WO2023183506A1

WO2023183506A1 - Diffractive gratings for optical elements of augmented reality and virtual reality head-mounted displays

Info

Publication number: WO2023183506A1
Application number: PCT/US2023/016111
Authority: WO
Inventors: Yuval Ofir
Original assignee: Rovi Guides, Inc.
Priority date: 2022-03-23
Filing date: 2023-03-23
Publication date: 2023-09-28

Abstract

Head-mounted displays with waveguides comprising buried diffractive gratings and methods for fabricating said waveguides are described herein. In an embodiment, a head-mounted display comprises an optical element and an image source that provides an image beam to an optical element. The optical element comprises a first flat surface, a second flat surface, and a buried diffractive grating spaced from and disposed between the first surface and the second surface. The buried diffractive grating comprises a high-refractive index material interspersed with a low-refractive index material or non-solid pockets, such as gas, air or vacuum. The diffractive grating may also diffract light into, through, or out of the waveguide and may absorb any light that is not polarized in the direction of the grating. Light diffracted by a first of two gratings may be further diffracted by a second grating without interacting with a surface of the optical element between diffractions.

Description

DIFFRACTIVE GRATINGS FOR OPTICAL ELEMENTS OF AUGMENTED REALITY

AND VIRTUAL REALITY HEAD-MOUNTED DISPLAYS

Background

[0001] This disclosure is generally directed to optical elements. In particular, the present disclosure relates to techniques for fabricating

[0002] diffractive gratings for use in optical equipment (e.g., for optical devices in augmented and virtual reality head-mounted displays) and the resultant waveguides fabricated through said techniques.

Summary

[0003] Virtual reality (VR) and augmented reality (AR) systems are becoming increasingly more common in the modern world. A large focus of modem technology is to create headmounted displays which provide near-eye displays of images. While head-mounted displays have become common for use with VR, head-mounted displays are less popular for AR implementations where their relative bulkiness creates difficulty in everyday environments. [0004] One of the reasons for the excessive bulkiness of head-mounted displays is the optical elements used to create them. In some approaches, head-mounted displays include waveguides made from glass or plastic which diffract light from an image source to an eye of the user. The waveguides may be implemented in lenses of glasses that are attached to the image source. To diffract the light, waveguides include diffractive gratings, such as an input coupler (incoupling) grating and an output coupler (outcoupling) grating. Approaches of gratings used in waveguides include volume Bragg gratings and surface relief gratings.

[0005] Volume Bragg gratings are useful in that they provide refractive index modulation, but can be difficult to fabricate as they involve irradiating photosensitive glass with ultraviolet light. Conversely, surface relief gratings can be mass produced with lithographic techniques as they involve creating surface structures that refract incoming light beams. [0006] Surface relief gratings, however, are limited in usability, as they can be extremely fragile. Any additional optical elements that touch the surface relief gratings can damage them. Additionally, any coatings placed on the optical elements can damage the surface relief gratings or ruin their ability to refract light by covering spaces in the structure that need to have a low refractive index. These issues are compounded when combined with headmounted displays which are often built with multiple waveguides for different frequencies of light. Additionally, for AR usage, people with vision problems may require additional corrective lenses in addition to the waveguides. As waveguides with surface relief gratings include fragile non-flat surfaces, and rely on total internal reflection, the addition of lenses or coatings is not a straightforward process.

[0007] To address the aforementioned problem, in one approach, multiple waveguides are mounted in a housing and spaced such that the lenses do not touch each other. If other types of lenses are required, they can also be mounted in the housing such that they do not touch the surface of the waveguides. To further protect the waveguides, the house may have an outer shell that fully surrounds all of the waveguides. While this approach protects the surface relief gratings, the resulting head-mounted display is bulky and cumbersome.

[0008] To overcome such deficiencies, head-mounted displays with waveguides comprising buried diffractive gratings and methods for fabricating said waveguides are described herein. In an embodiment, a head-mounted display comprises an optical element and an image source that provides an image beam to an optical element. The optical element comprises a first flat surface, a second flat surface, and a buried diffractive grating spaced from and disposed between the first surface and the second surface. The buried diffractive grating comprises a high-refractive index material interspersed with a low-refractive index material or non-solid pockets, such as gas, air or vacuum.

[0009] The high-refractive index material comprises a material with a refractive index above 1.6, such as a high refractive index glass, a high refractive index resin, cubic zirconium oxide, titanium oxide, aluminum oxide, diamond hafnium oxide, tantalum oxide, or zinc oxide. In some embodiments, the refractive index of the high-refractive index material is above is within a range of 1.8-2.1. The low-index material may include any of lithium fluoride, calcium fluoride, magnesium fluoride, or any other low refractive index optical resin. The buried diffractive grating may be used as one or more of an incoupling grating, an outcoupling grating, an expansion grating, or any other diffractive grating used in conjunction with a head-mounted display.

[0010] One method of producing the buried diffractive grating in the waveguide comprises patterning a sacrificial material on a surface of a transparent material, such as glass or plastic. The sacrificial material is then coated with a coating comprising a refractive index that is substantially equal to the refractive index of the transparent material, such as cubic zirconium oxide, titanium oxide, aluminum oxide, diamond hafnium oxide, tantalum oxide, or zinc oxide. The sacrificial material is then removed through a process of sintering or dissolution to form nonsolid pockets within the optical waveguide.

[0011] Another method of producing the buried diffractive grating in the waveguide comprises patterning a low-index material, such as lithium fluoride, calcium fluoride, magnesium fluoride, or any other low refractive index optical resin, on a surface of a transparent material, such as glass or plastic. The low-index material is then coated with a coating comprising a refractive index that is substantially equal to the refractive index of the transparent material, such as cubic zirconium oxide, titanium oxide, aluminum oxide, diamond hafnium oxide, tantalum oxide, or zinc oxide or a relevant high refractive index resin.

[0012] Another method of producing the buried diffractive grating in the waveguide comprises fabricating a grating onto a transparent material, such as through various lithographic techniques. A flat piece of transparent material of a desired thickness is then bonded to the top of the grating, such as through glass laser bonding. An additional processing step may include thinning the flat piece of glass to the desired thickness.

[0013] An aspect of the present disclosure addresses the problems of the fragility of surface relief gratings and the bulkiness caused by using additional optical elements or coatings in addition to optical elements with surface relief gratings. Buried surface relief gratings allow the optical elements to have flat surfaces, thereby allowing the optical elements to be coated with other materials, such as anti -reflective coating or relevant optical coatings, or to subsequently be bonded to other optical elements, such as bonding multiple waveguides together or bonding a waveguide to corrective lenses. Additionally, the buried diffractive gratings fabricated using the techniques described herein can be placed at any depth within the optical element, placed at varying depths within the optical element, and/or fabricated in any of a variety of shapes, spacings, or structures.

[0014] One further issue with VR and AR headsets is stray light within the waveguides. Stray light can be caused by a variety of sources, such as unpolarized light coming from the image source, changes in polarization to some of the light as it reflects through the waveguide, or light from an outside source, such as the sun, entering the waveguide. The issues of stray light are compounded with AR headsets which are intended to be used in diverse environments with different light conditions, including outdoor environments with greater amounts of incident light from the sun or that is bouncing off surfaces around the headset.

[0015] To help address the aforementioned problem, in one approach, wire grid polarizers are used to absorb light based on polarization. Wire grid polarizers use sub -wavelength width strips of silver to absorb light in specific polarizations, thereby removing any light that is not polarized in a desired direction. While this approach can remove any light that is not polarized in a specific direction, the fabrication of wire grid polarizers onto a waveguide can be excessively difficult. The wires themselves are very susceptible to warping and the act of applying an adhesive to the wires to attach them to the waveguide can ruin the orientation or straightness of the wires. Additionally, when a waveguide uses surface relief gratings, the available surface area for placing wire grid polarizers is severely reduced.

[0016] To overcome such deficiencies of wire grid polarizers and to more effectively address the issue of stray light, head-mounted displays with waveguides comprising surface relief gratings made of a transparent conductive material and methods for fabricating said waveguides are described herein. In an embodiment, a head-mounted display comprises an image source that provides an image beam to an optical element. In this embodiment, the optical element includes a waveguide that comprises a diffractive grating formed at least in part by a transparent conductive material. The diffractive grating diffracts light into, through, or out of the waveguide as well as absorbing light that is not polarized in the direction of the diffractive grating.

[0017] In one approach, the transparent conductive material comprises a material designed to absorb light while also comprising a high enough refractive index that it can be used to diffract light. Examples of transparent conductive materials include transparent conductive oxides, such as fluorinated tin oxide (FTO), indium tin oxide (ITO), aluminum zinc oxide (AZO), indium zinc oxide, indium tin zirconium oxide, indium gallium oxide, indium gallium zinc oxide, tin oxide, or zinc oxide, or conductive polymers, such as poly(3,4- ethylenedioxythiphene) (PEDOT) or poly(3,4-ethylenedi oxythiophene): poly(styrene sulfonate) (PEDOT:PSS). In some embodiments, the transparent conductive material is used to create an in-coupling grating to reduce incoming stray light from the image source. In some embodiments, the transparent conductive material is used to create an out-coupling grating to reduce stray light from external sources and/or to reduce the stray light diffracted to an eye of the user through the out-coupling grating. Other embodiments may use transparent conductive materials in any of the diffractive gratings of the waveguide and/or multiple diffractive gratings of the waveguide.

[0018] In some embodiments, the transparent conductive material is used to create a buried diffractive grating. The buried diffractive grating may be generated by patterning the transparent conductive material onto a substrate, such as glass or plastic, and interspersing the transparent conductive material with a resin comprising a low refractive index or a sacrificial material that is to be removed later through sintering or dissolution. The materials may then be covered with a resin that has a refractive index substantially equal to that of the substrate. [0019] The present disclosure addresses the problem of stray light in waveguides used in AR or VR displays by utilizing one or more diffractive gratings fabricated from a transparent conductive material. The thin lines of transparent conductive material used to create the diffractive gratings double as a polarizer, thereby absorbing light that is not polarized in the correct direction while not requiring a separate space on the waveguide on which to be fabricated. Additionally, the methods described herein are versatile to different implementations, such as being able to be fabricated in different patterns or fabricated with different compositions. For instance, ITO may be doped with more indium to create a more absorbent but less transparent diffractive grating or with more tin oxide to create a less absorbent but more transparent grating. This diversification allows the diffractive gratings to be used differently in different locations of a waveguide. For instance, in smaller locations, such as an in-coupling grating, a higher doping of indium would reduce the stray light in the system while having a relatively small effect on the usage of the waveguide as a lens. Conversely, in larger or more central locations, such as in the out-coupling grating, a higher doping of tin oxide would make it easier for a user to see through the lens but would decrease the overall absorption of non-polarized or differently polarized light.

[0020] One approach to diffractive gratings is the surface relief grating which uses structures on the surface of the waveguide to diffract light based on differences in the refractive indices between the structures and the surrounding air. As the difference in refractive indices between the structures and the air increase, the angle by which the image beam is diffracted increases as well, thereby reducing the number of times the beam reflects through the waveguide before being diffracted out through the out-coupling grating and increasing the quality of the produced image. Thus, manufactures of waveguides often work to increase the difference between the refractive indices of the structures and the surrounding air by using materials for the structures that have a high refractive index.

[0021] Unfortunately, materials with high refractive indices can be expensive, difficult to work with, rarer, or difficult to manufacture. Additionally, even the best materials currently available for creating diffractive gratings can be improved with respect to the diffraction of the image beam. Additionally, optimizing the difference in refractive indices through use of high-refractive index materials limits the types of materials that can be used, thereby removing materials which could provide different benefits, such as transparent conductive materials which can be used to absorb unpolarized or polarized light .

[0022] To improve the diffraction of light by the diffractive gratings, superimposed diffractive gratings are described herein. The superimposed diffractive gratings include two diffractive gratings including at least one buried diffractive grating and a second diffractive grating situated at least in part above the buried diffractive grating (e.g., located less deep into substrate of the optical element). In some embodiments, the second diffractive grating comprises a surface relief grating on a flat surface of the waveguide. In some embodiments, the second diffractive grating comprises a second buried diffractive grating. When light hits the first of the two diffractive gratings, it diffracts at a first angle. When used as an incoupling grating, light diffracted from the second diffractive grating is further diffracted by the first diffractive grating prior to being reflected from the surfaces of the waveguide. When used as an out-coupling grating, light diffracted from the first diffractive grating is further diffracted by the second diffractive grating prior to exiting the waveguide. By superimposing the two gratings, diffraction of light for high-refractive materials can be increased and/or materials with a lower refractive index can be used to produce a same effect. Additionally, by superimposing two diffractive gratings, different types of structures can be used in combination, such as a slanted structure used in combination with a straight structure. The superimposed diffractive gratings can be used for the in-coupling grating, out-coupling grating, expansion grating, other grating, or any combination thereof.

[0023] The buried diffractive grating comprises a high-refractive index material interspersed with a low-refractive index material or non-solid pockets, such as gas, air or vacuum. The high-refractive index material comprises a material with a refractive index above 1.6, such as a high refractive index glass, a high refractive index resin, cubic zirconium oxide, titanium oxide, aluminum oxide, diamond, hafnium oxide, tantalum oxide, or zinc oxide. In some embodiments, the refractive index of the high-refractive index material is within a range of 1.8-2.1. The low-index material may include any of lithium fluoride, calcium fluoride, magnesium fluoride, or any other low refractive index optical resin. The buried diffractive grating may be used as one or more of an in-coupling grating, an outcoupling grating, an expansion grating, or any other diffractive grating used in conjunction with a head-mounted display.

[0024] One method of producing the superimposed diffractive gratings comprises patterning a sacrificial material on a surface of a transparent material, such as glass or plastic. The sacrificial material is then coated with a coating comprising a refractive index that is substantially equal to the refractive index of the transparent material, such as cubic zirconium oxide, titanium oxide, aluminum oxide, diamond hafnium oxide, tantalum oxide, or zinc oxide. The sacrificial material is then removed through a process of sintering or dissolution to form nonsolid pockets within the optical waveguide. A second diffractive grating is then patterned on top of the coating.

[0025] Another method of producing the superimposed diffractive gratings in the waveguide comprises patterning a low-index material, such as lithium fluoride, calcium fluoride, magnesium fluoride, or any other low refractive index optical resin, on a surface of a transparent material, such as glass or plastic. The low-index material is then coated with a coating comprising a refractive index that is substantially equal to the refractive index of the transparent material, such as cubic zirconium oxide, titanium oxide, aluminum oxide, diamond hafnium oxide, tantalum oxide, or zinc oxide or a relevant high refractive index resin. A second diffractive grating is then patterned on top of the coating.

[0026] Another method of producing the superimposed diffractive gratings in the waveguide comprises fabricating a grating onto a transparent material, such as through various lithographic techniques. A flat piece of transparent material of a desired thickness is then bonded to the top of the grating, such as through glass laser bonding. An additional processing step may include thinning the flat piece of glass to the desired thickness. A second diffractive grating is then patterned on top of the transparent material.

[0027] In some embodiments, one or more of the surface relief grating or the buried diffractive grating is fabricated using a transparent conductive material. The transparent conductive material comprises a material designed to absorb certain polarization light while also comprising a high enough refractive index that it can be used to diffract light. Examples of transparent conduct materials include transparent conductive oxides, such as fluorinated tin oxide, indium tin oxide, aluminum zinc oxide, indium zinc oxide, indium tin zirconium oxide, indium gallium oxide, indium gallium zinc oxide, tin oxide, or zinc oxide, or conductive polymers, such as poly(3,4-ethylenedioxythiphene) (PEDOT) or poly(3,4- ethylenedi oxy thiophene): poly(styrene sulfonate) (PEDOT:PSS). In some embodiments, the transparent conductive material is used to create an in-coupling grating to reduce incoming stray light from the image source by absorbing unwanted polarizations. In some embodiments, the transparent conductive material is used to create an out-coupling grating to reduce stray light from external sources and/or to reduce the stray light diffracted to an eye of the user through the out-coupling grating, by absorbing unwanted polarizations. Other embodiments may use diffractive gratings as any of the diffractive gratings of the waveguide and/or multiple diffractive gratings of the waveguide.

[0028] An aspect of the present disclosure addresses the problems of difficulties diffracting light through waveguides. By using superimposed diffractive gratings, cheaper materials with lower diffractive indices can be used to the same effect as the higher refractive index materials and higher refractive index materials can be used to greater effect. Additionally, different materials which provide other benefits, such as transparent conductive materials which may act as a polarizer, can be used without negatively impacting the diffraction of light through the waveguide.

Description of the Drawings

[0029] The above and other objects and advantages of the disclosure will be apparent upon consideration of the following detailed description, taken in conjunction with the accompanying drawings, in which like reference characters refer to like parts throughout, and in which:

[0030] FIG. 1 A depicts a diagram of an optical element of a head-mounted display comprising a waveguide with a buried diffractive grating.

[0031] FIG. IB depicts a diagram of a plurality of diffraction gratings of an optical element of a head-mounted display.

[0032] FIG. 2 depicts an example of fabricating a buried diffractive grating in an optical element using sacrificial material.

[0033] FIG. 3 depicts an example of fabricating a buried diffractive grating in an optical element using direct laser bonding. [0034] FIG. 4 depicts an example of fabricating a buried diffractive grating in an optical element using a low-refractive index material.

[0035] FIG. 5 depicts an example of fabricating a buried diffractive grating at multiple depths in an optical element.

[0036] FIG. 6 depicts an optical element comprising a buried diffractive grating and an additional coating.

[0037] FIG. 7 depicts an optical element comprising a buried diffractive grating and additional coatings that allow other optical elements to be attached to the optical element comprising the buried diffractive grating.

[0038] FIG. 8 depicts an example of a plurality of stacked optical elements with buried diffractive gratings.

[0039] FIG. 9 depicts an example of a head-mounted display comprising one or more optical elements with a buried diffractive grating.

[0040] FIG. 10A depicts a diagram of an optical element of a head-mounted display comprising a waveguide with a superimposed diffractive grating.

[0041] FIG. 10B depicts a diagram of a plurality of diffraction gratings of an optical element of a head-mounted display.

[0042] FIG. 10C depicts a diagram of operation of optical elements of a head-mounted display comprising a superimposed diffractive grating.

[0043] FIG. 11 depicts an example of fabricating a superimposed diffractive grating in an optical element using sacrificial material.

[0044] FIG. 12 depicts an example of fabricating a superimposed diffractive grating in an optical element using direct laser bonding.

[0045] FIG. 13 depicts an example of fabricating a superimposed diffractive grating in an optical element using a low-refractive index material.

[0046] FIG. 14A depicts examples of fabricating a superimposed diffractive grating with a buried diffractive grating at multiple depths in an optical element.

[0047] FIG. 14B depicts example structures of superimposed gratings.

[0048] FIG. 15 depicts an example of a plurality of waveguides comprising superimposed diffractive gratings for use in a head-mounted display.

[0049] FIG. 16 depicts an optical element comprising a superimposed buried diffractive grating and an additional coating. [0050] FIG. 17 depicts an optical element comprising a superimposed buried diffractive grating and additional coatings that allow other optical elements to be attached to the optical element comprising the buried diffractive grating.

[0051] FIG. 18 depicts an example of a plurality of stacked optical elements with superimposed buried diffractive gratings.

[0052] FIG. 19 depicts an example of a head-mounted display comprising one or more optical elements with a superimposed diffractive grating.

[0053] FIG. 20A depicts a diagram of an optical element of a head-mounted display comprising a waveguide with a diffractive grating.

[0054] FIG. 20B depicts a diagram of a plurality of diffraction gratings of an optical element of a head-mounted display.

[0055] FIG. 20C depicts a diagram of operation of optical elements of a head-mounted display comprising a transparent conductive material in a diffractive grating.

[0056] FIG. 21 depicts an example of fabricating a diffractive grating in an optical element using a transparent conductive material.

[0057] FIG. 22 depicts an example of fabricating a buried diffractive grating in an optical element using a transparent conductive material and a low-refractive index material.

[0058] FIG. 23 depicts an example of fabricating a buried diffractive grating in an optical element using a transparent conductive material and a sacrificial material.

[0059] FIG. 24 depicts an example of a head-mounted display comprising one or more optical elements with a buried diffractive grating.

Detailed description

[0060] FIG. 1 A depicts a diagram of an optical element of a head-mounted display comprising a waveguide with a buried diffractive grating. Optical element 100 comprises a first flat surface 102 that is substantially parallel to a second flat surface 104. The first flat surface 102 and second flat surface 104 may comprise a same material, such as glass or plastic, or different materials with substantially equal refractive indices. As used herein, a first refractive index is substantially equal to a second refractive index if the refractive indices differ by less than 0.01. In some embodiments, the refractive indices differ by less than 0.001. In some embodiments, the refractive indices differ by less than 0.0001. In some embodiments, the material at the second surface 102 is a glass or plastic and the material at the first surface 104 is a coating that has a refractive index that is substantially equal to the refractive index of the first material.

[0061] Optical element 100 comprises a first buried diffractive grating 106 between the first flat surface 102 and the second flat surface 104, such that the buried diffractive grating is spaced from the first surface and the second surface. Methods for creating a buried diffractive grating 106 between the first flat surface 102 and the second flat surface 104 are described further herein. Optical element 100 additionally comprises a second buried diffractive grating 107 between the first flat surface 102 and the second flat surface 104, such that the buried diffractive grating is spaced from the first surface and the second surface. While FIG. 1 depicts both the in-coupling grating and out-coupling grating as buried diffractive gratings, embodiments may include an optical element that comprises a buried diffractive grating as an in-coupling grating, out-coupling grating, expansion grating, or any combination thereof. [0062] As shown in the cross section of buried diffractive grating 106 the buried diffractive gratings each comprise a plurality of pockets 112 of low refractive index material interspaced with high refractive index material 114. The low refractive index material may include solid materials with relatively low refractive indices, such as lithium fluoride, calcium fluoride, magnesium fluoride, or optical resins, or nonsolid pockets, such as pockets of air, vacuum, or gas. The high refractive index material comprises a material that has a refractive index substantially equal to the refractive index of the material of the first surface and/or the second surface. In some embodiments, the high refractive index material is a same material as the first surface and/or second surface, such as glass or plastic. In other embodiments, the high refractive index material is a different material than one or more of the surfaces, such as a coating material. The coating material may comprise any of cubic zirconium oxide, titanium oxide, aluminum oxide, diamond, hafnium oxide, tantalum oxide, zinc oxide, or other high- index resins.

[0063] FIG. 1 A depicts the buried diffractive grating as a uniform vertical structure of the pockets 112 with even spacing between adjacent pockets by the material 114, which is illustrated as such for the purpose of providing a clear example. FIG. 1-9 are provided as representations of the methods and systems described herein. The elements of FIG. 1-9 are not intended to provide to-scale examples of the methods and systems described herein and embodiments may include different orientations of elements, different sizing of elements, different spacing of elements, or other different configurations of elements. The methods described herein may be used to generate buried diffractive gratings in a variety of different shapes and structures, including blazed structures, slanted structures, binary structures, analogue structures, or varying depth structures. Additionally, the buried diffractive gratings may utilize different types of spacings, such that the low refractive index material and high refractive index material sections have different widths.

[0064] An image source 108 provides an image beam to the optical element 100. The image source may comprise a device configured to project an image beam 110 comprising beams of light corresponding to a plurality of pixels that are to be displayed as an image. The image beam 110 is diffracted by the in-coupling grating and guided through the waveguide through total internal reflection across the first flat surface 102 and the second flat surface 104. The image beam is then diffracted by an out-coupling grating to be displayed to an eye of a user. In this manner, the optical element propagates the image beam through the waveguide and directs the image beam through a surface of the waveguide towards an eye of a user, thereby converting the image beam into an image for viewing by the user. While FIG. 1 A depicts the image beam 110 being directed out through the same surface as the incoming beam (first flat surface 102), other embodiments of outcoupling gratings may direct the beam out through different surfaces, such as second flat surface 104 or other surfaces of the waveguide.

[0065] FIG. IB depicts a diagram of a plurality of diffraction gratings of an optical element of a head-mounted display. Optical element 120 comprises in-coupling grating 122, expansion grating 124, and out-coupling grating 126. Any of in-coupling grating 122, expansion grating 124, and out-coupling grating 126 may comprise buried diffractive gratings as described herein. While FIG. IB depicts three diffractive gratings, other embodiments may include more or less diffractive gratings. For example, an optical element may include a plurality of expansion gratings including a first expansion grating that expands the image beam in a first direction and a second expansion grating that expands the image beam in a second direction perpendicular to the first direction.

[0066] The image source 128 transmits the image beam 130 into optical element 120 at incoupling grating 122. In-coupling grating 122 diffracts the image beam along optical element 120 through total internal reflection towards expansion grating 124. Expansion grating 124 comprises a grating configured to expand an incoming beam in the plane of the waveguide. The expansion grating may also be configured to redirect the image beam to another direction. For example, in FIG. IB, the expansion grating redirects the incoming beam from the x-direction to the y-direction towards the out-coupling grating. The out-coupling grating is configured to diffract the expanded beam towards an eyeball of a wearer of the headmounted display, such as head-mounted display 900 of FIG. 9. In some embodiments, the out-coupling grating is further configured to expand the image beam, such as in a direction perpendicular to the direction expanded by the expansion grating. Thus, if the expansion grating expands the image beam in the x-direction, the out-coupling grating may be configured to expand the image beam in the y-direction, wherein the z-direction is perpendicular to the optical element in a direction of the user’s eye.

[0067] FIGS. 2-5 depict different methods of creating buried diffractive gratings for use in optical elements for head-mounted displays. The buried diffractive gratings of FIG. 1 may be generated using any of the methods described in FIGS. 2-5.

[0068] FIG. 2 depicts an example of fabricating a buried diffractive grating in an optical element using sacrificial material. Optical element 200 includes substrate 210. Substrate 210 may comprise a material with a high refractive index for fabricating the buried diffractive grating onto, such as glass or plastic. At step 202, sacrificial material 212 is patterned onto the substrate. Sacrificial material 212 may comprise a soluble or dissolvable material, such as a photoresist, a water-soluble polymer or material, or organic-solvent soluble polymer or material. One example of sacrificial material includes water soluble polyvinyl alcohol.

[0069] The sacrificial material 212 may be patterned onto substrate 210 in any of a plurality of designs, including blazed patterns, slanted patterns, binary patterns, analogue structures, or varying depth structures. Techniques for patterning sacrificial material 212 onto substrate 210 include any of electron beam lithography, interference lithography, nanoimprint lithography, hot embossing, photolithography, focused ion beams, or any other lithography or patterning techniques.

[0070] At step 204, a high index coating 214 is applied to cover sacrificial material 212 and substrate 210. High index coating 214 may comprise any of cubic zirconium oxide, titanium oxide, aluminum oxide, diamond, hafnium oxide, tantalum oxide, zinc oxide, or any other high-index resins. In some embodiments, high index coating 214 is selected to have a refractive index that is substantially equal to the refractive index of substrate 210. Processes for coating the high-index coating onto the sacrificial material 212 and substrate 210 include any of physical vapor deposition, atomic layer deposition, chemical vapor deposition, application of nanoparticle inks, spin casting, or dip-coating.

[0071] At step 206, after the high index coating has been applied to cover sacrificial material 212 and substrate 210, the sacrificial material is removed. For example, a sintering or dissolution process may be applied to optical element 200 to remove sacrificial material. [0072] While FIG. 2 depicts the sacrificial material being separately patterned on, other embodiments may include different processes for creating patterns of sacrificial materials on substrate 210. For example, the sacrificial material may be applied to cover optical element 200 in a single layer. A stamp may then be pressed onto the sacrificial material to create the pattern depicted in FIG. 2. While the stamp is in place, ultra-violent nanoimprint lithography or thermal nanoimprint lithography may be used to harden the sacrificial material into place. The stamp may then be removed, thereby leaving a structure with a pattern of sacrificial material similar to the structure made through patterning of sacrificial material. Steps 204 and 206 may then proceed in the same way as described with respect to FIG. 2.

[0073] FIG. 3 depicts an example of fabricating a buried diffractive grating in an optical element using laser bonding. Optical element 300 includes substrate 310. Substrate 310 may comprise a material with a high refractive index for fabricating the buried diffractive grating onto, such as glass or plastic. At step 302, grating 312 is fabricated onto substrate 310. For example, a standard surface relief grating may be fabricated onto the surface of substrate 310, such as through laser etching/ablation.

[0074] At step 304, a second surface 314 is bonded onto the grating to generate a buried diffractive grating via laser direct bonding in an adhesive-free process. The second surface 314 may comprise a material with a substantial equal refractive index as the first material. In some embodiments, the second surface 314 is a same material as substrate 310. For example, both materials may be glass with a same refractive index. The second surface may be attached to the first surface and grating through any bonding techniques, such as direct glass laser bonding.

[0075] FIG. 4 depicts an example of fabricating a buried diffractive grating in an optical element using a low-refractive index material. Optical element 400 includes substrate 410. Substrate 410 may comprise a material with a high refractive index for fabricating the buried diffractive grating onto, such as glass or plastic. At step 402, a low index material 412 is patterned onto the substrate. Low index material 412 may comprise a material with a refractive index that is substantially lower than the refractive index of substrate 410, such as lithium fluoride, calcium fluoride, magnesium fluoride, or any other low refractive index optical resin. Substantially lower, as used herein, refers to a difference of 0.5 or greater between the two indices.

[0076] The low index material 412 may be patterned onto substrate 410 in any of a plurality of designs, including blazed patterns, slanted patterns, binary patterns, analogue structures, or varying depth structures. Techniques for patterning low index material 412 onto substrate 410 include any of electron beam lithography, interference lithography, nanoimprint lithography, hot embossing, photolithography with a pattern transfer, reactive ion etching and deposition of the low index material, or any other lithography or patterning techniques.

[0077] At step 404, a high index coating 414 is applied to cover low index material 412 and substrate 410. High index coating 414 may comprise any of cubic zirconium oxide, titanium oxide, aluminum oxide, diamond, hafnium oxide, tantalum oxide, zinc oxide, or any other high-index resins. In some embodiments, high index coating 414 is selected to have a refractive index that is substantially equal to the refractive index of substrate 410. Processes for coating the high-index coating onto the low index material 412 and substrate 410 include any of physical vapor deposition, atomic layer deposition, chemical vapor deposition, application of nanoparticle inks, spin casting, or dip-coating.

[0078] The fabrication techniques described herein with respect to FIG. 2-4 provide a wide array of benefits. One such benefit is that buried diffractive gratings can be fabricated in different structures within the optical element, such as blazed structures, slanted structures, binary structures, analogue structures, or varying depth structures. In particular, varying depth structures are not possible with surface relief gratings. In contrast, buried diffractive gratings can be fabricated at different depths depending on need, with some embodiments including buried diffractive gratings in a center of the optical element and other embodiments including buried diffractive gratings closer to one surface of the optical element than the other. In addition, a single diffractive grating can be fabricated with portions of the buried diffractive grating at different depths and/or different buried diffractive gratings in a single optical element can be fabricated at different depths, such as an in-coupling grating at a first depth and an out-coupling grating at a second depth.

[0079] FIG. 5 depicts an example of fabricating a buried diffractive grating at multiple positional depths in an optical element. Optical element 500 includes substrate 510 at a plurality of varying heights. For instance, in FIG. 5, a first portion of substrate 510 comprises an initial height relative to a bottom surface of hl while a second portion of substrate 510 comprises an initial height relative to the bottom surface of h2. Substrate 510 may comprise a material with a high refractive index for fabricating the buried diffractive grating onto, such as glass or plastic. Substrate 510 may be fabricated to have varying heights through bonding of multiple pieces of substrate, such as laser bonding of glass, and/or through removal of a portion of the substrate, such as by laser etching or other glass cutting processes. The varying heights may be fabricated in a location designated for a single buried diffractive grating and/or in locations for different buried diffractive gratings such that a first location is a first height and a second location is a second height.

[0080] At step 502, sacrificial material 512 is patterned onto the substrate at the plurality of varying heights. For instance, sacrificial material 512 is patterned onto the first portion of substrate 510 at height hl and the second portion of substrate 510 at height h2. Sacrificial material 512 may comprise a soluble or dissolvable material, such as a photoresist, a water- soluble polymer or material, or organic- solvent soluble polymer or material. One example of sacrificial material includes water soluble polyvinyl alcohol.

[0081] The sacrificial material 512 may be patterned onto substrate 510 in any of a plurality of designs, including blazed patterns, slanted patterns, binary patterns, analogue structures, or varying depth structures. Techniques for patterning sacrificial material 512 onto substrate 510 include any of electron beam lithography, interference lithography, nanoimprint lithography, hot embossing, photolithography, focused ion beams, or any other lithography or patterning techniques.

[0082] At step 504, a high index coating 514 is applied to cover sacrificial material 512 and substrate 510. High index coating 514 may comprise any of cubic zirconium oxide, titanium oxide, aluminum oxide, diamond, hafnium oxide, tantalum oxide, zinc oxide, or any other high-index resins. In some embodiments, high index coating 514 is selected to have a refractive index that is substantially equal to the refractive index of substrate 510. Processes for coating the high-index coating onto the sacrificial material 512 and substrate 510 include any of physical vapor deposition, atomic layer deposition, chemical vapor deposition, application of nanoparticle inks, spin casting, or dip-coating. [0083] At step 506, after the high index coating has been applied to cover sacrificial material 512 and substrate 510, the sacrificial material is removed. For example, a sintering or dissolution process may be applied to optical element 500 to remove sacrificial material. [0084] Another benefit of the fabrication techniques described herein is that the resultant buried diffractive grating is separated from the surface of the optical element, thereby protecting the buried diffractive grating and allowing additional coatings or layers to be added to the optical element. FIG. 6 depicts an optical element comprising a buried diffractive grating and an additional coating. Optical element 600 comprises buried diffractive grating 602, surface 604, and surface 606. Surface 604 and surface 606 comprise substantially flat surfaces that are substantially parallel to each other. Surface 604 and surface 606 may comprise a same material, such as glass or plastic, or different materials, such as glass with a high-refractive index coating that has a substantially equal refractive index to the glass. Additional coating 608 comprises a material that is used to coat surface 604 after fabrication of the buried diffractive grating and/or is coated on top of an angular- sensitive reflective coating which is coated on top of the surface. Additional coating may comprise a wax or polish, an anti -reflective coating, such as magnesium fluoride, a high- reflection coating, such as a combination of zinc sulfide or titanium dioxide with magnesium fluoride or silicon dioxide, a transparent conductive coating, such as indium tin oxide, or any other coating material.

[0085] In some embodiments, a coating is used to provide a pseudo-air layer between different types of lenses. The coating may comprise an angular-sensitive reflective coating that maintain operation of the waveguide at the angles at which light is expected to strike the surface based on the diffractive gratings. The angular-sensitive layer effectively isolates the waveguide operation at the relevant angles allowing other optical elements to be attached to the surface over the coating. Examples of additional optical elements include ophthalmic lenses, photochromic or electrochromic lenses, dynamic or active operated lenses, polarized lenses, or other lenses. In some embodiments, the angular-sensitive reflective coating is used in conjunction with the additional coatings described above, such that the pseudo-air layer is placed between the optical element and the additional coating.

[0086] FIG. 7 depicts an optical element comprising a buried diffractive grating and additional coatings that allow other optical elements to be attached to the optical element comprising the buried diffractive grating. Optical element 700 comprises buried diffractive grating 702, surface 704, and surface 706. Surface 704 and surface 706 comprise substantially flat surfaces that are substantially parallel to each other. Surface 704 and surface 706 may comprise a same material, such as glass or plastic, or different materials, such as glass with a high-refractive index coating that has a substantially equal refractive index to the glass. Each of surface 704 and surface 706 is coated with an angular sensitive coating to provide pseudo-air layer 708. The pseudo-air layer 708 allows the waveguide to continue to operate as if surface 704 and surface 705 were surrounded with a substance with a low refractive index, such as air or gas.

[0087] Convex lens 710 and concave lens 712 comprise two examples of ophthalmic lenses that may be used in conjunction with optical element 700 that are attached to the flat surface of optical element 700 on the pseudo-air layer 708 made up of the angular-sensitive coating. Convex lens 710 comprises a lens with a rounded surface and a flat surface which is separated from surface 704 by pseudo-air layer 708. Similarly, concave lens 712 comprises a lens with a rounded surface and a flat surface which is separated from surface 706 by pseudoair layer 708. Other embodiments may include a single optical element attached to optical element 700 and separated by pseudo-air layer 708. Additionally, embodiments may include different types of optical elements attached to optical element 700 and separated by pseudoair layer 708, such as photochromic or electrochromic lenses, actively operated lenses, polarized lenses, or other lenses.

[0088] In some embodiments, an angular sensitive coating is used to separate waveguides comprising buried diffractive gratings. The use of the buried diffractive gratings allows the waveguides to be stacked on top of each other, thereby creating a compact set of waveguides which can be used to provide a plurality of images, such as images at different focal points or images at different frequencies. FIG. 8 depicts an example of a plurality of stacked optical elements with buried diffractive gratings. While FIG. 3 depicts three stacked optical elements for the purpose of providing a clear example, other embodiments may include fewer or more stacked optical elements. Additionally, while FIG. 3 depicts optical elements for different wavelengths, the combination of optical elements described herein may be used with other types of optical elements, such as optical elements for different focal points. The combination of waveguides described with respect to FIG. 8 may additionally be combined with other embodiments, including additional lenses, such as the ophthalmic lenses of FIG. 7, or other types of coatings, such as anti -reflective coatings. [0089] Optical element combination 800 comprises three optical elements 802, 804, and 806 separated by an angular sensitive pseudo-air layer coating 808. Each of optical elements 802, 804, and 806 comprise buried diffractive gratings. While the buried diffractive gratings in FIG. 8 are depicted as being equivalent, other embodiments may include buried diffractive gratings with different structures, at different depths, and/or with different spacings. Each of optical elements 802, 804, and 806 are configured to diffract image beams of different wavelengths. For example, optical element 802 may comprise a waveguide configured to diffract an image beam with a wavelength of 465nm, optical element 804 may comprise a waveguide configured to diffract an image beam with a wavelength of 530nm, and optical element 806 may comprise a waveguide configured to diffract an image beam with a wavelength of 630nm. Thus, a head-mounted display may provide each of the image beams to the different waveguides, thereby providing a full color image to an eye of a viewer.

[0090] FIG. 9 depicts an example of a head-mounted display comprising one or more optical elements with a buried diffractive grating. Head-mounted display 900 comprises optical element 902, image source 904, control circuitry 908, memory 910, network adaptor 912, and power source 914. Optical element 902 comprises an optical element, such as a lens, which sits in front of an eye of a user. Image source 904 provides an image beam 906 to the optical element which is diffracted by a buried diffractive grating and displayed to the eye of the user. Control circuitry 908 may be based on any suitable processing circuitry, such as one or more microprocessors, microcontrollers, digital signal processors, programmable logic devices, field-programmable gate arrays (FPGAs), application-specific integrated circuits (ASICs), etc., and may include a multi-core processor (e.g., quad-core). Control circuitry 908 may be configured to generate one or more images for display through the head-mounted display and instruct image source 904 to produce one or more image beams corresponding to the one or more images. Memory 910 may be any device for storing electronic data, such as random-access memory, solid state devices, quantum storage devices, hard disk drives, nonvolatile memory or any other suitable fixed or removable storage devices, and/or any combination of the same. Memory 910 may store data defining images for display by the head-mounted display. Network adaptor 912 comprises circuitry that connects the headmounted display to one or more other devices over a network. Network adaptor 912 may comprise wires and/or busses connected to a physical network port, e.g. an ethemet port, a wireless WiFi port, cellular communication port, or any other type of suitable physical port. Power source 914 comprises a source of power to the image source 904, control circuitry 908, memory 910, and/or network adaptor 912, such as a battery, solar generator, or wired power source.

[0091] FIG. 10A depicts a diagram of an optical element of a head-mounted display comprising a waveguide with a superimposed diffractive grating. Optical element 1000 comprises a first flat surface 1002 that is substantially parallel to a second flat surface 1004. The first flat surface 1002 and second flat surface 1004 may comprise a same material, such as glass or plastic, or different materials with substantially equal refractive indices. As used herein, a first refractive index is substantially equal to a second refractive index if the refractive indices differ by less than 0.01. In some embodiments, the refractive indices differ by less than 0.001. In some embodiments, the refractive indices differ by less than 0.0001. In some embodiments, the first material is a glass or plastic and the second material is a coating that has a refractive index that is substantially equal to the refractive index of the first material.

[0092] Optical element 1000 comprises a first superimposed diffractive grating 1006 comprising a surface relief grating 1006a on top of the first flat surface 1002 and a buried diffractive grating 1006b between the first flat surface 1002 and the second flat surface 1004, such that the buried diffractive grating is spaced from the first surface and the second surface. For example, the surface relief grating 1006a may overlap the buried diffractive grating 1006b (e.g., the surface relief grating 1006a may be placed less deep into the substrate of optical element 1000). Methods for creating a superimposed grating comprising a surface relief grating 1006a on top of the first flat surface 1002 and a buried diffractive grating 1006b between the first flat surface 1002 and the second flat surface 1004 are described further herein. Optical element 1000 additionally comprises a second superimposed diffractive grating 1007 comprising a surface relief grating 1007a and a buried diffractive grating 1007b between the first flat surface 1002 and the second flat surface 1004, such that the buried diffractive grating is spaced from the first surface and the second surface. For example, the surface relief grating 1007a may overlap the buried diffractive grating 1007b (e.g., the surface relief grating 1007a may be placed less deep into the substrate of optical element 1000).

[0093] While FIG. 10 depicts both the in-coupling grating and out-coupling grating as superimposed diffractive gratings, embodiments may include an optical element that comprises a superimposed diffractive grating as an in-coupling grating, out-coupling grating, expansion grating, or any combination thereof.

[0094] As shown in the cross section of buried diffractive grating 1006 the buried diffractive gratings of the superimposed gratings each comprise a plurality of pockets of low refractive index material (e.g., shaded sections of 1006, 1007) interspaced with high refractive index material (e.g., non-shaded sections of 1006, 1007). The low refractive index material may include solid materials with relatively low refractive indices, such as lithium fluoride, calcium fluoride, magnesium fluoride, or optical resins, or nonsolid pockets, such as pockets of air, vacuum, or gas. The high refractive index material comprises a material that has a refractive index substantially equal to the refractive index of the material of the first surface and/or the second surface. In some embodiments, the high refractive index material is a same material as the first surface and/or second surface, such as glass or plastic. In other embodiments, the high refractive index material is a different material than one or more of the surfaces, such as a coating material. The coating material may comprise any of cubic zirconium oxide, titanium oxide, aluminum oxide, diamond, hafnium oxide, tantalum oxide, zinc oxide, or other high-index resins.

[0095] In some embodiments, the high refractive index material of the buried diffractive grating comprises a transparent conductive material. The transparent conductive material may comprise a transparent conductive oxide, such as fluorinated tin oxide, indium tin oxide, aluminum zinc oxide, indium zinc oxide, indium tin zirconium oxide, indium gallium oxide, indium gallium zinc oxide, tin oxide, or zinc oxide, or a conductive polymer, such as poly(3,4-ethylenedioxythiphene) (PEDOT) or poly(3,4-ethylenedioxythiophene): poly(styrene sulfonate) (PEDOT:PSS). In some embodiments, the surface relief grating is fabricated using a transparent conductive material. In some embodiments, both the surface relief grating and the buried diffractive grating are manufactured using a transparent conductive material.

[0096] FIG. 10A depicts the surface relief grating as a uniform vertical structure with even spacing for the purpose of providing a clear example. FIG. 10-16 are provided as representations of the methods and systems described herein. The elements of FIG. 10-16 are not intended to provide to-scale examples of the methods and systems described herein and embodiments may include different orientations of elements, different sizing of elements, different spacing of elements, or other different configurations of elements. The methods described herein may be used to generate superimposed diffractive gratings in a variety of different shapes and structures, including blazed structures, slanted structures, binary structures, analogue structures, varying depth structures, or a combination thereof with a surface relief grating different in shape and/or structure from a buried diffractive grating beneath it. Additionally, the diffractive gratings may utilize different types of spacings, such that the low refractive index material and high refractive index material sections have different widths.

[0097] An image source 1008 provides an image beam to the optical element 1000. The image source may comprise a device configured to project an image beam 1010 comprising beams of light corresponding to a plurality of pixels that are to be displayed as an image. The image beam 1010 is diffracted by the in-coupling grating and guided through the waveguide through total internal reflection across the first flat surface 1002 and the second flat surface 1004. The image beam is then diffracted by an out-coupling grating to be displayed to an eye of a user. In this manner, the optical element propagates the image beam through the waveguide and directs the image beam through a surface of the waveguide towards an eye of a user, thereby converting the image beam into an image for viewing by the user.

[0098] FIG. 10B depicts a diagram of a plurality of diffraction gratings of an optical element of a head-mounted display. Optical element 1020 comprises in-coupling grating 1022, expansion grating 1024, and out-coupling grating 1026. Any of in-coupling grating 1022, expansion grating 1024, and out-coupling grating 1026 may comprise superimposed diffractive gratings, surface relief gratings, or buried diffractive gratings as described herein. While FIG. 10B depicts three diffractive gratings, other embodiments may include more or less diffractive gratings. For example, an optical element may include a plurality of expansion gratings including a first expansion grating that expands the image beam in a first direction and a second expansion grating that expands the image beam in a second direction perpendicular to the first direction.

[0099] The image source 1028 transmits the image beam 1030 into optical element 1020 at in-coupling grating 1022. In-coupling grating 1022 diffracts the image beam along optical element 1020 through total internal reflection towards expansion grating 1024. Expansion grating 1024 comprises a grating configured to expand an incoming beam in the plane of the waveguide. The expansion grating may also be configured to redirect the image beam to another direction. For example, in FIG. 10B, the expansion grating redirects the incoming beam from the x-direction to the y-direction towards the out-coupling grating. The out- coupling grating is configured to diffract the expanded beam towards an eyeball of a wearer of the head-mounted display, such as head-mounted display 700 of FIG. 7. In some embodiments, the out-coupling grating is further configured to expand the image beam, such as in a direction perpendicular to the direction expanded by the expansion grating. Thus, if the expansion grating expands the image beam in the x-direction, the out-coupling grating may be configured to expand the image beam in the y-direction, wherein the z-direction is perpendicular to the optical element in a direction of the user’s eye.

[0100] FIG. 10C depicts a diagram of operation of optical elements of a head-mounted display comprising a superimposed diffractive grating. Optical element 1050 comprises an optical element with a superimposed diffractive grating used as an in-coupling grating. Optical element 1060 comprises an optical element with a superimposed diffractive grating used as an out-coupling grating. The diagrams of optical element 1050 and optical 1060 are provided as a visualization of the optical elements and the location, size, and scaling of the diffractive gratings may differ in different implementations. Additionally, while optical element 1050 and optical element 1060 depict only an in-coupling grating and out-coupling grating respectively, the use of a superimposed diffractive grating as described herein may be applied to any of the diffractive gratings or combination of diffractive gratings in the waveguide.

[0101] Optical element 1050 comprises surface relief grating 1052 and buried diffractive grating 1054 as an in-coupling grating. Image beam 1056 is projected from an image source in a first direction. When the image beam 1056 reaches the surface relief grating 1052, the image beam 1056 is diffracted at an angle 0i from the first direction. When the image beam 1056 reaches the buried diffractive grating, the image beam is further diffracted to an angle 02 from the first direction. Optical element 1060 comprises surface relief grating 1062 and buried diffractive grating 1064 as an out-coupling grating. Image beam 1066 travels through the waveguide through total internal reflected and is reflected towards the out-coupling grating at a second direction. When the image beam 1066 reaches the buried diffractive grating 1064, the image beam 1066 is diffracted at an angle 03 from the second direction. When image beam 1066 reaches the surface relief grating 1062, the image beam is further diffracted to an angle 04 from the second direction. [0102] FIGS. 11-14 depict different methods of creating superimposed diffractive gratings for use in optical elements for head-mounted displays. The superimposed diffractive gratings of FIG. 10 may be generated using any of the methods described in FIGS. 11-14.

[0103] FIG. 11 depicts an example of fabricating a superimposed diffractive grating in an optical element using sacrificial material. Optical element 1100 includes substrate 1110. Substrate 1110 may comprise a material with a high refractive index for fabricating the buried diffractive grating onto, such as glass or plastic. At step 1102, sacrificial material 1112 is patterned onto the substrate. Sacrificial material 1112 may comprise a soluble or dissolvable material, such as a photoresist, a water-soluble polymer or material, or organic- solvent soluble polymer or material. One example of sacrificial material includes water soluble polyvinyl alcohol.

[0104] The sacrificial material 1112 may be patterned onto substrate 1110 in any of a plurality of designs, including blazed patterns, slanted patterns, binary patterns, analogue structures, or varying depth structures. Techniques for patterning sacrificial material 1112 onto substrate 1110 include any of electron beam lithography, interference lithography, nanoimprint lithography, hot embossing, photolithography, focused ion beams, or any other lithography or patterning techniques.

[0105] At step 1104, a high index coating 1114 is applied to cover sacrificial material 1112 and substrate 1110. High index coating 1114 may comprise any of cubic zirconium oxide, titanium oxide, aluminum oxide, diamond, hafnium oxide, tantalum oxide, zinc oxide, or any other high-index resins. In some embodiments, high index coating 1114 is selected to have a refractive index that is substantially equal to the refractive index of substrate 1110. Processes for coating the high-index coating onto the sacrificial material 1112 and substrate 1110 include any of physical vapor deposition, atomic layer deposition, chemical vapor deposition, application of nanoparticle inks, spin coating, or dip-coating.

[0106] At step 1106, after the high index coating has been applied to cover sacrificial material 1112 and substrate 1110, the sacrificial material is removed. For example, a sintering or dissolution process may be applied to optical element 1100 to remove sacrificial material. [0107] At step 1108, a new diffractive grating 1116 is patterned on top of the coating 1114. The new diffractive grating 1116 may be patterned onto coating 1114 in any of a plurality of designs, including blazed patterns, slanted patterns, or binary patterns, analogue structures. The design of the new diffractive grating 1116 may be a same design as the design of the sacrificial material 1112 or a different design. Techniques for patterning new diffractive grating 1116 onto coating 1114 include any of electron beam lithography, interference lithography, nanoimprint lithography, hot embossing, photolithography, focused ion beams, or any other lithography or patterning techniques. In some embodiments, the new diffractive grating 1116 is fabricated using a transparent conductive material.

[0108] While FIG. 11 depicts the sacrificial material being separately patterned on, other embodiments may include different processes for creating patterns of sacrificial materials on substrate 1110. For example, the sacrificial material may be applied to cover optical element 1100 in a single layer. A stamp may then be pressed onto the sacrificial material to create the pattern depicted in FIG. 11. While the stamp is in place, ultra-violent nanoimprint lithography or thermal nanoimprint lithography may be used to harden the sacrificial material into place. The stamp may then be removed, thereby leaving a structure with a pattern of sacrificial material similar to the structure made through patterning of sacrificial material. Steps 1104 and 1106 may then proceed in the same way as described with respect to FIG. 11.

[0109] FIG. 12 depicts an example of fabricating a superimposed diffractive grating in an optical element using laser bonding. Optical element 1200 includes substrate 1210. Substrate 1210 may comprise a material with a high refractive index for fabricating the buried diffractive grating onto, such as glass or plastic. At step 1202, grating 1212 is fabricated onto substrate 1210. For example, a standard surface relief grating may be fabricated onto the surface of substrate 1210, such as through laser etching/ablation.

[0110] At step 1204, a second surface 1214 is bonded onto the grating to generate a buried diffractive grating via laser direct bonding in an adhesive-free process. The second surface 1214 may comprise a material with a substantial equal refractive index as the first material. In some embodiments, the second surface 1214 is a same material as substrate 1210. For example, both materials may be glass with a same refractive index. The second surface may be attached to the first surface and grating through any bonding techniques, such as direct glass laser bonding.

[0111] At step 1206, a new diffractive grating 1216 is fabricated onto the second surface 1214. For example, a standard surface relief grating may be fabricated onto the surface of second surface 1214, such as through laser etching/ablation. In some embodiments, the surface relief grating comprises a high index material that is patterned onto the second surface using any of the techniques previously described herein. In some embodiments, the high index material comprises a transparent conductive material. The design of the new diffractive grating 1216 may be a same design as the design of the grating 1212 and/or a different design.

[0112] FIG. 13 depicts an example of fabricating a superimposed diffractive grating in an optical element using a low-refractive index material. Optical element 1300 includes substrate 1310. Substrate 1310 may comprise a material with a high refractive index for fabricating the buried diffractive grating onto, such as glass or plastic. At step 1302, a low index material 1312 is patterned onto the substrate. Low index material 1312 may comprise a material with a refractive index that is substantially lower than the refractive index of substrate 1310, such as lithium fluoride, calcium fluoride, magnesium fluoride, or any other low refractive index optical resin. Substantially lower, as used herein, refers to a difference of 0.5 or greater between the two indices.

[0113] The low index material 1312 may be patterned onto substrate 1310 in any of a plurality of designs, including blazed patterns, slanted patterns, binary patterns, analogue structures, or varying depth structures. Techniques for patterning low index material 1312 onto substrate 1310 include any of electron beam lithography, interference lithography, nanoimprint lithography, hot embossing, photolithography with a pattern transfer, reactive ion etching and deposition of the low index material, or any other lithography or patterning techniques.

[0114] At step 1304, a high index coating 1314 is applied to cover low index material 1312 and substrate 1310. High index coating 1314 may comprise any of cubic zirconium oxide, titanium oxide, aluminum oxide, diamond, hafnium oxide, tantalum oxide, zinc oxide, or any other high-index resins. In some embodiments, high index coating 1314 is selected to have a refractive index that is substantially equal to the refractive index of substrate 1310. Processes for coating the high-index coating onto the low index material 1312 and substrate 1310 include any of physical vapor deposition, atomic layer deposition, chemical vapor deposition, application of nanoparticle inks, spin coating, or dip-coating.

[0115] At step 1306, a new diffractive grating 1316 is patterned onto the high index coating 1314. The new diffractive grating 1316 may be patterned onto high index coating 1314 in any of a plurality of designs, including blazed patterns, slanted patterns, binary patterns, or analogue structures. The design of the new diffractive grating 1316 may be a same design as the design of the low index material 1312 and/or a different design. Techniques for patterning new diffractive grating 1316 onto high index coating 1314 include any of electron beam lithography, interference lithography, nanoimprint lithography, hot embossing, photolithography, focused ion beams, or any other lithography or patterning techniques. In some embodiments, the new diffractive grating 1316 is fabricated using a transparent conductive material.

[0116] The fabrication techniques described herein with respect to FIG. 11-13 provide a wide array of benefits. One such benefit is that buried diffractive gratings can be fabricated in different structures within the optical element, such as blazed structures, slanted structures, binary structures, analogue structures, or varying depth structures. In particular, varying depth structures are not possible with surface relief gratings. In contrast, buried diffractive gratings can be fabricated at different depths depending on need, with some embodiments including buried diffractive gratings in a center of the optical element and other embodiments including buried diffractive gratings closer to one surface of the optical element than the other. In addition, a single diffractive grating can be fabricated with portions of the buried diffractive grating at different depths and/or different buried diffractive gratings in a single optical element can be fabricated at different depths, such as an in-coupling grating at a first depth and an out-coupling grating at a second depth. By combining the buried diffractive grating with a surface relief grating, different types of structures can be used in combination to produce different effects.

[0117] FIG. 14A depicts an example of fabricating a superimposed diffractive grating with a buried diffractive grating at multiple depths in an optical element. Optical element 1400 includes substrate 1410 at a plurality of varying heights. Substrate 1410 may comprise a material with a high refractive index for fabricating the buried diffractive grating onto, such as glass or plastic. Substrate 1410 may be fabricated to have varying heights through bonding of multiple pieces of substrate, such as laser bonding of glass, and/or through removal of a portion of the substrate, such as by laser etching or other glass cutting processes. The varying heights may be fabricated in a location designated for a single buried diffractive grating and/or in locations for different buried diffractive gratings such that a first location is a first height and a second location is a second height.

[0118] At step 1402, sacrificial material 1412 is patterned onto the substrate at the plurality of varying heights. Sacrificial material 1412 may comprise a soluble or dissolvable material, such as a photoresist, a water-soluble polymer or material, or organic-solvent soluble polymer or material. One example of sacrificial material includes water soluble polyvinyl alcohol. [0119] The sacrificial material 1412 may be patterned onto substrate 1410 in any of a plurality of designs, including blazed patterns, slanted patterns, binary patterns, analogue structures, or varying depth structures. Techniques for patterning sacrificial material 1412 onto substrate 1410 include any of electron beam lithography, interference lithography, nanoimprint lithography, hot embossing, photolithography, focused ion beams, or any other lithography or patterning techniques.

[0120] At step 1404, a high index coating 1414 is applied to cover sacrificial material 1412 and substrate 1410. High index coating 1414 may comprise any of cubic zirconium oxide, titanium oxide, aluminum oxide, diamond, hafnium oxide, tantalum oxide, zinc oxide, or any other high-index resins. In some embodiments, high index coating 1414 is selected to have a refractive index that is substantially equal to the refractive index of substrate 1410. Processes for coating the high-index coating onto the sacrificial material 1412 and substrate 1410 include any of physical vapor deposition, atomic layer deposition, chemical vapor deposition, application of nanoparticle inks, spin casting, or dip-coating.

[0121] At step 1406, after the high index coating has been applied to cover sacrificial material 1412 and substrate 1410, the sacrificial material is removed. For example, a sintering or dissolution process may be applied to optical element 1400 to remove sacrificial material. [0122] At step 1408, a new diffractive grating 1416 is patterned onto the high index coating 1414. The new diffractive grating 1416 may be patterned onto high index coating 1414 in any of a plurality of designs, including blazed patterns, slanted patterns, binary patterns, or analogue structures. The design of the new diffractive grating 416 may be a same design as the design of the sacrificial material 1412 and/or a different design. Techniques for patterning new diffractive grating 1416 onto high index coating 1414 include any of electron beam lithography, interference lithography, nanoimprint lithography, hot embossing, photolithography, focused ion beams, or any other lithography or patterning techniques. In some embodiments, the new diffractive grating 1416 is fabricated using a transparent conductive material.

[0123] While FIG. 14A depicts a method of fabricating superimposed gratings with varying depth buried diffractive gratings, other embodiments of fabricating a superimposed grating with a varying depth buried diffractive gratings may include laser bonding of glass at different widths, using methods described in respect to FIG. 12, or coating a low index material with a high index material, using methods described in respect to FIG. 4. Additionally, while FIG. 14A depicts a superimposed grating with buried diffractive gratings at different depths, in some embodiments only a strict subset of the different depths is used to create a superimposed diffractive grating. Thus, the superimposed diffractive grating may include a buried diffractive grating that is situated at a different depth than a buried diffractive grating that is not beneath a surface relief grating.

[0124] FIG. 14B depicts example structures of superimposed gratings. Structure 1450 comprises a superimposed grating comprising a slanted buried diffractive grating with a straight surface relief grating. Structure 1460 comprises a straight buried diffractive grating with a slanted surface relief grating. Structure 1470 comprises a varying depth diffractive grating with a single depth surface relief grating. Structure 1480 comprises a varying depth slanted diffractive grating a single depth straight surface relief grating. As shown in the examples of FIG. 14B, the methods described herein may be used to generate superimposed grating structures with any combination of a variety of surface relief structures and a variety of buried depth structures.

[0125] FIG. 15 depicts an example of a plurality of optical elements with superimposed diffractive gratings. While FIG. 15 depicts three optical elements for the purpose of providing a clear example, other embodiments may include fewer or more stacked optical elements. Additionally, while FIG. 15 depicts optical elements for different wavelengths, the combination of optical elements described herein may be used with other types of optical elements, such as optical elements for different focal points.

[0126] Optical element combination 1500 comprises three optical elements 1502, 1504, and 1506 separated from each other using an air spacing. Each of optical elements 1502, 1504, and 1506 comprise superimposed diffractive gratings. While the superimposed diffractive gratings in FIG. 15 are depicted as being equivalent, other embodiments may include buried diffractive gratings with different structures, at different depths, and/or with different spacings. Each of optical elements 1502, 1504, and 1506 are configured to diffract image beams of different wavelengths. For example, optical element 1502 may comprise a waveguide configured to diffract an image beam with a wavelength of 465nm, optical element 1504 may comprise a waveguide configured to diffract an image beam with a wavelength of 530nm, and optical element 1506 may comprise a waveguide configured to diffract an image beam with a wavelength of 630nm. Thus, a head-mounted display may provide each of the image beams to the different waveguides, thereby providing a full color image to an eye of a viewer.

[0127] In some embodiments, a superimposed diffractive grating comprises two buried diffractive gratings at different depths. For example, a first buried diffractive grating may be fabricated onto a substrate using any of steps 1102-1106 of FIG. 11, 1202-1204 of FIG. 12, or 1302-1304 of FIG. 13. A second buried diffractive grating may then be fabricated on top of the first buried diffractive grating using any of the aforementioned methods. With both diffractive gratings of the superimposed grating comprising buried diffractive gratings, the surfaces of the waveguide may comprise flat surfaces onto which an angular sensitive coating and one or more other coatings or lenses may be applied. Additionally, or alternatively, a third diffractive grating comprising a surface relief grating may be fabricated onto the surface of the waveguide, thereby creating a triple superimposed grating.

[0128] FIG. 16 depicts an optical element comprising a superimposed diffractive grating comprising two buried diffractive gratings and an additional coating. Optical element 1600 comprises superimposed buried diffractive gratings 1602, surface 1604, and surface 1606. Surface 1604 and surface 1606 comprise substantially flat surfaces that are substantially parallel to each other. Surface 1604 and surface 1606 may comprise a same material, such as glass or plastic, or different materials, such as glass with a high-refractive index coating that has a substantially equal refractive index to the glass. Additional coating 1608 comprises a material that is used to coat surface 1604 after fabrication of the buried diffractive grating and/or is coated on top of an angular-sensitive reflective coating which is coated on top of the surface. Additional coating may comprise a wax or polish, an anti-reflective coating, such as magnesium fluoride, a high-reflection coating, such as a combination of zinc sulfide or titanium dioxide with magnesium fluoride or silicon dioxide, a transparent conductive coating, such as indium tin oxide, or any other coating material.

[0129] In some embodiments, a coating is used to provide an optical isolation of the waveguide and other appended optical elements, such as lenses. The coating may comprise an angular-sensitive reflective coating that maintain operation of the waveguide at the angles at which light is expected to strike the surface based on the diffractive gratings. The angular- sensitive layer effectively isolates the waveguide operation at the relevant angles allowing other optical elements to be attached to the surface over the coating. Examples of additional optical elements include ophthalmic lenses, photochromic or electrochromic lenses, dynamic or active operated lenses, polarized lenses, or other lenses. In some embodiments, the angular-sensitive reflective coating is used in conjunction with the additional coatings described above, such that the pseudo-air layer is placed between the optical element and the additional coating.

[0130] FIG. 17 depicts an optical element comprising superimposed buried diffractive gratings and additional coatings that allow other optical elements to be attached to the optical element comprising the superimposed buried diffractive gratings. Optical element 1700 comprises superimposed buried diffractive grating 1702, surface 1704, and surface 1706. Surface 1704 and surface 1706 comprise substantially flat surfaces that are substantially parallel to each other. Surface 1704 and surface 1706 may comprise a same material, such as glass or plastic, or different materials, such as glass with a high-refractive index coating that has a substantially equal refractive index to the glass. Each of surface 1704 and surface 1706 is coated with an angular sensitive coating to provide layers 1708 and 1714 (respectively), acting as a pseudo-air layers. The pseudo-air layers 1708 and 1714 allow the waveguide to continue to operate as if surface 1704 and surface 1706 were surrounded with a substance with a low refractive index, such as air or gas.

[0131] Convex lens 1710 and concave lens 1712 comprise two examples of ophthalmic lenses that may be used in conjunction with optical element 1700 that are attached to the flat surface of optical element 1700 on the pseudo-air layers 1708 and 1714 made up of the angular-sensitive coating. Convex lens 1710 comprises a lens with a rounded surface and a flat surface which is separated from surface 1704 by pseudo-air layer 1708. Similarly, concave lens 1712 comprises a lens with a rounded surface and a flat surface which is separated from surface 1706 by pseudo-air layer 1714. Other embodiments may include a single optical element attached to optical element 1700 and separated by one of pseudo-air layers 1708 or 1714. Additionally, embodiments may include different types of optical elements attached to optical element 1700 and separated by one of pseudo-air layers 1708 or 1714, such as photochromic or electrochromic lenses, actively operated lenses, polarized lenses, or other lenses.

[0132] In some embodiments, an angular sensitive coating is used to separate waveguides comprising superimposed buried diffractive gratings. The use of the buried diffractive gratings in the superimposed diffractive gratings allows the waveguides to be stacked on top of each other, thereby creating a compact set of waveguides which can be used to provide a plurality of images, such as images at different focal points or images at different frequencies. FIG. 18 depicts an example of a plurality of stacked optical elements with buried diffractive gratings. While FIG. 18 depicts three stacked optical elements for the purpose of providing a clear example, other embodiments may include fewer or more stacked optical elements. Additionally, while FIG. 18 depicts optical elements for different wavelengths, the combination of optical elements described herein may be used with other types of optical elements, such as optical elements for different focal points. The combination of waveguides described with respect to FIG. 18 may additionally be combined with other embodiments, including additional lenses, such as the ophthalmic lenses of FIG. 17, or other types of coatings, such as anti -reflective coatings.

[0133] Optical element combination 1800 comprises three optical elements 1802, 1804, and 1806 separated by an angular sensitive layer coating 1808. Each of optical elements 1802, 1804, and 1806 comprise superimposed buried diffractive gratings. While the buried diffractive gratings in FIG. 18 are depicted as being equivalent, other embodiments may include buried diffractive gratings with different structures, at different depths, and/or with different spacings. Each of optical elements 1802, 1804, and 1806 are configured to diffract image beams of different wavelengths. For example, optical element 1802 may comprise a waveguide configured to diffract an image beam with a wavelength of 465nm, optical element 1804 may comprise a waveguide configured to diffract an image beam with a wavelength of 530nm, and optical element 1806 may comprise a waveguide configured to diffract an image beam with a wavelength of 630nm. Thus, a head-mounted display may provide each of the image beams to the different waveguides, thereby providing a full color image to an eye of a viewer.

[0134] FIG. 19 depicts an example of a head-mounted display comprising one or more optical elements with a buried diffractive grating. Head-mounted display 1900 comprises optical element 1902, image source 1904, control circuitry 1908, memory 1910, network adaptor 1912, and power source 1914. Optical element 1902 comprises an optical element, such as a lens, which sits in front of an eye of a user. Image source 1904 provides an image beam 1906 to the optical element which is diffracted by a buried diffractive grating and displayed to the eye of the user. Control circuitry 1908 may be based on any suitable processing circuitry, such as one or more microprocessors, microcontrollers, digital signal processors, programmable logic devices, field-programmable gate arrays (FPGAs), application-specific integrated circuits (ASICs), etc., and may include a multi-core processor (e.g., quad-core). Control circuitry 1908 may be configured to generate one or more images for display through the head-mounted display and instruct image source 1904 to produce one or more image beams corresponding to the one or more images. Memory 1910 may be any device for storing electronic data, such as random-access memory, solid state devices, quantum storage devices, hard disk drives, non-volatile memory or any other suitable fixed or removable storage devices, and/or any combination of the same. Memory 1910 may store data defining images for display by the head-mounted display. Network adaptor 1912 comprises circuitry that connects the head-mounted display to one or more other devices over a network. Network adaptor 1912 may comprise wires and/or busses connected to a physical network port, e.g. an ethernet port, a wireless WiFi port, cellular communication port, or any other type of suitable physical port. Power source 1914 comprises a source of power to the image source 1904, control circuitry 1908, memory 1910, and/or network adaptor 1912, such as a battery, solar generator, or wired power source.

[0135] FIG. 20A depicts a diagram of an optical element of a head-mounted display comprising a waveguide with a diffractive grating fabricated using a transparent conductive material. Optical element 2000 comprises a first flat surface 2002 that is substantially parallel to a second flat surface 2004. The first flat surface 2002 and second flat surface 2004 may comprise a same material, such as glass or plastic, or different materials with substantially equal refractive indices. As used herein, a first refractive index is substantially equal to a second refractive index if the refractive indices differ by less than 0.01. In some embodiments, the refractive indices differ by less than 0.001. In some embodiments, the refractive indices differ by less than 0.0001. In some embodiments, the first material at the first surface is a glass or plastic and the second material at the second surface is a coating that has a refractive index that is substantially equal to the refractive index of the first material. [0136] Optical element 2000 comprises a first surface relief grating 2006 on top of the first flat surface 2002. Optical element 2000 additionally comprises a second surface relief grating 2007 on top of the first flat surface 2002. The second surface relief grating 2007 may be fabricated from a different material than the first surface relief grating 2006. For instance, the first surface relief grating may be fabricated using laser etching of glass while the second surface relief grating may be fabricated from a transparent conductive material. Other embodiments may include an optical element that comprises a transparent conductive material in the in-coupling grating, out-coupling grating, expansion grating, or any combination thereof. While FIG. 20 depicts the grating comprising the transparent conductive material as a surface relief grating, in other embodiments the transparent conductive material may be used in a buried diffractive grating as described further herein. [0137] FIG. 20 A depicts the diffractive grating as a uniform vertical structure with even spacing for the purpose of providing a clear example. FIG. 20-24 are provided as representations of the methods and systems described herein. The elements of FIG. 20-24 are not intended to provide to-scale examples of the methods and systems described herein and embodiments may include different orientations of elements, different sizing of elements, different spacing of elements, or other different configurations of elements. The methods described herein may be used to generate diffraction gratings in a variety of different shapes and structures, including blazed structures, slanted structures, binary structures, analogue structures, or varying depth structures. Additionally, the diffractive gratings may utilize different types of spacings and/or may comprise buried diffractive gratings, as described further herein.

[0138] An image source 2008 provides an image beam to the optical element 2000. The image source may comprise a device configured to project an image beam 2010 comprising beams of light corresponding to a plurality of pixels that are to be displayed as an image. The image beam 2010 is diffracted by the in-coupling grating and guided through the waveguide through total internal reflection across the first flat surface 2002 and the second flat surface 2004. The image beam is then diffracted by an out-coupling grating to be displayed to an eye of a user. In this manner, the optical element propagates the image beam through the waveguide and directs the image beam through a surface of the waveguide towards an eye of a user, thereby converting the image beam into an image for viewing by the user.

[0139] FIG. 20B depicts a diagram of a plurality of diffraction gratings of an optical element of a head-mounted display. Optical element 2020 comprises in-coupling grating 2022, expansion grating 2024, and out-coupling grating 2026. Any of in-coupling grating 2022, expansion grating 2024, and out-coupling grating 2026 may comprise a transparent conductive material as described herein. While FIG. 20B depicts three diffractive gratings, other embodiments may include more or less diffractive gratings. For example, an optical element may include a plurality of expansion gratings including a first expansion grating that expands the image beam in a first direction and a second expansion grating that expands the image beam in a second direction perpendicular to the first direction.

[0140] The image source 2028 transmits the image beam 2030 into optical element 2020 at in-coupling grating 2022. In-coupling grating 2022 diffracts the image beam along optical element 2020 through total internal reflection towards expansion grating 2024. Expansion grating 2024 comprises a grating configured to expand an incoming beam in the plane of the waveguide. The expansion grating may also be configured to redirect the image beam to another direction. For example, in FIG. 20B, the expansion grating redirects the incoming beam from the x-direction to the y-direction towards the out-coupling grating. The out- coupling grating is configured to diffract the expanded beam towards an eyeball of a wearer of the head-mounted display, such as head-mounted display 500 of FIG. 24. In some embodiments, the out-coupling grating is further configured to expand the image beam, such as in a direction perpendicular to the direction expanded by the expansion grating. Thus, if the expansion grating expands the image beam in the x-direction, the out-coupling grating may be configured to expand the image beam in the y-direction, wherein the z-direction is perpendicular to the optical element in a direction of the user’s eye.

[0141] FIG. 20C depicts a diagram of operation of optical elements of a head-mounted display comprising a transparent conductive material in a diffractive grating. Optical element 2050 comprises an optical element that uses the transparent conductive material in the out- coupling grating. Optical element 2060 comprises an optical element that uses the transparent conductive material in the in-coupling grating. The diagrams of optical element 2050 and optical 2060 are provided as a visualization of the optical elements and the location, size, and scaling of the diffractive gratings may differ in different implementations. Additionally, while optical element 2050 and optical element 2060 depict only an out-coupling grating and in-coupling grating respectively, the use of a transparent conductive material as described herein may be applied to any of the diffractive gratings or combination of diffractive gratings in the waveguide.

[0142] Optical element 2050 comprises diffractive grating 2052 as an out-coupling grating fabricated from a transparent conductive material. Light beams 2054 comprise light within the waveguide that is polarized in an intended direction, polarized in a different direction, and/or unpolarized. The light may become polarized in a different direction through reflection in the waveguide. Unpolarized light may be a product of external light sources and/or stray light from the image source. When the light beams 2054 reach the diffractive grating 2052, the light beams 2054 are diffracted towards an eye of a user. Additionally, due to the use of the transparent conductive material, parts of light beams 2054 that are polarized in an unintended direction or are unpolarized are absorbed by the transparent conductive material, leaving only light beams 2056 comprising light beams that are polarized in the intended direction.

[0143] As used herein, the intended direction of polarization comprises a designed and/or selected polarization direction. The image source may initially provide an image beam that is polarized in the intended direction. Thus, any light that is not polarized in the intended direction comprises light that has been distorted from the initial image beam or is stray light separate from the image beam. By using the transparent conductive material in the out- coupling grating, the excess light is removed from the image beam that is provided to the eye of the user.

[0144] While diffractive grating 2052 is depicted as absorbing a beam bouncing through the waveguide through total internal reflection prior to diffraction towards an eye of the user, the diffractive grating 2052 may additionally absorb light bouncing through the waveguide in different directions and/or external light prior to the light bouncing through the waveguide. Thus, sunlight may be absorbed by the polarizing of the diffractive grating 2052 even prior to entering the waveguide.

[0145] Optical element 2060 comprises diffractive grating 2062 as an in-coupling grating fabricated from a transparent conductive material. Light beams 2064 comprise light being projected to the waveguide that is polarized in an intended direction, polarized in a different direction, and/or unpolarized. The light may become unpolarized due to imperfections in the light source or reflection off other materials. Unpolarized light may be a product of external light sources and/or stray light from the image source. When the light beams 2064 reach diffractive grating 2062, light beams 2064 are diffracted into the waveguide. Additionally, due to the use of the transparent conductive material, parts of light beams 2064 that are polarized in an unintended direction or are unpolarized are absorbed by the transparent conductive material, leaving only light beams 2066 comprising light beams that are polarized in the intended direction.

[0146] While diffractive grating 2062 is depicted as absorbing a beam prior to the beam entering the waveguide, the diffractive grating 2062 may additionally absorb light bouncing through the waveguide in different directions. Thus, stray light bouncing through the waveguide may still be absorbed by diffractive grating 2062 in addition to diffractive grating 2062 acting as an in-coupling grating.

[0147] FIGS. 22-24 depict different methods of creating diffractive gratings using a transparent conductive material for use in optical elements for head-mounted displays. The diffractive gratings of FIG. 20 may be generated using any of the methods described in FIGS. 22-24.

[0148] FIG. 21 depicts an example of fabricating a diffractive grating in an optical element using a transparent conductive material. Optical element 2100 includes substrate 2110. Substrate 2110 may comprise a material with a high refractive index for fabricating the diffractive grating onto, such as glass or plastic. At step 2102, transparent conductive material 2112 is patterned onto the substrate. Transparent conductive material 2112 may comprise a transparent conductive oxide, such as fluorinated tin oxide, indium tin oxide, aluminum zinc oxide, indium zinc oxide, indium tin zirconium oxide, indium gallium oxide, indium gallium zinc oxide, tin oxide, or zinc oxide, or a conductive polymer, such as poly(3,4- ethylenedioxythiphene) (PEDOT) or poly(3,4-ethylenedi oxythiophene): poly(styrene sulfonate) (PEDOT:PSS).

[0149] In some embodiments, the transparent conductive material is tuned to a particular implementation. For instance, indium tin oxide may be doped with more indium to create a more absorbent but less transparent diffractive grating or with more tin oxide to create a less absorbent but more transparent grating. This diversification allows the diffractive gratings to be used differently in different locations. For instance, in smaller locations, such as an incoupling grating, a higher doping of indium would reduce the stray light in the system while having a relatively small effect on the usage of the waveguide as a lens. Conversely, in larger or more central locations, such as in the out-coupling grating, a higher doping of tin oxide would make it easier for one to see through the lens but would decrease the overall absorption of non-polarized or differently polarized light. In some embodiments, different diffractive gratings on a waveguide include different transparent conductive materials or different chemical makeups of a same transparent conductive material. For instance, if both the incoupling and out-coupling gratings are fabricated with indium tin oxide, the in-coupling grating may be fabricated with indium tin oxide that is doped with indium to give the indium tin oxide in the in-coupling grating a higher percentage of indium than the indium tin oxide coupling grating. Additionally or alternatively, the out-coupling grating may be fabricated with indium tin oxide that is doped with tin oxide for the same or similar effect.

[0150] The transparent conductive material 2112 may be patterned onto substrate 2110 in any of a plurality of designs, including blazed patterns, slanted patterns, binary patterns, analogue structures, or varying depth structures. Techniques for patterning transparent conductive material 2112 onto substrate 2110 include any of electron beam lithography, interference lithography, nanoimprint lithography, hot embossing, photolithography, focused ion beams, laser ablation, physical vapor deposition, atomic layer deposition, chemical vapor deposition, nanoparticle inks, spin-casting, dip-coating, or any other lithography, patterning, coating techniques or combination thereof. For instance, a coating technique, such as chemical vapor deposition may be used to coat the substrate with the transparent conductive material. Following the coating of the substrate, strips of the transparent conductive material may be removed through a lithographic technique, such as electron beam lithography of a resist followed by an etching and lift-off step.

[0151] In some embodiments, the transparent conductive material is patterned onto the substrate in a manner that produces gaps between each strip. For instance, some lithographic techniques provide a thin layer of the material between each of the strips to provide additional support. The transparent conductive material may be applied without the thin layer and/or the thin layer may be removed through ablation techniques to ensure that the spaces between the transparent conductive material do not include any of the transparent conductive material.

[0152] FIG. 22 depicts an example of fabricating a buried diffractive grating in an optical element using a transparent conductive material and a low-refractive index material. Optical element 2200 includes substrate 2210. Substrate 2210 may comprise a material with a high refractive index for fabricating the transparent conductive material as a buried diffractive grating onto, such as glass or plastic. At step 2202, transparent conductive material 2212 is patterned onto the substrate using the techniques described herein. Transparent conductive material 2212 may comprise a transparent conductive oxide, such as fluorinated tin oxide, indium tin oxide, aluminum zinc oxide, indium zinc oxide, indium tin zirconium oxide, indium gallium oxide, indium gallium zinc oxide, tin oxide, or zinc oxide, or a conductive polymer, such as poly(3,4-ethylenedioxythiphene) (PEDOT) or poly(3,4- ethylenedi oxy thiophene): poly(styrene sulfonate) (PEDOT:PSS). At step 2204, a low index material 2214 is patterned onto the substrate. Low index material 2214 may comprise a material with a refractive index that is substantially lower than the refractive index of substrate 2210, such as lithium fluoride, calcium fluoride, magnesium fluoride, or any other low refractive index optical resin. Substantially lower, as used herein, refers to a difference of 0.5 or greater between the two indices.

[0153] The low index material 2214 may be patterned onto substrate 2210 in between the strips of transparent conductive material 2212 using lithographic and/or patterning techniques, such as electron beam lithography, interference lithography, nanoimprint lithography, hot embossing, photolithography with a pattern transfer, reactive ion etching and deposition of the low index material, physical vapor deposition, atomic layer deposition, chemical vapor deposition, application of nanoparticle inks, spin casting, dip-coating, or any combination of techniques, such as coating the low index material 2214 onto the substrate on top of the transparent conductive material 2212 and removing excess material through lithographic techniques.

[0154] At step 2206, a high index coating 2216 or separate sheet of glass or plastic is applied to cover transparent conductive material 2212, low index material 2214, and substrate 2216. High index coating 2216 may comprise any of cubic zirconium oxide, titanium oxide, aluminum oxide, diamond, hafnium oxide, tantalum oxide, zinc oxide, or any other high- index resins. In some embodiments, high index coating 2216 is selected to have a refractive index that is substantially equal to the refractive index of substrate 2210. Processes for coating the high-index coating 2216 onto the transparent conductive material 2212, low index material 2214, and substrate 2216 include any of physical vapor deposition, atomic layer deposition, chemical vapor deposition, application of nanoparticle inks, spin casting, or dipcoating. Additionally or alternatively, a sheet of glass or plastic may be laser bonded to the transparent conductive material 2212 and low index material 2214. An additional processing step may include polishing or otherwise thinning the sheet of glass to a desired thickness.

[0155] FIG. 13 depicts an example of fabricating a buried diffractive grating in an optical element using a transparent conductive material and a sacrificial material. Optical element 1300 includes substrate 1310. Substrate 1310 may comprise a material with a high refractive index for fabricating the buried diffractive grating onto, such as glass or plastic. At step 1302, transparent conductive material 1312 is patterned onto the substrate. Transparent conductive material 1312 may comprise a transparent conductive oxide, such as fluorinated tin oxide, indium tin oxide, aluminum zinc oxide, indium zinc oxide, indium tin zirconium oxide, indium gallium oxide, indium gallium zinc oxide, tin oxide, or zinc oxide, or a conductive polymer, such as poly(3,4-ethylenedioxythiphene) (PEDOT) or poly(3,4- ethylenedi oxy thiophene): poly(styrene sulfonate) (PEDOT:PSS).

[0156] At step 2304, sacrificial material 2314 is patterned onto the substrate in between the strips of transparent conductive material. Sacrificial material 2314 may comprise a soluble or dissolvable material, such as a photoresist, a water-soluble polymer or material, or organic- solvent soluble polymer or material. One example of sacrificial material includes water soluble polyvinyl alcohol.

[0157] At step 2306, a high index coating 2316 is applied to cover transparent conductive material 2312, sacrificial material 2314, and substrate 2310. High index coating 2316 may comprise any of cubic zirconium oxide, titanium oxide, aluminum oxide, diamond, hafnium oxide, tantalum oxide, zinc oxide, or any other high-index resins. In some embodiments, high index coating 2316 is selected to have a refractive index that is substantially equal to the refractive index of substrate 2310. Processes for coating the high-index coating onto the transparent conductive material 2312, sacrificial material 2314, and substrate 2140 include any of physical vapor deposition, atomic layer deposition, chemical vapor deposition, application of nanoparticle inks, spin casting, or dip-coating.

[0158] At step 2308, after the high index coating has been applied to cover transparent conductive material 2312, sacrificial material 2314, and substrate 2140, the sacrificial material is removed. For example, a sintering or dissolution process may be applied to optical element 2300 to remove sacrificial material.

[0159] The fabrication techniques described herein with respect to FIG. 22-23 provide a wide array of benefits. One such benefit is that buried diffractive gratings can be fabricated in different structures within the optical element, such as blazed structures, slanted structures, binary structures, analogue structures, or varying depth structures. In particular, varying depth structures are not possible with surface relief gratings. In contrast, buried diffractive gratings can be fabricated at different depths depending on need, with some embodiments including buried diffractive gratings in a center of the optical element and other embodiments including buried diffractive gratings closer to one surface of the optical element than the other. In addition, a single diffractive grating can be fabricated with portions of the buried diffractive grating at different depths and/or different buried diffractive gratings in a single optical element can be fabricated at different depths, such as an in-coupling grating at a first depth and an out-coupling grating at a second depth.

[0160] Additionally, the use of a transparent conductive material to fabricate diffractive gratings provides absorption of stray light in the waveguide without requiring additional structures to be fabricated onto the surface of the waveguide which would be limited in location and may negatively impact visibility through the waveguide. While transparent conductive materials would block light if they covered the waveguide, the thin strips of material used in a diffractive grating has minimal impact on the transparency of the waveguide. FIG. 24 depicts an example of a head-mounted display comprising one or more optical elements with a diffractive grating fabricated using a transparent conductive material. Head-mounted display 2400 comprises optical element 2402, image source 2404, control circuitry 2408, memory 2410, network adaptor 2412, and power source 2414. Optical element 2402 comprises an optical element, such as a lens, which sits in front of an eye of a user. Image source 2404 provides an image beam 2406 to the optical element which is diffracted by a buried diffractive grating and displayed to the eye of the user. Control circuitry 2408 may be based on any suitable processing circuitry, such as one or more microprocessors, microcontrollers, digital signal processors, programmable logic devices, field-programmable gate arrays (FPGAs), application-specific integrated circuits (ASICs), etc., and may include a multi-core processor (e.g., quad-core). Control circuitry 2408 may be configured to generate one or more images for display through the head-mounted display and instruct image source 2404 to produce one or more image beams corresponding to the one or more images. Memory 2410 may be any device for storing electronic data, such as randomaccess memory, solid state devices, quantum storage devices, hard disk drives, non-volatile memory or any other suitable fixed or removable storage devices, and/or any combination of the same. Memory 2410 may store data defining images for display by the head-mounted display. Network adaptor 2412 comprises circuitry that connects the head-mounted display to one or more other devices over a network. Network adaptor 2412 may comprise wires and/or busses connected to a physical network port, e.g. an ethemet port, a wireless WiFi port, cellular communication port, or any other type of suitable physical port. Power source 2414 comprises a source of power to the image source 2404, control circuitry 2408, memory 2410, and/or network adaptor 2412, such as a battery, solar generator, or wired power source. The following sets of items are contemplated as part of the invention:

The following set of items (all numbers of this set referring to this set of items):

1. A head-mounted display, comprising: an image source configured to provide an image beam; an optical element comprising a flat first surface, a flat second surface opposing the first surface, and a buried diffractive grating spaced from and disposed between the first surface and the second surface, the buried diffractive grating comprising a plurality of nonsolid pockets interspaced with a material, wherein the material has a refractive index that is substantially equal to a refractive index of the first surface and a refractive index of the second surface; wherein the optical element is configured to convert the image beam into an output image by diffracting the beam through the buried diffractive grating, propagating the image beam through the optical element through reflection off the first and second surfaces, and directing the image beam through at least one of the first or second surfaces of the optical element.

2. The head-mounted display of item 1, further comprising a lens coupled to the optical element, wherein the lens is separated from the first surface by an angular-sensitive reflective coating.

3. The head-mounted display of item 1, wherein the optical element is a first optical element, the buried diffractive grating is a first buried diffractive grating, and the headmounted display further comprises: a second optical element comprising a third surface, a fourth surface opposing the third surface, and a second buried diffractive grating spaced from and disposed between the third surface and the fourth surface; wherein the fourth surface of the second optical element is separated from the first surface of the first optical element by an angular-sensitive reflective coating; wherein the image source is configured to provide a first image beam at a first wavelength to the first optical element and a second image beam at a second wavelength to the second optical element.

4. The head-mounted display of item 1, wherein the optical element is a first optical element, the buried diffractive grating is a first buried diffractive grating, and the headmounted display further comprises: a second optical element comprising a third surface, a fourth surface opposing the third surface, and a second buried diffractive grating spaced from and disposed between the third surface and the fourth surface; wherein the fourth surface of the second optical element is separated from the first surface of the first optical element by an angular-sensitive reflective coating; wherein the first optical element is configured to output the image at a first focus and the second optical element is configured to output the image at a second focus.

5. The head-mounted display of item 1, wherein the buried diffractive grating comprises two or more of an incoupling grating, an outcoupling grating, or an expansion grating.

6. The head-mounted display of item 1, wherein the optical element is configured to convert the image beam into an image by: receiving the image beam at an incoupling grating of the optical element which diffracts the image beam towards an expansion grating of the optical element; expanding the image beam by the expansion grating of the optical element and transmitting the beam from the expansion grating to an outcoupling grating; diffracting the expanded image beam by the outcoupling grating towards an eyeball of a wearer of the head-mounted display.

7. The head-mounted display of item 1, wherein the material comprises a coating of one or more of cubic zirconium oxide, titanium oxide, aluminum oxide, diamond hafnium oxide, tantalum oxide, or zinc oxide, and wherein the first surface comprises the coating. 8. The head-mounted display of item 1, wherein the material comprises glass of the first surface.

9. The head-mounted display of item 1, wherein the buried diffractive grating comprises a first grating and a second grating, wherein the first grating is spaced closer to the first surface than the second grating.

10. The head-mounted display of item 1, wherein one or more of the first surface or the second surface are coated with an antireflective coating.

11. The head-mounted display of item 1, wherein the buried diffractive grating comprises a grating structure that is blazed or slanted.

12. The head-mounted display of item 1, wherein the nonsolid pockets and the material of the buried diffractive grating have different widths.

13. A method for producing a waveguide for a head-mounted display comprising: patterning a sacrificial material on a surface of transparent material; coating the sacrificial material with a coating comprising a refractive index substantially equal to a refractive index of the transparent material; performing sintering or dissolution to form nonsolid pockets in place of the sacrificial material.

14. The method of item 13, wherein the transparent material comprises glass or plastic.

15. The method of item 13, wherein the transparent material is disposed at a plurality of heights, and wherein patterning the sacrificial material on the surface of the transparent material comprises patterning a first portion of the sacrificial material at a first height and patterning a second portion of the sacrificial material at a second height.

16. The method of item 13, wherein the sacrificial material comprises one or more of a photoresist, a water-soluble material, or organic-solvent soluble material. 17. The method of item 13, wherein the coating comprises one or more of cubic zirconium oxide, titanium oxide, aluminum oxide, diamond hafnium oxide, tantalum oxide, or zinc oxide.

18. A waveguide for a head-mounted display produced by: patterning a sacrificial material on a surface of a transparent material; coating the sacrificial material with a coating comprising a refractive index substantially equal to a refractive index of the transparent material; performing sintering or dissolution to form a plurality of pockets of nonsolid pockets in place of the sacrificial material.

19. The waveguide of item 18, wherein the transparent material comprises glass or plastic.

20. The waveguide of item 18, wherein the transparent material is disposed at a plurality of heights, and wherein patterning the sacrificial material on the surface of the transparent material comprises patterning a first portion of the sacrificial material at a first height and patterning a second portion of the sacrificial material at a second height.

21. The waveguide of item 18, wherein the sacrificial material comprises one or more of a photoresist, a water-soluble material, or organic-solvent soluble material.

22. The waveguide of item 18, wherein the coating comprises one or more of cubic zirconium oxide, titanium oxide, aluminum oxide, diamond hafnium oxide, tantalum oxide, or zinc oxide.

23. A method for producing a waveguide for a head-mounted display comprising: patterning a first material on a surface of transparent material; coating the first material with a second material comprising a refractive index substantially equal to a refractive index of the transparent material and substantially higher than a refractive index of the first material. 24. The method of item 23, wherein the transparent material comprises glass or plastic.

25. The method of item 23, wherein the transparent material is disposed at a plurality of heights and wherein patterning the first material on the surface of the transparent material comprises patterning a first portion of the first material at a first height and patterning a second portion of the first material at a second height.

26. The method of item 23, wherein the first material comprises one or more of lithium fluoride, calcium fluoride, or magnesium fluoride.

27. The method of item 23, wherein the second material comprises one or more of cubic zirconium oxide, titanium oxide, aluminum oxide, diamond hafnium oxide, tantalum oxide, or zinc oxide.

28. A waveguide for a head-mounted display produced by: patterning a first material on a surface of transparent material; coating the first material with a coating comprising a refractive index substantially equal to a refractive index of the transparent material and substantially higher than a refractive index of the first material.

29. The waveguide of item 28, wherein the transparent material comprises glass or plastic.

30. The waveguide of item 28, wherein the transparent material is disposed at a plurality of heights and wherein patterning the first material on the surface of the transparent material comprises patterning a first portion of the first material at a first height and patterning a second portion of the first material at a second height.

31. The waveguide of item 28, wherein the first material comprises one or more of lithium fluoride, calcium fluoride, or magnesium fluoride. 32. The waveguide of item 28, wherein the coating comprises one or more of cubic zirconium oxide, titanium oxide, aluminum oxide, diamond hafnium oxide, tantalum oxide, or zinc oxide.

And also the following set of items (all numbers of this set referring to this set of items):

1. A display comprising: an image source configured to provide an image beam; an optical element comprising an in-coupling grating and an out-coupling grating, the incoupling grating comprising:

(a) a first diffractive grating superimposed at least in part over a second diffractive grating, wherein the image beam diffracted by the first diffractive grating is further diffracted by the second diffractive grating before further propagating through the optical element; wherein the optical element is configured to convert the image beam into an output image by diffracting the beam through the in-coupling grating, propagating the image beam through the optical element through internal reflection, and diffracting the image beam by the out-coupling grating through a surface of the optical element.

2. The display item 1, wherein a pattern of the first diffractive grating differs from a pattern of the second diffractive grating.

3. The display of item 1, wherein the first diffractive grating comprises a surface relief grating fabricated with a transparent conductive material.

4. The display of item 1, wherein the second diffractive grating comprises a first buried diffractive grating at a first depth within the optical element and a second buried diffractive grating at a second depth, different than the first depth, within the optical element.

5. The display of item 1, further comprising a lens coupled to the optical element, wherein the lens is separated from a flat surface of the optical element by an angular-sensitive reflective coating. 6. The display of item 1, wherein the optical element is a first optical element, and the display further comprises: a second optical element; wherein a flat surface of the first optical element is separated from a flat surface of the second optical element by an angular-sensitive reflective coating; wherein the image source is configured to provide a first image beam at a first wavelength to the first optical element and a second image beam at a second wavelength to the second optical element.

7. The display of item 1, wherein the optical element is a first optical element, and the display further comprises: a second optical element; wherein a flat surface of the first optical element is separated from a flat surface of the second optical element by an angular-sensitive reflective coating; wherein the first optical element is configured to output the image at a first focal focus and the second optical element is configured to output the image at a second focal focus.

8. The display of item 1, wherein the out-coupling grating comprises: a third diffractive grating superimposed at least in part over the fourth diffractive grating such that the image beam is diffracted by the third diffractive grating and the fourth diffractive grating before exiting the optical element.

9. The display of item 1, wherein the optical element is configured to convert the image beam into an output image by: receiving the image beam at the in-coupling grating of the optical element which diffracts the image beam towards an expansion grating of the optical element; expanding the image beam by the expansion grating of the optical element and transmitting the beam from the expansion grating to an out-coupling grating; diffracting the expanded image beam by the out-coupling grating towards an eyeball of a user of the display. 10. The display of item 1, wherein the second diffractive grating comprises a plurality of nonsolid pockets interspaced with a material, wherein the material has a refractive index that is substantially equal to a refractive index of a surface of the optical element through which the image beam enters the optical element or is substantially equal to a refractive index of a surface of the optical element through which the image beam leaves the optical element.

11. The display of item 10, wherein the material comprises a coating of one or more of cubic zirconium oxide, titanium oxide, aluminum oxide, diamond hafnium oxide, tantalum oxide, zinc oxide, or glass of the one or more surfaces.

12. The display of item 1, wherein at least one of the first diffractive grating or the second diffractive grating comprises a grating structure that is blazed or slanted.

13. The display of item 1, wherein the first diffractive grating and the second diffractive grating diffract the image beam at different angles.

14. A display comprising: an image source configured to provide an image beam; an optical element comprising an in-coupling grating and an out-coupling grating, the out-coupling grating comprising:

(a) a first diffractive grating superimposed at least in part over a second diffractive grating wherein the image beam diffracted by the second diffractive grating is further diffracted by the first diffractive grating before exiting the optical element; wherein the optical element is configured to convert the image beam into an output image by diffracting the beam through the in-coupling grating, propagating the image beam through the optical element through internal reflection, and diffracting the image beam by the out-coupling grating through a surface of the optical element.

15. The display of item 14, wherein a pattern of the first diffractive grating differs from a pattern of the second diffractive grating. 16. The display of item 15, wherein the first diffractive grating comprises a surface relief grating fabricated with a transparent conductive material.

17. The display of item 14, wherein the second diffractive grating comprises a first buried diffractive grating at a first depth within the optical element and a second buried diffractive grating at a second depth, different than the first depth, within the optical element.

18. The display of item 14, further comprising a lens coupled to the optical element, wherein the lens is separated from a flat surface of the optical element by an angular-sensitive reflective coating.

19. The display of item 14, wherein the optical element is a first optical element and the display further comprises: a second optical element; wherein a flat surface of the first optical element is separated from a flat surface of the second optical element by an angular-sensitive reflective coating; wherein the image source is configured to provide a first image beam at a first wavelength to the first optical element and a second image beam at a second wavelength to the second optical element.

20. The display of item 14, wherein the optical element is a first optical element and the display further comprises: a second optical element; wherein a flat surface of the first optical element is separated from a flat surface of the second optical element by an angular-sensitive reflective coating; wherein the first optical element is configured to output the image at a first focal focus and the second optical element is configured to output the image at a second focal focus.

21. The display of item 14, wherein the optical element is configured to convert the image beam into an output mage by: receiving the image beam at the in-coupling grating of the optical element which diffracts the image beam towards an expansion grating of the optical element; expanding the image beam by the expansion grating of the optical element and transmitting the beam from the expansion grating to an out-coupling grating; diffracting the expanded image beam by the out-coupling grating towards an eyeball of a user of the display.

22. The display of item 14, wherein the buried diffractive grating comprises a plurality of nonsolid pockets interspaced with a material, wherein the material has a refractive index that is substantially equal to a refractive index of a surface of the optical element through which the image beam enters the optical element or is substantially equal to a refractive index of a surface of the optical element through which the image beam leaves the optical element.

23. The display of item 22, wherein the material comprises a coating of one or more of cubic zirconium oxide, titanium oxide, aluminum oxide, diamond hafnium oxide, tantalum oxide, zinc oxide, or glass of the one or more surfaces.

24. The display of item 14, wherein at least one of the first diffractive grating or the second diffractive grating comprises a grating structure that is blazed or slanted.

25. The display of item 14, wherein the first diffractive grating and the second diffractive grating diffract the image beam at different angles.

26. A method for producing a waveguide for a display comprising: fabricating an optical element with a first diffractive grating comprising a buried diffractive grating; patterning a second diffractive grating over a portion of the optical element comprising the first diffractive grating.

27. The method of item 26, wherein the fabricating the optical element with the first diffractive grating comprising the buried diffractive grating comprises: patterning a sacrificial material on a surface of a transparent material; coating the sacrificial material with a coating comprising a refractive index substantially equal to a refractive index of the transparent material; performing sintering or dissolution to form nonsolid pockets in place of the sacrificial material.

28. The method of item 26, wherein the fabricating the optical element with the first diffractive grating comprising the buried diffractive grating comprises: patterning a first material on a surface of transparent material; coating the first material with a coating comprising a refractive index substantially equal to a refractive index of the transparent material and substantially higher than a refractive index of the first material.

29. The method of item 26, wherein the fabricating the optical element with the first diffractive grating comprising the buried diffractive grating comprises: fabricating a first grating onto a surface of a first material; laser bonding a second surface to the first grating.

30. The method of item 26, wherein the patterning the second grating over the portion of the optical element comprising the first diffractive grating comprises patterning a transparent conductive material over the portion of the optical element.

31. The method of item 26, wherein the patterning the second grating over the portion of the optical element comprises fabricating a second buried diffractive grating over the portion of the optical element.

32. The method of item 31, wherein the fabricating the second buried diffractive grating comprises: patterning a sacrificial material on a surface of the portion of the optical element; coating the sacrificial material with a coating comprising a refractive index substantially equal to a refractive index of the portion of the optical element; performing sintering or dissolution to form nonsolid pockets in place of the sacrificial material. 33. The method of item 31, wherein the fabricating the second buried diffractive grating comprises: patterning a first material on a surface of the portion of the optical element; coating the first material with a coating comprising a refractive index substantially equal to a refractive index of the portion of the optical element and substantially higher than a refractive index of the first material.

34. The method of item 31, wherein the fabricating the second buried diffractive grating comprises: fabricating a grating onto a surface of the portion of the optical element; laser bonding a second surface to the grating.

35. A wavegui de for a di spl ay produced by : fabricating an optical element with a first diffractive grating comprising a buried diffractive grating; patterning a second diffractive grating over a portion of the optical element comprising the first diffractive grating.

36. The waveguide of item 35, wherein the fabricating the optical element with the first diffractive grating comprising the buried diffractive grating comprises: patterning a sacrificial material on a surface of a transparent material; coating the sacrificial material with a coating comprising a refractive index substantially equal to a refractive index of the transparent material; performing sintering or dissolution to form nonsolid pockets in place of the sacrificial material.

37. The waveguide of item 35, wherein the fabricating the optical element with the first diffractive grating comprising the buried diffractive grating comprises: patterning a first material on a surface of transparent material; coating the first material with a coating comprising a refractive index substantially equal to a refractive index of the transparent material and substantially higher than a refractive index of the first material. 38. The waveguide of item 35, wherein the fabricating the optical element with the first diffractive grating comprising the buried diffractive grating comprises: fabricating a first grating onto a surface of a first material; laser bonding a second surface to the first grating.

39. The waveguide of item 35, wherein the patterning the second grating over the portion of the optical element comprising the first diffractive grating comprises patterning a transparent conductive material over the portion of the optical element.

40. The waveguide of item 35, wherein the patterning the second grating over the portion of the optical element comprises fabricating a second buried diffractive grating over the portion of the optical element.

41. The waveguide of item 40, wherein the fabricating the second buried diffractive grating comprises: patterning a sacrificial material on a surface of the portion of the optical element; coating the sacrificial material with a coating comprising a refractive index substantially equal to a refractive index of the portion of the optical element; performing sintering or dissolution to form nonsolid pockets in place of the sacrificial material.

42. The waveguide of item 40, wherein the fabricating the second buried diffractive grating comprises: patterning a first material on a surface of the portion of the optical element; coating the first material with a coating comprising a refractive index substantially equal to a refractive index of the portion of the optical element and substantially higher than a refractive index of the first material.

43. The waveguide of item 40, wherein the fabricating the second buried diffractive grating comprises: fabricating a grating onto a surface of the portion of the optical element; laser bonding a second surface to the grating.

1. A head-mounted display, comprising: an image source configured to provide an image beam; an optical element comprising a diffractive grating, the diffractive grating comprising a transparent conductive material, wherein the diffractive grating comprising the transparent conductive material absorbs light that is not polarized in a particular direction; wherein the optical element is configured to convert the image beam into an output image by diffracting the beam through an in-coupling grating, propagating the image beam through the optical element through internal reflection, and directing the image beam through a surface of the optical element by an out-coupling grating.

2. The head-mounted display of item 1, wherein the diffractive grating comprising the transparent conductive material is the out-coupling grating.

3. The head-mounted display of item 1, wherein the transparent conductive material comprises fluorinated tin oxide, indium tin oxide, aluminum zinc oxide, indium zinc oxide, indium tin zirconium oxide, indium gallium oxide, indium gallium zinc oxide, tin oxide, or zinc tin oxide.

4. The head-mounted display of item 1, wherein the diffractive grating comprises an in-coupling grating and an out-coupling grating.

5. The head-mounted display of item 4, wherein the transparent conductive material comprises a first transparent conductive material in the in-coupling grating and a second transparent conductive material in the out-coupling grating that comprises a different chemical makeup than the first transparent conductive material.

6. The head-mounted display of item 5, wherein the first transparent conductive material and the second transparent conductive material comprise indium tin oxide and wherein the first transparent conductive material comprises a higher percentage of indium than the second transparent conductive material.

7. The head-mounted display of item 1, wherein the diffractive grating comprises a buried diffractive grating spaced from and disposed between a first flat surface of the optical element and a second flat surface of the optical element opposing the first flat surface.

8. A method for producing a waveguide for a head-mounted display comprising patterning a diffractive grating comprising a transparent conductive material onto a second transparent material, wherein the diffractive grating comprising the transparent conductive material is configured to absorb light that is not polarized in a particular direction.

9. The method of item 8, further comprising: patterning, in between the transparent conductive material, a third material comprising a refractive index substantially lower than the refractive index of the second transparent material; covering the transparent conductive material and the third material with a fourth material comprising a refractive index substantially equal to the second transparent material.

10. The method of item 8, further comprising: patterning, in between the transparent conductive material, a sacrificial material; covering the transparent conductive material and the sacrificial material with a third material comprising a refractive index substantially equal to the second transparent material; performing sintering or dissolution to form pockets of nonsolid pockets in place of the sacrificial material.

11. The method of item 8, wherein the diffractive grating comprising the transparent conductive material is the out-coupling grating.

12. The method of item 8, wherein the transparent conductive material comprises fluorinated tin oxide, indium tin oxide, aluminum zinc oxide, indium zinc oxide, indium tin zirconium oxide, indium gallium oxide, indium gallium zinc oxide, tin oxide, or zinc tin oxide.

13. The method of item 8, wherein the diffractive grating comprises an in-coupling grating and an out-coupling grating.

14. The method of item 13, wherein the transparent conductive material comprises a first transparent conductive material in the in-coupling grating and a second transparent conductive material in the out-coupling grating that comprises a different chemical makeup than the first transparent conductive material.

15. The method of item 14, wherein the first transparent conductive material and the second transparent conductive material comprise indium tin oxide and wherein the first transparent conductive material comprises a higher percentage of indium than the second transparent conductive material.

16. A waveguide for a head-mounted display produced by patterning diffractive grating comprising a transparent conductive material onto a second transparent material, wherein the diffractive grating comprising the transparent conductive material is configured to absorb light that is not polarized in a particular direction.

17. The waveguide of item 16, wherein the waveguide is further produced by: patterning, in between the transparent conductive material, a third material comprising a refractive index substantially lower than the refractive index of the second transparent material; covering the transparent conductive material and the third material with a fourth material comprising a refractive index substantially equal to the second transparent material.

18. The waveguide of item 16, wherein the waveguide is further produced by: patterning, in between the transparent conductive material, a sacrificial material; covering the transparent conductive material and the sacrificial material with a third material comprising a refractive index substantially equal to the second transparent material; performing sintering or dissolution to form pockets of nonsolid pockets in place of the sacrificial material.

19. The waveguide of item 16, wherein the diffractive grating comprising the transparent conductive material is the out-coupling grating.

20. The waveguide of item 16, wherein the transparent conductive material comprises fluorinated tin oxide, indium tin oxide, aluminum zinc oxide, indium zinc oxide, indium tin zirconium oxide, indium gallium oxide, indium gallium zinc oxide, tin oxide, or zinc tin oxide.

21. The waveguide of item 16, wherein the diffractive grating comprises an in-coupling grating and an out-coupling grating.

22. The waveguide of item 21, wherein the transparent conductive material comprises a first transparent conductive material in the in-coupling grating and a second transparent conductive material in the out-coupling grating that comprises a different chemical makeup than the first transparent conductive material.

23. The waveguide of item 22, wherein the first transparent conductive material and the second transparent conductive material comprise indium tin oxide and wherein the first transparent conductive material comprises a higher percentage of indium than the second transparent conductive material.

Claims

What is claimed is:

2. The head-mounted display of claim 1, further comprising a lens coupled to the optical element, wherein the lens is separated from the first surface by an angular-sensitive reflective coating.

3. The head-mounted display of claim 1, wherein the optical element is a first optical element, the buried diffractive grating is a first buried diffractive grating, and the headmounted display further comprises: a second optical element comprising a third surface, a fourth surface opposing the third surface, and a second buried diffractive grating spaced from and disposed between the third surface and the fourth surface; wherein the fourth surface of the second optical element is separated from the first surface of the first optical element by an angular-sensitive reflective coating; wherein the image source is configured to provide a first image beam at a first wavelength to the first optical element and a second image beam at a second wavelength to the second optical element.

4. The head-mounted display of claim 1, wherein the optical element is a first optical element, the buried diffractive grating is a first buried diffractive grating, and the headmounted display further comprises: a second optical element comprising a third surface, a fourth surface opposing the third surface, and a second buried diffractive grating spaced from and disposed between the third surface and the fourth surface; wherein the fourth surface of the second optical element is separated from the first surface of the first optical element by an angular-sensitive reflective coating; wherein the first optical element is configured to output the image at a first focus and the second optical element is configured to output the image at a second focus.

5. The head-mounted display of claim 1, wherein the buried diffractive grating comprises two or more of an incoupling grating, an outcoupling grating, or an expansion grating.

6. The head-mounted display of claim 1, wherein the optical element is configured to convert the image beam into an image by: receiving the image beam at an incoupling grating of the optical element which diffracts the image beam towards an expansion grating of the optical element; expanding the image beam by the expansion grating of the optical element and transmitting the beam from the expansion grating to an outcoupling grating; diffracting the expanded image beam by the outcoupling grating towards an eyeball of a wearer of the head-mounted display.

7. The head-mounted display of claim 1, wherein the material comprises a coating of one or more of cubic zirconium oxide, titanium oxide, aluminum oxide, diamond hafnium oxide, tantalum oxide, or zinc oxide, and wherein the first surface comprises the coating.

8. The head-mounted display of claim 1, wherein the material comprises glass of the first surface.

9. The head-mounted display of claim 1, wherein the buried diffractive grating comprises a first grating and a second grating, wherein the first grating is spaced closer to the first surface than the second grating.

10. The head-mounted display of claim 1, wherein one or more of the first surface or the second surface are coated with an antireflective coating.

11. The head-mounted display of claim 1, wherein the buried diffractive grating comprises a grating structure that is blazed or slanted.

12. The head-mounted display of claim 1, wherein the nonsolid pockets and the material of the buried diffractive grating have different widths.

14. The method of claim 13, wherein the transparent material is disposed at a plurality of heights, and wherein patterning the sacrificial material on the surface of the transparent material comprises patterning a first portion of the sacrificial material at a first height and patterning a second portion of the sacrificial material at a second height.

15. A method for producing a waveguide for a head-mounted display comprising: patterning a first material on a surface of transparent material; coating the first material with a second material comprising a refractive index substantially equal to a refractive index of the transparent material and substantially higher than a refractive index of the first material.