Holographic photopolymers and methods for making the same

Abstract

The present disclosure relates to a novel holographic photopolymer and method for making the same. In some embodiments, the holographic photopolymer may comprise a polyurethane matrix, a writing monomer, and a dye system. In some embodiments, the writing monomer may comprise a polycyclic heteroaryl writing monomer In some embodiments, the holographic photopolymer may comprise a anti-oxygen inhibitor. In some embodiments, an augmented reality / virtual reality (AR / VR) apparatus comprising the holographic photopolymer is described.

Description

[0001] HOLOGRAPHIC PHOTOPOLYMERS AND METHODS FOR MAKING THE SAME

[0002] Inventors: Nan Hu, Sergey Simavoryan, Masanori Hosoyamada, Hongxi Zhang, Yuyang Wang, Peng Wang, Cheng-Kang Mai, and Liye Fu

[0003] CROSS-REFERENCE TO RELATED APPLICATIONS

[0004] This application claims the benefit of U.S. Provisional Patent Application No. 63 / 518,258, filed on August 8, 2023, and U.S. Provisional Application No. 63 / 627,381 , filed January 31 , 2024, both of which are incorporated by reference in their entirety.

[0005] FIELD

[0006] The present disclosure relates to holographic photopolymers which may be used in AR / VR glasses, optical displays, and other holographic applications, amongst other possibilities. The present disclosure also relates to methods of making holographic photopolymers.

[0007] BACKGROUND

[0008] As the use of augmented reality glasses and optical displays becomes more common, the demand for improved holographic media has increased. Holographic media or materials can be produced from holographic gratings such as a volume holographic grating. For example, U.S. Patent Nos. 9,281 ,000 B2 and 8,771 ,904 B2 relate to a photopolymer formulation which includes chemically crosslinked matrix polymers, writing monomers, and a photoinitiator system for producing holographic media via volume holographic gratings. However, the process to produce this photopolymer formulation requires an expensive nitrogen purge to remove oxygen. If the oxygen is not completely removed, there may be deficient polymerization of the polymers due to radicals being oxidized.

[0009] Furthermore, highly visible holograms may be achievable through the combination of a high refractive index and a low film thickness. Existing volume grating hologram films used for AR / VR glasses may only achieve 50% reflection efficiency but have a thickness greater than 12 pm which results in a low refractive index modulation. By way of example, increasing the refractive index modulation to greater than 0.03 may facilitate production of holographic films having a thickness between 5-12 pm without inhibiting wider field of view for AR / VR glasses.

[0010] In view of the foregoing, additional contributions in this area of technology are needed. SUMMARY

[0011] The present disclosure generally relates to holographic photopolymers and methods for making the same. In one embodiment, a holographic photopolymer may include a polyurethane matrix, a writing monomer and a dye system. In some forms of this embodiment, the holographic photopolymer may further include an oxygen scavenger additive. In some forms, the polyurethane matrix may be according to one of the following formulae:

[0012] In some forms, the polyurethane matrix may be linked to the writing monomer via a bond, such as a covalent bond or a noncovalent bond. In one non-limiting form, the bond may be a hydrogen bond.

[0013] In one or more forms, the polyurethane matrix may have a refractive index of about 1.44 to about 1.48. In some forms, the writing monomer may have a refractive index of about 1.45 to about 1.6 resulting in a refractive index modulation (An) greater than about 0.02.

[0014] In some forms, the polyurethane matrix may include one or more of polypropylene polyol, 2-ethyl-2-(hydroxymethyl)-1 ,3-propanediol, aliphatic polyether isocyanate, and dimethyltin dineodecanoate. In one or more forms, the polyurethane matrix may have a glass transition temperature (Tg) of less than about 0 °C, although other variations are possible. In some forms, the ratio by weight of the polyurethane matrix to the writing monomer may be in the range of 19:1 to 1 :9, although other variations are contemplated and possible.

[0015] In one or more forms, the writing monomer may include a difunctional writing monomer or a monofunctional writing monomer. In some forms, the writing monomer may include one or more of bisphenol a diglycidyl ether diacrylate, bisphenol A ethoxylated diacrylate / dimethacrylate, and trimethylolpropane triacrylate. In some forms where the bisphenol A di(meth)acrylate is present, it may be according to one or more of the following formulae:

[0016] In some forms, the writing monomer may include a polycyclic heteroaryl writing monomer. In some forms, the polycyclic heteroaryl writing monomer may include a difunctional writing monomer or a monofunctional writing monomer. When present, the polycyclic heteroaryl writing monomer may include one or more of bisphenol a diglycidyl ether diacrylate, bisphenol A ethoxylated di(meth)acrylate, trimethylolpropane triacrylate, ethoxylated fluorene diacrylate, a liquid carbazole acrylate / methylacrylate including an alkyl sulfide chain or ether chain, m-phenoxybenzyl acrylate, and a hydroxyfunctional acrylate, just to provide a few non-limiting examples. In some forms, the polycyclic heteroaryl writing monomer may include one of the following structures:

[0017]

[0018] In some forms, the polycyclic heteroaryl writing monomer may be according to the following formula: , wherein X is C, Si, or Ge.

[0019] In some forms, the polycyclic heteroaryl writing monomer may be according to one of the following formulas: , wherein X is O, In some forms, the polycyclic heteroaryl writing monomer further includes a diluent such as 1-vinyl-2-pyrrolidone, although other variations are possible.

[0020] In some forms, the dye system includes a dye and a co-initiator. By way of example, the dye may include one or more of safranine O, methylene blue, astrazon orange G, and ethyl violet and the co-initiator may include one or more of n-phenylglycine and hexaaryl-bisimidazolyl, just to provide a few non-limiting examples.

[0021] In forms in which the oxygen scavenger additive is present, it may be an antioxygen inhibitor such as triphenyl phosphine, although other variations are possible. In some forms, the holographic photopolymer may also include a reactive diluent such as a [C1-C3 alkyl pyrrolidone]N-ethyl-2-pyrrolidone, although other forms of the reactive diluent are possible and contemplated.

[0022] In some forms, the holographic photopolymer may have a transparency greater than 90%. Additionally or alternatively, in some forms the holographic photopolymer may have a haze less than about 1%.

[0023] In some embodiments, an augmented reality / virtual reality (AR / VR) apparatus includes a holographic photopolymer as described herein.

[0024] In one embodiment, a method for making a holographic photopolymer includes: providing a polyurethane matrix including at least one hydroxyl group, a writing monomer including at least one ether group, a dye system, and an oxidation or oxygen removing additive; mixing the polyurethane matrix, the writing monomer, the dye system, and the oxidation oxygen additive to form a mixture; and drying the mixture at room temperature. In some forms, the method may further include applying a vacuum to the mixture for about 1 hour. In some forms, the method involves the formation of a bond between the hydroxyl group and the ether group. In some forms, the method may further include polymerizing the mixture to form a photopolymer film. In some forms, the method may also further include exposing the photopolymer film to a laser and bleaching the photopolymer film.

[0025] These and other embodiments are described in greater detail below.

[0026] BRIEF DESCRIPTION OF THE DRAWINGS

[0027] FIG. 1 is a graphical illustration showing the infrared spectrum of a holographic photopolymer described herein.

[0028] FIG. 2 is a schematic illustration of an experimental setup to measure characteristics of a holographic photopolymer described herein.

[0029] FIG. 3 is a graphical illustration showing the stability of refraction efficiency over time of a holographic photopolymer described herein.

[0030] FIG. 4 is a schematic illustration of an experimental setup to measure the reflection efficiency of a holographic photopolymer described herein. FIG. 5 is a graphical illustration showing the measured data of reflection efficiency and diffraction efficiency of a holographic photopolymer described herein.

[0031] FIG. 6 is a graphical illustrated showing the thickness of a holographic photopolymer described herein.

[0032] FIGS. 7A and 7B are graphical illustrations showing the reflectance and transmittance and diffraction efficiency of a holographic photopolymer described herein.

[0033] FIG. 8 is a graphical illustration showing the transparency of a holographic photopolymer described herein.

[0034] FIG. 9 is a graphical illustration showing the reflectance and transmittance of a holographic photopolymer described herein.

[0035] FIGS. 10A and 10B are graphical illustrations showing the reflectance, transmittance and diffraction efficiency of a holographic photopolymer described herein.

[0036] FIG. 11 is a graphical illustration showing the reflectance and transmittance of a holographic photopolymer described herein.

[0037] FIGS. 12A and 12B are graphical illustrations showing the reflectance, transmittance and diffraction efficiency of a holographic photopolymer described herein.

[0038] FIGS. 13A and 13B are graphical illustrations showing the reflectance, transmittance and diffraction efficiency of a holographic photopolymer described herein.

[0039] FIG. 14 is a schematic illustration of an experimental setup to measure the diffraction efficiency of a holographic photopolymer described herein.

[0040] DETAILED DESCRIPTION

[0041] In some aspects, the current disclosure relates to a holographic media which may be used in an augmented reality / virtual reality (AR / VR) apparatus and / or optical displays, although other uses are contemplated. In one or more forms, a holographic photopolymer includes a polyurethane matrix, a writing monomer, a dye system, and optionally an oxygen scavenger additive. In some aspects, the holographic photopolymer may provide highly visible holograms for use in A / R 3-dimensional glasses which may improve the viewing experience of a user. Methods for making a holographic photopolymer are also described.

[0042] The term “bond” or “bonded” as used herein means a chemical bond between two atoms or to two moieties when the atoms joined by the bond are considered to be part of a larger structure.

[0043] The term “moiety” as used herein refers to a specific segment or functional group of a molecule.

[0044] The term “ester” as used herein refers to a chemical moiety with the formula RCOOR’, where R may be an alkyl, cycloalkyl, aryl, heteroaryl (bonded through a ring carbon), heterocyclic (bonded through a ring carbon), or a hydrogen atom, and R’ may be an alkyl or an aryl.

[0045] The term “purgeless” as used herein refers to a process or method of making a holographic photopolymer described herein where purging a gas from the local atmosphere in which the holographic photopolymer is made is not required.

[0046] The present disclosure generally relates to a holographic photopolymer and a method for making the same. In one or more forms, a holographic photopolymer includes a polyurethane matrix, a writing monomer, a dye system, and, optionally, an oxygen scavenger additive. In some embodiments, the holographic photopolymer may further include a reactive diluent and / or may be solvent-free.

[0047] In one or more forms, the polyurethane matrix may be a chemically crosslinked aliphatic polyether polyurethane matrix. More particular but non-limiting examples of the polyurethane matrix include one or more of polypropylene polyol, 2-ethyl-2- (hydroxymethyl)-1 ,3-propanediol, aliphatic polyether isocyanate, and dimethyltin dineodecanoate. In some forms, the polyurethane matrix be according to one of the following formulae: , wherein n=

[0048] 5-70. Other variations of the polyurethane matrix are possible and contemplated. For example, in some forms where the polyurethane matrix includes dimethyltin dineodecanoate, it may include one or more aliphatic ether (having 2 to 5 carbon atoms) groups which may improve compatibility with polypropylene polyol when it is also present in the polyurethane matrix. More particular but non-limiting examples of a polyurethane matrix that can be used in the holographic photopolymers described herein include Sannix (Sanyo Chemical, Japan), A13084.0C (ThermoFisher Scientific, USA), Desmodur ultra E 30600, (Covestro, Germany), and Fomrez UL-28 (Galata Chemicals, USA).

[0049] In some forms, the components of the polyurethane matrix may be selected based on their molecular weight which may affect the viscosity, flexibility, and mechanical strength of the polyurethane matrix. In some forms for example where the polyurethane matrix includes polypropylene polyol, it may have a molecular weight of about 2,000, and in forms in which the polyurethane matrix additionally or alternatively includes 2-ethyl-2- (hydroxymethyl)-1 ,3-propanediol it may have a molecular weight of about 178. Without being bound to any particular theory, selecting the components of the polyurethane matrix based on their molecular weight may have the unexpected result of optimizing the reactivity and curing time of the polyurethane matrix. In one or more forms, the polyurethane matrix may have a glass transition temperature (Tg) of less than about 0 °C, about -5 °C, about -10 °C, about -20 °C, about -30 °C, about -40 °C, about -50 °C, about -50 °C, about -70 °C, or any temperature bounded by these values, e.g., -52 °C. In some forms, the ratio by weight of the polyurethane matrix to the writing monomer may be in the range of 19:1 to 1 :9, although other variations are possible and contemplated. In some embodiments, the polyurethane matrix may have a free volume defined by unoccupied space between the molecules of the matrix. It is believed that a polyurethane matrix having a free volume may improve shear modulus and, in turn, diffusivity of the writing monomer.

[0050] In one or more forms, the writing monomer may include one or more of bisphenol a diglycidyl ether diacrylate, bisphenol A ethoxylated diacrylate / dimethacrylate, trimethylolpropane triacrylate (TMPTA), and N-ethyl pyrrolidone, just to provide a few nonlimiting examples. In some embodiments, the bisphenol A di(meth)acrylate may be according to one or more of the following formulae:

[0051] In one or more forms, the writing monomer may be a polycyclic heteroaryl writing monomer. In these forms, the polycyclic heteroaryl writing monomer may include a difunctional writing monomer or a monofunctional writing monomer. More particular but non-limiting examples of polycyclic heteroaryl writing monomers which may be used include one or more of bisphenol a diglycidyl ether diacrylate, bisphenol A ethoxylated di(meth)acrylate, trimethylolpropane triacrylate, ethoxylated cumylphenol acrylate, ethoxylated fluorene diacrylate, a liquid ethoxylated fluorene diacrylate, 2-hydroxy-3- phenoxypropylacrylate, o-phenylphenolethyl acrylate, o-phenylphenol (EO)2 acrylate, biphenylmethyl acrylate, a liquid carbazole acrylate / methylacrylate which includes an alkyl sulfide chain or ether chain, m-phenoxybenzyl acrylate, and a hydroxyfunctional acrylate. In one or more forms, the monofunctional writing monomer may include a secondary hydroxy-functional acrylate which may include the same organic ether unit as the polyurethane matrix. In some forms, the polycyclic heteroaryl writing monomer may include one of the following structures:

[0052] In some forms, the polycyclic heteroaryl writing monomer may be according to the following formula: , wherein X is C, Si, or Ge.

[0053] In some forms, the polycyclic heteroaryl writing monomer may be according to one or more of the following formulas:

[0054] wherein X is

[0055] O, S, Se, or Te.

[0056] In one or more forms where the heteroaryl writing monomer includes bisphenol A di(meth)acrylate and TMPTA, the ratio by weight of the bisphenol A di(meth)acrylate to the TMPTA may be about 2:3 to about 3:1 , or any ratio bounded by this range. Other variations of the writing monomer are possible and contemplated, and more particular but non-limiting forms of the writing monomer include Ebercryl 3700 (Allnex, Germany) and TMPTA (Allnex, Germany). Without being bound to any particular theory, selecting a writing monomer and a polyurethane matrix with similar structures and forming a hydrogen bond between the writing monomer and the polyurethane matrix may provide increased miscibility.

[0057] In forms in which the writing monomer includes bisphenol a diglycidyl ether diacrylate, it may have a molecular weight of about 484. In forms in which the writing monomer includes trimethylolpropane triacrylate, it may have a molecular weight of about 296. In forms in which the writing monomer includes N-ethyl pyrrolidone, it may have a molecular weight of about 113.

[0058] Without being bound to any particular theory, use of a monofunctional writing monomer having a refractive index greater than 1.5 having a glass transition temperature less than 40 °C, and a viscosity below 2000 cP at 25°C, below 1000 cP at 25°C, below 500 cP at 25°C or below 200 cP at 25°C and a mobility associated with a particular viscosity (e.g., as viscosity decreases mobility increases such that materials have a low resistance and shear easily, and molecules flow quickly) may increase diffusivity and facilitate production of a holographic photopolymer having a An > 0.03. In some forms, the monofunctional writing monomer may have a refractive index greater than about 1.5, about 1.51 , about 1.52, about 1.53, about 1.54, about 1.55, about 1.56, about 1.57, about 1 .58, about 1.59, about 1.6, or any refractive index bounded by this range, e.g., 1.523. In some embodiments, the monofunctional writing monomer can have a glass transition temperature (Tg) of less than about 40 °C, about 40 °C, about 35 °C, about 30 °C, about 25 °C, about 20 °C, about 15 °C, about 10 °C, about 5 °C, or any temperature bounded by these values, e.g., 31 °C. In some embodiments, the monofunctional writing monomer can have a viscosity less than 200 cP at 25 °C.

[0059] The polyurethane matrix may be linked to the writing monomer by a bond in one or more forms. By way of example, the bond may be a covalent bond or a noncovalent bond. In some forms where the writing monomer includes bisphenol a diglycidyl ether diacrylate, the bond may be a hydrogen bond which is formed between the hydroxyl groups of the bisphenol a diglycidyl ether diacrylate and ether groups of the polyurethane matrix. In other forms, hydrogen bonds may be formed between hydroxyl groups of other (meth)acrylates of the writing monomers and ether groups of the polyurethane matrix. By way of example, in forms which include polypropylene polyol and aliphatic polyether isocyanate, hydrogen bond formation may be facilitated by having a ratio of the polypropylene polyol to the aliphatic polyether isocyanate which ensures they fully react to each other. By way of non-limiting example, the ratio of the polypropylene polyol to the aliphatic polyether isocyanate may be about 1.4:1. In one non-limiting form which includes bisphenol a diglycidyl ether diacrylate and trimethylolpropane triacrylate for example, it may include about 1 gram to about 2 grams of the bisphenol a diglycidyl ether diacrylate and about 1 gram to about 2 grams of the trimethylolpropane triacrylate.

[0060] Hydrogen bonding between the polyurethane matrix and the writing monomer of a holographic photopolymer may be evaluated using Fourier-transform infrared (FTIR) spectroscopy. By way of example, hydrogen bonding was evaluated for a holographic photopolymer described herein after holographic grating was applied (Thermo Scientific FTIR-ATR Spectrophotometer). FTIR results are shown in FIG. 1. Absorbance peaks at 2920 cm-1 and 2850 cm-1 correlates to OH stretching vibration. The decreasing intensity and weakening phenomena of OH peaks are indicative of hydrogen bonds. (Y. Wang et. al., Macromol. Mater. Eng. 2023, 5, 2200440).

[0061] In some forms, the polyurethane matrix can have a refractive index of about 1.4 to about 1.5, about 1 .40, about 1.41 , about 1.42, about 1.43, about 1 .44, about 1.45, about 1.46, about 1.47, about 1.48, about 1.49, about 1.5, or any value bounded by this range, e.g., 1.468. In some forms, the writing monomer can have a refractive index of about 1.5 to about 1.6, about 1 .5, about 1 .51 , about 1.52, about 1 .53, about 1.54, about 1 .55, about 1.56, about 1.57, about 1.58, about 1.59, about 1.6, or any value bounded by this range, e.g., 1.523. Without being bound to any particular theory, it is believed that hydrogen bonding between the writing monomer and the polyurethane matrix and the selection of a writing monomer and a polyurethane matrix with similar polymer chains may increase refractive index modulation (An) of a holographic photopolymer to greater than about 0.02. In some forms, the refractive index modulation of the holographic photopolymer may be greater than about 0.02, about 0.025, about 0.026, about 0.027, about 0.028, about 0.029, about 0.030, about 0.031 , about 0.032, about 0.033, about 0.034, about 0.035, about 0.036, about 0.037, about 0.038, about 0.039, about 0.04, or any refractive index modulation bounded by this range, e.g., 0.0293 or 0.0387. By achieving a refractive index modulation of about 0.02 to about 0.033, a holographic photopolymer film may be provided with a thickness of less than 12 pm. A schematic illustration of a setup which may be used to measure the refractive index modulation of a holographic photopolymer is illustrated in FIG. 2, and was used in connection with evaluation of the holographic photopolymers described in the Examples below. In some forms, a holographic photopolymer described herein may have a haze less than about 1%, about 0.75%, about 0.5%, about 0.25%, or any haze bounded by this range, e.g., 0.41.

[0062] In one or more forms, the writing monomer and the polyurethane matrix may include two methyl groups between two phenyl groups. Without being limited to any particular theory, it is believed that mechanical stability of the holographic photopolymer may be increased by selecting a writing monomer and a polyurethane matrix with similar structures.

[0063] In one or more forms, the dye system includes a dye and a co-initiator. The dye may include safranine O, methylene blue, astrazon orange G, ethyl violet, or a combination thereof, just to provide a few non-limiting examples. The co-initiator may include n-phenylglycine, hexaaryl-bisimidazolyl, or a combination thereof, just to provide a few non-limiting examples. Other variations of the dye system are possible and contemplated. More particular but non-limiting examples of a dye system that may be used in the present holographic photopolymer include Safranine O available as product no. B21674.09 from ThermoFisher Scientific, USA (CAS no. 477-73-6) and N- Phenylglycine available as product no. 330469 from Sigma-Aldrich, Germany)(CAS no. 103-01-5). In one or more forms, the dye may have a molecular weight of about 300 to about 400, about 300, about 325, about 350, about 375, about 400, or any value bound by this range. In one or more forms, the co-initiator may have a molecular weight of about 100 to about 200, about 100, about 125, about 150, about 175, about 200, or any value bound by this range.

[0064] Controlling the ratio of the dye to the co-initiator may result in an increased polymerization rate and degree of polymerization. By way of non-limiting example, the ratio by weight of the dye to the co-initiator may be about 1 :2 to about 1 :20, about 1 :2, about 1 :5 about 1 :10, about 1 :15, about 1 :20, about 1 :10 to about 1 :14 or any ratio or range of ratios bounded by this range. For example, in one form the ratio by weight of dye to the co-initiator may be 1 :10 to about 1 :14. As indicated above, in some forms the holographic photopolymer may optionally comprise an oxygen scavenger additive which may facilitate polymerization of the holographic photopolymer in a local atmosphere without the need to deaerate, degas, or purge the local atmosphere of oxygen. By way of example, the oxygen scavenger additive may include an anti-oxygen inhibitor such as triphenyl phosphine available as product no. T84409 from Sigma-Aldrich, Germany. Variations in the oxygen scavenger additive are possible and contemplated, and one more particular but non-limiting example of an oxygen scavenger additive that may be used in a holographic photopolymer described herein includes Triphenylphosphine available as product no. T84409 from Sigma-Aldrich, Germany (CAS no. 603-35-0). Without intending to being bound by any particular theory, it is contemplated that the type of oxygen scavenger additive may unexpectedly result in reducing the oxidation of radicals to regenerate active radicals, thus eliminating the need to remove oxygen from the local atmosphere by deaerating, degassing, or purging with a gas, e.g., nitrogen, and thus improving time and cost efficiency. In one or more forms, the oxygen scavenger additive may have a molecular weight of about 200 to about 300, about 200, about 225, about 250, about 275, about 300, or any value bound by this range.

[0065] When the oxygen scavenger additive is triphenylphosphine it may reduce oxygen inhibition during the free radical polymerization of writing monomer according to the following reaction scheme:

[0066] As indicated above, in some embodiments a holographic photopolymer may also include a diluent. For example, in some forms the writing monomer of the holographic photopolymer may include a reactive diluent. In some forms, the reactive diluent may be a solvent such as a low volatility aprotic solvent. One non-limiting example of a low volatility aprotic solvent which may be used is [C1-C3 alkyl pyrrolidone]N-ethyl-2- pyrrolidone. In some forms, the diluent may include a polymerizable diluent such as 1- vinyl-2-pyrrolidone. Amongst other things, the polymerizable diluent may facilitate dissolving and mixing of the dye system without the need for a solvent. Other variations for the diluent are possible and contemplated, and one more particular but non-limiting example of a reactive diluent that may be used in a holographic photopolymer includes 1- Ethyl-2-pyrrolidone available as product no. 146358 from Sigma-Aldrich, Germany.

[0067] In some forms, a holographic photopolymer described herein may have a reflection efficiency greater than about 50%, about 55%, about 60%, about 65%, or any reflection efficiency bounded by these values, e.g., 61%. In one or more forms, a holographic photopolymer described herein may have a diffraction efficiency greater than about 70%, about 75%, about 80%, about 85%, or any diffraction efficiency bounded by these values, e.g., 79%.

[0068] In one or more forms, a holographic photopolymer described herein may have a thickness from about 3 pm to about 12 pm, about 3 pm, about 4 pm, about 5 pm, about 6 pm, about 7 pm, about 8 pm, about 9 pm, about 10 pm, about 11 pm, about 12 pm, or any value bounded by this range.

[0069] In some forms, a holographic photopolymer may also include a tin catalyst. The tin catalyst may include an organotin catalyst, 1-Vinyl-2-Pyrrolidone, or a combination thereof. The ratio of the organotin catalyst to 1-Vinyl-2-Pyrrolidone may be about 0:10, about 1 :10, about 2:10, about 3:10, about 0:9, about 1 :9, about 2:9, about 3:9, about 0:8, about 1 :8, about 2:8, about 3:8, or any value bound by this range, e.g., 1.1 :9.2.

[0070] As indicated above, in one or more forms the dye system may include a dye such as safranine O, and a co-initiator such as hexaaryl-bisimidazolyl, n-phenyl glycine, or a combination thereof. When n-phenyl glycine is present for example, it may act as a hydrogen donor which is compatible with the tin catalyst (when present) and an aliphatic polyether isocyanate of the polyurethane matrix (as applicable). Combining the n-phenyl glycine with a visible light-absorbing photosensitizer, such as safranine O, and a coinitiator, such as hexaaryl-bisimidazolyl, had the unexpected result of improving the transparency to over 95%.

[0071] In one non-limiting form, a holographic photopolymer may include liquid ethoxylated fluorene diacrylate, liquid carbazole acrylate / methylacrylate, a monofunctional writing monomer, a secondary hydroxy-functional acrylate, an aliphatic polyether isocyanate, and a polypropylene polyol. In this form, there may be high compatibility between the components due to the components having similar organic ether units and covalent bonds formed between isocyanate and the secondary hydroxyfunctional acrylate. This compatibility may provide the unexpected result of the holographic photopolymer having a high transparency greater than 95% and a low haze of less than 1 %.

[0072] In some embodiments, a holographic photopolymer described herein may have a transmittance greater than about 90%. In some embodiments, a holographic photopolymer described herein may have a transmittance greater than about 90% at 457nm, greater than 91% at 457nm, greater than 92% at 457nm, greater than 93% at 457nm, greater than 94% at 457nm, greater than 95% at 457nm, greater than 96% at 457nm, or greater than 97% at 457nm. In some embodiments, a holographic photopolymer described herein may have a transmittance greater than about 90% at 532nm, greater than 91% at 532nm, greater than 92% at 532nm, greater than 93% at 532nm, greater than 94% at 532nm, greater than 95% at 532nm, greater than 96% at 532nm, greater than 97% at 532nm, greater than 98% at 532nm, or greater than 99% at 532nm. In some embodiments, a holographic photopolymer described herein may have a transmittance greater than about 90% at 600nm, greater than 91% at 600nm, greater than 92% at 600nm, greater than 93% at 600nm, greater than 94% at 600nm, greater than 95% at 600nm, greater than 96% at 600nm, greater than 97% at 600nm, greater than 98% at 600nm, or greater than 99% at 600nm.

[0073] A method for making a holographic photopolymer described herein may include: providing a polyurethane matrix including at least one hydroxyl group; providing a writing monomer including at least one ether group; providing a dye system; providing an oxygen scavenger additive; mixing the polyurethane matrix, the writing monomer, the dye system, and the oxygen scavenger additive at a temperature of about 50°C to about 70°C to form a mixture; and drying the mixture at room temperature. In some forms, the method may further include applying a vacuum to the mixture for about 30 minutes or about 1 hour at a pressure of about -0.09 MPa to about 0.09 MPa. In some forms, application of a vacuum removes imperfections in the mixture and is performed prior to polymerization. In some forms, the method may further include polymerizing the mixture to form a photopolymer film. In some aspects, the method involves forming a hydrogen bond between a hydroxyl group and an ether group. In some embodiments, the method for making the holographic photopolymer may be purgeless. In some forms, the method may further include exposing the photopolymer film to a laser and bleaching the photopolymer film.

[0074] In some forms, a method for making a holographic photopolymer may include: preparing a dye system; preparing a writing monomer; preparing a catalyst solution; mixing the dye system, writing monomer, and polypropylene polyol to form a first mixture; mixing the first mixture with polyether isocyanate to form a second mixture; mixing the second mixture with the catalyst solution to form a third mixture; and applying a vacuum to the third mixture. In some forms, the step for preparing the dye system may include mixing n-ethyl pyrrolidinone, safranine O, n-phenylglycine, and triphenyl phosphine at a temperature of about 40 °C to about 70 °C for about 2 to about 3 hours. In some forms, the step for preparing the writing monomer may include mixing bisphenol a diglycidyl ether diacrylate and TMPTA at a temperature of about 50 °C to about 70 °C for about 5 to about 7 hours. In some forms, the step for preparing the catalyst solution may include mixing a catalyst with 1 -ethyl-2 -pyrrolidone at a ratio of about 1 :9 (catalyst: 1 -ethyl-2 -pyrrolidone). In some forms, the ratio of the catalyst to 1 -ethyl-2 -pyrrolidone can be about 1 :2 to about 1 :20, about 1 :2, about 1 :5 about 1 :10, about 1 :15, about 1 :20, or any ratio bounded by this range, e.g., about 1 :9. In some forms, the step for mixing the dye system, writing monomer, and polyurethane matrix to form a first mixture may further include mixing the dye system, writing monomer, and polyurethane matrix at a temperature of about 50 °C to about 70 °C for about 7 to about 9 hours. In some forms, the first mixture may comprise about 5 wt. % to about 20 wt. % of the dye system, about 40 wt. % to about 60 wt. % of polypropylene polyol, and about 30 wt. % to about 40 wt. % of polyether isocyanate. In some forms, the method may further include cooling the first mixture to about 30 °C to about 40 °C.

[0075] In some forms, the vacuum may be applied to the third mixture for about 10 minutes to about 1 hour at a temperature of about 20 °C to about 50 °C, although various alternatives are possible and contemplated. For example, in one non-limiting form the vacuum may be applied to the third mixture for about 30 minutes. The vacuum may be applied at a pressure of about 0.01 MPa to about 0.09 MPa.

[0076] In some forms, a method for making a holographic photopolymer may further include performing a two-stage chemistry approach. The first stage of the approach may include the step of polymerizing the third mixture to form a photopolymer film. In some forms, the photopolymer film may include a polymerized polyurethane matrix and a nonpolymerized writing monomer dispersed within the polymerized polyurethane matrix. In some forms, the step of polymerizing the third mixture to form a photopolymer film may further include curing the third mixture at room temperature for about 6 hours to about 12 hours.

[0077] In some embodiments, the second stage may include the step of exposing the photopolymer film to a laser having a wavelength of about 532 nm at a power density of about 1 mW / cm2to about 20 mW / cm2for about 1 second to about 30 seconds. In some embodiments, the step of exposing the photopolymer film to a laser may further include polymerizing the writing monomer dispersed within the polymerized polyurethane matrix. In some embodiments, the second stage may further include the step of bleaching the photopolymer film which may include exposing the photopolymer to an ultraviolet light having a wavelength of about 340 nm to about 400 nm.

[0078] Utilizing a two-stage chemistry may improve upon the holographic photopolymer because the two-stage chemistry process provides the ability to achieve precise control over the sequence, timing, and selectivity of different chemical transformations, enabling the synthesis of complex molecules or materials with high efficiency and accuracy. In some embodiments, a solid host matrix including a polyurethane matrix and additives can be formed to produce a first photopolymer, and then followed by recording into a second photopolymer including the dye system and the writing monomer. Additionally, the writing monomer and the polyurethane matrix may be selected for their similar ether groups to increase the transparency of the holographic photopolymer. In some embodiments, a holographic photopolymer may have a transparency greater than 80%, 85%, 90% or 95%, or any transparency bounded by these values, e.g., 91%. In some forms, high transparency may be observed in the UV - visible light spectrum.

[0079] In some embodiments, a film may be made according to a method described herein in an atmosphere including about 25% oxygen to about 0% oxygen, less than about 25% oxygen, less than about 20% oxygen, less than about 15% oxygen, less than about 10% oxygen, less than about 5% oxygen, less than about 1% oxygen, or any percentage of oxygen bounded by this range.

[0080] In some embodiments, a holographic photopolymer described herein may be a film or coating applied to a glass substrate. In some embodiments, a holographic photopolymer described herein may be applied as a film or coating to a plastic substrate. In some embodiments, a holographic photopolymer described herein may be applied as a coating to a glass substrate via a vacuum lamination coating method. In some embodiments, a holographic photopolymer described herein may be applied as a coating to a plastic substrate via a roll-to-roll coating method.

[0081] In some embodiments, an AR / VR apparatus may include a holographic photopolymer described herein. In some forms, the AR / VR apparatus may be an AR / VR headset. In some forms, the AR / VR apparatus may include a holographic display.

[0082] Hereinafter, embodiments and methods will be described in more detail.

[0083] EXAMPLES

[0084] The following examples are intended to be illustrative of the disclosure only and are not intended to limit the scope or underlying principles in any way.

[0085] Example 1

[0086] Preparation of photopolymer solution 1

[0087] N-ethyl pyrrolidinone (CAS No.: 2687-91-4) - 0.35 mg, Safranine O (SFH+) (CAS 477-73-6) - 10 mg, N-phenylglycine (CAS No.: 103-01-5) - 100 mg, and triphenyl phosphine (CAS 603-35-0) - 50 mg were mixed together for 2-3 hours at a temperature of 55 °C to form a dye stock solution. Equal weights of Ebecryl 3700 (Allnex, Inc.) and TMPTA (CAS Number: 15625-89-5) were then mixed together under safe light conditions for 6 hours at a temperature of 60 °C to form a writing monomer solution. 1 part of catalyst (FOMREZ® UL-28) was mixed with 9 parts of 1-Ethyl-2 -pyrrolidone to form a 10% catalyst stock solution.

[0088] 0.51 grams of the dye stock solution was mixed with 2.5 grams of the writing monomer solution and 1.4 grams of polyol under safe light conditions for 8 hours at a temperature of 60 °C. The dye stock / writing monomer solution was then cooled down to 35 °C and 1 gram of isocyanate Desmodur ultra E 36000 (Covestro company) was mixed vigorously into the dye stock / writing monomer solution for 5-10 minutes.

[0089] 0.018 grams of the catalyst stock solution was mixed into the dye stock / writing monomer solution for 1 minute. The final solution was then placed under a vacuum for 30 minutes at 35 °C to remove imperfections and air bubbles in the solution.

[0090] Stability of the holographic photopolymer of this Example was tested. After the polyurethane matrix was cured, the non-grating photopolymer sample films were stored in ambient environment. Every 2 days, a sample was hologram recorded, UV-bleached, and then reflection efficiency was measured. FIG. 3 provides a graphical illustration depicting the stability of the holographic photopolymer after being stored in an ambient environment at room temperature for two weeks.

[0091] Reflection efficiency and diffraction efficiency of the holographic photopolymer of this Example were evaluated. The setup to measure the refractive index modulation of a holographic photopolymer illustrated in FIG. 2 was used and the diffraction efficiency was measured using the set-up illustrated in FIG. 4. In the evaluation, a signal beam is going through a recorded and developed hologram. By adjusting the angle of the hologram the position when reflected beam -PR shows its max value may be found. In the meantime, the transmitted beam PT will show its min value. Reflection efficiency can be calculated using formula (PR I PT+ PR) x 100%. FIG. 5 provides a graphical illustration of measured reflection efficiency and diffraction efficiency of a holographic photopolymer.

[0092] Photopolymer film thickness may be measured by positioning a recorded and UV bleached sample of the photopolymer between upper and lower glass substrates. A blade may be carefully used to detach the substrates from each other, leaving the photopolymer on one of the substrates. A blade may be used to remove the film from the hologram area. A Dektak stylus profiler (Bruker.com) may be used to measure thickness of the PP layer. FIG. 6 provides a graphical illustration of the measured thickness of a holographic photopolymer described herein.

[0093] Example 2

[0094] Preparation of writing monomers mixture

[0095] 2.25 g of EA-F5710 (mixture of ethoxylated fluorene diacrylate and m- phenoxybenzyl acrylate) (OSAKA GAS Chemicals, Japan), and 0.25 g of 2-Hydroxy-3- phenoxypropylacrylate (CAS No.: 16969-10-1) were mixed in the dark for 6 hours at 60 °C to provide a writing monomer mixture.

[0096] Preparation of photoinitiation mixture solution 50 mg of N-phenylglycine (CAS No.: 103-01-5, coinitiator 1), 5.4 mg of Safranine O (Photosensitizer or dye) and 16.2 mg of 2,2'-Bis(2-chlorophenyl)-4,4',5,5'-tetraphenyl- 1 ,2'-biimidazole (HABI, CAS No.: 7189-82-4, coinitiator 2) were dissolved and mixed in 0.35 g of 1-Ethyl-2-Pyrrolidone (CAS No.: 88-12-0) at 50-55 °C for 2 hours in the dark.

[0097] Preparation of Tin catalyst solution (for urethane polymerization)

[0098] 1 part of an organotin catalyst (FOMREZ UL-28, Dimethyltin dineodecanoate) was mixed with 9 parts of 1-Ethyl-2-Pyrrolidone (CAS No.: 88-12-0) at room temperature to form a 10% catalyst stock solution.

[0099] Preparation of photopolymer formulation 2

[0100] 2.5 g of the writing monomers mixture of this Example 2, 1.4 g of polypropylene polyol (SANNIX PP-2000, Sanyo Chemical), and 0.42 g of the photoinitiation mixture solution were mixed in the dark at about 35 °C for 1 hour. This was followed by the addition of 1 g of isocyanate (Desmodur ultra E 30600, Covestro). The mixture was mixed vigorously for 10 minutes at 35 °C. Then 0.018 g of the Tin catalyst solution was added into the resulting mixture, and mixed for 3 minutes at 35 °C. The resulting liquid solution was ready to place under a vacuum at 35 °C to remove air bubbles caused by mixing. Reflectance and transmittance performance data were tested and the resultant spectral plot is illustrated in FIGS. 7A and 7B. The transmission of the grating area with zero angle of incidence was measured with a LIV-VIS spectrometer. The resulting Bragg curve was analyzed according to the Kogelnik theory to deduce the refractive index modulation An.

[0101] The transmittance of the photopolymer of this Example was measured with a UV- Vis spectrophotometer. The photopolymer film was sandwiched between two glass substrates. After being UV-bleached, the transmittance spectra of the film was measured. FIG. 8 illustrates the transmittance spectra of the UV-bleached film with a thickness of 13 pm.

[0102] Example 3

[0103] Preparation of photoinitiation mixture solution

[0104] 80 mg of Borate salt tetrabutylammonium tris(3-chloro-4-methylphenyl) (hexyl)borate (CAS No.: 1147315, coinitiator), 8.0 mg of Safranine O (Photosensitizer or dye) were dissolved and mixed in 0.35 g of 1-Vinyl-2-Pyrrolidone (CAS No.: 88-12-0) at 55 °C for 2 hours in the dark.

[0105] Preparation of Tin catalyst solution (for urethane polymerization) 1 part of an organotin catalyst (FOMREZ UL-28, Dimethyltin dineodecanoate) was mixed with 9 parts of 1-Vinyl-2-Pyrrolidone (CAS No.: 88-12-0) at room temperature to form a 10% catalyst stock solution.

[0106] Preparation of photopolymer formulation 3

[0107] 2.5 g of EA-F5710 (see writing monomers mixture in Example 2), 1.4 g of polypropylene polyol (SANNIX PP-2000, Sanyo Chemical), and 0.44 g of the photoinitiation mixture solution were mixed in the dark at about 60 °C for 2 hours. This was followed by cooling the above solution to 35 °C, and the addition of 1 g of isocyanate (Desmodur ultra E 30600, Covestro). The mixture was mixed vigorously for 10 minutes at 35 °C. Then 0.018 g of Tin catalyst solution was added into the resulting mixture, and mixed for 3 minutes at 35 °C. The resulting liquid solution was ready to place under a vacuum at 35 °C to remove air bubbles trapped by mixing. Reflectance and transmittance performance data were tested and the resultant transmittance spectral plot of nonexposed film, grating area and UV bleached film is illustrated in FIG. 9.

[0108] Example 4

[0109] Preparation of writing monomers mixture

[0110] 1.25 g of carbazole methylacrylate (synthesized according to literature ACS Applied Materials & Interfaces 2023, 15, 24827-24835) and 1.25 g of o-phenylphenolethyl acrylate (MIRAMER M1142) were mixed in the dark for 6 hours at 60 °C to provide a writing monomer mixture.

[0111] Preparation of photoinitiation mixture solution

[0112] 80 mg of Borate salt tetrabutylammonium butyltriphenylborate (CAS No.: 120307- 06-4, coinitiator) and 8.0 mg of Safranine O (Photosensitizer or dye) were dissolved and mixed in 0.35 g of 1-Ethyl-2-Pyrrolidone (CAS No.: 88-12-0) at 55 °C for 2 hours in the dark.

[0113] Preparation of Tin catalyst solution (for urethane polymerization)

[0114] 1 part of an organotin catalyst (FOMREZ UL-28, Dimethyltin dineodecanoate) was mixed with 9 parts of 1-Ethyl-2-Pyrrolidone (CAS No.: 88-12-0) at room temperature to form a 10% catalyst stock solution.

[0115] Preparation of photopolymer formulation 4

[0116] 2.5 g of the writing monomers mixture of this Example 4, 1.4 g of polypropylene polyol (SANNIX PP-2000, Sanyo Chemical), and 0.42 g of the photoinitiation mixture solution were mixed in dark at about 35 °C for 1 hour. This was followed by the addition of 1 g of isocyanate (Desmodur ultra E 30600, Covestro). The mixture was mixed vigorously for 10 minutes at 35 °C. Then 0.018 g of Tin catalyst solution was added into the resulting mixture, and mixed for 3 minutes at 35 °C. The resulting liquid solution was ready to place under a vacuum at 35 °C to remove air bubbles caused by mixing. Reflectance and transmittance performance were tested and the resultant spectral plots are illustrated in FIGS. 10A and B. The transmission of the grating area with zero angle of incidence was measured with a LIV-VIS spectrometer. The resulting Bragg curve was analyzed according to the Kogelnik theory to deduce the refractive index modulation An.

[0117] Example 5

[0118] Preparation of photoinitiation mixture stock solution

[0119] 1.5 g of Borate salt tetrabutylammonium tris(3-chloro-4-methylphenyl) (hexyl)borate (CAS No.: 1147315, coinitiator), 0.05 g of Astrazon Orange G (CAS No.: 3056-93-7) (Photosensitizer or dye at 492 nm wavelength), 0.05g of new methylene blue (CAS No.: 1934-16-3) (Photosensitizer or dye at 632 nm wavelength), and 0.05 g of ethyl violet (CAS No.: 2390-59-2) (Photosensitizer or dye at 592 nm wavelength) were dissolved completely and mixed in 3.5 g of 1-Ethyl-2-Pyrrolidone (CAS No.: 88-12-0) at 55 °C for 2 hours in dark.

[0120] Preparation of Tin catalyst solution (for urethane polymerization)

[0121] 1 part of an organotin catalyst (FOMREZ UL-28, Dimethyltin dineodecanoate) was mixed with 9 parts of 1-Ethyl-2-Pyrrolidone (CAS No.: 88-12-0) at room temperature to form a 10% catalyst stock solution.

[0122] Preparation of photopolymer formulation 5

[0123] 2.5 g of EA-F5710 (see writing monomers mixture in Example 2), 1.4 g of polypropylene polyol (SANNIX PP-2000, Sanyo Chemical), and 0.39 g of the photoinitiation mixture solution from this Example 5 were mixed in the dark at about 60 °C for 1 hour. This was followed by the addition of 1.2 g of isocyanate (Desmodur ultra E 30600, Covestro). The mixture was mixed vigorously for 10 minutes at 35 °C. Then 0.018 g of Tin catalyst solution was added into the resulting mixture, and mixed for 3 minutes at 35 °C. The resulting liquid solution was ready to place under a vacuum at 35 °C to remove air bubbles caused by mixing. Reflectance and transmittance were tested and the resultant spectral plot is illustrated in FIG. 11.

[0124] Example 6

[0125] Preparation of photoinitiation mixture stock solution 1.5 g of Borate salt tetrabutylammonium tris(3-chloro-4-methylphenyl) (hexyl)borate (CAS No.: 1147315, coinitiator), 0.05 g of Safranine O (CAS No.: 477-73- 6) (Photosensitizer or dye at 532 nm wavelength), 0.05g of new methylene blue (CAS No.: 1934-16-3) (Photosensitizer or dye at 632 nm wavelength), and 0.05 g of ethyl violet (CAS No.: 2390-59-2) (Photosensitizer or dye at 592 nm wavelength) were dissolved completely and mixed in 3.5 g of 1-Vinyl-2-Pyrrolidone (CAS No.: 88-12-0) at 55 °C for 2 hours in dark.

[0126] Preparation of Tin catalyst solution (for urethane polymerization)

[0127] 1 part of an organotin catalyst (FOMREZ UL-28, Dimethyltin dineodecanoate) was mixed with 9 parts of 1-Vinyl-2-Pyrrolidone (CAS No.: 88-12-0) at room temperature to form a 10% catalyst stock solution.

[0128] Preparation of photopolymer formulation 6

[0129] 2.5 g of EA-F5710 (see writing monomers mixture in Example 2)(writing monomers mixture), 1.4 g of polypropylene polyol (SANNIX PP-2000, Sanyo Chemical), and 0.39 g of the photoinitiation mixture solution from this Example 6 were mixed in the dark at about 60 °C for 1 hour. This was followed by the addition of 1.2 g of isocyanate (Desmodur ultra E 30600, Covestro). The mixture was mixed vigorously for 10 minutes at 35 °C. Then 0.018 g of Tin catalyst solution was added into the resulting mixture, and mixed for 3 minutes at 35 °C. The resulting liquid solution was ready to place under a vacuum at 35 °C to remove air bubbles caused by mixing. Reflectance and transmittance performance were tested and the resultant spectral plot is illustrated in FIG 11.

[0130] Example 7

[0131] Preparation of photoinitiation mixture stock solution

[0132] 500 mg of N-phenylglycine (CAS No.: 103-01-5), 50 mg of Safranine O (CAS No.: 477-73-6) (Photosensitizer or dye at 532 nm wavelength), and 250 mg of triphenyl phosphine (CAS 603-35-0) were dissolved completely and mixed in 1.75 g of N-ethyl pyrrolidinone (CAS No.: 2687-91-4) at 55 °C for 2 hours in the dark.

[0133] Preparation of Tin catalyst solution (for urethane polymerization)

[0134] 1 part of an organotin catalyst (FOMREZ UL-28, Dimethyltin dineodecanoate) was mixed with 9 parts of N-ethyl pyrrolidinone (CAS No.: 2687-91-4) at room temperature to form a 10% catalyst stock solution.

[0135] Preparation of writing monomers mixture 1 .25 g of Ebecryl 3700 ( Allnex, Inc.), 0.625 g of SR590 (ethoxylated cumylphenol acrylate, Arkema, inc.) and 0.625 g of TMPTA (CAS Number: 15625-89-5) were mixed in the dark for 6 hours at 60 °C to provide a writing monomers mixture.

[0136] Preparation of photopolymer formulation 7

[0137] 2.5 g of the writing monomers mixture from this Example 7, 1 .4 g of polypropylene polyol (SANNIX PP-2000, Sanyo Chemical), and 0.26 g of the photoinitiation mixture solution from this Example 7 were mixed in the dark at about 60 °C for 1 hour. This was followed by the addition of 1.0 g of isocyanate (Desmodur ultra E 30600, Covestro). The mixture was mixed vigorously for 10 minutes at 35 °C. Then 0.018 g of Tin catalyst solution was added into the resulting mixture, and mixed for 3 minutes at 35 °C. The resulting liquid solution was ready to place under a vacuum at 35 °C to remove air bubbles caused by mixing. Reflectance and transmittance performance were tested and the resultant spectral plots are illustrated in FIGS. 12A and 12B.

[0138] Example 8

[0139] Preparation of photoinitiation mixture stock solution

[0140] 500 mg of N-phenylglycine (CAS No.: 103-01-5), 50 mg of Safranine O (CAS No.: 477-73-6) (Photosensitizer or dye at 532 nm wavelength), and 250 mg triphenyl phosphine (CAS 603-35-0) were dissolved completely and mixed in 1.75 g of N-ethyl pyrrolidinone (CAS No.: 2687-91-4) at 55 °C for 2 hours in the dark.

[0141] Preparation of Tin catalyst solution (for urethane polymerization)

[0142] 1 part of an organotin catalyst (FOMREZ UL-28, Dimethyltin dineodecanoate) was mixed with 9 parts of N-ethyl pyrrolidinone (CAS No.: 2687-91-4) at room temperature to form a 10% catalyst stock solution.

[0143] Preparation of writing monomers mixture

[0144] 1.25 g of SR590 (ethoxylated cumylphenol acrylate, Arkema, Inc.), 0.625 g of SR349 (ethoxylated bisphenol A diacrylate, Arkema, Inc.), 0.375 g of 2-hydroxy-3- pehnoxypropylacrylate and 0.25 g of TMPTA (CAS Number: 15625-89-5) were mixed in the dark for 6 hours at 60 °C to provide a writing monomers mixture.

[0145] Preparation of photopolymer formulation 8

[0146] 2.5 g of the writing monomers mixture from this Example 8, 1 .4 g of polypropylene polyol (SANNIX PP-2000, Sanyo Chemical), and 0.51 g of the photoinitiation mixture solution from this Example 8 were mixed in the dark at about 60 °C for 1 hour. This was followed by the addition of 1.0 g of isocyanate (Desmodur ultra E 30600, Covestro). The mixture was mixed vigorously for 10 minutes at 35 °C. Then 0.018 g of the Tin catalyst solution was added into the resulting mixture, and mixed for 3 minutes at 35 °C. The resulting liquid solution was ready to place under a vacuum at 35 °C to remove air bubbles caused by mixing. Reflectance and transmittance performance were tested and the resultant spectral plot is illustrated in FIG. 13.

[0147] Glass substrate coating (Sample preparation for exposure)

[0148] In a light controlled (safe red-light lamp) environment, a glass substrate was cleaned and dried. The glass substrate was cut to an about 2” x 2” square. The cut glass substrate was cleaned with soap (washing detergent) and water, and then dried by nitrogen gas (N2) at room temperature for about 30 seconds and then heated to about 60 °C. A drop of synthesized photopolymer material was then applied to the glass substrate, and then covered with a second glass plate, which was kept at distance of about 4 pm thickness by applying silica microsphere beads. The glass sandwiched samples were vacuumed and laminated using NPC vacuum chamber bonding machine (https: / / www.npcgroup.net / eng / solarcell / vacuum-bonding). After lamination, the sample specimens were left at 35 °C. The samples were left to cure (polymerize) for about 8 to 60 hours at room temperature.

[0149] Hologram-mirror recording

[0150] The polymerized photopolymer film was exposed to a Denisyuk (mirror type hologram) arrangement with a laser at 532 nm and exposed with a power density of 5- 10 mW / cm2with an exposure time of 3-10 seconds. After exposure, holograms were examined visually and a bleaching step under UV light was applied. The hologram laminated between two glass slides was exposed under a lamp (UVASPOT 1000 RF2 from Honle UV Technology, Germany). The UVASPOT is a modular, high-intensity UV unit and system that can achieve very high uniformity throughout the irradiation field. Through various lamp and filter configurations, different spectra can be produced for applications in the ranges of UVA (340 nm - 400 nm).

[0151] Reflection efficiency measurement

[0152] The experimental setup to measure the reflection efficiency is shown in FIG. 4 The signal beam is going through recorded and developed hologram. By adjusting the angle of the hologram find position when reflected beam -PR shows its max value. In the meantime, transmitted beam PT will show its min value. The Reflection efficiency can be calculated using formula (PR / PT+ PR) X 100%. Film thickness measurement

[0153] A recorded and UV bleached sample photopolymer sandwiched between an upper glass substrate and lower glass substrate was provided. Using a blade, the upper and lower glass substrates were carefully detached from each other, leaving the photopolymer to be left on one of them. Using a blade, a film was removed from the hologram area. Using a Dektak profiler (Bruker.com), the thickness of the PP layer was measured.

[0154] Refractive Index Modulation calculation

[0155] The amplitude of refractive index modulation is measured using the transmission hologram as follows: a) Using collimated double-beam, the hologram shall be recorded while assuming the incident angle of object wave as 0 and the incident angle of reference wave as 2TT-0, b) The highest diffraction efficiency value (or the diffraction efficiency value recognized for saturation) shall be determined in the exposure characteristics curve and measured according to set up shown in FIG. 14.

[0156] The amplitude of refractive index modulation (An) shall be calculated from Ar? = - cos arcsin . I — — T 100

[0157] Equation: i J. The relationship between the Bragg diffraction angle 0'B and double-beam incident angle 0 can be expressed as follows according to the Snell’s law: Mean refractive index of hologram.

[0158] Use of the term “may” or “may be” or “can” should be construed as shorthand for “is” or “is not” or, alternatively, “does” or “does not” or “will” or “will not,” etc. For example, the statement “a thermally conductive composite may further comprise a backing layer” should be interpreted as, for example, “In some embodiments, a thermally conductive composite further comprises a backing layer,” or “In some embodiments, a thermally conductive composite does not further comprise a backing layer.”

[0159] Unless otherwise indicated, all numbers expressing quantities of ingredients, properties, such as, molecular weight, reaction conditions, and so forth used in the specification and embodiments are to be understood as being modified in all instances by the term “about.” The term “about” as used herein, can include any numerical value that can vary without changing the basic function of that value. When used with a range, “about” also discloses the range defined by the absolute values of the two endpoints. The term “about” may refer to plus or minus 10% of the indicated number. Accordingly, unless indicated to the contrary, the numerical parameters set forth in the specification and attached embodiments are approximations that may vary depending upon the desired properties sought to be obtained. At the very least, and not as an attempt to limit the application of the doctrine of equivalents. To the scope of the embodiments, each numerical parameter should at least be construed in light of the number of reported significant digits and by applying ordinary rounding techniques.

[0160] For the processes and / or methods disclosed, the functions performed in the processes and methods may be implemented in differing order, as may be indicated by context. Furthermore, the outlined steps and operations are only provided as examples, and some of the steps and operations may be optional, combined into fewer steps and operations, or expanded into additional steps and operations.

[0161] This disclosure may sometimes illustrate different components contained within, or connected with, different other components. Such depicted architectures are merely examples, and many other architectures can be implemented which achieve the same or similar functionality.

[0162] The terms used in this disclosure, and in the appended embodiments, are generally intended as “open” terms (e.g., the term “including” should be interpreted as “including, but not limited to,” the term “having” should be interpreted as “having at least,” the term “includes” should be interpreted as “includes, but is not limited to,” etc.). In addition, if a specific number of elements is introduced, this may be interpreted to include at least the recited number, as may be indicated by context (e.g., the bare recitation of "two recitations," without other modifiers, includes at least two recitations, or two or more recitations). As used in this disclosure, any disjunctive word and / or phrase presenting two or more alternative terms should be understood to contemplate the possibilities of including one of the terms, either of the terms, or both terms. For example, the phrase “A or B” will be understood to include the possibilities of “A” or “B” or “A and B.”

[0163] The terms and words used are not limited to the bibliographical meanings but are merely used to enable a clear and consistent understanding of the disclosure. The terms “a,” “an,” “the” and similar referents used in the context of describing the present disclosure (especially in the context of the following embodiments) are to be construed to cover both the singular and the plural, unless otherwise indicated herein or clearly contradicted by context. The use of any and all examples, or representative language (e.g., “such as”) provided herein is intended merely to better illuminate the present disclosure and does not pose a limitation on the scope of any embodiments. No language in the specification should be construed as indicating any non-embodied element essential to the practice of the present disclosure. Groupings of alternative elements or embodiments disclosed herein are not to be construed as limitations. Each group member may be referred to and embodied individually or in any combination with other members of the group or other elements found herein. It is anticipated that one or more members of a group may be included in, or deleted from, a group for reasons of convenience and / or patentability. When any such inclusion or deletion occurs, the specification is deemed to contain the group as modified thus fulfilling the written description of all Markush groups used in the appended embodiments.

[0164] Certain embodiments are described herein, including the best mode known to the inventors for carrying out the present disclosure. Of course, variations on these described embodiments, will become apparent to those of ordinary skill in the art upon reading the foregoing description. The inventor expects skilled artisans to employ such variations as appropriate, and the inventors intend for the present disclosure to be practiced otherwise than specifically described herein. Accordingly, the embodiments include all modifications and equivalents of the subject matter recited in the embodiments as permitted by applicable law. Moreover, any combination of the above-described elements in all possible variations thereof is contemplated unless otherwise indicated herein or otherwise clearly contradicted by context. In closing, it is to be understood that the embodiments disclosed herein are illustrative of the principles of the embodiments. Other modifications that may be employed are within the scope of the embodiments. Thus, by way of example, but not of limitation, alternative embodiments may be utilized in accordance with the teachings herein. Accordingly, the embodiments are not limited to the embodiments precisely as shown and described.

[0165] By the term "substantially" it is meant that the recited characteristic, parameter, or value need not be achieved exactly, but that deviations or variations, including for example, tolerances, measurement error, measurement accuracy limitations and other suitable factors may occur in amounts that do not preclude the effect the characteristic was intended to provide.

[0166] Aspects of the present disclosure may be embodied in other forms without departing from its spirit or essential characteristics. The described aspects are to be considered in all respects illustrative and not restrictive. The embodied subject matter is indicated by the appended embodiments rather than by the foregoing description. All changes, which come within the meaning and range of equivalency of the embodiments, are to be embraced within their scope.

Claims

CLAIMSWhat is claimed is:

1. A holographic photopolymer, comprising: a polyurethane matrix, a polycyclic heteroaryl writing monomer, and a dye system.

2. The holographic photopolymer of claim 1 , further comprising an oxygen scavenger additive.

3. The holographic photopolymer of claim 1 , wherein the polyurethane matrix is linked to the polycyclic heteroaryl writing monomer by a bond.

4. The holographic photopolymer of claim 3, wherein the bond is a covalent bond or a noncovalent hydrogen bond.

5. The holographic photopolymer of claim 1 , wherein the polyurethane matrix comprises at least one of polypropylene polyol, 2-ethyl-2-(hydroxymethyl)-1 ,3- propanediol, aliphatic polyether isocyanate, and dimethyltin dineodecanoate.

6. The holographic photopolymer of claim 1 , wherein the polycyclic heteroaryl writing monomer comprises a difunctional writing monomer or a monofunctional writing monomer.

7. The holographic photopolymer of claim 1 , wherein the polycyclic heteroaryl writing monomer comprises one of the following structures:

8. The holographic photopolymer of claim 1 , wherein the polycyclic heteroaryl writing monomer is according to the following formula:

9. The holographic photopolymer of claim 1 , wherein the polycyclic heteroarylO, S, Se, or Te.

10. The holographic photopolymer of claim 1 , wherein the polycyclic heteroaryl writing monomer comprises at least one of bisphenol A diglycidyl ether diacrylate, bisphenol A ethoxylated di(meth)acrylate, trimethylolpropane triacrylate, ethoxylatedfluorene diacrylate, a liquid carbazole acrylate / methylacrylate comprising an alkyl sulfide chain or ether chain, m-phenoxybenzyl acrylate, and a hydroxyfunctional acrylate.

11. The holographic photopolymer of claim 10, wherein the bisphenol A diglycidyl ether diacrylate comprises one of the following structures:

12. The holographic photopolymer of claim 1 , wherein the polycyclic heteroaryl writing monomer further comprises a diluent.

13. The holographic photopolymer of claim 12, wherein the diluent comprises 1 -vinyl-2-pyrrolidone.

14. The holographic photopolymer of claim 1 , wherein the dye system comprises a dye and a co-initiator.

15. The holographic photopolymer of claim 14, wherein the dye comprises at least one of safranine O, methylene blue, astrazon orange G, and ethyl violet, and wherein the co-initiator comprises a least one of n-phenylglycine and hexaaryl-bisimidazolyl.

16. The holographic photopolymer of claim 14, wherein the ratio by weight of the dye to the co-initiator is about 1 :10 to about 1 :14.

17. The holographic photopolymer of claim 2, wherein the oxygen scavenger additive comprises triphenyl phosphine.

18. The holographic photopolymer of claim 1 , further comprising a reactive diluent, the reactive diluent comprising a [C1-C3 alkyl pyrrolidone]N-ethyl-2-pyrrolidone.

19. The holographic photopolymer of claim 1 , wherein the ratio by weight of the polyurethane matrix to the polycyclic heteroaryl writing monomer is in the range of 19: to 1 :3.

20. The holographic photopolymer of claim 1 , wherein the polyurethane matrix is according to the following formula:

21. An augmented reality / virtual reality apparatus comprising a holographic photopolymer according to any one of claims 1-20.

22. A film comprising a holographic photopolymer according to any one of claims 1-20.

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