TW202424584A - Fabrication of high-index encapsulated grating designs - Google Patents

Abstract

Embodiments described herein relate to an outcoupler of a waveguide combiner includes a plurality of optical device structures and an overburden layer. The optical structures are disposed in or on an optical device substrate. The plurality of optical device structures have a structure refractive index less than or equal to 2.0. The overburden layer is disposed over a top surface and sidewalls of each optical device structure of the plurality of optical device structures. The overburden layer has an overburden refractive index greater than or equal to 2.0. A refractive index contrast between the overburden refractive index and the structure refractive index is about 0.3 to about 0.5. An overburden thickness variation is less than or equal to 20 nm. The overburden thickness variation is a difference between a maximum point and a minimum point of an uppermost surface of the overburden layer.

Description

Fabrication of high refractive index encapsulated grating design

Embodiments of the present disclosure generally relate to waveguide combiners for augmented reality, virtual reality, and mixed reality. More specifically, embodiments described herein provide an output coupler of a waveguide combiner and a method of forming an output coupler of a waveguide combiner.

Virtual reality is generally considered to be a computer-generated simulated environment in which the user has a distinct sense of physical presence. The virtual reality experience can be generated in 3D and viewed with a head-mounted display (HMD), such as glasses or other wearable display elements with near-eye display panels as lenses to display a virtual reality environment that replaces the real environment.

However, the augmented reality experience is that the user can still view the surrounding environment through the display lens of the glasses or other HMD components, and can also see the generated images of virtual objects that appear as part of the environment. Augmented reality can include any type of input, such as audio and tactile input, as well as virtual images, graphics, and video of the environment that enhance or enhance the user's experience. As an emerging technology, augmented reality has many challenges and design limitations.

One of the challenges is to overlay a virtual image on the surrounding environment. An output coupler of a waveguide combiner, such as an augmented reality waveguide combiner, is used to assist in overlaying the image. The light generated propagates through the waveguide combiner until the output coupler and is overlaid on the surrounding environment. Therefore, what is needed in the art is an output coupler of a waveguide combiner and a method of forming an output coupler of a waveguide combiner.

In one embodiment, the output coupler of the waveguide combiner includes a plurality of optical element structures and a covering layer. The optical structure is disposed in or on an optical element substrate. The plurality of optical element structures have a structural refractive index less than or equal to 2.0. The covering layer is disposed above the top surface and sidewalls of each of the plurality of optical element structures. The covering layer has a covering refractive index greater than or equal to 2.0. The refractive index contrast between the covering refractive index and the structural refractive index is about 0.3 to about 0.5. The covering thickness variation is less than or equal to 20 nm. The covering thickness variation is the difference between the maximum point and the minimum point of the uppermost surface of the covering layer.

In another embodiment, a method of forming an output coupler of a waveguide combiner is disclosed. The method includes forming a plurality of optical element structures in or on an optical element substrate, the plurality of optical element structures having a structural refractive index less than or equal to 2.0; and depositing a capping layer over a top surface and sidewalls of each of the plurality of optical element structures, the capping layer having a capping refractive index greater than or equal to 2.0. The refractive index contrast between the capping refractive index and the structural refractive index is about 0.3 to about 0.5. The capping thickness variation is less than or equal to 20 nm. The capping thickness variation is the difference between a maximum point and a minimum point of the uppermost surface of the capping layer.

In another embodiment, the output coupler of the waveguide combiner includes a plurality of optical element structures, a packaging layer and a covering layer. The plurality of optical element structures are formed in or on an optical element substrate. The plurality of optical element structures have a structural refractive index greater than or equal to 2.0. The packaging layer is disposed on the top surface and side walls of each of the plurality of optical element structures. The packaging layer has a packaging refractive index less than or equal to 2.0. A first refractive index contrast between the packaging refractive index and the structural refractive index is about 0.3 to about 0.5. The covering layer is disposed above the packaging layer. The covering layer has a covering refractive index greater than or equal to 2.0. A second refractive index contrast between the packaging refractive index and the covering refractive index is about 0.3 and about 0.5. The coverage thickness variation is less than or equal to 20nm. The coverage thickness variation is the difference between the maximum point and the minimum point on the top surface of the coverage layer.

In another embodiment, a method of forming an output coupler of a waveguide combiner is disclosed. The method includes forming a plurality of optical element structures disposed on or in an optical element substrate; depositing a packaging layer over a top surface and a side wall of each of the plurality of optical element structures; and depositing a covering layer on the packaging layer. The plurality of optical element structures have a structural refractive index greater than or equal to 2.0. The packaging layer has a packaging refractive index less than or equal to 2.0. A first refractive index contrast between the packaging refractive index and the structural refractive index is about 0.3 to about 0.5. The covering refractive index is greater than or equal to 2.0. A second refractive index contrast between the covering refractive index and the packaging refractive index is about 0.3 to about 0.5. The coverage thickness variation is less than 20 nm, where the coverage thickness variation is the difference between the maximum point and the minimum point on the top surface of the coverage layer.

Embodiments of the present disclosure generally relate to waveguide combiners for augmented reality, virtual reality, and mixed reality. More specifically, embodiments described herein provide an output coupler of a waveguide combiner and a method of forming an output coupler of a waveguide combiner.

FIG. 1A is a schematic top view of a waveguide combiner 100. The waveguide combiner 100 is used for augmented reality, virtual reality, and mixed reality. The waveguide combiner 100 includes a plurality of optical element structures 102 disposed on a surface 103 of an optical element substrate 101. The optical element structures 102 may be nanostructures having sub-micrometer dimensions (e.g., nanometer-sized dimensions). The waveguide combiner 100 includes at least an input coupler 104A and an output coupler 104C. The waveguide combiner 100 according to an embodiment that may be combined with other embodiments described herein includes a pupil dilator 104B.

The optical element substrate 101 may be formed of any suitable material, as long as the optical element substrate 101 is capable of sufficiently transmitting light within a desired wavelength or wavelength range and is capable of providing sufficient support for the waveguide combiner 100 described herein. In some embodiments that may be combined with other embodiments described herein, the material of the optical element substrate 101 has a relatively low refractive index compared to the refractive index of the plurality of optical element structures 102. The optical element substrate 101 may be selected to include a substrate of any suitable material, including but not limited to amorphous dielectrics, non-amorphous dielectrics, crystalline dielectrics, silicon oxide, polymers, and combinations thereof. In some embodiments that may be combined with other embodiments described herein, the optical element substrate 101 includes a transparent material. In one example, the optical device substrate 101 includes a high refractive index transparent material such as silicon (Si), silicon dioxide (SiO ₂ ), silicon carbide (SiC), germanium (Ge), silicon germanium (SiGe), indium phosphide (InP), gallium arsenide (GaAs), gallium nitride (GaN), fused silicon dioxide, quartz, sapphire, and high refractive index glass. The optical device structure 102 formed in or on the optical device substrate 101 includes silicon nitride (SiN), silicon oxynitride (SiON), aluminum oxide (Al ₂ O ₃ ), silicon oxide (SiO ₂ ), titanium dioxide (TiO ₂ ), niobium (V) oxide (Ni ₂ O ₅ ), silicon nitride (SiN), tantalum (V) oxide (Ta ₂ O ₅ ) or a combination thereof.

1B is a schematic cross-sectional view of a portion of an output coupler 104C of the waveguide combiner 100. The output coupler 104C includes a plurality of optical element structures 102 disposed in or on an optical element substrate 101. The plurality of optical element structures 102 have a structural refractive index less than or equal to 2.0. In one embodiment, the material of the plurality of optical element structures 102 may include silicon nitride (SiN), silicon oxynitride (SiON), aluminum oxide (Al ₂ O ₃ ), silicon oxide (SiO ₂ ) or a combination thereof. The optical element structure 102 has a depth d of about 80 nm to about 200 nm. The optical element structure 102 has a critical dimension less than 150 nm, which corresponds to the width or diameter of a cross section of each optical element structure 102. The gap g corresponds to the distance between adjacent optical element structures of the plurality of optical element structures 102. The aspect ratio of the gap g to the depth d is between about 4:1 and about 1:1.

The cover layer 110 is disposed over the top surface 120 and the sidewalls 122 of each of the plurality of optical element structures 102. The cover layer 110 has a cover refractive index greater than or equal to 2.0. The refractive index contrast, i.e., the difference between the cover refractive index and the structure refractive index, is about 0.3 to about 0.5. In some embodiments, the refractive index contrast is about 0.3. The refractive index contrast enables red light, green light, and blue light propagating through the waveguide combiner 100 to be output coupled from the output coupler 104C at the same rate. For example, red light propagates at a larger angle than blue light. This causes the blue light to have more interactions with the plurality of optical element structures 102. More interactions with the plurality of optical element structures 102 result in more output coupling events for the blue light, but at a lower efficiency. In contrast, red light interacts with the optical element structure 102 at a lower frequency but with a higher efficiency. As a result, the image produced by the waveguide combiner 100 is clearer in the user's field of vision. The cover layer 110 includes titanium dioxide (TiO ₂ ), niobium (V) oxide (Ni ₂ O ₅ ), silicon nitride (SiN), tantalum (V) oxide (Ta ₂ O ₅ ), or a combination thereof. The cover layer 110 has a thickness t ₁ between about 150 nm and about 250 nm. The cover layer 110 has a cover thickness variation 130. The cover thickness variation 130 is the difference between a maximum point 132 and a minimum point 134 of an uppermost surface 136 of the cover layer 110. The cover thickness variation 130 is less than or equal to 20 nm. Minimizing the cover thickness variations 130 prevents the thickness variations from passing through any layers deposited on top of the cover layer 110. Additionally, variations such as the cover thickness variations 130 can act as additional optical element structures. Such additional optical element structures can distort and reflect light propagating through the waveguide combiner 100, resulting in a less clear image in the user's field of view.

In one embodiment, additional layers may be disposed over the capping layer 110. For example, an additional low refractive index layer (not shown) may be disposed over the capping layer 110, the low refractive index layer having a refractive index less than 2.0. In another embodiment, other layers are disposed over the capping layer 110, such as a gradient refractive index layer 150, an anti-reflective coating (ARC) layer 160, or a mirror layer 170, as shown and described below in FIG. 1C.

1C is a schematic cross-sectional view of a portion of an output coupler 104C of the waveguide combiner 100. The output coupler 104C includes a plurality of optical element structures 102 formed in or on an optical element substrate 101. The plurality of optical element structures 102 have a structural refractive index greater than or equal to 2.0. In one embodiment, the material of the plurality of optical element structures 102 may include titanium dioxide (TiO ₂ ), niobium (V) oxide (Ni ₂ O ₅ ), silicon nitride (SiN), tantalum (V) oxide (Ta ₂ O ₅ ) or a combination thereof. The optical element structure 102 has a depth d of about 80 nm to about 200 nm. The optical element structure 102 has a critical dimension of less than 150 nm, which corresponds to the width or diameter of a cross section of each optical element structure 102. In addition, the gap g corresponds to the distance between adjacent optical element structures of the plurality of optical element structures 102. The aspect ratio of the gap g to the depth d is between about 4:1 and about 1:1.

The encapsulation layer 112 is disposed above the top surface 120 and the sidewall 122 of each of the plurality of optical element structures 102. The encapsulation layer 112 has an encapsulation refractive index less than or equal to 2.0. The first refractive index contrast, i.e., the difference between the encapsulation refractive index and the structure refractive index, is about 0.3 to about 0.5. In some embodiments, the first refractive index contrast is about 0.3. The refractive index contrast enables red light, green light, and blue light propagating through the waveguide combiner 100 to be output coupled from the output coupler 104C with the same efficiency. For example, red light propagates at a larger angle than blue light. This causes the blue light to have more interactions with the plurality of optical element structures 102. More interactions with the plurality of optical element structures 102 result in more output coupling events for the blue light, but with lower efficiency. In contrast, red light interacts with the optical element structure 102 at a lower frequency but with a higher efficiency. As a result, the image produced by the waveguide combiner 100 is clearer in the user's field of vision. The material of the encapsulation layer 112 includes silicon nitride (SiN), silicon oxynitride (SiON), aluminum oxide (Al ₂ O ₃ ), silicon oxide (SiO ₂ ) or a combination thereof. In one embodiment, the encapsulation layer 112 may have an encapsulation thickness t ₂ between about 100 nm and about 110 nm. In another embodiment, after the encapsulation layer 112 is disposed above the optical element structure 102, the encapsulation thickness t ₂ is removed by etching or chemical mechanical planarization (CMP).

The cover layer 110 is disposed above the encapsulation layer 112. The cover layer 110 has a cover refractive index greater than or equal to 2.0. The second refractive index contrast, i.e., the difference between the encapsulation refractive index and the cover refractive index, is about 0.3 and about 0.5. In some embodiments, the second refractive index contrast is about 0.3. As described above, the refractive index contrast enables the red light, green light, and blue light propagating through the waveguide combiner 100 to be output coupled from the output coupler 104C with the same efficiency. As a result, the image generated by the waveguide combiner 100 is clearer in the user's field of view.

The cover layer 110 has a cover thickness variation 130. The cover thickness variation 130 is the difference between a maximum point 132 and a minimum point 134 of an uppermost surface 136 of the cover layer 110. The cover thickness variation 130 is less than or equal to 20 nm. Minimizing the cover thickness variation 130 prevents the thickness variation from passing through any layer deposited on top of the cover layer 110. In addition, variations such as the cover thickness variation 130 can effectively act as additional optical element structures. Such additional optical element structures can distort and reflect light propagating through the waveguide combiner 100, resulting in a less clear image in the user's field of view. The material of the capping layer 110 may include titanium dioxide (TiO ₂ ), niobium (V) oxide (Ni ₂ O ₅ ), silicon nitride (SiN), tantalum (V) oxide (Ta ₂ O ₅ ) or a combination thereof. In one embodiment, the material of the capping layer 110 and the material of the optical device structure 102 are the same material.

In one embodiment, an additional layer may be disposed on the cover layer 110. For example, an additional low refractive index layer (not shown) may be disposed on the cover layer 110, the refractive index of the low refractive index layer being less than or equal to 2.0. In another embodiment, as shown in FIG. 1C , other layers are disposed on the cover layer 110, such as a gradient refractive index layer 150, an anti-reflective coating (ARC) layer 160, or a mirror layer 170.

Fig. 2 is a flow chart of a method 200 for forming the output coupler 104C of the waveguide combiner 100 shown in Fig. 1B. Fig. 3A and Fig. 3B are schematic cross-sectional views of a portion of the optical device substrate 101 during the method of forming the output coupler 104C.

At operation 202, as shown in FIG. 3A, an optical element structure 102 is formed on or in an optical element substrate 101. In one embodiment, a pattern resist is deposited on the optical element substrate 101 and the optical element substrate 101 is etched. In another embodiment, an optical element layer is deposited on the optical element substrate 101, a pattern resist is deposited on the optical element layer, and the optical element layer is etched. In another embodiment, an optical element layer is deposited on the optical element substrate 101, a nanoimprint stamp uses nanoimprint lithography (NIL) to imprint an optical element pattern on the optical element layer, and the optical element layer is cured. The plurality of optical element structures 102 have a structural refractive index less than 2.0. The material of the optical element structure 102 may include silicon nitride (SiN), silicon oxynitride (SiON), aluminum oxide (Al ₂ O ₃ ), silicon oxide (SiO ₂ ) or a combination thereof. The optical element structure 102 has a depth d of about 80 nm to about 150 nm. The optical element structure 102 has a critical dimension less than 150 nm, which corresponds to the width or diameter of the cross section of each optical element structure 102. The gap g corresponds to the distance between adjacent optical element structures in the plurality of optical element structures 102. The aspect ratio of the gap g to the depth d is between about 4:1 and about 1:1.

At operation 204, as shown in FIG. 3B, a capping layer 110 is deposited over the top surface 120 and the sidewalls 122 of each of the plurality of optical element structures 102. The capping layer 110 is deposited using furnace chemical vapor deposition (FCVD) or ink jet printing (IJP). The capping layer 110 has a capping refractive index greater than or equal to 2.0. The refractive index contrast, i.e., the difference between the capping refractive index and the structure refractive index, is about 0.3 to about 0.5. In another embodiment, the refractive index contrast is about 0.3. The capping layer 110 has a thickness t ₁ between about 150 nm and about 250 nm. In one embodiment, the capping layer 110 has a capping thickness variation 130 less than or equal to 20 nm. The capping thickness variation 130 is measured as the difference between a maximum point 132 and a minimum point 134 of an uppermost surface 136 of the capping layer 110. In one embodiment, the capping thickness variation 130 is achieved by using etching, chemical mechanical planarization (CMP), or cyclic deposition and etching. The material of the capping layer 110 may include titanium dioxide (TiO ₂ ), niobium (V) oxide (Ni ₂ O ₅ ), silicon nitride (SiN), tantalum (V) oxide (Ta ₂ O ₅ ), or a combination thereof.

In one embodiment, additional layers may be disposed over the capping layer 110. For example, an additional low refractive index layer (not shown) may be disposed over the capping layer 110, the low refractive index layer having a refractive index less than 2.0. In another embodiment, as shown and described in FIG. 1C , other layers are disposed over the capping layer 110, such as a gradient refractive index layer 150, an anti-reflective coating (ARC) layer 160, or a mirror layer 170.

Figure 4 is a flow chart of a method 400 for forming an output coupler 104C of the waveguide combiner 100. Figures 5A-5C are schematic cross-sectional views of a portion of the optical device substrate 101 during the method 400 of forming the output coupler 104C.

At operation 402, as shown in FIG5A, a plurality of optical element structures 102 are formed on or in an optical element substrate 101. In one embodiment, a pattern resist is deposited on the optical element substrate 101 and the optical element substrate 101 is etched. In another embodiment, an optical element layer is deposited on the optical element substrate 101, a pattern resist is deposited on the optical element layer, and the optical element layer is etched. In another embodiment, an optical element layer is deposited on the optical element substrate 101, a nanoimprint stamp uses nanoimprint lithography (NIL) to imprint an optical element pattern on the optical element layer, and the optical element layer is cured. The optical element structures in the plurality of optical element structures 102 have a structural refractive index greater than or equal to 2.0. The optical element structures 102 have a depth d of about 80 nm to about 150 nm. The optical element structures 102 have a critical dimension less than 150 nm, which corresponds to a width or diameter of a cross section of each optical element structure 102. In addition, the gap g corresponds to a distance between adjacent optical element structures of the plurality of optical element structures 102. The aspect ratio of the gap g to the depth d is between about 4:1 and about 1:1.

At operation 404, as shown in FIG. 5B, an encapsulation layer 112 is deposited over the top surface 120 and the sidewalls 122 of the plurality of optical element structures 102. The encapsulation layer 112 is deposited using furnace chemical vapor deposition (FCVD) or ink jet printing (IJP). The encapsulation refractive index of the encapsulation layer 112 is less than or equal to 2.0. The first refractive index contrast, i.e., the difference between the structural refractive index and the encapsulation refractive index, is about 0.3 to about 0.5. In another embodiment, the first refractive index contrast is about 0.3. The material of the encapsulation layer 112 may include silicon nitride (SiN), silicon oxynitride (SiON), aluminum oxide (Al ₂ O ₃ ), silicon oxide (SiO ₂ ) or a combination thereof. In one embodiment, the encapsulation layer 112 may have an encapsulation thickness t ₂ between about 100 nm and about 110 nm. In another embodiment, after the encapsulation layer 112 is disposed over the optical device structure 102 , the encapsulation thickness t ₂ is removed by etching or chemical mechanical planarization (CMP).

At operation 406, as shown in Figure 5C, a capping layer 110 is deposited over the encapsulation layer 112 to form the output coupler 104C. The capping layer 110 has a capping refractive index greater than or equal to 2.0. The second refractive index contrast, i.e., the difference between the capping refractive index and the encapsulation refractive index, is about 0.3 to about 0.5. In another embodiment, the second refractive index contrast is about 0.3. The capping layer 110 has a capping thickness variation 130 that is less than 20 nm. The capping thickness variation 130 is the difference between a maximum point 132 and a minimum point 134 of the uppermost surface 136 of the capping layer 110. The capping thickness variation 130 is achieved by using etching or chemical mechanical planarization (CMP). The material of the capping layer 110 may include titanium dioxide (TiO ₂ ), niobium (V) oxide (Ni ₂ O ₅ ), silicon nitride (SiN), tantalum (V) oxide (Ta ₂ O ₅ ) or a combination thereof. In one embodiment, the material of the capping layer 110 and the material of the optical device structure 102 are the same material.

In one embodiment, additional layers may be disposed on the cover layer 110. For example, an additional low refractive index layer (not shown) may be disposed on the cover layer 110, the refractive index of the low refractive index layer being less than or equal to 2.0. In another embodiment, as shown in FIG. 1C , other layers are disposed on the cover layer 110, such as a gradient refractive index layer 150, an anti-reflective coating (ARC) layer 160, or a mirror layer 170.

In summary, the embodiments described herein provide an output coupler of a waveguide combiner and a method of forming an output coupler of a waveguide combiner. In one embodiment, the output coupler and the method of forming the output coupler have a refractive index contrast between an optical component structure and a cover layer between 0.3 and 0.5. In another embodiment, the output coupler and the method of forming the output coupler have a first refractive index contrast between an optical component structure and a packaging layer between 0.3 and 0.5, and a second refractive index contrast between a packaging layer and a cover layer between 0.3 and 0.5.

Although the foregoing is directed to examples of the present disclosure, other and further examples of the present disclosure may be derived without departing from the basic scope thereof, and the scope thereof shall be determined by the appended claims.

100: waveguide combiner 101: optical element substrate 102: optical element structure 103: surface 110: layer 112: packaging layer 120: top surface 122: side wall 130: thickness variation 132: maximum point 134: minimum point 136: surface 150: graded refractive index layer 160: antireflection coating (ARC) layer 170: mirror layer 200: method 202: operation 204: operation 400: method 402: operation 404: operation 406: operation 104A: input coupler 104B: pupil dilator 104C: output coupler

In order to understand the above features of the present disclosure in detail, the present disclosure briefly summarized above can be described in more detail with reference to the embodiments, some of which are shown in the accompanying drawings. However, it should be noted that the accompanying drawings only show exemplary embodiments and should not be considered as limiting the scope thereof, and other equally effective embodiments may be allowed.

FIG. 1A is a schematic top view of a waveguide combiner according to an embodiment.

Figure 1B is a schematic cross-sectional view of a portion of an output coupler of a waveguide combiner according to an embodiment.

Figure 1C is a schematic cross-sectional view of a portion of an output coupler of a waveguide combiner according to an embodiment.

FIG. 2 is a flow chart of a method of forming an output coupler of a waveguide combiner according to an embodiment.

3A and 3B are schematic cross-sectional views of a portion of an optical element substrate during a method of forming an output coupler of a waveguide combiner according to an embodiment.

4 is a flow chart of a method for forming an output coupler of a waveguide combiner according to an embodiment.

5A-5C are schematic cross-sectional views of a portion of an optical element substrate during a method of forming an output coupler of a waveguide combiner according to an embodiment.

To facilitate understanding, identical reference numerals have been used, where possible, to designate identical elements that are common to the figures. It is anticipated that elements and features of one embodiment may be beneficially incorporated in other embodiments without further recitation.

Domestic storage information (please note in the order of storage institution, date, and number) None Overseas storage information (please note in the order of storage country, institution, date, and number) None

100: Waveguide combiner

101: Optical component substrate

102: Optical element structure

103: Surface

104A: Input coupler

104B:Pupil dilator

104C: Output coupler

Claims (20)

An output coupler of a waveguide combiner, comprising: A plurality of optical element structures disposed in or on an optical element substrate, the plurality of optical element structures having a structural refractive index less than or equal to 2.0; and A covering layer disposed above a top surface and sidewalls of each of the plurality of optical element structures, the covering layer having: A covering refractive index greater than or equal to 2.0, wherein a refractive index contrast between the covering refractive index and the structural refractive index is about 0.3 to about 0.5; and A covering thickness variation less than or equal to 20 nm, wherein the covering thickness variation is a difference between a maximum point and a minimum point of an uppermost surface of the covering layer. The output coupler of claim 1, wherein the plurality of optical element structures comprises silicon nitride (SiN), silicon oxynitride (SiON), aluminum oxide (Al ₂ O ₃ ), silicon oxide (SiO ₂ ) or a combination thereof. The output coupler of claim 1, wherein the capping layer comprises titanium dioxide (TiO ₂ ), niobium (V) oxide (Ni ₂ O ₅ ), silicon nitride (SiN), tantalum (V) oxide (Ta ₂ O ₅ ) or a combination thereof. An output coupler as described in claim 1, wherein the capping layer has a thickness of about 150 nm to about 250 nm. An output coupler as described in claim 1, wherein the optical element structure has a depth d of about 80 nm to about 150 nm. An output coupler as described in claim 1, wherein a critical dimension corresponding to a width or a diameter of a cross section of each optical element structure is less than 150 nm. An output coupler as described in claim 1, wherein a gap corresponds to a distance between adjacent optical element structures of a plurality of optical element structures, wherein an aspect ratio of the gap to a depth d is between about 4:1 and about 1:1. An output coupler as described in claim 1, wherein the refractive index contrast is 0.3. A method of forming an output coupler of a waveguide combiner, comprising the steps of: forming a plurality of optical element structures in or on an optical element substrate, the plurality of optical element structures having a structural refractive index less than or equal to 2.0; and depositing a covering layer over a top surface and sidewalls of each of the plurality of optical element structures, the covering layer having: a covering refractive index greater than or equal to 2.0, wherein a refractive index contrast between the covering refractive index and the structural refractive index is about 0.3 to about 0.5; and a covering thickness variation less than or equal to 20 nm, wherein the covering thickness variation is a difference between a maximum point and a minimum point of a topmost surface of the covering layer. The method of claim 9, wherein the plurality of optical element structures comprises silicon nitride (SiN), silicon oxynitride (SiON), aluminum oxide (Al ₂ O ₃ ), silicon oxide (SiO ₂ ) or a combination thereof. The method of claim 9, wherein the capping layer comprises titanium dioxide (TiO ₂ ), niobium (V) oxide (Ni ₂ O ₅ ), silicon nitride (SiN), tantalum (V) oxide (Ta ₂ O ₅ ) or a combination thereof. An output coupler of a waveguide combiner comprises: A plurality of optical element structures formed in or on an optical element substrate, the plurality of optical element structures having a structural refractive index greater than or equal to 2.0; A packaging layer disposed above a top surface and a side wall of each of the plurality of optical element structures, the packaging layer having: A packaging refractive index less than or equal to 2.0, wherein a first refractive index contrast between the packaging refractive index and the structural refractive index is about 0.3 to about 0.5; and A covering layer disposed above the packaging layer, the covering layer having: A covering refractive index greater than or equal to 2.0, wherein a second refractive index contrast between the packaging refractive index and the covering refractive index is about 0.3 to about 0.5; and A coating thickness variation less than or equal to 20 nm, wherein the coating thickness variation is a difference between a maximum point and a minimum point of an uppermost surface of the coating layer. The output coupler of claim 12, wherein the encapsulation layer comprises silicon nitride (SiN), silicon oxynitride (SiON), aluminum oxide (Al ₂ O ₃ ), silicon oxide (SiO ₂ ) or a combination thereof. An output coupler as described in claim 12, wherein the plurality of optical element structures and the cover layer comprise titanium dioxide (TiO ₂ ), niobium (V) oxide (Ni ₂ O ₅ ), silicon nitride (SiN), tantalum (V) oxide (Ta ₂ O ₅ ) or a combination thereof. An output coupler as described in claim 12, wherein the capping layer has a thickness of about 150 nm to about 250 nm. An output coupler as described in claim 12, wherein the optical element structure has a depth of about 80nm to about 150nm. An output coupler as described in claim 12, wherein a critical dimension corresponding to a width or a diameter of a cross-section of each optical element structure is less than 150 nm. An output coupler as described in claim 12, wherein a gap g corresponds to a distance between adjacent optical element structures of the plurality of optical element structures, wherein an aspect ratio of the gap g to a depth d is between approximately 4:1 and approximately 1:1. An output coupler as described in claim 12, wherein the first refractive index contrast is 0.3 and wherein the second refractive index contrast is 0.3. A method for forming an output coupler of a waveguide combiner, comprising the following steps: Forming a plurality of optical element structures in or on an optical element substrate, the plurality of optical element structures having a structural refractive index greater than or equal to 2.0; and Depositing a packaging layer over a top surface and sidewalls of each of the plurality of optical element structures, the packaging layer having: A packaging refractive index less than or equal to 2.0, wherein a first refractive index contrast between the packaging refractive index and the structural refractive index is about 0.3 to about 0.5; and Depositing a covering layer over the packaging layer, having: A covering refractive index greater than or equal to 2.0, wherein a second refractive index contrast between the covering refractive index and the packaging refractive index is about 0.3 to about 0.5; and A coating thickness variation of less than 20 nm, wherein the coating thickness variation is a difference between a maximum point and a minimum point of an uppermost surface of the coating layer.