiments, memory 2220 may additionally include secure memory, which may include additional security controls to prevent copying or other unauthorized access to secure information.

[0175] In some embodiments, memory 2220 may include an operating system 2225 loaded therein. Operating system 2225 may be operable to initiate the execution of the instructions provided by application modules 2222-2224 and/or manage other hardware modules 2280 as well as interfaces with a wireless communication subsystem 2230 which may include one or more wireless transceivers. Operating system 2225 may be adapted to perform other operations across the components of electronic system 2200 including threading, resource management, data storage control and other similar functionality.

[0176] Wireless communication subsystem 2230 may include, for example, an infrared communication device, a wireless communication device and/or chipset (such as a Bluetooth® device, an IEEE 802.11 device, a Wi-Fi device, a WiMax device, cellular communication facilities, etc.), and/or similar communication interfaces. Electronic system 2200 may include one or more antennas 2234 for wireless communication as part of wireless communication subsystem 2230 or as a separate component coupled to any portion of the system. Depending on desired functionality, wireless communication subsystem 2230 may include separate transceivers to communicate with base transceiver stations and other wireless devices and access points, which may include communicating with different data networks and/or network types, such as wireless wide-area networks (WWANs), wireless local area networks (WLANs), or wireless personal area networks (WPANs). A WWAN may be, for example, a WiMax (IEEE 802.16) network. A WLAN may be, for example, an IEEE 802.1 lx network. A WPAN may be, for example, a Bluetooth network, an IEEE 802.15x, or some other types of network. The techniques described herein may also be used for any combination of WWAN, WLAN, and/or WPAN. Wireless communications subsystem 2230 may permit data to be exchanged with a network, other computer systems, and/or any other devices described herein. Wireless communication subsystem 2230 may include a means for transmitting or receiving data, such as identifiers of HMD devices, position data, a geographic map, a heat map, photos, or videos, using antenna(s) 2234 and wireless link(s) 2232.

[0177] Embodiments of electronic system 2200 may also include one or more sensors 2290. Sensor(s) 2290 may include, for example, an image sensor, an accelerometer, a pressure sensor, a temperature sensor, a proximity sensor, a magnetometer, a gyroscope, an inertial sensor (e.g., a module that combines an accelerometer and a gyroscope), an ambient light sensor, or any other similar module operable to provide sensory output and/or receive sensory input, such as a depth sensor or a position sensor. For example, in some implementations, sensor(s) 2290 may include one or more inertial measurement units (IMUs) and/or one or more position sensors. An IMU may generate calibration data indicating an estimated position of the HMD device relative to an initial position of the HMD device, based on measurement signals received from one or more of the position sensors. A position sensor may generate one or more measurement signals in response to motion of the HMD device. Examples of the position sensors may include, but are not limited to, one or more accelerometers, one or more gyroscopes, one or more magnetometers, another suitable type of sensor that detects motion, a type of sensor used for error correction of the IMU, or some combination thereof. The position sensors may be located external to the IMU, internal to the IMU, or some combination thereof. At least some sensors may use a structured light pattern for sensing.

[0178] Electronic system 2200 may include a display module 2260. Display module 2260 may be a near-eye display, and may graphically present information, such as images, videos, and various instructions, from electronic system 2200 to a user. Such information may be derived from one or more application modules 2222-2224, virtual reality engine 2226, one or more other hardware modules 2280, a combination thereof, or any other suitable means for resolving graphical content for the user (e.g., by operating system 2225). Display module 2260 may use liquid crystal display (LCD) technology, light-emitting diode (LED) technology (including, for example, OLED, ILED, pLED, AMOLED, TOLED, etc.), light emitting polymer display (LPD) technology, or some other display technology.

[0179] Electronic system 2200 may include a user input/output module 2270. User input/output module 2270 may allow a user to send action requests to electronic system 2200. An action request may be a request to perform a particular action. For example, an action request may be to start or end an application or to perform a particular action within the application. User input/output module 2270 may include one or more input devices. Example input devices may include a touchscreen, a touch pad, microphone(s), button(s), dial(s), switch(es), a keyboard, a mouse, a game controller, or any other suitable device for receiving action requests and communicating the received action requests to electronic system 2200. In some embodiments, user input/output module 2270 may provide haptic feedback to the user in accordance with instructions received from electronic system 2200. For example, the haptic feedback may be provided when an action request is received or has been performed. [0180] Electronic system 2200 may include a camera 2250 that may be used to take photos or videos of a user, for example, for tracking the user’s eye position. Camera 2250 may also be used to take photos or videos of the environment, for example, for VR, AR, or MR applications. Camera 2250 may include, for example, a complementary metal-oxide- semiconductor (CMOS) image sensor with a few millions or tens of millions of pixels. In some implementations, camera 2250 may include two or more cameras that may be used to capture 3-D images.

[0181] In some embodiments, electronic system 2200 may include a plurality of other hardware modules 2280. Each of other hardware modules 2280 may be a physical module within electronic system 2200. While each of other hardware modules 2280 may be permanently configured as a structure, some of other hardware modules 2280 may be temporarily configured to perform specific functions or temporarily activated. Examples of other hardware modules 2280 may include, for example, an audio output and/or input module (e.g., a microphone or speaker), a near field communication (NFC) module, a rechargeable battery, a battery management system, a wired/wireless battery charging system, etc. In some embodiments, one or more functions of other hardware modules 2280 may be implemented in software.

[0182] In some embodiments, memory 2220 of electronic system 2200 may also store a virtual reality engine 2226. Virtual reality engine 2226 may execute applications within electronic system 2200 and receive position information, acceleration information, velocity information, predicted future positions, or some combination thereof of the HMD device from the various sensors. In some embodiments, the information received by virtual reality engine 2226 may be used for producing a signal (e.g., display instructions) to display module 2260. For example, if the received information indicates that the user has looked to the left, virtual reality engine 2226 may generate content for the HMD device that mirrors the user’s movement in a virtual environment. Additionally, virtual reality engine 2226 may perform an action within an application in response to an action request received from user input/output module 2270 and provide feedback to the user. The provided feedback may be visual, audible, or haptic feedback. In some implementations, processor(s) 2210 may include one or more GPUs that may execute virtual reality engine 2226.

[0183] In various implementations, the above-described hardware and modules may be implemented on a single device or on multiple devices that can communicate with one another using wired or wireless connections. For example, in some implementations, some components or modules, such as GPUs, virtual reality engine 2226, and applications (e.g., tracking application), may be implemented on a console separate from the head-mounted display device. In some implementations, one console may be connected to or support more than one HMD.

[0184] In alternative configurations, different and/or additional components may be included in electronic system 2200. Similarly, functionality of one or more of the components can be distributed among the components in a manner different from the manner described above. For example, in some embodiments, electronic system 2200 may be modified to include other system environments, such as an AR system environment and/or an MR environment.

[0185] The methods, systems, and devices discussed above are examples. Various embodiments may omit, substitute, or add various procedures or components as appropriate. For instance, in alternative configurations, the methods described may be performed in an order different from that described, and/or various stages may be added, omitted, and/or combined. Also, features described with respect to certain embodiments may be combined in various other embodiments. Different aspects and elements of the embodiments may be combined in a similar manner. Also, technology evolves and, thus, many of the elements are examples that do not limit the scope of the disclosure to those specific examples.

[0186] Specific details are given in the description to provide a thorough understanding of the embodiments. However, embodiments may be practiced without these specific details.

For example, well-known circuits, processes, systems, structures, and techniques have been shown without unnecessary detail in order to avoid obscuring the embodiments. This description provides example embodiments only, and is not intended to limit the scope, applicability, or configuration of the invention. Rather, the preceding description of the embodiments will provide those skilled in the art with an enabling description for implementing various embodiments. Various changes may be made in the function and arrangement of elements without departing from the scope of the claims.

[0187] Also, some embodiments were described as processes depicted as flow diagrams or block diagrams. Although each may describe the operations as a sequential process, many of the operations may be performed in parallel or concurrently. In addition, the order of the operations may be rearranged. A process may have additional steps not included in the figure. Furthermore, embodiments of the methods may be implemented by hardware, software, firmware, middleware, microcode, hardware description languages, or any combination thereof. When implemented in software, firmware, middleware, or microcode, the program code or code segments to perform the associated tasks may be stored in a computer-readable medium such as a storage medium. Processors may perform the associated tasks.

[0188] It will be apparent to those skilled in the art that substantial variations may be made in accordance with specific requirements. For example, customized or special-purpose hardware might also be used, and/or particular elements might be implemented in hardware, software (including portable software, such as applets, etc.), or both. Further, connection to other computing devices such as network input/output devices may be employed.

[0189] With reference to the appended figures, components that can include memory can include non-transitory machine-readable media. The term “machine-readable medium” and “computer-readable medium,” as used herein, refer to any storage medium that participates in providing data that causes a machine to operate in a specific fashion. In embodiments provided hereinabove, various machine-readable media might be involved in providing instructions/code to processing units and/or other device(s) for execution. Additionally or alternatively, the machine-readable media might be used to store and/or carry such instructions/code. In many implementations, a computer-readable medium is a physical and/or tangible storage medium. Such a medium may take many forms, including, but not limited to, non-volatile media, volatile media, and transmission media. Common forms of computer-readable media include, for example, magnetic and/or optical media such as compact disk (CD) or digital versatile disk (DVD), punch cards, paper tape, any other physical medium with patterns of holes, a RAM, a programmable read-only memory (PROM), an erasable programmable read-only memory (EPROM), a FLASH-EPROM, any other memory chip or cartridge, a carrier wave as described hereinafter, or any other medium from which a computer can read instructions and/or code. A computer program product may include code and/or machine-executable instructions that may represent a procedure, a function, a subprogram, a program, a routine, an application (App), a subroutine, a module, a software package, a class, or any combination of instructions, data structures, or program statements.

[0190] Those of skill in the art will appreciate that information and signals used to communicate the messages described herein may be represented using any of a variety of different technologies and techniques. For example, data, instructions, commands, information, signals, bits, symbols, and chips that may be referenced throughout the above description may be represented by voltages, currents, electromagnetic waves, magnetic fields or particles, optical fields or particles, or any combination thereof.

[0191] Terms, “and” and “or” as used herein, may include a variety of meanings that are also expected to depend at least in part upon the context in which such terms are used. Typically, “or” if used to associate a list, such as A, B, or C, is intended to mean A, B, and C, here used in the inclusive sense, as well as A, B, or C, here used in the exclusive sense. In addition, the term “one or more” as used herein may be used to describe any feature, structure, or characteristic in the singular or may be used to describe some combination of features, structures, or characteristics. However, it should be noted that this is merely an illustrative example and claimed subject matter is not limited to this example. Furthermore, the term “at least one of’ if used to associate a list, such as A, B, or C, can be interpreted to mean A, B, C, or a combination of A, B, and/or C, such as AB, AC, BC, AA, ABC, AAB, ACC, AABBCCC, or the like.

[0192] Further, while certain embodiments have been described using a particular combination of hardware and software, it should be recognized that other combinations of hardware and software are also possible. Certain embodiments may be implemented only in hardware, or only in software, or using combinations thereof. In one example, software may be implemented with a computer program product containing computer program code or instructions executable by one or more processors for performing any or all of the steps, operations, or processes described in this disclosure, where the computer program may be stored on a non-transitory computer readable medium. The various processes described herein can be implemented on the same processor or different processors in any combination. [0193] Where devices, systems, components or modules are described as being configured to perform certain operations or functions, such configuration can be accomplished, for example, by designing electronic circuits to perform the operation, by programming programmable electronic circuits (such as microprocessors) to perform the operation such as by executing computer instructions or code, or processors or cores programmed to execute code or instructions stored on a non-transitory memory medium, or any combination thereof. Processes can communicate using a variety of techniques, including, but not limited to, conventional techniques for inter-process communications, and different pairs of processes may use different techniques, or the same pair of processes may use different techniques at different times.

[0194] The specification and drawings are, accordingly, to be regarded in an illustrative rather than a restrictive sense. It will, however, be evident that additions, subtractions, deletions, and other modifications and changes may be made thereunto without departing from the broader scope as set forth in the claims. Thus, although specific embodiments have been described, these are not intended to be limiting. Various modifications and equivalents are within the scope of the following claims.

Claims

CLAIMS:

1. An optical device comprising: a substrate; a first surface-relief grating including grooves and ridges formed on or in the substrate; a first overcoat layer in the grooves of the first surface-relief grating; and a first antireflective layer on the first overcoat layer, wherein: the ridges of the first surface-relief grating comprise metal oxide nanoparticles and a material of the first overcoat layer in regions between the metal oxide nanoparticles, wherein the metal oxide nanoparticles are photoactive, or the first overcoat layer comprises the metal oxide nanoparticles and a material of the first antireflective layer in regions between the metal oxide nanoparticles.

2. The optical device of claim 1, wherein the metal oxide nanoparticles include TiOx nanoparticles, NbOx nanoparticles, or a combination thereof.

3. The optical device of claim 1 or claim 2, wherein the ridges of the first surface-relief grating or the first overcoat layer is characterized by a refractive index greater than 2.0 for visible light.

4. The optical device of claim 1, claim 2 or claim 3, wherein a total optical absorption of the first surface-relief grating and the first overcoat layer is lower than 0.1% for visible light.

5. The optical device of any one of the preceding claims, wherein the material in the regions between the metal oxide nanoparticles in the ridges of the first surface-relief grating or the material between the metal oxide nanoparticles in the first overcoat layer comprises acrylate, polystyrenics, epoxy, siloxane, AI2O3, S1O2, ZrCh, Nb20s, T1O2, Ta20s, or a combination thereof.

6. The optical device of any one of the preceding claims, wherein the ridges of the first surface-relief grating or the first overcoat layer is characterized by a carbon atomic concentration less than 10%.

7. The optical device of any one of the preceding claims, further comprising: a second surface-relief grating including grooves and ridges formed on or in the substrate, the second surface-relief grating and the first surface-relief grating on opposite sides of the substrate; a second overcoat layer in the grooves of the second surface-relief grating; and a second antireflective layer on the second overcoat layer, wherein: the ridges of the second surface-relief grating comprise the metal oxide nanoparticles and a material of the second overcoat layer in regions between the metal oxide nanoparticles, or the second overcoat layer comprises the metal oxide nanoparticles and a material of the second antireflective layer in regions between the metal oxide nanoparticles.

8. A method comprising: depositing an overcoat layer on a surface-relief grating to at least partially fill grating grooves of the surface-relief grating, the overcoat layer including: a sol-gel material including metal oxide precursors; or a resin layer including metal oxide nanoparticles; curing the overcoat layer by heat or electromagnetic radiation; performing a high-energy treatment on the overcoat layer to remove light-absorbing organics and form porous structures in the overcoat layer; and depositing a first layer of an organic material, an inorganic material, or a combination thereof on the overcoat layer to fill the porous structures in the overcoat layer.

9. The method of claim 8, wherein the high-energy treatment includes rapid thermal annealing (RTA), laser spike annealing (LSA), or ultraviolet (UV) light irradiation.

10. The method of claim 8 or claim 9, wherein depositing the first layer of the organic material, the inorganic material, or a combination thereof on the overcoat layer comprises an inkjet coating process, a spin coating process, an atomic layer deposition (ALD) process, a sequential infiltration synthesis (SIS) process, a physical vapor deposition (PVD) process, an initiated chemical vapor deposition (iCVD) process, or a combination thereof; and preferably wherein ALD precursors used in the ALD process include:

TiCU and H₂0; tetrakis (dimethylamino) titanium(IV) (TDMAT) and CL; tetrakis (dimethylamido) zirconium (IV) (TDMAZ) / trimethylaluminum (TMA) and LLO; or a combination thereof.

11. The method of claim 8, claim 9 or claim 10, wherein the organic material comprises acrylate, poly sty renics, epoxy, siloxane, or a combination thereof; and/or preferably wherein the inorganic material comprises AI2O3, S1O2, ZrCh. MnCL, T1O2, Ta₂05, or a combination thereof; and/or preferably wherein the first layer of the organic material, the inorganic material, or a combination thereof forms an antireflective coating layer on the overcoat layer

12. The method of any one of claims 8 to 11, further comprising, before curing the overcoat layer, imprinting the overcoat layer using a planar imprint stamp to planarize a top surface of the overcoat layer.

13. The method of any one of claims 8 to 12, wherein the sol-gel material includes: organic solvent soluble Titania with organic ligands; and a solvent including propylene glycol methyl ether acetate (PGMEA), dipropylene glycol methyl ether (DPGME)/tripropylene glycol monomethyl ether (TPM), or a combination thereof; and/or preferably wherein the resin layer includes: an acrylate resin including photo radical generator or thermal radical generator;

TiOx or NbOx nanoparticles decorated with organic ligands; and a solvent including PGMEA, DPGME/TPM, or a combination thereof.

14. A method comprising: depositing, on a substrate, a resin layer that includes metal oxide nanoparticles; imprinting a surface-relief grating in the resin layer; performing a high-energy treatment on the surface-relief grating to remove light absorbing organics and form porous structures in the surface-relief grating; and depositing a layer of an organic material, an inorganic material, or a combination thereof on the surface-relief grating to fill grating grooves of the surface-relief grating and the porous structures in the surface-relief grating.

15. The method of claim 14, wherein the high-energy treatment includes rapid thermal annealing (RTA), laser spike annealing (LSA), or ultraviolet (UV) light irradiation; and/or preferably wherein: the resin layer includes: an acrylate resin including photo radical generator or thermal radical generator;

TiCh or NbOx nanoparticles decorated with organic ligands; and a solvent including propylene glycol methyl ether acetate (PGMEA), dipropylene glycol methyl ether (DPGME)/tripropylene glycol monomethyl ether (TPM), or a combination thereof; and the layer of the organic material, the inorganic material, or a combination thereof includes acrylate, polystyrenics, epoxy, siloxane, AI2O3, SiO₂, Z1O2, Nb₂O₅, TiO₂, Ta₂O₅, or a combination thereof.

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