Facebook Patent | Photosensitive Polymers For Volume Holography

Patent: Photosensitive Polymers For Volume Holography

Publication Number: 20200081398

Publication Date: 20200312

Applicants: Facebook

Abstract

Photosensitive polymers for recording volume holograms, anisotropic volume holograms, and corresponding volume holographic elements are described herein.

RELATED APPLICATIONS

[0001] This application claims the benefit of, and priority to, U.S. Provisional Patent Application Ser. No. 62/728,053, filed Sep. 6, 2018, which is incorporated by reference herein in its entirety. This application is related to U.S. patent application Ser. No. 16/273,068, filed Feb. 11, 2019, which claims the benefit of, and priority to, U.S. Provisional Patent Application Ser. No. 62/659,104, filed Apr. 17, 2018 and U.S. Provisional Patent Application Ser. No. 62/728,053, filed Sep. 6, 2018. All of these applications are incorporated by reference herein in their entireties.

FIELD OF THE INVENTION

[0002] The invention described herein relates generally to recording materials for volume holographic gratings, volume holograms, volume holographic elements, and photosensitive polymers for use in volume holography applications as well as the volume holographic gratings, volume holograms, volume holographic elements produced through writing to such recording materials.

BACKGROUND

[0003] Polarization volume holographic elements (also called herein polarization volume holograms) have gained increasing interest for applications in optics, such as beam steering devices, waveguides, and display technologies. Conventionally, polarization volume holographic elements were made by using a photoalignment layer. For example, the photoalignment layer includes directional molecules arranged in a particular pattern, and liquid crystals are deposited on the photoalignment layer so that the liquid crystals are aligned along the direction of the directional molecules. However, when the photoalignment layer is used, liquid crystals can be arranged in only certain three-dimensional configurations (e.g., only in the plane defined by the photoalignment layer).

SUMMARY

[0004] One embodiment of the invention relates to a recording material for a volume hologram, wherein the recording material is characterized by a thickness and includes a compound including a moiety of Formula 1:

STR00001

[0005] wherein the moiety of Formula 1 includes a polymer backbone, and a side chain including a linker L.sub.1 and a photosensitive group PG. In some embodiments, the moiety of Formula 1 further includes a terminal group T.sub.1 as in Formula 2:

STR00002

[0006] In some embodiments, the photosensitive group includes a cycloaddition precursor (CAP). In some embodiments, the photosensitive group includes a cinnamate moiety. In some embodiments, the linker L.sub.2 includes one or more of a single bond, a double bond, a triple bond, a carbonyl group, a carboxyl group, a methylene group, an ether group, and an optionally substituted phenylene group. In some embodiments, the terminal group T.sub.1 includes one or more of a single bond, a double bond, a triple bond, an optionally substituted phenylene group, a carbonyl group, a carboxyl group, a methylene group, an ether group, –H, –OH, –CH.sub.3, –N(CH.sub.3).sub.2, –C.sub.2H.sub.5, –C.sub.3H.sub.7, –CN, –NO.sub.2, –OCH.sub.3, –OC.sub.2H.sub.5, and –OC.sub.3H.sub.7. In some embodiments, the phenylene group is a 1,4-phenylene group. In some embodiments, the polymer backbone includes a homopolymer fragment or a copolymer fragment. In some embodiments, the polymer backbone includes one or more of a polysiloxane fragment, a polymethylmethacrylate (PMMA) fragment, a polyacrylate fragment, a polymethacrylate fragment, a cellulose fragment, a polyvinyl fragment, a polyurethane fragment, a polyamic acid fragment, or a polyimide fragment.

[0007] Another embodiment of the invention relates to a recording material for a volume hologram, where the recording material is characterized by a thickness and includes a compound including a moiety of Formula 1, wherein the moiety of Formula 1 includes a polymer backbone, and a side chain, wherein the side chain is of any one of Formulas 101 to 105:

STR00003

[0008] wherein in Formulas 101 to 105: n is independently an integer from 0 to 12, m is independently an integer from 1 to 6, k is independently 0 or 1, l is independently 0 or 1, x is independently 0 or 1, y is independently 0 or 1, and z is independently at each occurrence 0, 1, 2, 3, or 4; X and Y are independently selected from a single bond, a double bond, a triple bond, –C(O)–, –C(O)O–, –OC(O)–, and –C(O)NR.sup.a; R.sub.1 is independently selected from –H, –OH, –CH.sub.3, –NR.sup.aR.sup.b, –C.sub.2H.sub.5, –C.sub.3H.sub.7, –CN, –NO.sub.2, –OCH.sub.3, –OC.sub.2H.sub.5, and –OC.sub.3H.sub.7; R.sub.2 and R.sub.3 are independently selected at each occurrence from hydrogen, alkyl, heteroalkyl, alkenyl, alkynyl, cycloalkyl, heterocycloalkyl, aryl, arylalkyl, heteroaryl, heteroarylalkyl, hydroxy, halo, cyano, fluoroalkyl, trifluoromethyl, trifluoromethoxy, nitro, trimethylsilanyl, –OR.sup.a, –SR.sup.a, –OC(O)R.sup.a, –N(R.sup.a)R.sup.b, –C(O)R.sup.a, –C(O)OR.sup.a, –OC(O)N(R.sup.a)R.sup.b, –C(O)N(R.sup.a)R.sup.b, –N(R.sup.a)C(O)OR.sup.a, –N(R.sup.a)C(O)R.sup.a, –N(R.sup.a)C(O)N(R.sup.a)R.sup.b, –N(R.sup.a)C(NR.sup.a)N(R.sup.a)R.sup.b, –N(R.sup.a)S(O).sub.tR.sup.a, –S(O).sub.tOR.sup.a, –S(O).sub.tN(R.sup.a)R.sup.b, –S(O).sub.tN(R.sup.a)C(O)R.sup.b, and –P(O)(OR.sup.a)(OR.sup.b); and R.sup.a and R.sup.b are independently selected at each occurrence from hydrogen, alkyl, fluoroalkyl, carbocyclyl, carbocyclylalkyl, aryl, aralkyl, heterocycloalkyl, heterocycloalkylalkyl, heteroaryl, and heteroarylalkyl.

[0009] Another embodiment of the invention relates to a recording material for a volume hologram, where the recording material is characterized by a thickness and includes a compound including a moiety of Formula 1, wherein the moiety of Formula 1 includes a polymer backbone, and a side chain, wherein the side chain is of any one of Formulas 106 to 117:

STR00004## ##STR00005

[0010] wherein in Formulas 106 to 117: n is independently an integer from 0 to 12, m is independently an integer from 1 to 6, k is independently 0 or 1, l is independently 0 or 1, x is independently 0 or 1, y is independently 0 or 1, and z is independently at each occurrence 0, 1, 2, 3, or 4; R.sub.1 is independently selected from –H, –OH, –CH.sub.3, –NR.sup.aR.sup.b, –C.sub.2H.sub.5, –C.sub.3H.sub.7, –CN, –NO.sub.2, –OCH.sub.3, –OC.sub.2H.sub.5, and –OC.sub.3H.sub.7; R.sub.2 and R.sub.3 are independently selected at each occurrence from hydrogen, alkyl, heteroalkyl, alkenyl, alkynyl, cycloalkyl, heterocycloalkyl, aryl, arylalkyl, heteroaryl, heteroarylalkyl, hydroxy, halo, cyano, fluoroalkyl, trifluoromethyl, trifluoromethoxy, nitro, trimethylsilanyl, –OR.sup.a, –SR.sup.a, –OC(O)R.sup.a, –N(R.sup.a)R.sup.b, –C(O)R.sup.a, –C(O)OR.sup.a, –OC(O)N(R.sup.a)R.sup.b, –C(O)N(R.sup.a)R.sup.b, –N(R.sup.a)C(O)OR.sup.a, –N(R.sup.a)C(O)R.sup.a, –N(R.sup.a)C(O)N(R.sup.a)R.sup.b, –N(R.sup.a)C(NR.sup.a)N(R.sup.a)R.sup.b, –N(R.sup.a)S(O).sub.tR.sup.a, –S(O).sub.tOR.sup.a, –S(O).sub.tN(R.sup.a)R.sup.b, –S(O).sub.tN(R.sup.a)C(O)R.sup.b, and –P(O)(OR.sup.a)(OR.sup.b); and R.sup.a and R.sup.b are independently selected at each occurrence from hydrogen, alkyl, fluoroalkyl, carbocyclyl, carbocyclylalkyl, aryl, aralkyl, heterocycloalkyl, heterocycloalkylalkyl, heteroaryl, and heteroarylalkyl.

[0011] Another embodiment of the invention relates to a recording material for a volume hologram, wherein the recording material is characterized by a thickness and includes a compound including a moiety of Formula 1, wherein the moiety of Formula 1 includes a polymer backbone, and a side chain, wherein the side chain is of any one of Formulas 118 to 123:

STR00006

[0012] wherein in Formulas 118 to 123: n is independently an integer from 0 to 12, m is independently an integer from 1 to 6, k is independently 0 or 1, l is independently 0 or 1, x is independently 0 or 1, y is independently 0 or 1, and z is independently at each occurrence 0, 1, 2, 3, or 4; R.sub.1 is independently selected from –H, –OH, –CH.sub.3, –NR.sup.aR.sup.b, –C.sub.2H.sub.5, –C.sub.3H.sub.7, –CN, –NO.sub.2, –OCH.sub.3, –OC.sub.2H.sub.5, and –OC.sub.3H.sub.7; R.sub.2 and R.sub.3 are independently selected at each occurrence from hydrogen, alkyl, heteroalkyl, alkenyl, alkynyl, cycloalkyl, heterocycloalkyl, aryl, arylalkyl, heteroaryl, heteroarylalkyl, hydroxy, halo, cyano, fluoroalkyl, trifluoromethyl, trifluoromethoxy, nitro, trimethylsilanyl, –OR.sup.a, –SR.sup.a, –OC(O)R.sup.a, –N(R.sup.a)R.sup.b, –C(O)R.sup.a, –C(O)OR.sup.a, –OC(O)N(R.sup.a)R.sup.b, –C(O)N(R.sup.a)R.sup.b, –N(R.sup.a)C(O)OR.sup.a, –N(R.sup.a)C(O)R.sup.a, –N(R.sup.a)C(O)N(R.sup.a)R.sup.b, –N(R.sup.a)C(NR.sup.a)N(R.sup.a)R.sup.b, –N(R.sup.a)S(O).sub.tR.sup.a, –S(O).sub.tOR.sup.a, –S(O).sub.tN(R.sup.a)R.sup.b, –S(O).sub.tN(R.sup.a)C(O)R.sup.b, and –P(O)(OR.sup.a)(OR.sup.b); and R.sup.a and R.sup.b are independently selected at each occurrence from hydrogen, alkyl, fluoroalkyl, carbocyclyl, carbocyclylalkyl, aryl, aralkyl, heterocycloalkyl, heterocycloalkylalkyl, heteroaryl, and heteroarylalkyl.

[0013] Another embodiment of the invention relates to a recording material for a volume hologram, wherein the recording material is characterized by a thickness and includes a compound including a moiety of Formula 1, wherein the moiety of Formula 1 includes a polymer backbone, and a side chain, wherein the side chain is of any one of Formulas 124 to 131:

STR00007

wherein in Formulas 124 to 131: z is independently at each occurrence 0, 1, 2, 3, or 4; n is independently an integer from 0 to 12; m is independently 0 or 1;

[0014] R.sub.1, R.sub.2, and R.sub.3 are independently selected at each occurrence from hydrogen, alkyl, heteroalkyl, alkenyl, alkynyl, cycloalkyl, heterocycloalkyl, aryl, arylalkyl, heteroaryl, heteroarylalkyl, hydroxy, halo, cyano, fluoroalkyl, trifluoromethyl, trifluoromethoxy, nitro, trimethylsilanyl, –OR.sup.a, –SR.sup.a, –OC(O)R.sup.a, –N(R.sup.a)R.sup.b, –C(O)R.sup.a, –C(O)OR.sup.a, –OC(O)N(R.sup.a)R.sup.b, –C(O)N(R.sup.a)R.sup.b, –N(R.sup.a)C(O)OR.sup.a, –N(R.sup.a)C(O)R.sup.a, –N(R.sup.a)C(O)N(R.sup.a)R.sup.b, –N(R.sup.a)C(NR.sup.a)N(R.sup.a)R.sup.b, –N(R.sup.a)S(O).sub.tR.sup.a, –S(O).sub.tOR.sup.a, –S(O).sub.tN(R.sup.a)R.sup.b, –S(O).sub.tN(R.sup.a)C(O)R.sup.b, and –P(O)(OR.sup.a)(OR.sup.b); and R.sup.a and R.sup.b are independently selected at each occurrence from hydrogen, alkyl, fluoroalkyl, carbocyclyl, carbocyclylalkyl, aryl, aralkyl, heterocycloalkyl, heterocycloalkylalkyl, heteroaryl, and heteroarylalkyl.

[0015] Another embodiment of the invention relates to a recording material for a volume hologram, wherein the recording material is characterized by a thickness and includes a compound including a moiety of Formula 1, wherein the moiety of Formula 1 includes a polymer backbone, and a side chain, wherein the side chain is of any one of Formulas 1001 to 1011:

STR00008

[0016] Another embodiment of the invention relates to a recording material for a volume hologram, wherein the recording material is characterized by a thickness and includes a compound including a moiety of Formula 1, wherein the moiety of Formula 1 is of any one of Formulas 2001 to 2014:

STR00009## ##STR00010

[0017] Another embodiment of the invention relates to a recording material for a volume hologram, wherein the recording material is characterized by a thickness and includes a compound including a moiety of Formula 1, wherein a volume hologram is recorded in the recording material and is characterized by a Q parameter that is equal to or greater than 10,* wherein*

Q = 2 .pi. .lamda. d n .LAMBDA. 2 , ##EQU00001##

[0018] and wherein: .lamda. is a wavelength of diffracting light, d is the thickness of the volume hologram, n is an averaged refractive index of the recording material for a wavelength .lamda., and .LAMBDA. is a grating period (fringe spacing) of the volume hologram.

[0019] Another embodiment of the invention relates to a recording material for a volume hologram, wherein a volume hologram is recorded in the recording material and is characterized by a thickness and includes a compound including a moiety of Formula 1, wherein the volume hologram is characterized by a Q parameter that is equal to or greater than 5,* wherein*

Q = 2 .pi. .lamda. d n .LAMBDA. 2 , ##EQU00002##

[0020] and wherein: .lamda. is a wavelength of diffracting light, d is the thickness of the volume hologram, n is an averaged refractive index of the recording material for a wavelength .lamda., and .LAMBDA. is a grating period (fringe spacing) of the volume hologram. In some embodiments, the thickness of the volume hologram is between 0.05 .mu.m and 500 .mu.m. In some embodiments, the recording material is liquid crystalline, and the thickness of the volume hologram is between 0.05 .mu.m and 500 .mu.m. In some embodiments, the recording material has liquid crystal property, and the thickness of the volume hologram is between 0.05 .mu.m and 500 .mu.m. In some embodiments, the recording material is amorphous and a thickness of the volume hologram is between 0.05 m and 500 .mu.m. In some embodiments, the thickness of the volume hologram is between 0.5 m and 1000 m. In some embodiments, the recording material is liquid crystalline, and the thickness of the volume hologram is between 0.5 m and 1000 m. In some embodiments, the recording material has liquid crystal property, and the thickness of the volume hologram is between 0.5 m and 1000 m. In some embodiments, the recording material is amorphous and a thickness of the volume hologram is between 0.5 m and 1000 m. In some embodiments, the thickness of the volume hologram is between 1 m and 2500 m. In some embodiments, the recording material is liquid crystalline, and the thickness of the volume hologram is between 1 m and 2500 m. In some embodiments, the recording material has liquid crystal property, and the thickness of the volume hologram is between 1 m and 2500 m. In some embodiments, the recording material is amorphous and a thickness of the volume hologram is between 1 m and 2500 m.

[0021] Another embodiment of the invention relates to a recording material for a volume hologram, wherein the recording material is characterized by a thickness and includes a compound including a moiety of Formula 1, wherein: the recording material is characterized by induced birefringence of greater than 0.002, greater than 0.04, or greater than 0.10. In some embodiments, the material is optically transparent to visible light.

[0022] Another embodiment of the invention relates to a recording material for a volume hologram, wherein the recording material is characterized by a thickness and includes a compound including a moiety of Formula 1, wherein: the recording material is amorphous and characterized by induced birefringence of greater than 0.002.

[0023] Another embodiment of the invention relates to a recording material for a volume hologram, wherein the recording material is characterized by a thickness and includes a compound including a moiety of Formula 1, wherein the recording material is liquid crystal or amorphous polymer. In some embodiments, the compound aligns parallel in relation to the substrate. In some embodiments, the compound aligns homeotropically in relation to the substrate. In some embodiments, the compound is arranged in tilted alignment in relation to the substrate. In some embodiments, the compound is arranged in twisted alignment.

[0024] Another embodiment of the invention relates to a recording material for a volume hologram, wherein the recording material is characterized by a thickness and includes a compound including a moiety of Formula 1, and wherein a volume hologram in the recording material is a polarization grating.

[0025] Another embodiment of the invention relates to a recording material for a volume hologram, wherein a volume hologram recorded in the recording material is characterized by a thickness and includes a compound including a moiety of Formula 1, and wherein the volume hologram is an intensity grating.

[0026] Another embodiment of the invention relates to a process for writing a data structure including information into a recording material, thereby forming a volume hologram, wherein the recording material for the volume hologram is characterized by a thickness and includes a compound including a moiety of Formula 1, the process including: orienting a coherent light source toward the recording material; and cross-linking through a cycloaddition reaction a portion of the total number of photosensitive groups in the recording material using the coherent light source; wherein the information of the data structure is stored in the form of selective cross-linking of the photosensitive groups in a pattern corresponding to the information. In some embodiments, the writing results in between two percent and thirty percent of the photosensitive groups of the recording material being cross-linked. In some embodiments, the writing results in between three percent and twenty percent of the photosensitive groups of the recording material being cross-linked.

[0027] In some embodiments, the coherent light source includes: a first incident light beam and a second incident light beam originating from the coherent light source, where the beams partially or completely overlap and interfere in recording material. In some embodiments, the incident light beams are linearly polarized and their polarization directions are parallel. In some embodiments, the incident light beams are linearly polarized and their polarization directions are perpendicular. In some embodiments, the first incident light beam is circularly polarized in a first direction, and the second incident light beam is circularly polarized in the first direction. In some embodiments, the first incident light beam is circularly polarized in a first direction, and the second incident light beam is circularly polarized in an opposite (orthogonal) direction. In one embodiment, the first incident light beam and the second incident light beam impinge upon the same surface of a recording layer. In another embodiment, the first incident light beam and the second incident light beam impinge upon opposite surfaces of a recording layer.

[0028] In some embodiments, the writing results in between four percent and fifteen percent of the photosensitive groups being cross-linked. In some embodiments, the writing results in between four percent and fifteen percent of the photosensitive groups being cross-linked. In some embodiments, the writing results in about five percent, or about six percent, or about seven percent, or about eight percent, or about nine percent, or about ten percent of the photosensitive groups being cross-linked. In some embodiments, the recording material is an amorphous polymer and the writing results in between five percent and ninety percent, or about ten percent and eighty percent, or about 20 percent and 60 percent of the photosensitive groups being cross-linked. In some embodiments, the recording material is a liquid crystal polymer and the writing results in between two percent and fifty percent, or between five percent and forty percent, or between ten percent and thirty percent of the photosensitive groups being cross-linked. In some embodiments, the recording material is an amorphous polymer and the writing results in about 1%, about 2%, about 3%, about 4%, about 5%, about 6%, about 7%, about 8%, about 9%, about 10%, about 11%, about 12%, about 13%, about 14%, about 15%, about 16%, about 17%, about 18%, about 19%, about 20%, about 21%, about 22%, about 23%, about 24%, about 25%, about 26%, about 27%, about 28%, about 29%, about 30%, about 31%, about 32%, about 33%, about 34%, about 35%, about 36%, about 37%, about 38%, about 39%, about 40%, about 41%, about 42%, about 43%, about 44%, about 45%, about 46%, about 47%, about 48%, about 49%, about 50%, about 51%, about 52%, about 53%, about 54%, about 55%, about 56%, about 57%, about 58%, about 59%, about 60%, about 61%, about 62%, about 63%, about 64%, about 65%, about 66%, about 67%, about 68%, about 69%, about 70%, about 71%, about 72%, about 73%, about 74%, about 75%, about 76%, about 77%, about 78%, about 79%, about 80%, about 81%, about 82%, about 83%, about 84%, about 85%, about 86%, about 87%, about 88%, about 89%, about 90%, about 91%, about 92%, about 93%, about 94%, about 95%, about 96%, about 97%, about 98%, about 99%, or about 100% of the photosensitive groups being cross-linked. In some embodiments, the recording material is a liquid crystal polymer and the writing results in about 1%, about 2%, about 3%, about 4%, about 5%, about 6%, about 7%, about 8%, about 9%, about 10%, about 11%, about 12%, about 13%, about 14%, about 15%, about 16%, about 17%, about 18%, about 19%, about 20%, about 21%, about 22%, about 23%, about 24%, about 25%, about 26%, about 27%, about 28%, about 29%, about 30%, about 31%, about 32%, about 33%, about 34%, about 35%, about 36%, about 37%, about 38%, about 39%, about 40%, about 41%, about 42%, about 43%, about 44%, about 45%, about 46%, about 47%, about 48%, about 49%, about 50%, about 51%, about 52%, about 53%, about 54%, about 55%, about 56%, about 57%, about 58%, about 59%, about 60%, about 61%, about 62%, about 63%, about 64%, about 65%, about 66%, about 67%, about 68%, about 69%, about 70%, about 71%, about 72%, about 73%, about 74%, about 75%, about 76%, about 77%, about 78%, about 79%, about 80%, about 81%, about 82%, about 83%, about 84%, about 85%, about 86%, about 87%, about 88%, about 89%, about 90%, about 91%, about 92%, about 93%, about 94%, about 95%, about 96%, about 97%, about 98%, about 99%, or about 100% of the photosensitive groups being cross-linked.

[0029] In some embodiments, cross-linking photosensitive groups results in formation of a cyclobutane moiety of Formula A or Formula B:

STR00011

[0030] wherein in Formulas A and B: z is independently at each occurrence 0, 1, 2, 3, or 4; R.sub.2 is independently selected at each occurrence from hydrogen, alkyl, heteroalkyl, alkenyl, alkynyl, cycloalkyl, heterocycloalkyl, aryl, arylalkyl, heteroaryl, heteroarylalkyl, hydroxy, halo, cyano, fluoroalkyl, trifluoromethyl, trifluoromethoxy, nitro, trimethylsilanyl, –OR.sup.a, –SR.sup.a, –OC(O)R.sup.a, –N(R.sup.a)R.sup.b, –C(O)R.sup.a, –C(O)OR.sup.a, –OC(O)N(R.sup.a)R.sup.b, –C(O)N(R.sup.a)R.sup.b, –N(R.sup.a)C(O)OR.sup.a, –N(R.sup.a)C(O)R.sup.a, –N(R.sup.a)C(O)N(R.sup.a)R.sup.b, –N(R.sup.a)C(NR.sup.a)N(R.sup.a)R.sup.b, –N(R.sup.a)S(O).sub.tR.sup.a, –S(O).sub.tOR.sup.a, –S(O).sub.tN(R.sup.a)R.sup.b, –S(O).sub.tN(R.sup.a)C(O)R.sup.b, and –P(O)(OR.sup.a)(OR.sup.b); and R.sup.a and R.sup.b are independently selected at each occurrence from hydrogen, alkyl, fluoroalkyl, carbocyclyl, carbocyclylalkyl, aryl, aralkyl, heterocycloalkyl, heterocycloalkylalkyl, heteroaryl, and heteroarylalkyl.

BRIEF DESCRIPTION OF THE DRAWINGS

[0031] The foregoing summary, as well as the following detailed description of the invention, will be better understood when read in conjunction with the appended drawings.

[0032] FIGS. 1A-1C illustrate light patterns created by two interfering beams of different polarizations; FIG. 1A illustrates intensity patterns for same linear polarization of the beams; FIG. 1B illustrates intensity patterns for same circular polarization of the beams; FIG. 1C illustrates polarization patterns corresponding to orthogonal circular polarizations of the beams.

[0033] FIGS. 2A-2C are a schematic representation of molecular photo-orientation in a photosensitive material (shaded ellipses: photoproducts; white ellipses: non-reacted photosensitive fragments and other fragments); FIG. 2A illustrates random alignment of molecular fragments before exposure; FIG. 2B illustrates molecular orientation in the direction of light polarization E; FIG. 2C illustrates molecular orientation in the direction perpendicular to light polarization E.

DETAILED DESCRIPTION OF THE INVENTION

[0034] Volume gratings, usually produced by holographic technique and known as volume holographic gratings (VHG) or volume holograms, are diffractive optical elements based on material with periodic phase or absorption modulation throughout the entire volume of the material. When an incident light satisfies Bragg condition it is diffracted by the grating. The diffraction occurs within a narrow range of wavelength and incidence angles. In turn, the grating has no effect on the light from the off-Bragg angular and spectral range. These gratings also demonstrate excellent multiplexing ability. Due to these properties, VHG are of great interest for various applications in optics such as data storage and diffractive optical elements for displays, fiber optic communication, spectroscopy, etc.

[0035] The conventional holographic materials for VHG are sensitive only to intensity of recording light and corresponding gratings are polarization insensitive. There is however other group of materials sensitive to both intensity and polarization of the recording light. Use of these materials opens way to anisotropic VHG characterized by polarization sensitivity that gives important degree of freedom for design and extends the field of possible applications. Depending on exposure geometry and polarization of recording beams, the resulted VHG can be sensitive to linear polarization or circular polarization. The latter known as polarization volume gratings (PVG) currently attract special research interest primarily due to potential application in beam steering devices and AR headsets.

[0036] The materials currently used as recording media for polarization sensitive VHG have different practical limitations, primarily, insufficient stability and coloring. The alternative method for recording of such gratings based on the surface mediated photoalignment of polymerizable liquid crystals has another set of limitations. When the photoalignment layer is used, liquid crystal can be arranged in only certain three-dimensional configurations. In addition, this preparation method is rather cumbersome because of deposition and processing of two organic films.

[0037] The disclosure relates to a class of holographic materials which are transparent in the entire visible range and provide enhanced stability of the recorded VHG.

[0038] Volume gratings are based on periodic phase or absorption modulation throughout the entire volume of recording material and characterized by selective diffraction at the wavelength and incidence angle of light satisfying Bragg law. Usually these gratings are produced by holographic technique and known as volume holographic gratings (VHG) or volume holograms.

[0039] Achieving of the Bragg regime of a diffraction grating is usually determined by Klein parameter Q:

Q = 2 .pi. .lamda. d n .LAMBDA. 2 , ##EQU00003##

where d is a thickness of the grating, .lamda. is a wavelength of light, .LAMBDA. is a grating period, and n is an average refractive index of the recording medium. As a rule, Bragg property is achieved if Q>>1, typically, Q.gtoreq.10. Thus, to meet Bragg regime, thickness of diffraction grating should be higher than some value determined by parameters of grating, recording medium and light. Because of this, VHG is also called a thick grating. On the contrary, the grating with Q<1 is usually considered as a thin one, which typically demonstrates many diffraction orders (Raman-Nath diffraction regime).

[0040] Depending on the light patterns used in recording process, the corresponding VHG are divided into two groups. The gratings induced by only periodic modulation of light intensity are usually called intensity gratings. In turn, the gratings induced by periodic modulation of light polarization and constant intensity are called polarization gratings. Some holographic geometries for generation of intensity and polarization patterns of light corresponding to one-side incidence of two recording beams are presented in FIG. 1. FIGS. 1A and 1B show intensity patterns in case of linear and circular polarization of recording beams, respectively. As shown, light polarization of the recording beams is preserved and not spatially varied. FIG. 1C shows polarization patterns generated by two coherent beams with orthogonal circular polarizations. In this case light intensity is kept constant, while light of circular polarization is transformed to the patterns of linear polarization.

[0041] Typically, holographic materials are optically isotropic photo-polymeric compositions responsive only to intensity of recording light. In these materials, only the intensity grating can be recorded. The induced contrast of refractive index achieves 0.03. The other class of materials for optical recording demonstrates effect of photoinduced anisotropy. The anisotropic photosensitive units of these materials arrange in some preferential direction, usually perpendicular or parallel to polarization direction of recording light, see FIG. 2. Thus, in contrast to the above-mentioned photopolymers sensitive only to the intensity of recording light, these materials are sensitive to both light intensity and polarization. This allows to record both intensity and polarization gratings characterized by the induced birefringence modulated by magnitude or orientation, respectively. These anisotropic gratings may be sensitive to linear or circular polarization that gives new options in optical design. In case of Q>1, the Bragg condition is met and thus the above anisotropic gratings may be considered as the intensity or polarization volume gratings with unique optical responses.

[0042] The majority of materials demonstrating effect of photoinduced anisotropy and thus suitable for recording of anisotropic VHG sufferers from different practical limitations such as low photoinduced birefringence, insufficient stability and absorption in a visible range. This invention relates to the class of holographic polymers which are transparent in the entire visible range, provide high photoinduced birefringence and much enhanced stability of the recorded volume holograms.

[0043] The disclosure relates generally to photosensitive polymers for use in volume holography applications. Materials suitable for anisotropic volume holograms include, but are not limited to, bacteriorhodopsin, dichromated gelatin, and polymers with polarization sensitive groups. Polymers with polarization sensitive groups include, but are not limited to, homopolymers, copolymers, and polymer blends containing dichroic dyes, cinnamates, and coumarines. In some embodiments, materials include polymers and copolymers with chemically linked photosensitive fragments, for example polymers with azo dye fragments (azopolymers) undergoing trans-cis isomerization, and polymers with cinnamoyl fragments undergoing both trans-cis isomerization and (2+2)-cycloaddition reaction under irradiation. Azopolymers are capable of high photoinduced optical birefringence reaching 0.3-0.35. The cross-linking fragments occurring as a result of (2+2)-cycloaddition reaction fix the photoinduced molecular order providing high stability of the recorded orientational structures.

[0044] In some embodiments, materials for VHG include polymers containing fragments capable of (2+2)-cycloaddition, for example cinnamates, because they are colorless and not sensitive to visible light, the photoinduced anisotropy is induced only with UV light, and the photoreaction of cycloaddition fixes the induced orientational structure. In some embodiments, the induced structure, e.g. polarization grating, is photo- and thermally stable. In some embodiments, the materials include liquid crystalline polymers undergoing cycloaddition, because in these polymers the photoinduced order is enhanced by liquid crystal orientational ordering.

Definitions

[0045] Unless defined otherwise, all technical and scientific terms used herein have the same meaning as is commonly understood by one of skill in the art to which this invention belongs. All patents and publications referred to herein are incorporated by reference in their entireties.

[0046] When ranges are used herein to describe, for example, physical or chemical properties such as molecular weight or chemical formulae, all combinations and subcombinations of ranges and specific embodiments therein are intended to be included. Use of the term “about” when referring to a number or a numerical range means that the number or numerical range referred to is an approximation within experimental variability (or within statistical experimental error), and thus the number or numerical range may vary. The variation is typically from 0% to 15%, or from 0% to 10%, or from 0% to 5% of the stated number or numerical range. The term “including” (and related terms such as “comprise” or “comprises” or “having” or “including”) includes those embodiments such as, for example, an embodiment of any composition of matter, method or process that “consist of” or “consist essentially of” the described features.

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