WO2015029025A1

WO2015029025A1 - Multi-spectral anti-reflective sub-wavelength structure for optical devices

Info

Publication number: WO2015029025A1
Application number: PCT/IL2014/050763
Authority: WO
Inventors: Michael Rosenblit; Erez Golan
Original assignee: B. G. Negev Technologies And Applications Ltd. (Ben-Gurion University)
Priority date: 2013-08-26
Filing date: 2014-08-26
Publication date: 2015-03-05

Abstract

A device comprising a substrate having two sides, the substrate comprising an anti-reflective side structure on each side of the two sides, the anti-reflective side structure comprising a surface, the surface comprising a plurality of elements arranged in a grating form, wherein the elements are of a sub-wavelength scale.

Description

MULTI-SPECTRAL ANTI-REFLECTIVE SUB-WAVELENGTH

STRUCTURE FOR OPTICAL DEVICES

FIELD OF THE INVENTION

The invention relates to optical device structures.

BACKGROUND

The field of anti-reflective (AR) sub-wavelength structure for solid state devices for light was motivated historically by the need of the use of visible and, especially, infrared (IR) light in industrial, space, telecommunication and research and commercial applications for faster and more complex devices. Laser communication systems, active and passive imaging sensors, industrial cutting, welding, and marking lasers, and a variety of devices, typically require durable infrared transmitting windows and optics made of materials such as germanium , sapphire, silicon, gallium arsenide, etc. In most applications, the region of the IR light spectrum employed is not absorbed by these materials. However, reflected IR light is a major problem particularly with IR cameras and laser radars. For example, just one surface of a Zinc selenide (ZnSe) window will reflect 17% of the long wave IR (LWIR) light incident on-axis, a cadmium zinc telluride (CZT) window reflects 21%, and a Germanium (Ge) window or optic will reflect over 36%. The problem gets worse for IR light incident at higher angles off the normal to the window. Such large reflections produce stray light and can lead to superimposed images that can reduce the contrast or even blind security cameras.

Surface relief microstructures, from a simpler type of anti-reflective surface structure called a sub-wavelength surface (or "SWS"), up to commonly known as Motheye textures, have been shown to be an effective alternative to thin-film anti-reflective coatings in many infrared and visible -band applications where durability, radiation resistance, wide viewing angle, or broad-band performance are critical. These microstructures are built into the surface of the device window or optic material, imparting an optical function that minimizes reflections without compromising the inherent properties of the material. An array of pyramidal or conic surface structures or their combination provides a gradual change of the refractive index for light propagating from air into the bulk optical material. Reflection losses are reduced to a minimum for broadband light incident over a wide angular range. In general, these surface relief structures will exhibit similar characteristics, as the bulk material with respect to durability, thermal issues, and radiation resistance. The problems associated with thin-film coating adhesion, stress, abrasion resistance and lifetime, are eliminated.

The foregoing examples of the related art and limitations related therewith are intended to be illustrative and not exclusive. Other limitations of the related art will become apparent to those of skill in the art upon a reading of the specification and a study of the figures.

SUMMARY

The following embodiments and aspects thereof are described and illustrated in conjunction with systems, tools and methods which are meant to be exemplary and illustrative, not limiting in scope.

There is provided in accordance with an embodiment, a device comprising a substrate having two sides, the substrate comprising an anti-reflective side structure on each side of the two sides, the anti-reflective side structure comprising a surface, the surface comprising a plurality of elements arranged in a grating form, wherein the elements are of a sub-wavelength scale.

There is provided in accordance with another embodiment, an anti-reflective device having two sides, each side of the two sides comprising a surface, the surface being formed as protruding elements arranged in a grating form, wherein the protruding elements are of a sub -wavelength scale.

In some embodiments, the substrate comprises one or more layers.

In some embodiments, the elements are of a shape selected from the group consisting of: columnar, pyramidal, cylindrical or conic.

In some embodiments, the anti-reflective side structure is made of materials selected from the group consisting of: silicon, fused silica, sapphire, Zinc sulfide (ZnS), Zinc selenide (ZnSe), cadmium zinc telluride and germanium.

In some embodiments, the anti-reflective side structure have parameters predefined to allow transmission of electromagnetic (EM) waves of one or more predetermined wavelengths.

In some embodiments, the parameters are selected from the group consisting of: period, width, duty cycle, height and side wall angle. In some embodiments, one or more parameters of the parameters of the anti- reflective side structure on one side of the two sides of the substrate differ from the one or more parameters of the anti-reflective side structure on the other side of the two sides of the substrate by one or more predetermined ratios, correspondingly.

In some embodiments, one or more parameters of the parameters of the anti- reflective side structure on each side of the two sides are the same, correspondingly.

In some embodiments, the one or more predetermined wavelengths are selected from the group consisting of: a single wavelength and a wavelength band.

In some embodiments, the one or more predetermined wavelengths are selected from the group consisting of: visible and near- infrared wavelength range, short-wave infrared wavelength range, mid-wave infrared wavelength range and long-wave infrared wavelength range.

In some embodiments, the protruding elements are of a shape selected from the group consisting of: columnar, pyramidal, cylindrical or conic.

In some embodiments, the anti-reflective device is made of a material selected from the group consisting of: silicon, fused silica, sapphire, Zinc sulfide (ZnS), Zinc selenide (ZnSe), cadmium zinc telluride and germanium.

In some embodiments, the anti-reflective device have parameters predefined to allow transmission of electromagnetic (EM) waves of one or more predetermined wavelengths.

In addition to the exemplary aspects and embodiments described above, further aspects and embodiments will become apparent by reference to the figures and by study of the following detailed description.

BRIEF DESCRIPTION OF THE FIGURES

Exemplary embodiments are illustrated in referenced figures. Dimensions of components and features shown in the figures are generally chosen for convenience and clarity of presentation and are not necessarily shown to scale. The figures are listed below.

Fig. 1 shows graphs and SEM (Scanning Electron Microscope) images of a motheye texture, in accordance with prior art;

Fig. 2 shows an SEM image of an SWS anti-reflective structure, in accordance with prior art; Figs. 3 A, 3B, 3C and 3D show examples of columnar, cylindrical, pyramidal and conic devices, in accordance with prior art;

Fig. 4A shows a perspective top view of a an exemplary anti-reflective device, which is conic, in accordance with an embodiment of the disclosed devices;

Fig. 4B shows a perspective bottom view of the anti-reflective device of Fig. 4A, in accordance with an embodiment of the disclosed devices;

Figs. 5 A and 5B show graphs illustrating simulations of diffraction efficiency as a function of wavelength for set of bands in the Visible and Near Infra-Red (VNIR), Short- Wavelength Infra-Red (SWIR) and Mid-Wavelngth Infra-Red (MWIR) portions of the electromagnetic spectrum (or simply 'spectrum');

Figs. 6A and 6B show graphs illustrating simulations of diffraction efficiency as a function of wavelength for sets of bands in the SWIR and MWIR spectrum portions;

Figs. 7 A and 7B show graphs illustrating simulations of diffraction efficiency as a function of wavelength for sets of bands in the MWIR and Long-Wavelength Infra-Red (LWIR) spectrum portions; and

Figs. 8 A, 8B and 8C show graphs illustrating simulations of diffraction efficiency as a function of wavelength for sets of bands in the MWIR and LWIR spectrum portions.

DETAILED DESCRIPTION

Figs. 1-3D relate to anti-reflective devices of the prior art. Fig. 1 shows graphs and SEM (Scanning Electron Microscope) images 100, 110 and 120 of a motheye texture, in accordance with prior art. The graphs relate to wavelengths between 7 μιη and 14 μιη. As shown in the Figure, an estimate of the maximum percentage of transmission of EM waves according to the prior art does not reach 84%. An exemplary device including a motheye anti-reflective surface structure only on one side made of ZnSe may provide up to approximately 83% of transmission. For comparison purposes, a graph for an untreated substrate of ZnSe is also shown, according to which it may provide approximately 70% of transmission at most. SEM images 100, 110 and 120 show various views of a motheye texture.

Fig. 2 shows an SEM image of an SWS anti -reflective structure 130, in accordance with prior art. SWS anti-reflective structure 130 may include depressions 140 arranged in a grating form. As shown in the image, the length of a portion of SWS anti-reflective structure 130 which includes three depressions 140 may be around only two μηι. Figs. 3A, 3B, 3C and 3D show examples of columnar, cylindrical, pyramidal and conic surface structures, in accordance with prior art. Fig. 3A shows a columnar (i.e., quadrangular) surface structure 150 which may include a single surface on which a plurality of columnar elements 160 (i.e., shaped as a column) are arranged in a grating form. Fig. 3B shows a pyramidal surface structure 170 which may include a single surface on which a plurality of pyramidal elements 180 (i.e., shaped as a pyramid) are arranged in a grating form. Fig. 3C shows a cylindrical surface structure 200 which may include a single surface on which a plurality of cylindrical elements 210 (i.e., shaped as a cylinder) are arranged in a grating form. Fig. 3D shows a conic surface structure 220 which may include a single surface on which a plurality of conic elements (i.e., shaped as a conic) are arranged in a grating form.

An aspect of some embodiments of the disclosed devices relates to a double sided, sub-wavelength scale, anti-reflective surface grating structure design for high transmission of electromagnetic (EM) waves of one or more predetermined wavelengths. The one or more predetermined wavelengths may include single wavelengths and/or wavelength bands such as narrow, broad or wide wavelength bands or any of their combinations in VNIR (i.e., between approximately 0.4 and 1.4 micrometer (μιη)), SWIR (i.e., between approximately 1.4 and 3 μιη), MWIR (i.e., between approximately 3 and 8 μιη) and LWIR (i.e., between approximately 8 and 15 μιη) spectral ranges.

The disclosed anti-reflective devices can be column, cylinder, pyramidal, conic, etc. (i.e., including elements in the shape of column, cylinder, pyramid or a cone) or any of their combinations on one side and the same or any of their combinations on the other side.

In some embodiments, the disclosed devices may include a substrate. The substrate may include two sides (e.g., up and bottom) and an anti-reflective side structure on each side of the two sides. The anti-reflective side structure may include a surface. The surface may include a plurality of elements arranged in a grating form. Optionally, the elements are of a sub-wavelength scale. The elements may be, for example, columnar, pyramidal, cylindrical or conic. One or more parameters of the anti-reflective side structure such as period (i.e., the length of a base portion of an element with respect to the surface and of a gap between the element to the next element on the surface), width (i.e., the width of the base portion of an element with respect to the surface), duty cycle (i.e., the ratio between the length of the base potion of the element and the period), height (i.e., the height of the element with respect to the surface) and side wall angle (i.e., a base angle of the element with respect to the surface) may be the same for both anti-reflective side structures, or differ by one or more specific ratios, depends on specific wavelength and width of spectral bands needed. Such parameters may be predefined to allow transmission of electromagnetic (EM) waves of one or more predetermined wavelengths.

The substrate may include one or more layers. A substrate which includes a single layer may embed the two anti-reflective side structures, each on each of its sides. Thus, the disclosed anti-reflective structures may be formed in a single material as silicon, fused silica, sapphire, Zinc sulfide (ZnS), ZnSe, cadmium zinc telluride, germanium, etc. In other words, the etching may be conducted in the substrate material itself (i.e., a substrate including a single layer), avoiding any mismatch issue, adhesion or stress induced delamination of the coating. Generally, it may be formed on two sides of a single material substrate or the substrate may be covered by another material (i.e., including more than one layer) and the anti-reflective structures may be made of and/or designed and formed on different materials, such as silicon, fused silica, sapphire, Zinc sulfide (ZnS), Zinc selenide (ZnSe), cadmium zinc telluride and germanium to achieve the desired optical property.

Fig. 4A shows a perspective top view of an exemplary anti-reflective device 250 which is conic. Fig. 4B shows a perspective bottom view of anti-reflective device 250 of Fig. 4A. Anti-reflective device 250 may be similar to the above disclosed devices which include a substrate of a single layer. Anti-reflective device 250 has a top side 260 and a bottom side 270. Top side 260 may include a surface (not indicated) formed as conic protruding elements 280 arranged in a grating form. Bottom side 270 may include a surface (not indicated) formed as conic protruding elements 290 arranged in a grating form. Light may impinge bottom side 290 (as indicated by an arrow and a caption: "input light" in Fig. 4B), pass through anti-reflective device 250 and exit anti- reflective device 250 through top side 260 (as indicated by an arrow and a caption: "output light" in Fig. 4B).

Reference is now made to Figs. 5 A and 5B, which show graphs illustrating simulations of diffraction efficiency as a function of wavelength for set of bands in the VNIR, SWIR and MWIR portions of the spectrum. The simulation of Fig. 5A is made for a design of the disclosed anti-reflective device, characterized by parameters as described in Table 1 below: Parameters\Surfaces Top Surface Bottom Surface Top-Bottom Ratio

Ratio

Period (Microns) 0.7 1 0.7

Height (Microns) 0.45 0.45 1

Duty Cycle 0.85 0.85 1

Width (Microns) 0.595 0.85 0.7

Side Wall Angle 69 61 1.13

(Degrees)

Table 1 : Parameters of the anti -reflective design of Fig. 5 A

The simulation of Fig. 5B is made for a design of the disclosed anti-reflective device, characterized by parameters as described in Table 2 below:

Table 2: Parameters of the anti-reflective design of Fig. 5B

As shown in these figures, the total transmission for these specific sets of bands is over 96%.

Reference is now made to Figs. 6A and 6B, which show graphs illustrating simulations of diffraction efficiency as a function of wavelength for sets of bands in the SWIR and MWIR spectrum portions.

The simulation of Fig. 6A is made for a design of the disclosed anti-reflective device, characterized by parameters as described in Table 3 below:

Table 3: Parameters of the anti-reflective design of Fig. 6A

The simulation of Fig. 6B is made for a design of the disclosed anti-reflective device, characterized by parameters as described in Table 4 below: Parameters\Surfaces Top Surface Bottom Surface Ratio

& Ratio

Period (Microns) 1.2 1.5 0.8

Height (Microns) 0.5 0.5 1

Duty Cycle 0.8 0.8 1

Width (Microns) 0.96 1.2 0.8

Side Wall Angle 69 64 1.19

(Degrees)

Table 4: Parameters of the anti-reflective design of Fig. 6B

As shown in these figures, the total transmission for these specific sets of bands is over 98%.

Reference is now made to Figs. 7 A and 7B, which show graphs illustrating simulations of diffraction efficiency as a function of wavelength for sets of bands in the MWIR and Long- Wavelength Infra-Red (LWIR) spectrum portions.

The simulation of Fig. 7A is made for a design of the disclosed anti-reflective device, characterized by parameters as described in Table 5 below:

Table 5: Parameters of the anti-reflective design of Fig. 7A

The simulation of Fig. 7B is made for a design of the disclosed anti-reflective device, characterized by parameters as described in Table 6 below:

Table 6: Parameters of the anti-reflective design of Fig. 7B

Reference is now made to Figs. 8A, 8B and 8C, which show graphs illustrating simulations of diffraction efficiency as a function of wavelength for sets of bands in the MWIR and LWIR spectrum portions. The simulation of Fig. 8A is made for a design of the disclosed anti-reflective device, characterized by parameters as described in Table 7 below:

Table 7: Parameters of the anti-reflective design of Fig. 8A

The simulation of Fig. 8B is made for a design of the disclosed anti -reflective device, characterized by parameters as described in Table 8 below:

Table 8: Parameters of the anti-reflective design of Fig. 8B

The simulation of Fig. 8C is made for a design of the disclosed anti -reflective device, characterized by parameters as described in Table 9 below:

Table 9: Parameters of the anti-reflective design of Fig. 8C

Designs such as the ones used in the simulations of Figs. 5A-8C may enable high transmission of at least 96-99% or more for predetermined single wavelengths and narrow (for example, of 20 to 100 nanometer) broad or wide wavelength bands (for example, of 0.1 to 2.0 micrometer) or any of its combinations in VNIR, SWIR, MWIR and LWIR spectral ranges. The disclosed embodiments may be useful for optical device applications requiring high optical transmission, durability, survivability, and radiation resistance, which, in turn, may enable a high transmission performance. Examples for such optical devices may include multi-spectral windows, reflectors, lenses, refractive elements, polarizer, prism, beam-splitter, filters, laser components, etc. Thus, the disclosed devices may serve as optical devices such as the above examples or may be embedded in such optical devices, for example by embedding anti-reflective device 250 according to Figs. 4 A and 4B.

The polarization and angular dependence of the disclosed devices and optical devices embedding the disclosed devices may be controlled by the design of the sub- wavelength structures, allowing additional degrees of freedom not offered by traditional multi-layer coatings.

The optical properties of the disclosed devices may be simulated using an effective medium theory, finite difference time domain (FDTD) and rigorous coupled wave analysis (RCWA). Optimal combinations of parameters for the anti-reflective SWS on both sides of the disclosed devices, such as period, width, duty cycle, height, side wall angle, etc. may allow desired functionality for wide range of materials. Combining the tuning of such structure parameters of anti-reflective surface grating structures on both sides of the disclosed devices may achieve transmission for a desired combination of single wavelengths and wavelength bands (for example, 0.2- 2.0 micron) of more than 99%.

In the description and claims of the application, each of the words "comprise" "include" and "have", and forms thereof, are not necessarily limited to members in a list with which the words may be associated. In addition, where there are inconsistencies between this application and any document incorporated by reference, it is hereby intended that the present application controls.

The descriptions of the various embodiments of the present invention have been presented for purposes of illustration, but are not intended to be exhaustive or limited to the embodiments disclosed. Many modifications and variations will be apparent to those of ordinary skill in the art without departing from the scope and spirit of the described embodiments. The terminology used herein was chosen to best explain the principles of the embodiments, the practical application or technical improvement over technologies found in the marketplace, or to enable others of ordinary skill in the art to understand the embodiments disclosed herein.

Claims

CLAIMS What is claimed is:

1. A device comprising a substrate having two sides, the substrate comprising an anti-reflective side structure on each side of the two sides, the anti-reflective side structure comprising a surface, the surface comprising a plurality of elements arranged in a grating form, wherein the elements are of a sub-wavelength scale.

2. The device of claim 1, wherein the substrate comprises one or more layers.

3. The device of claim 1, wherein the elements are of a shape selected from the group consisting of: columnar, pyramidal, cylindrical or conic.

4. The device of claim 1, wherein the anti-reflective side structure is made of materials selected from the group consisting of: silicon, fused silica, sapphire, Zinc sulfide (ZnS), Zinc selenide (ZnSe), cadmium zinc telluride and germanium.

5. The device of claim 1, wherein the anti-reflective side structure have parameters predefined to allow transmission of electromagnetic (EM) waves of one or more predetermined wavelengths.

6. The device of claim 5, wherein the parameters are selected from the group consisting of: period, width, duty cycle, height and side wall angle.

7. The device of claim 6, wherein one or more parameters of the parameters of the anti-reflective side structure on one side of the two sides of the substrate differ from the one or more parameters of the anti-reflective side structure on the other side of the two sides of the substrate by one or more predetermined ratios, correspondingly.

8. The device of claim 6, wherein one or more parameters of the parameters of the anti-reflective side structure on each side of the two sides are the same, correspondingly.

9. The device of claim 5, wherein the one or more predetermined wavelengths are selected from the group consisting of: a single wavelength and a wavelength band.

10. The device of claim 5, wherein the one or more predetermined wavelengths are selected from the group consisting of: visible and near- infrared wavelength range, short-wave infrared wavelength range, mid-wave infrared wavelength range and longwave infrared wavelength range.

11. An anti-reflective device having two sides, each side of the two sides comprising a surface, the surface being formed as protruding elements arranged in a grating form, wherein the protruding elements are of a sub-wavelength scale.

12. The anti-reflective device of claim 11, wherein the protruding elements are of a shape selected from the group consisting of: columnar, pyramidal, cylindrical or conic.

13. The anti-reflective device of claim 11, wherein the anti-reflective device is made of a material selected from the group consisting of: silicon, fused silica, sapphire, Zinc sulfide (ZnS), Zinc selenide (ZnSe), cadmium zinc telluride and germanium.

14. The anti-reflective device of claim 11, wherein the anti-reflective device have parameters predefined to allow transmission of electromagnetic (EM) waves of one or more predetermined wavelengths.

15. The anti-reflective device of claim 14, wherein the parameters are selected from the group consisting of: period, width, duty cycle, height and side wall angle.

16. The anti-reflective device of claim 14, wherein the one or more predetermined wavelengths are selected from the group consisting of: a single wavelength and a wavelength band.

17. The anti-reflective device of claim 17, wherein the one or more predetermined wavelengths are selected from the group consisting of: visible and near-infrared wavelength range, short-wave infrared wavelength range, mid-wave infrared wavelength range and long-wave infrared wavelength range.