阅读说明：本技术 用于促进波导内的全内反射的气穴结构 (Cavitation structure for promoting total internal reflection within a waveguide ) 是由 邓晓培 V·辛格 杨书强 K·罗 N-W·皮 F·Y·徐 于 2019-12-20 设计创作，主要内容包括：凹槽形成在波导的前侧和后侧。将固体致孔剂材料旋涂到前侧和后侧并填充凹槽。然后在波导的凸起构造和固体致孔剂材料上形成第一前盖层和第一后盖层。然后加热整个结构并且固体致孔剂材料分解成致孔剂气体。第一前盖层和第一后盖层是多孔的,以允许致孔剂气体逸出并且空气进入凹槽中。空气使波导的高折射率透明材料和空气之间的折射率差异最大化,以促进在波导中的从波导和空气之间的界面的反射。(Grooves are formed on the front and rear sides of the waveguide. A solid porogen material is spin coated onto the front side and back side and fills the recesses. A first front cap layer and a first back cap layer are then formed over the raised features of the waveguides and the solid porogen material. The entire structure is then heated and the solid porogen material decomposes into porogen gases. The first front cap layer and the first back cap layer are porous to allow porogen gas to escape and air to enter the recesses. Air maximizes the refractive index difference between the high index transparent material of the waveguide and air to promote reflection in the waveguide from the interface between the waveguide and the air.)

1. A method of manufacturing an optical system, comprising:

securing a cover layer of a selected transparent material to a waveguide of a high index of refraction transparent material having a front side and a back side, defining a cavity between the cover layer and the waveguide with an optical gas in the cavity such that if an ambient light source is located at the front side of the waveguide, an ambient light beam is transmitted in the selected transparent material of the cover layer, in the cavity containing the optical gas, and in the high index of refraction transparent material of the waveguide.

2. The method of claim 1, wherein the cover layer is a front cover layer positioned between the ambient light source and the front side of the waveguide, and the ambient light beam is sequentially transmitted through the selected transparent material of the front cover layer, through the cavity containing the optical gas, and into the high index transparent material of the waveguide.

3. The method of claim 2, wherein the selected transparent material of the front cover layer is an anti-reflective material that increases absorption and reduces reflection of the ambient light by the front surface of the waveguide.

4. The method of claim 3, wherein the high index material is one of a high index glass, a high index lithium niobate, a lithium tantalate, and a silicon carbide.

5. The method of claim 3, wherein the high index material has a refractive index of at least 1.74.

6. The method of claim 1, wherein the optical gas has a refractive index of less than 1.3.

7. The method of claim 1, wherein the optical gas is air having a refractive index of 1.

8. The method of claim 1, further comprising:

forming a stack comprising the waveguide, solid porogen material, and the cap layer; and

replacing the porogen material with the optical gas.

9. The method of claim 8, wherein the porogen material is removed by:

heating the porogen material to a decomposition temperature, wherein the porogen material becomes a sacrificial gas; and

removing the sacrificial gas from the cavity.

10. The method of claim 9, wherein the selected material of the cap layer is porous and the sacrificial gas is outgassed through the selected material of the cap layer.

11. The method of claim 9, wherein the porogen material decomposes at a decomposition temperature between 120 ℃ and 230 ℃.

12. The method of claim 10, wherein the capping layer is made of SiOx with a thickness of at least 12nm, where x is variable.

13. The method of claim 1, wherein a plurality of cavities are defined between the cover layer and the waveguide, and an optical gas is present in each respective cavity.

14. The method of claim 13, further comprising:

forming the front side of the waveguide to have a plurality of grooves and a plurality of raised formations, each raised formation being located between two grooves; and

supporting first portions of the cover layer with the raised formations, wherein second portions of the cover layer located between the first portions of the cover layer are located over the recesses such that a respective one of the cavities is defined by a respective one of the second portions of the cover layer and a respective one of the trenches in the front side of the waveguide.

15. The method of claim 14, wherein each groove has a depth and a width, and the width is less than 300 microns.

16. The method of claim 14, wherein the grooves are embossed on the front side.

17. The method of claim 14, further comprising:

forming a conformal layer on the front side of the waveguide, the conformal layer being made of a transparent material.

18. The method of claim 1, wherein the cover layer is a front cover layer positioned between the ambient light source and the front side of the waveguide, and the cavity is a front cavity positioned between the front cover layer and the front side of the waveguide, further comprising;

securing a back cover layer of a selected transparent material to the waveguide, defining a back cavity between the back cover layer and a back portion of the waveguide, with an optical gas in the back cavity, such that if an ambient light source is located on the front side of the waveguide, an ambient light beam is transmitted in the high index transparent material of the waveguide, the back cavity containing the optical gas, and the selected transparent material of the back cover layer.

19. The method of claim 1, wherein the cap layer is a first cap layer and the selected transparent material is a first selected transparent material, further comprising:

securing a second cover layer of a second selected transparent material to the first cover layer.

20. The method of claim 19, wherein said second cap layer is harder than said first cap layer.

21. The method of claim 19, wherein at least one of the selected transparent materials of the cover layer is an anti-reflective material that increases absorption and reduces reflection of the ambient light by the front surface of the waveguide.

22. The method of claim 21, further comprising:

forming a stack of the cap layer having a magnitude-changed refractive index.

23. The method of claim 22, wherein the capping layer is made of SiOx with a refractive index of 1.45 and TiOx with a refractive index between 2.2 and 2.3, where x is variable.

24. An optical system, comprising:

a waveguide of high refractive index transparent material having a front side and a back side;

a cover layer of a selected transparent material secured to the waveguide defining a cavity between the cover layer and the waveguide; and

an optical gas in the cavity such that if an ambient light source is located on the front side of the waveguide, an ambient light beam is transmitted in the selected transparent material of the cover layer, in the cavity containing the optical gas, and in the high index of refraction transparent material of the waveguide.

Technical Field

The present invention generally relates to an optical system and a method of manufacturing an optical system.

Background

Modern computing and display technology has facilitated the development of so-called "augmented reality" viewing devices. Such viewing devices typically have a frame that is mountable to the head of a user and typically include two waveguides, one in front of each eye of the observer. The waveguide is transparent so that ambient light from the object can be transmitted through the waveguide and the user can see the object. Each waveguide is also used to transmit the projected light from the projector to a respective eye of the user. The projected light forms an image on the retina of the eye. The retina of the eye thus receives ambient light and projected light. The user sees both the real object and the image produced by the projected light.

Projection light typically enters the waveguide at the edge of the waveguide, then reflects within the waveguide, and then exits the waveguide through the pupil of the waveguide towards the user's eye. Total Internal Reflection (TIR) is an ideal case where the light projected out of the waveguide is not lost and 100% of the projected light reaches the user's eye.

Disclosure of Invention

The present invention provides a method of manufacturing an optical system comprising securing a cover layer of a selected transparent material to a waveguide of a high refractive index transparent material having a front side and a back side, defining a cavity between the cover layer and the waveguide having an optical gas therein such that if an ambient light source is located at the front side of the waveguide, an ambient light beam is transmitted in the selected transparent material of the cover layer, in the cavity containing the optical gas, and in the high refractive index transparent material of the waveguide.

The present invention also provides an optical system comprising a waveguide of high refractive index transparent material having a front side and a back side; a cover layer of a selected transparent material secured to the waveguide; defining a cavity between the cap layer and the waveguide; and an optical gas in the cavity such that if the ambient light source is located on the front side of the waveguide, the ambient light beam is transmitted in the selected transparent material of the cover layer, in the cavity containing the optical gas, and in the high index transparent material of the waveguide.

Drawings

The invention is further described, by way of example, with reference to the accompanying drawings, in which:

FIG. 1A is a cross-sectional side view of a waveguide having a high index of refraction transparent material;

FIG. 1B is a view similar to FIG. 1A after the waveguide has been patterned with a plurality of concave and convex configurations on the front and back sides;

FIG. 1C is a view similar to FIG. 1B after filling the recess with a solid porogen material during spinning;

FIG. 1D is a view similar to FIG. 1C after forming a first front cap layer on the front side and a first back cap layer on the back side;

FIG. 1E is a view similar to FIG. 1D after the structure has been heated to remove solid porogen material and replace it with air;

FIG. 1F is a view similar to FIG. 1E after forming further cap layers on the first front cap layer and the first back cap layer to complete fabrication of the optical system;

FIG. 1G is a view of an optical system similar to FIG. 1F and showing its function;

FIG. 1H is a cross-sectional side view of an optical system in accordance with an alternative embodiment of the present invention, wherein the patterned layer is formed of a photoresist material;

FIG. 2 is a cross-sectional side view of an optical system having nanostructures to facilitate absorption of ambient light according to an alternative embodiment of the invention;

FIG. 3 is a cross-sectional side view similar to FIG. 1F, wherein the nanostructures have variable feature heights or variable duty cycles;

FIG. 4 is a cross-sectional side view similar to FIG. 2, with nanostructures of variable feature height or variable duty cycle;

FIG. 5 is a cross-sectional side view of an optical system having air pockets in different layers;

FIG. 6A is a Scanning Electron Microscope (SEM) image with a single coating;

FIG. 6B is an SEM image of a capped air cavity with a multilayer coating;

FIG. 7A is a cross-sectional side view of an air pocket capped with a silicon oxide layer and then spin coated with an optical polymer;

FIG. 7B is a view similar to FIG. 7A at a lower magnification level;

FIG. 8 is a 0 ° transmission plot from experimental measurements;

FIG. 9 is a model for simulation purposes;

FIG. 10A is a side view of a waveguide without any coating for simulation;

FIG. 10B is a side view of a waveguide with an optical polymer coating for simulation;

FIG. 10C is a side view of a waveguide with air pockets for simulation;

FIG. 10D is a side view of a waveguide with a polymer replacing the air gap for simulation;

FIG. 11 is a graph showing transmission data from a simulation; and

fig. 12 is a graph showing the user-side diffraction efficiency from the simulation.

Detailed Description

An optical system and a method for manufacturing the optical system are described. Grooves are formed on the front and rear sides of the waveguide. A solid porogen material is spin coated onto the front side and back side and fills the recesses. A first front cap layer and a first back cap layer are then formed over the raised features of the waveguides and the solid porogen material. The entire structure is then heated and the solid porogen material decomposes into porogen gases. The first front cap layer and the first back cap layer are porous to allow porogen gas to escape and air to enter the recesses. Air maximizes the refractive index difference between the high index transparent material of the waveguide and air to promote reflection in the waveguide from the interface between the waveguide and the air. Second front and rear cap layers are formed on the first front and rear cap layers, respectively, and then further front and rear cap layers are formed on the second front and rear cap layers. The cap layer has a refractive index that promotes absorption of ambient light through the cap layer and into the waveguide.

Fig. 1A to 1F illustrate a method of manufacturing an optical system according to an embodiment of the present invention.

Fig. 1A illustrates a waveguide 20 used as a primary substrate for subsequent fabrication. The waveguide 20 is made of a high refractive index transparent material. It is generally contemplated that the refractive index of waveguide 20 is at least 1.5. In the present embodiment, the waveguide 20 is made of high refractive index glass having a refractive index of 1.73. In another embodiment, the waveguide may be made of lithium niobate, lithium tantalate, or silicon carbide with a refractive index greater than 2.0. A high index of refraction transparent material is preferred because it maximizes the field of view of the final product.

Waveguide 20 has a front side 22 and a back side 24. The front side 22 and the back side 24 are spaced apart from each other by a thickness 26 of less than 3 mm. The front side 22 and the back side 24 each have a width 28 of between 50 and 70mm and a depth in the paper of between 50 and 70 mm. The front side 22 and the back side 24 are planes parallel to each other. The material of the waveguide 20 is soft enough to allow the front side 22 and the back side 24 to be formed at room temperature of 22 c or at moderately high temperatures of 50 c without forming microcracks or optical distortions within the material of the waveguide 20.

Fig. 1B illustrates waveguide 20 after front side 22 and back side 24 have been shaped. The front side 22 is shaped with a plurality of recesses 30 and a plurality of raised formations 32, wherein each raised formation 32 is located between two recesses 30. The side walls 34 of the recess 30 form the side walls of the raised formation 32. The raised formations 32 have outer surfaces 36 that are in the same plane. The recess 30 has a grooved surface 38 in the same plane and parallel to the plane of the outer surface 36. Each groove 30 has a width 40 between 10nm and 500 nm. The back side 24 is shaped with a plurality of recesses 44 and a plurality of raised formations 46, wherein each raised formation 46 is located between two recesses 44. The side walls 48 of the recess 44 form the side walls of the raised formation 46. The raised formation 46 has an outer surface 50 in the same plane. The groove 44 has a groove surface 52 in the same plane parallel to the plane of the outer surface 50. Each groove 44 has a width 54 between 10nm and 500 nm.

The front side 22 and the back side 24 are simultaneously formed with a tool that embosses the grooves 30 and 44 and the raised formations 32 and 46. The tool has a front portion and a rear portion made of hardened metal. The front portion has a shape complementary to the profile formed on the front side 22 and the rear portion has a shape complementary to the shape formed on the rear side 24. The waveguide 20 is inserted between the front and rear and an actuator is used to move the front and rear parts towards each other while the surfaces of the components exert pressure on the front side 22 and the rear side 24 of the waveguide 20. Waveguide 20 is then removed from the tool. The front side 22 and back side 24 are then etched. The etching process removes microscopic artifacts from trench surfaces 38 and 52 and planarizes trench surfaces 38 and 52.

A thickness 58 of waveguide 20 measured between outer surfaces 36 and 50 is greater than thickness 26 of the substrate in fig. 1A and a thickness 60 measured between trench surfaces 38 and 52 is less than thickness 26. The thickness of the waveguide 20 is between 200 microns and 1 nm. Each groove 30 or 44 has a depth 62 between 10nm and 500 nm.

Figure 1C illustrates the waveguide 20 after deposition of a porogen (sacrificial) material. The porogen material may be spin coated on the front side 22 and back side 24 of the waveguide 20. Porogen material fills recesses 30 and 44. The porogen material forms a plurality of separate porogen portions 64 within recesses 30 on frontside 22 and forms a plurality of separate porogen portions 66 within recesses 44 on backside 24. Each porogen portion 64 fills a respective recess 30 until the outer surfaces 68 of the porogen portions 64 are coplanar with the outer surfaces 36 of the raised features 32. Porogen portions 66 fill recesses 44 until the outer surfaces 70 of porogen portions 66 are coplanar with the outer surfaces 50 of raised features 46.

FIG. 1D illustrates the structure of FIG. 1C after forming a first front cap layer 74 and a first back cap layer 76. The capping layers 74 and 76 may be formed, for example, in a chemical vapor deposition process.

The first front cover layer 74 is made of a selected solid transparent material. A first cap layer 74 is formed directly on the outer surface 36 of the raised feature 32 and the outer surface 68 of the porogen component 64. First front cover layer 74 is also adhered to outer surface 36 of raised formation 32 and is thus secured to waveguide 20.

The first front cover layer 74 is shown as ultimately manufactured and made of a relatively strong solid material. However, the first front cover layer 74 is initially a thin and unstable film during its manufacture. Such a membrane is fragile and would collapse without the support provided by the solid material of porogen component 64. The first cap layer 74 becomes more stable as it becomes thicker and eventually thick enough so that it does not rely on the support provided by the porogen component 64 for its structural integrity. The first cap layer 74 has a plurality of first portions 80 formed on the raised features 32 and a plurality of second portions 82 formed on the porogen portions 64.

Similarly, the first back cap layer 76 relies on the solid material of the porogen portions 66 as support during its initial fabrication, but does not require support of the porogen portions 66 after it has been finally fabricated and obtained to a thickness suitable to support itself without the need for porogen portions 66. First cap layer 76 has a plurality of first portions 84 formed on raised feature 46 and a plurality of second portions 86 formed on porogen portion 66.

FIG. 1E illustrates the structure of FIG. 1D after porogen portions 64 and 66 are removed to leave respective cavities 88 and 90. Each cavity 88 and 90 has the same dimensions as the corresponding porogen portion that has been removed. Each of the cavities 88 and 90 is filled with an optical gas in the form of air.

The solid porogen materials of porogen portions 64 and 66 are thermally decomposable materials or mixtures of materials that decompose at temperatures that do not cause damage to waveguide 20, first front cap layer 74 or first back cap layer 76. The entire structure of figure 1D is heated to a decomposition temperature, which results in the conversion of the solid porogen material to porogen gas. The material of the first front and back cap layers 74 and 76 is sufficiently porous to allow porogen gas to pass through the first front and back cap layers 74 and 76 such that porogen gas exits the cavities 88 and 90 and air passes through the first front and back cap layers 74 and 76 into the cavities 88 and 90. For example, propylene carbonate (PPC) can be decomposed in an inert atmosphere or air without leaving a significant residue behind. Generally, a decomposition temperature between 120 ℃ and 230 ℃ is expected. If decomposition temperatures between 200 ℃ and 300 ℃ are used, porogen portions 64 and 66 can be replaced with air in a short time. If the decomposition temperature has to be lowered, additives may be added or the baking time may be extended. Decomposition temperatures between 120 ℃ and 160 ℃ are possible by suitable combinations of materials, film thickness and baking time. The baking temperature and temperature ramp rate need to be carefully controlled so that no significant residue is left and so that the release rate of the porogen gas is controlled so as not to damage the first front cap layer 74 and the first back cap layer 76, such as popping, sagging, and cracking.

When cavities 88 and 90 are ultimately formed, first front cover layer 74 and first portion 80 of first back cover layer 76 are secured to and supported by raised formations 32 and 46. Each of the cavities 88 is defined on three sides by a surface of a respective one of the grooves 30 and on a fourth side by one of the second portions 82 of the first front cover layer 74. Similarly, each of the cavities 90 is defined on three sides by the surface of the groove 44 and on the fourth side by one of the second portions 86 of the first back cover layer 76. It should be noted that second portions 82 and 86 of first front cap layer 74 and first back cap layer 76 are no longer supported by porogen portions 64 and 66. However, the first front cap layer 74 and the first back cap layer 76 are still supported by the raised formations 32 and 46, and given that the widths 40 and 54 of the cavities 88 and 90 are both less than 500nm, the structural integrity of the first front cap layer 74 and the first back cap layer 76 can be preserved during and after degassing of the solid porogen material.

Fig. 1F illustrates the structure of fig. 1E after a second cap layer 94 is formed on first cap layer 74 and a further cap layer 96 is sequentially formed on second cap layer 94. Second front cover layer 94 provides additional strength to first front cover layer 74. For better adhesion, a material such as aluminum may be used between first and second front cover layers 74 and 94 and between second and further front cover layers 94 and 96OrThe adhesion promoter of (1).

The second and further front cover layers 94, 96 are made of different selected transparent materials. One or more of the materials of the first, second and further front cover layers 74, 94 and 96 are selected to have a refractive index that promotes light absorption and reduces light reflection. In a practical example, the first front cover layer 74 is made of SiOx with a refractive index of 1.45, the second front cover layer 94 is made of TiOx with a refractive index between 2.2 and 2.3, the third front cover layer is made of SiOx, and the fourth front cover layer is made of TiOx, where "x" is variable.

FIG. 1F also illustrates the structure of FIG. 1E after a second back cap layer 98 is formed on the first back cap layer 76 and further back cap layers 100 are sequentially formed on the second back cap layer 98. The second back cover layer 98 provides additional strength to the first back cover layer 76. For better adhesion, a material such as aluminum may be used between the first back cover layer 76 and the second back cover layer 98 and between the second back cover layer 98 and the further back cover layer 100OrThe adhesion promoter of (1).

The second 98 and further 100 back cover layers are made of different selected transparent materials. One or more of the materials of the first, second and further back cover layers 76, 98 and 100 are selected to have a refractive index that promotes light absorption and reduces light reflection. In a practical example, the first back cover layer 76 is made of SiOx with a refractive index of 1.45, the second back cover layer 98 is made of TiOx with a refractive index between 2.2 and 2.3, the third back cover layer is made of SiOx, and the fourth front and back layers are made of TiOx, where "x" is variable.

FIG. 1G further illustrates the source 102 of ambient light and the projector 106. The source 102 of ambient light may for example be an object reflecting ambient light. Ambient light is represented by beams 104A and 104B. Each light beam 104A and 104B propagates through the ambient air and then passes through the front cover layers 96, 94 and 74, through the waveguide 20 and through the back cover layers 76, 98 and 100 in turn. The refractive index between adjacent cap layers is minimized to minimize reflection of ambient light and to facilitate absorption of ambient light into waveguide 20. The light beam 104A also passes through the air in one of the recesses 44 of the back side 24 of the waveguide 20. The light beam 104B passes through the air in one of the grooves 30 of the front side 22 of the waveguide 20.

The projector 106 produces projected light represented by the beam 104C. The light beam 104C is inserted into the waveguide 20. The light beam 104C may, for example, be inserted through the back cover layers 100, 98, and 76, and their refractive indices are selected to promote absorption and limit reflection of the light beam 104C. The light beam 104C is directed to one of the grooves 30 in the front side 22. The difference between the refractive index of the waveguide 20 and the refractive index of the air in the groove 30 is maximized to promote reflection of the light beam 104C and limit transmission of the light beam 104C into the air in the groove 30. Air has a refractive index of 1 and waveguide 20 may have a refractive index of at least 1.74. Thus, the refractive indices differ from each other by at least 0.74. In another embodiment, another optical gas may be used instead of air, as long as such optical gas has a refractive index of less than 1.3. Ideally, the refractive index between the material of the waveguide 20 and the optical gas should be at least 0.50. The light beam 104C reflected from the air in one of the grooves 30 is then transmitted to one of the grooves 44 in the back side 24 of the waveguide 20. The light beam 104C is reflected from the interface between the air in the groove 44 towards the other groove 30 in the front side 22 of the waveguide 20. An alternative structure could be to use a direct imprint pattern of a Si-containing resist on a spin-coated paraben material, and then evaporate the spin-coated paraben material. The Si-containing resist may be plasma treated to form the SiOx polymer structure.

The reflection at the air interface significantly improves the optical image quality by altering the optical ghosting in the following way: such as 1) increasing the overall transmission of world light through a "transparent" eyepiece, making world-side objects clearer and brighter; 2) maintaining a refractive index difference between the relief structure grooves and the grating height, thereby allowing high diffraction efficiency of the grating constituting the functional waveguide relief structure; 3) reducing ghosting from reflections of light exiting the eyepiece and reflected back from different lens or stacked waveguide interfaces; and 4) reducing diffraction from the eye-box to the user and producing rainbow defects that would be stronger without the nano-features and film stack structure.

Fig. 1A to 1G illustrate one example of producing an antireflection cover structure. Alternative methods may also be used to produce the anti-reflective properties. Fig. 1H illustrates an optical system in which a front patterned layer 120 and a rear patterned layer 122 are formed on the front and rear sides of the waveguide 20. Layers 120 and 122 may be patterned using conventional photolithographic techniques and formed from a polymer or photoresist material suitable for patterning using photolithography. No additional etching step is required. These layers are then coated with a front conformal layer 124 and a back conformal layer 126, respectively. The conformal layer is made of inorganic SiOx and is formed using chemical vapor deposition. Conformal layers 124 and 126 define recesses 30 and 44, and recesses 30 and 44 are covered by front cover layer 74 and back cover layer 76.

Fig. 2 illustrates an alternative structure in which the nano-patterns 110 are made on the outer surface rather than multiple cap layers as described in fig. 1F. The nano-pattern 110 reduces reflection of ambient light and facilitates absorption of ambient light. Fig. 2 has like reference numerals to those used in fig. 1F, and like reference numerals designate like or similar components.

Fig. 3 and 4 are similar to fig. 1F and 2. The optical systems shown in fig. 3 and 4 have waveguides with variable heights or "duty cycles". Porogen material may be formed in such structures in a spin coating operation as previously described.

Fig. 5 illustrates a further optical system with different layers of different three-dimensional nanostructure stacks. The three-dimensional nanostructure stack can be designed differently for different waveguide purposes. The material composition, thickness, and nanopatterns having various spatial and geometric configurations for each cap layer may be different or the same as each other.

Fig. 6A illustrates a Scanning Electron Microscope (SEM) image of a capped air cavity with a single layer of SiOx on an etched grating in high index glass. Fig. 6B shows an SEM image of a capped air cavity with a multilayer coating. The multilayer coating alternates between SiOx and TiOx, each layer having a different thickness. The composition and thickness of each layer on the top of the grating are 20nm porous SiOx, 15nm TiOx, 65nm SiOx, 34nm TiOx, 18nm SiOx, 59nm TiOx and 97nm SiOx from bottom to top in sequence. The multilayer coating on top of the cavitation structure may be applied by chemical and/or physical vapour deposition or spin coating or a combination of different coating techniques.

Fig. 7A and 7B show samples with air pockets first capped with a SiOx layer and then spin-coated with an optical polymer (Teflon AF1600 from Chemours corporation) with a refractive index of 1.31. The air pockets lower the effective refractive index of the nanostructured grating region, resulting in a gradual change in refractive index from the bulk substrate to the surface grating region, to the SiOx cap layer, to the spin-coated optical polymer layer, and finally to air. Such a gradual change in refractive index is advantageous for antireflection purposes and may significantly enhance the transmission of ambient light.

FIG. 8 is a 0 transmission plot from experimental measurements showing a significant increase in the combined transmission through the air pocket and coating in FIGS. 7A and 7B. The nanostructured substrate here is a high refractive index lithium niobate substrate that is etched to form a surface grating.

Fig. 9 illustrates a model for the purpose of simulating reflection characteristics. Fig. 10A through 10D show four different antireflection coating stack configurations simulated within the structure of fig. 9. Fig. 10A is a side view of a waveguide without any coating for simulation. FIG. 10B is a side view of a waveguide with an optical polymer coating for simulation. FIG. 10C is a side view of a waveguide with air pockets for simulation. FIG. 10D is a side view of a polymer waveguide with an alternative air gap for simulation. Fig. 11 is a graph illustrating transmission data based on a simulation. Figure 12 shows the user-side diffraction efficiency from simulations using high refractive index lithium niobate for waveguides. It can be seen that for the simulated transmission data, a direct spin-coated low index optical polymer (AF 2400 from chemiurs, refractive index 1.29) has a similar effect in enhancing transmission compared to the configuration with air pockets. However, for single bounce diffraction efficiency, the air pocket configuration is significantly better than the configuration with only spin-on low index polymer or stacks with PPC filled grating trenches. Simulations show that the diffraction efficiency is significantly higher than in the case of filling the trenches with a low refractive index material, but still lower than in the case without any antireflective coating applied. To further improve efficiency, the grating geometry needs to be changed accordingly.

While certain exemplary embodiments have been described and shown in the accompanying drawings, it is to be understood that such embodiments are merely illustrative of and not restrictive on the broad invention, and that this invention not be limited to the specific constructions and arrangements shown and described, since modifications may occur to those ordinarily skilled in the art.