CROSS-REFERENCED APPLICATIONS

[0001] This application claims priority to U.S. Provisional Application No. 63/382,413, entitled “Holographic Grating Formulations” to Abraham et al., filed on Nov. 4, 2022, the disclosure of which is incorporated by reference in its entirety.

FIELD OF THE INVENTION

[0002] The present invention generally relates to holographic mixtures including one or more moisture absorbing materials.

BACKGROUND

[0003] Waveguides can be referred to as structures with the capability of confining and guiding waves (i.e., restricting the spatial region in which waves can propagate). One subclass includes optical waveguides, which are structures that can guide electromagnetic waves, typically those in the visible spectrum. Waveguide structures can be designed to control the propagation path of waves using a number of different mechanisms. For example, planar waveguides can be designed to utilize diffraction gratings to diffract and couple incident light into the waveguide structure such that the incoupled light can proceed to travel within the planar structure via total internal reflection (TIR).

[0004] Fabrication of waveguides can include the use of material systems that allow for the recording of holographic optical elements within or on the surface of the waveguides. One class of such material includes polymer dispersed liquid crystal (PDLC) mixtures, which are mixtures containing photopolymerizable monomers and liquid crystals. A further subclass of such mixtures includes holographic polymer dispersed liquid crystal (HPDLC) mixtures. Holographic optical elements, such as volume phase gratings, can be recorded in such a liquid mixture by illuminating the material with two mutually coherent laser beams. During the recording process, the monomers polymerize, and the mixture undergoes a photopolymerization-induced phase separation, creating regions densely populated by liquid crystal (LC) micro-droplets, interspersed with regions of clear polymer. The alternating liquid crystal-rich and liquid crystal-depleted regions form the fringe planes of the grating.

[0005] Waveguide optics, such as those described above, can be considered for a range of display systems and sensor applications. In many applications, waveguides containing one or more grating layers encoding multiple optical functions can be realized using various waveguide architectures and material systems, enabling new innovations in near-eye displays for Augmented Reality (AR) and Virtual Reality (VR), compact Heads Up Displays (HUDs) for aviation and road transport, and sensors for biometric and laser radar (LIDAR) applications. As many of these applications are directed at consumer products, there is a growing requirement for efficient low cost means for manufacturing holographic waveguides in large volumes.

[0006] In near-eye displays and display devices it may be beneficial that the overall system including a waveguide and a projector be compact and light weight to enable the user to wear the near-eye display comfortably and to enable the user to perform different tasks in environments where the user moves.

SUMMARY OF THE INVENTION

[0007] In some aspects, the techniques described herein relate to a holographic mixture for manufacturing a waveguide grating including: a photoinitiator; reagents that promote adhesion to a substrate; photopolymerizable monomers; inert material; and an acid scavenger to minimize the effect of moisture build up with the inert material and reagents.

[0008] In some aspects, the techniques described herein relate to a holographic mixture, wherein the reagents include a silane coupling agent.

[0009] In some aspects, the techniques described herein relate to a holographic mixture, wherein the reagents include a formula of (R'O)3-Si-R, where R'O- is an alkoxy group and -R is an organofunctional group.

[0010] In some aspects, the techniques described herein relate to a holographic mixture, wherein the inert material is a liquid crystal.

[0011] In some aspects, the techniques described herein relate to a holographic mixture, wherein the inert material is nanoparticles. [0012] In some aspects, the techniques described herein relate to a holographic mixture, wherein the acid scavenger is low viscosity such that the holographic mixture is inkjet printable.

[0013] In some aspects, the techniques described herein relate to a holographic mixture, wherein the acid scavenger includes carbodiimides.

[0014] In some aspects, the techniques described herein relate to a holographic mixture, wherein the carbodiimides include one or more functionalized carbodiimides at a molecular level or incorporated into an oligomer or a polymer structure.

[0015] In some aspects, the techniques described herein relate to a holographic mixture, wherein the functionalized carbodiimides include dicyclohexylcarbodiimide (DCC) and/or diisopropylcarbodiimide (DICDI).

[0016] In some aspects, the techniques described herein relate to a holographic mixture, wherein the acid scavenger includes polymers or oligomers.

[0017] In some aspects, the techniques described herein relate to a holographic mixture, wherein the acid scavenger includes oligomers and makes up 2% or less weight of the holographic mixture.

[0018] In some aspects, the techniques described herein relate to a holographic mixture, wherein the acid scavenger is a viscous liquid with high solubility in the holographic mixture.

[0019] In some aspects, the techniques described herein relate to a holographic mixture, wherein the acid scavenger is a solid with high solubility in photopolymer formulations.

[0020] In some aspects, the techniques described herein relate to a holographic mixture, further including wetting agents.

[0021] In some aspects, the techniques described herein relate to a waveguide for a display device including: one or more gratings fabricated from a holographic mixture containing liquid crystal; an edge sealant surrounding the one or more gratings to prevent water ingress into the one or more gratings.

[0022] In some aspects, the techniques described herein relate to a waveguide, wherein the edge sealant includes acid scavengers. [0023] In some aspects, the techniques described herein relate to a waveguide, wherein the edge sealant includes a UV initiator and/or black dye for light absorption.

[0024] In some aspects, the techniques described herein relate to a waveguide, wherein the edge sealant includes a mixture of aliphatic and aromatic monomers, wetting agents, photoinitiators, and black dye.

[0025] In some aspects, the techniques described herein relate to a waveguide, wherein the edge sealant includes an inkjet printable material.

[0026] In some aspects, the techniques described herein relate to a waveguide, further includes a coating separate from the edge sealant and the one or more gratings which prevents water ingress into the one or more gratings.

[0027] In some aspects, the techniques described herein relate to a method for manufacturing a waveguide grating including: depositing a holographic mixture on a substrate, the holographic mixture including: a photoinitiator; reagents that promote adhesion to the substrate; photopolymerizable monomers; inert material; and an acid scavenger to minimize the effect of moisture build up with the inert material and reagents; and exposing the holographic mixture with a holographic recording beam to form a periodic structure including a polymer matrix separated by inert material rich regions to form a waveguide grating.

[0028] In some aspects, the techniques described herein relate to a method, further including evacuating the inert material from the inert material rich regions to form evacuated periodic structures.

[0029] In some aspects, the techniques described herein relate to a method, wherein the inert material is a liquid crystal.

[0030] In some aspects, the techniques described herein relate to a method, wherein the acid scavenger includes carbodiimides.

[0031] In some aspects, the techniques described herein relate to a method mixture, wherein the carbodiimides include one or more functionalized carbodiimides at a molecular level or incorporated into an oligomer or a polymer structure.

[0032] In some aspects, the techniques described herein relate to a method, wherein the functionalized carbodiimides include dicyclohexylcarbodiimide (DCC) and/or diisopropylcarbodiimide (DICDI). [0033] In some aspects, the techniques described herein relate to a method, wherein the acid scavenger includes polymers or oligomers.

[0034] In some aspects, the techniques described herein relate to a method, wherein the acid scavenger includes oligomers and makes up 2% or less weight of the holographic mixture.

[0035] In some aspects, the techniques described herein relate to a method, wherein the reagents include a silane coupling agent.

[0036] In some aspects, the techniques described herein relate to a method, wherein the reagents include a formula of (R'O)3-Si-R, where R'O- is an alkoxy group and -R is an organofunctional group.

[0037] In some aspects, the techniques described herein relate to a method, wherein the substrate includes glass.

[0038] In some aspects, the techniques described herein relate to a method, wherein the substrate includes hydroxyl groups which condense with the reagents and promote adherence of the holographic mixture to the substrate.

BRIEF DESCRIPTION OF THE DRAWINGS

[0039] The description will be more fully understood with reference to the following figures and data graphs, which are presented as various embodiment of the disclosure and should not be construed as a complete recitation of the scope of the disclosure, wherein:

[0040] Fig. 1 illustrates an example reaction of carbodiimides with water.

[0041] Figs. 2A-2G illustrate an example process flow for fabricating deep SRGs in accordance with an embodiment of the invention.

[0042] Fig. 3 illustrates an example reaction where the base substrate is exposed to reagents in accordance with an embodiment of the invention.

[0043] Fig. 4 illustrates an example reaction where reagents are exposed to the base substrate.

[0044] Fig. 5 illustrates an example process for forming the release layer. [0045] Figs. 6A and 6B illustrates example plots illustrating changes in the optical haze on exposure to moisture over time at a higher temperature (70 C & 90 % RH) for an optical device manufactured using an HPDLC not including an acid scavenger.

[0046] Figs. 7A and 7B illustrate example plots illustrating changes in the optical haze on exposure to moisture over time for an optical device manufactured using an HPDLC mixture including an acid scavenger.

[0047] Fig. 8 illustrates various plots for various different configurations of HPDLC mixtures including various concentrations of different acid scavengers.

[0048] Fig. 9 illustrates various plots comparing a control mixture without acid scavenger and mixture containing AS over a period of time.

[0049] Fig. 10 illustrates an example schematic of a waveguide in accordance with an embodiment of the invention.

[0050] Fig. 11 is an example method for forming a waveguide grating in accordance with an embodiment of the invention.

DETAILED DESCRIPTION

[0051] Waveguide displays may include a waveguide with various gratings which may be manufactured using a holographic exposure process to create various grating structures such as volume Bragg gratings (VBG) and/or evacuated periodic structures (EPS). A holographic polymer dispersed liquid crystal (HPDLC) mixture or reactive monomer liquid crystal mixture (RMLCM) may be exposed to a holographic recording beam to form the grating. The HPDLC may include a mixture of monomer and liquid crystal (LC). Examples of this process and the formed gratings are discussed in U.S. Pat. Pub. No. 2021/0063634, entitled “Evacuating Bragg gratings and methods of manufacturing” and filed on Aug. 28, 2020, which is hereby incorporated by reference in its entirety for all purposes. After exposure, the formed grating includes alternating LC rich regions and polymer rich regions. Both the LC rich regions and the polymer rich regions contain at least some LC. LC may be used in HPDLCs to provide index difference between LC rich regions and polymer rich regions. Differences in effective refractive indices may define grating structures and thereby Bragg response or optical diffraction efficiency in waveguide-based displays. Two commonly used techniques in obtaining higher diffraction efficiency are increase in the concentration of components like LC or by using LC singles with higher extra ordinary index. For example, conjugated aromatics such as tolanes and terphenyls may be included. HPDLC mixtures including conjugated aromatics are described in U.S. Pat. App. Pub. No. 20200271973, entitled “Holographic Polymer Dispersed Liquid Crystal Mixtures with High Diffraction Efficiency and Low Haze” and filed Feb. 24, 2020 which is hereby incorporated by reference in its entirety. One other technique used to increase the index of grating is by using high refractive index materials such as nano-particles or nano-crystals made up of functionalized Ti, Zr, etc. [0052] Water molecules can react with LCs or inorganic ligand coordination sites in nanoparticles and their derivatives. These reactions can be permanent or temporary resulting in physical transformation. External molecules like water can affect inter/intra molecular interactions in LCs leading to significant modification of optical performance of nematic droplets. For example, aggregation or precipitation of liquid crystalline molecules can increase haze; therefore, LCs need to be shielded from contamination of water molecules. For example, the HPDLC mixture may include aromatic components such as biphenyls, tolanes or terphenyls which can precipitate on exposure to moisture, due to insolubility of these components in aqueous medium. Moisture can result in hazy or opaque material hindering the see-through transparency of devices. Molecularly dissolved desiccants in formulations can preferentially react with moisture before any reaction between water & liquid crystals and thereby providing nano-barrier protection for sensitive optical components. One example is high modulation domains of LC droplets in their nematic phase in HPDLCs. Such desiccants in grating regions can stop water vapors. Hence, chemically reactive materials like desiccants can protect sensitive optical components such as domains of HPDLCs from degradation due to moisture. While described in the context of liquid crystal, the invention also includes holographic mixtures including photopolymerizable monomers mixed with an inert fluid which may be sensitive to moisture.

[0053] Disclosed herein are waveguide displays which perform even when exposed to harsh conditions which includes one or many external stimuli such as high temperature, humidity & harmful radiations or their combinations. The HPDLC mixture may include desiccants (e.g. acid scavengers, aka hydrolysis stabilizers) which may be used to absorb or react with water and/or react with hydrolysis byproducts such as carboxylic acids of polymer matrices. Formulations with such desiccants can be used to manufacture stable displays with no degradation or acceptable reduction in performance under various conditions including continuous usage at ambient environments, during extended storage, and upon exposure to high humidity, and temperature, and light.

[0054] The stable displays may be integrated into lenses or head worn displays. Acceptable loss in optical efficiency under operation temperature; very low reduction or change in diffraction efficiency at elevated temperatures is desirable in such devices. In some embodiments, edge sealant can be applied around lenses to protect waveguide (WG) materials from contaminants such as water molecules (moisture ingress).

[0055] In some embodiments, the HPDLC mixture may include dyes, photoinitiators, monomers, high refractive index components (e.g. liquid crystals), and/or nanoparticles. The HPDLC mixture may further includes wetting agents. The HPDLC mixture may further include desiccants such as acid scavengers and hydrolysis stabilizers which may be used to absorb moisture which may decease diffraction efficiency of the display. Molecular desiccants are reagents capable of adsorbing or reacting with water. In some embodiments, the acid scavengers include carbodiimides. An example of such reagent is substituted carbodiimides where substituents can be aromatic or aliphatic including oligomers, polymers, or their derivatives. Carbodiimides are a unique class of reactive organic compounds having the heterocumulene structure R-N=C=N-R. Examples of acid scavengers including carbodiimides include Hydrostab 1 manufactured by LUBIO headquartered in Ludwigshafen, Germany. Carbodiimides can be formally considered to be the diimides of carbon dioxide or the anhydrides of 1 ,3-substituted ureas, and they are closely related to the monoimides of carbon dioxide, the isocyanates. The substituent R can be alkyl, aryl, acyl, aroyl, imidoyl, or sulfonyl. Carbodiimides may be used in organic synthesis. Carbodiimides may be a dehydrating agent. In some embodiments, the carbodiimides may contain dicyclohexylcarbodiimide (DCC) and/or diisopropylcarbodiimide (DICDI) type functionalities. Carbodiimides with primary alkyl substituents may be less stable. The most stable aliphatic carbodiimide is di-t- butylcarbodiimide. Examples of carbodiimides are discussed in European Pat. No. 2388284 which is hereby incorporated by reference in its entirety. [0056] During the lifetime of polylactic acid (PLA) products, the hydrolytic chain scission of ester bonds that randomly takes place in the polymer might be considered a primary cause of reduction in molecular weight and the related deterioration of mechanical and physical properties of PLA. The aqueous medium penetrates the polymer matrix and simultaneously converts the long polymer chain to low-molecular-weight, water-soluble oligomers and the given monomer. It follows that an effective suppression of the degradation has to aim to retard the hydrolysis process, either to prevent the water penetration or to slow down the rate of hydrolytical reactions in conjunction with the autocatalysis phenomenon. Investigation was made on the stabilization effects of various concentrations of the anti-hydrolysis agent (BDICDI) in PLA film. Bis(2,6- diisopropylphenyl)carbodiimide (BDICDI) was shown to be an efficient stabilizer of PLA- suppressing chain scissions of esterbonds during abiotic hydrolysis, especially at concentrations above 1.5% w/w.

[0057] Poly[(R)-3-hydroxy butyrate] (PHB) belongs to the family of poly[(R)-3-hydroxy alkanoate]s (PHAs), which are essentially biopolymers synthesized by a broad range of microorganisms as carbon and energy storage materials. PHBs and PHA family tend to degrade during processing at temperatures above their melting point via a hydrolytic mechanism induced by moisture attack and a concerted reaction mechanism induced by temperature. This polymer is especially versatile due to its biocompatibility, total biodegradability in different environmental compartments. It is needed to overcome some of its inherent limitations such as brittleness; sensitivity to thermal and hydrolytic degradation during processing and post crystallization of processed items. The presence of moisture even at levels as low as 0.1-0.2 % eventually adsorbed upon storages, can contribute in a dramatic load to its hydrolytic degradation. In one example, Stabaxol manufactured by Lanxess headquartered in Germany may be an example of an acid scavenger. Stabaxol contains carbodiimide and used as an acid scavenger. Although thermal and mechanical properties decreased with increasing Stabaxol® types amount, formulations of PHB with carbodiimide compounds may be utilized in the melt processing of PHAs.

[0058] Fig. 1 illustrates an example reaction of carbodiimides with water. As illustrated, water (H2O) 152 may react with the carbodiimide 154. The carbodiimide 154 may absorb the water 152 to form a combined carbodiim ide-water molecule 156 which may isolate the water 152 from the liquid crystal. The R1/R2 may be substituted aliphatic or aromatic groups. When exposed to moisture, molecular desiccants in HPDLC mixture reacts with water thereby protecting sensitive high refractive components like liquid crystals.

[0059] Acid scavengers may neutralize any acidic products and catalytic residue. Acid scavengers may stabilize polymers damaging effects of acids such as halogen, carboxylic acids, mineral acids from decomposition and corrosion. Examples of acid scavengers include inorganic materials such as calcium stearate, calcium lactate, and zinc acid. One particular acid scavenger is based on monomer and highly molecular carbodiimides. Such an acid scavenger may be utilized in plastics, adhesives, paints, varnishes, rubber, and other polymer systems.

[0060] Another example of an acid scavenger includes synthetic hydrotalcite in polyolefins. The acid scavenger may include polyethylenes. The basis of a typical additive package for polyolefins include primary (phenolic-) and secondary antioxidants (phosphites or phosphonites) and acid scavengers. A common acid scavenger used in polyolefins is synthetic hydrotalcite, a material that is being used as stabilizer in polyolefins. The key functionality of hydrotalcites is the irreversible adsorption of acidic catalytic residues. By doing so, hydrotalcites prevent many damaging side effects, most notably corrosion of processing equipment and degradation of the polymer itself. Furthermore, hydrotalcites, and especially high-quality materials such as DHT- 4A, can synergistically improve the performance of other additives and pigments in the formulation. A good example of this effect is the improved performance of Hindered Amine Light Stabilizers (HALS) in the presence of hydrotalcite, resulting in increased weatherability of the polyolefin article.

[0061] In some embodiments, the molecular desiccants may be compatible with the HPDLC mixture for miscibility with no precipitation or phase separation over time. The molecular desiccants may be organic desiccants. Inorganic desiccants or many acid scavengers are less compatible with HPDLC mixtures due to poor solubility. Two acid scavengers manufactured by LUBIO headquartered in Ludwigshafen, Germany named AS14 & AS15 have shown superior results in photopolymer inks (RMLCM) for moisture resistance without significantly reducing optical, print & shelf-life characteristics at various concentrations, up to about 2%, but not limited to. Concentrations of acid scavengers may be based on their chemical/physical nature such as viscosity, melting point or reactivity. For example, AS & AS15 are viscous liquids whereas AS10 is a different acid scavenger material but solid at room temperature. Acid scavengers may be mixed with other reactive ingredients of the RMLCM, which includes photo-initiators and monomers such as acrylates, but not limited to, as mentioned before. In other embodiments, concentrations may exceed 2% based on desired protection against moisture ingress. These acid scavengers are polymeric in nature containing one or more carbodiimide functionalities. In some embodiments, carbodiimide functionality may be a nonreactive dopant or part of the monomer/polymeric chain but dissolved or homogenized into the formulations. In addition to carbodiimides, there are other functional groups or chains including, but not limited to, aliphatic groups such as oligomethylenes oxyoligomethylenes, and cyclic alkanes, aromatics, acrylates or any other linking groups which makes acid scavengers/hydrolysis stabilizers compatible in the ‘host’ polymer matrices. Thus, the carbodiimides may include one or more functionalized carbodiimides at a molecular level or incorporated into an oligomer or a polymer structure. In some embodiments, the molecular desiccants may be oligomer or a polymer which may be a viscous liquid. In some embodiments, the molecular desiccants may include a low viscosity liquid. It has been discovered that a liquid molecular desiccant may provide better results than a powder. The powder may include a small molecule.

[0062] In various embodiments, a pair of substrates may sandwich an unexposed holographic mixture. The pair of substrates may include a base substrate and a cover substrate. Advantageously, the cover substrate may have different properties than the base substrate to allow for the cover substrate to adhere to the unexposed holographic mixture layer while capable of being removed from the formed holographic polymer dispersed liquid crystal periodic structure after exposure. The formed holographic polymer dispersed liquid crystal grating may remain on the base substrate after the cover substrate is removed.

[0063] Figs. 2A-2G illustrate an example process flow for fabricating deep SRGs in accordance with an embodiment of the invention. In Fig. 2A, a pair of substrate 212, 1502 sandwiches an unexposed holographic mixture layer 211 . The pair of substrate 212, 1502 may include a base substrate 212 and a cover substrate 1502. The cover substrate 1502 may have different properties than the base substrate 212 to allow for the cover substrate to adhere to the unexposed holographic mixture layer 211 while capable of being removed from the formed holographic polymer dispersed liquid crystal grating after exposure.

[0064] In Fig. 2B, the holographic mixture layer 211 is exposed by a pair of holographic recording beams 213, 214. As illustrated in Fig. 2C, the holographic recording beams 213,214 expose the holographic mixture layer 211 to form a holographic polymer dispersed liquid crystal grating 215. The holographic polymer dispersed liquid crystal grating 215 may include alternating polymer rich regions and liquid crystal rich regions. In Fig. 2D, the cover substrate 1502 may be removed exposing the holographic polymer dispersed liquid crystal grating 215.

[0065] Advantageously, the cover substrate 1502 may have different properties than the base substrate 212 such as different materials or different surface properties. For example, the base substrate 212 may be made out of plastic whereas the cover substrate 1502 may be made out of glass. The cover substrate 1502 may be removed allowing the holographic polymer dispersed liquid crystal grating 215 to remain on the base substrate without damaging the holographic polymer dispersed liquid crystal grating 215 during removal.

[0066] In some embodiments, the base substrate 212 may be treated on the surface contacting the holographic mixture layer 211 with an adhesion promotion layer such as reagents.

[0067] As illustrated in Fig. 2E, the liquid crystal may be removed or evacuated from the liquid crystal rich regions between the polymer rich regions leaving air regions. The polymer rich regions and the air regions form polymer-air SRGs 216. In Fig. 2F, a material of different refractive index from the polymer rich regions may be refilled into the air regions to form hybrid SRGs 217. In some embodiments, the material may be a liquid crystal material. The liquid crystal material may be different from the liquid crystal material removed or evacuated from the liquid crystal rich regions. In some embodiments, portions of the liquid crystal in the liquid crystal rich regions may be left between the polymer rich regions to form the hybrid SRGs 217. [0068] In Fig. 2G, a protective substrate 218 may be positioned such that the hybrid SRGs 217 are between the protective substrate 218 and the base substrate 212. The protective substrate 218 may be used to protect the hybrid SRGs 217. The protective substrate 218 may be omitted in some instances. The protective substrate 218 and the cover substrate 1502 may have different properties where the protective substrate 218 may add more protection when the grating is implemented into a usable device than the cover substrate 1502.

[0069] In some embodiments, the polymer-air SRGs 216 may be manufactured. In these embodiments, the protective substrate 218 may be used to protect the polymer-air SRGs 216.

[0070] Water molecules can also react with specific surface coordination sites in polymer matrices. In some examples, the holographic mixture may include reagents. Examples of reagents are described in U.S. Pat. Pub. No. 2022/0283376, entitled “Evacuated Periodic Structures and Methods of Manufacturing” and filed Mar. 7, 2022, which is hereby incorporated by reference in its entirety for all purposes. For example, '- Si-O-‘ groups in silanes in HPDLC formulations may be utilized as reagents which may promote adhesion to the substrate. Water can hydrolyze '-Si-O-‘ bonds effectively cleaving attachments on substrates like glass, thereby influencing polymer structures. This breakdown can cause change in optical properties such as diffraction efficiency and haze. Other mechanism of degradation may include selective hydrolysis of groups sensitive to moisture in polymers such as acrylates, esters, etc. Thus, molecular desiccants may be beneficial for holographic mixtures including reagents that interact with adhesion promotion materials.

[0071] In some embodiments the base substrate 212 may be a glass, quartz, or silica substrate including a glass surface. In some embodiments, the base substrate 212 may be a plastic substrate and may be coated with a silicon oxide coating (e.g. SiC ) which may act similar to a glass surface. The silicon oxide coating or the glass surface may include hydroxyl groups on the top surface. The adhesion promotion material may be coated on top of the silicon oxide coating. The hydroxyl groups may be beneficial in allowing the adhesion promotion material to adhere to the base substrate 212. [0072] In some embodiments, the base substrate 212 may include a glass surface including hydroxyl groups and may be reacted with reagents such that the reagents react with the hydroxyl groups. Fig. 3 illustrates an example reaction where the base substrate 212 is exposed to reagents 1604 in accordance with an embodiment of the invention. The base substrate 212 may include hydroxyl groups 1608 on the surface. The base substrate 212 is exposed to reagents 1604 and a holographic mixture 1602 including polymer. The reagents 1604 may be included in the holographic mixture 1602. The reagents 1604 may be a silane coupling agent. In some embodiments, the reagents 1604 include (R’O)s-Si- R where R’O- is an alkoxy group and -R is an organofunctional group. The alkoxy groups may condense with the hydroxyl groups 1608 available on the surface resulting in surfaces decorated with organofunctional -R groups, which may promote formation of covalent bonding of the coupling agent with polymeric networks within the holographic polymer dispersed liquid crystal grating 215. The reagents 1604 may adhere to the hydroxyl groups 1608 and to the holographic mixture 1602 creating improved adhesion when compared to the adhesion of the holographic mixture material 1602 without the reagents 1604. The holographic mixture 1602 may form a holographic mixture layer 1610 on the surface of the base substrate 212.

[0073] Fig. 4 illustrates an example reaction where reagents 1704 are exposed to the base substrate 212. The reagents 1704 may include a silane coupling agent as illustrated and may couple to hydroxyl groups on the surface of a glass surface of the base substrate 212.

[0074] In some embodiments the cover substrate 1502 may be a glass, quartz, or silica substrate including a glass surface. In some embodiments, the cover substrate 1502 may be a plastic substrate and may be coated with a silicon oxide coating (e.g. SiCh) which may act similar to a glass surface. A release layer may be coated on top of the glass surface. In some embodiments, similar to the base substrate 212 discussed above, the cover substrate 1502 may include a glass surface including hydroxyl groups and may be reacted with reactants such that the reactants bond with the hydroxyl groups to form the release layer.

[0075] Fig. 5 illustrates an example process for forming the release layer. The cover substrate 1502 may be exposed to a release material 1804. The release material 1804 may include a silane based fluoropolymer or fluoro monomer reactant as illustrated. The release material 1804 may include a fluoropolymer such as OPTOOL UD509 (produced by Daikin Chemicals), Dow Corning 2634, Fluoropel (produced by Cytonix), and EC200 (produced by PPG Industries, Inc) or a fluoro monomer. In some embodiments, the release material 1804 may include a polysiloxane coating. A polysiloxane coating may adhere better to materials such as plastic that do not have hydroxyl groups on the surface. A polysiloxane coating may be more robust and processable, and may be more environmentally-friendly to produce than a fluoropolymer. The release material 1804 may be applied through vapor deposition, spin coating, or spraying. In some embodiments, the cover substrate 1502 may be reusable and thus after removal after holographic exposure, the cover substrate 1502 may be placed on another holographic mixture layer which may be exposed with holographic beams.

[0076] In some embodiments, the cover substrate 1502 and/or the base substrate 212 may be a substrate that does not include SiO2 as discussed above. In these instances, a very thin layer of SiO2 may be applied to the surface to facilitate bonding/adhesion of the applied reagent hence enabling silane chemistry. When the cover substrate 1502 and/or the base substrate 212 is a substrate that does not include SiO2 any surface modification followed by bonding can provide adhesion. Surface modification may include treating with reagents to introduce reactive functional groups including but not limited to hydroxyl groups. In some embodiments, the cover substrate 1502 and/or the base substrate 212 may not be a glass substrate but may still include hydroxyl groups on the surface. For example, the cover substrate 1502 and/or the base substrate 212 may be sapphire or silicate which may include hydroxyl groups on the surface. In this case, the hydroxyl groups may help facilitate adhesion of the reagent and thus the thin layer of SiO2 would not be present. Examples of silicate substrates are manufactured by: Corning Inc. of Corning, NY, Schott AG of Mainz, Germany, Ohara Inc. of Chuo-ku, Sagamihara, Kanagawa, Japan, Hoya Inc. of Japan, AGC Inc. of Marunouchi, Chiyoda-ku, Tokyo, Japan, and CDGM Glass of Central Islip, NY.

[0077] In some embodiments, the cover substrate 1502 and/or the base substrate 212 may include Cleartran which is a form of chemical vapor deposited (CVD) zinc sulfide. A thin layer of SiO2 may be applied to the Cleartran substrate to facilitate bonding/adhesion of the applied reagent. In some embodiments, the cover substrate 1502 and/or the base substrate 212 may be a transparent ceramic such as aluminum oxynitride or magnesium aluminate. A thin layer of SiO2 may be applied to the transparent ceramic substrate to facilitate bonding/adhesion of the applied reagent. In some embodiments, the cover substrate 1502 and/or the base substrate 212 may include plastic such as PMMA, acrylic, polystyrene, polycarbonate, cyclic olefin copolymer, cyclo olefin polymer, polyester. A thin layer of SiO2 may be applied to the plastic substrate to facilitate bonding/adhesion of the applied reagent.

[0078] Figs. 6A and 6B illustrates example plots illustrating changes in the optical haze on exposure to moisture over time at a higher temperature (70 C & 90 % RH) for an optical device manufactured using an HPDLC not including an acid scavenger. Fig. 6A illustrates the device kept at room temperature and standard humidity at 0 hours 202a, 90 hours 204a, and 247 hours 206a. Fig. 6B illustrates the device kept at 70 degrees C and 90% humidity at 0 hours 202b, 90 hours 204b, and 247 hours 206b. As illustrated in Fig. 6A, at room temperature and standard humidity, there is not a significant change in optical haze over time. Whereas, as illustrated in Fig. 6B, at 70 degrees C and 90% humidity, there is a significant increase in haze over time. This change results in significant reduction in optical contrast of waveguide displays.

[0079] Figs. 7A and 7B illustrate example plots illustrating changes in the optical haze on exposure to moisture over time for an optical device manufactured using an HPDLC mixture including an acid scavenger. These plots are all of a device including an HPDLC mixture including an acid scavenger. Fig. 7A illustrates the device kept at room temperature and standard humidity at 0 hours 302a, 90 hours 304a, and 247 hours 306a. Fig. 7B illustrates the device kept at 70 degrees C and 90% humidity at 0 hours 302b, 90 hours 304b, and 247 hours 306b. As illustrated, both at room temperature and standard humidity and at 70 degrees C and 90% humidity, the diffraction efficiency of the device remains stable over time. This temperature and humidity stability is critical to the device. [0080] Fig. 8 illustrates various plots for various different configurations of HPDLC mixtures including various concentrations of different acid scavengers. Keeping diffraction efficiency high and haze low is advantageous. The top graphs include various peak diffraction efficiencies and the bottom graphs include various haze measurements. AS14 is a viscous liquid with oligomers. AS15 is a viscous liquid with polymers. Hystabl is a low viscosity liquid. The control 402 is without acid scavenger. As illustrated, it has been discovered that AS is a compatible additive for photopolymer formulations with almost identical or better diffraction efficiency without significantly increasing haze compared to control 402 with no ASM. For example, ASM at 1 % concentration 404 has a higher diffraction efficiency than the control 402 with a minimal increase in haze. ASM may also increase the resistance to temperature and/or humidity. ASM may contain aromatic and reactive groups, along with one or more carbodiimide functionalities.

[0081] Fig. 9 illustrates various plots comparing a control mixture 502 without acid scavenger and mixture containing ASM 504 over a period of time. The plots on the right side for each mixture are at room temperature and standard humidity whereas the plots on the left side for each mixture are at 70 degrees C and 90% humidity. As illustrated, when kept at 70 degrees C and 90% humidity, the haze increases dramatically for the control mixture 502 whereas the haze for mixture containing ASM 504 remains fairly stable.

[0082] Various embodiments include the use of an edge sealant around the waveguide in order to protect the grating material from contaminants such as water molecules. The edge sealant may include acid scavengers. The edge sealant may protect the waveguide materials such as the LC in the HPDLC which forms the gratings. The edge sealant may prevent moisture ingress into the waveguide materials.

[0083] Fig. 10 illustrates an example schematic of a waveguide in accordance with an embodiment of the invention. The waveguide includes various gratings 102, 104 manufactured between two substrates 106a, 106b. The gratings may include an input grating 102 and an output grating 104. An edge sealant 108 may be applied around the outside edge of the substrates 106a, 106b. As discussed previously, the edge sealant may isolate the gratings 102, 104 from environmental effects including but not limited to moisture. Moisture may have detrimental effects to gratings including LC. In some embodiments, the edge sealant 108 may include acid scavengers.

[0084] The edge sealant 108 may be made out of an inkjet printable material. The edge sealant 108 may include a mixture of aliphatic and aromatic monomers, wetting agents, photoinitiators, and black dye. The edge sealant 108 may include an acid scavenger.

[0085] In some embodiments, there may be a separate coating on either of the two substrate 106a, 106b containing acid scavengers which may reduce water ingress into the various gratings 102, 104. The separate coating may be separate from the edge sealant 108 and the various gratings 102, 104.

[0086] Fig. 11 is an example method for forming a waveguide grating in accordance with an embodiment of the invention. The method 1100 includes depositing (1102) a holographic mixture on a substrate. The holographic mixture includes: a photoinitiator; reagents that promote adhesion to the substrate; photopolymerizable monomers; inert material; and an acid scavenger to minimize the effect of moisture build up with the inert material and reagents. The inert fluid may be liquid crystal. The method 1100 further includes exposing (1104) the holographic mixture with a holographic recording beam to form a periodic structure including a polymer matrix separated by inert material rich regions to form a waveguide grating.

[0087] The method may also include evacuating (1106) the inert material from the inert material rich regions to form an evacuated periodic structure. Examples of formations of the acid scavenger have been discussed above. The substrate may include hydroxyl groups which condense with the reagents and promote adherence of the holographic mixture to the substrate. The substrate may be glass.

DOCTRINE OF EQUIVALENTS

[0088] While the above description contains many specific embodiments of the invention, these should not be construed as limitations on the scope of the invention, but rather as an example of one embodiment thereof. It is therefore to be understood that the present invention may be practiced in ways other than specifically described, without departing from the scope and spirit of the present invention. Thus, embodiments of the present invention should be considered in all respects as illustrative and not restrictive. Accordingly, the scope of the invention should be determined not by the embodiments illustrated, but by the appended claims and their equivalents.