Photopolymer composition, reflective diffraction grating and preparation method thereof
阅读说明：本技术 光致聚合物组合物、反射式衍射光栅及其制备方法 (Photopolymer composition, reflective diffraction grating and preparation method thereof ) 是由 邱毅伟 魏一振 张卓鹏 于 2019-10-21 设计创作，主要内容包括：本发明涉及光致聚合物组合物、反射式衍射光栅及其制备方法,所述组合物包括如下组分：书写单体,基质单体,以及光引发剂体系,所述书写单体包括折射率为1.55以上的丙烯酸酯类单体和/或环氧类化合物,并且,所述书写单体的平均官能团数为大于2；所述基质单体包括折射率为1.5以下的可聚合单体。(The invention relates to a photopolymer composition, a reflective diffraction grating and a preparation method thereof, wherein the composition comprises the following components: the writing monomer comprises an acrylate monomer and/or an epoxy compound with the refractive index of more than 1.55, and the average functional group number of the writing monomer is more than 2; the matrix monomer includes a polymerizable monomer having a refractive index of 1.5 or less.)
1. A photopolymer composition for a diffraction grating, comprising the following components:
the writing unit is a single body for writing,
a matrix monomer, and
a photoinitiator system, wherein the photoinitiator system comprises a photoinitiator,
the writing monomer comprises an acrylate monomer and/or an epoxy compound with the refractive index of more than 1.55, and the average functional group number of the writing monomer is more than 2;
the matrix monomer includes a polymerizable monomer having a refractive index of 1.5 or less.
2. The composition as claimed in claim 1, wherein the writing monomer further comprises a trifunctional or higher acrylate monomer having a refractive index of 1.42 to 1.50.
3. Composition according to claim 1 or 2, characterized in that the polymerizable monomer is chosen from fluoroacrylate monomers, and/or vinyl esters of substituted or unsubstituted fatty acids.
4. The composition according to any one of claims 1 to 3, wherein the writing monomer is contained in an amount of 30 to 60% and the matrix monomer is contained in an amount of 20 to 50% based on the total weight of the composition.
5. The composition according to any one of claims 1 to 4, wherein the composition further comprises a plasticizer.
6. The composition of any one of claims 1 to 5, wherein the photoinitiator system comprises a photosensitive dye compound and a co-initiator.
7. A reflective diffraction grating comprising a resin film having a grating structure, wherein the resin film is obtained by curing the composition according to any one of claims 1 to 6.
8. A preparation method of a reflection type diffraction grating is characterized by comprising the following steps:
a mixing step, mixing the components of the composition according to any one of claims 1 to 6 to obtain a mixture;
a step of forming a grating structure by forming a film of the mixture and forming a grating structure on at least a part of the film,
wherein the step of forming a grating structure includes a step of exposing the film with coherent light.
9. The method of claim 8, wherein the step of forming a grating structure comprises the step of using spacers to compound the hybrid.
10. The method of claim 8 or 9, wherein the coherent light is derived from visible light.
11. A holographic optical waveguide display element comprising a reflective diffraction grating according to claim 7 or a reflective diffraction grating obtained by a method according to any one of claims 8 to 10.
Technical Field
The invention belongs to the field of optical materials and equipment, and particularly relates to a photoinduced recording material and a reflective diffraction grating formed by the photoinduced recording material, in particular to a photoinduced polymer composition for holographic recording, a reflective diffraction grating and a preparation method of the reflective diffraction grating or a holographic recording system formed by using a photoinduced polymer.
Background
Holographic photopolymer is widely applied to high-tech fields such as 3D display, security anti-counterfeiting and data storage. The rise of Augmented Reality (AR) devices has stimulated interest in the use of photopolymers for the production of holograms.
Optical waveguide devices are a key technology in the AR field. Currently, optical waveguide devices are commonly manufactured by the industry using photolithography techniques. The requirement of photoetching technology equipment is high, the development period is long, and the cost is high. Compared with the prior art, the photopolymer can be completed by only one-step holographic exposure, and has simple process and low cost, thereby having wide application prospect.
The photopolymer composition consists essentially of a monomer, a matrix, and an initiator. During holographic recording, the two beams of coherent light produce alternate light and dark fringes in the photopolymer. The writing monomers in the bright area are polymerized under the action of an initiator, at the moment, the writing monomers are gathered to the bright area, and the matrix monomers are gradually gathered to the dark area; the difference in the concentration of the different monomers in the bright and dark regions causes phase separation, and as the monomers migrate, a refractive index difference, i.e., a degree of refractive index modulation (Δ n), is formed between the bright and dark regions. As some documents indicate, the magnitude of Δ n determines the performance of the photopolymer grating product and is an important performance parameter of the photopolymer grating.
It is generally believed that the Δ n of the grating obtained by curing the film of the photopolymer composition is determined by both the degree of monomer phase separation in the composition and the refractive index. The degree of monomer phase separation is directly related to the viscosity of the system, and the smaller the viscosity of the system is, the more favorable the monomer migration is during photopolymerization. Further, the gratings formed using the photopolymer composition in the art are classified into a transmissive diffraction grating and a reflective diffraction grating. Typically, a reflective grating has a smaller period than a transmissive grating. For example, the period of the transmissive diffraction grating is 230nm or more in 532nm exposure, and the period of the diffraction grating is 230nm or less in reflection. That is, in the exposure of the transmission type diffraction grating, since the grating period is relatively long, the distance over which the monomer causing phase separation migrates (for example, the monomer for writing migrates from a dark region to a bright region) is long, resulting in a final grating having a smaller Δ n than that of the reflection type diffraction grating.
In addition, although reflective diffraction gratings can theoretically achieve a higher degree of refractive index modulation relative to transmissive gratings, the composition of existing photopolymer compositions used to make these reflective diffraction gratings can be more complex and typically require the use of film formers or matrix polymers to achieve the grating structure. The presence of the film and matrix polymer also limits the migration of writing monomers.
DuPont (US5013632, US5098803, US4950567, US4959284, US4987230, US4994347, US5292620, US5402514), Baolilai, Canon, Fuji and Cocisco (CN107223121A, CN102667934B, CN102667936B) and the like have proposed respective photosensitive/photopolymer holographic recording materials.
Cited document 1 discloses a photopolymer material developed by Bayer corporation, which is a polyurethane composition comprising a writing monomer component comprising at least 10% by weight of one or more structurally specific unsaturated urethanes as writing monomers, based on the total weight of the polyurethane composition, and a polymer or corresponding matrix precursor as a writing monomer matrix.
Citation 2 discloses a method of producing a photopolymer by cationic ring-opening polymerization.
In addition, the liquid phase system photopolymer facilitates monomer diffusion. As described in cited reference 3, dipentaerythritol penta-/hexa-acrylic acid (DPHA) and tris (4-hydroxyphenyl) methane triglycidyl ether (TPMTG) were used as writing monomers, and no film-forming component was contained. After holographic exposure, a DPHA-rich phase and TPMTG-rich phase reflection type grating with a period distribution is formed, and the spatial resolution reaches 7400 line/mm. However, Δ n is small and is only 0.0014.
It can be seen that although some research has been conducted on reflective diffraction gratings in the art, most of the above products still suffer from insufficient refractive index modulation and/or insufficient angular selectivity.
Cited documents:
cited document 1: EP 2172505B1
Cited document 2: US 9874811B2
Cited document 3: castagna, R., et al, Superior-Performance Polymeric Composite Materials for High-Density Optical Data storage. advanced Materials,2009.21(5): p.589-592.
Disclosure of Invention
Problems to be solved by the invention
In view of the problem in the art of producing diffraction gratings, particularly reflective diffraction gratings, from photopolymer compositions because the degree of modulation of the refractive index of the grating is still not ideal during photocuring, the present invention provides a photopolymer composition that does not require the use of cross-linking polymers or film-forming components and enables the resulting diffraction gratings, particularly reflective diffraction gratings, to have excellent improved degrees of modulation of the refractive index and diffraction efficiency through the use of suitable compositions of writing monomers and matrix monomers.
In addition, the invention also provides a method for preparing a diffraction grating by using the photopolymer composition and an optical waveguide element obtained by using the diffraction grating.
Means for solving the problems
After intensive research by the inventor, the technical problems can be solved by using the following technical scheme:
[1] the present invention firstly provides a photopolymer composition for a diffraction grating, said composition comprising the following components:
the writing unit is a single body for writing,
a matrix monomer, and
a photoinitiator system, wherein the photoinitiator system comprises a photoinitiator,
the writing monomer comprises an acrylate monomer and/or an epoxy compound with the refractive index of more than 1.55, and the average functional group number of the writing monomer is more than 2;
the matrix monomer comprises fluorine-containing acrylate monomer with the refractive index of less than 1.5 and/or substituted or unsubstituted fatty acid vinyl ester.
[2] The composition according to [1], wherein the writing monomer further comprises a trifunctional or higher acrylate monomer having a refractive index of 1.42 to 1.50.
[3] The composition according to the item [1] or [2], wherein the fluorine-containing acrylate monomer is selected from perfluoroalkyl acrylate, and/or the fatty acid part in the substituted or unsubstituted fatty acid vinyl ester is derived from fatty acid with 2-25 carbon atoms.
[4] The composition according to any one of [1] to [3], wherein the content of the writing monomer is 30 to 60% and the content of the matrix monomer is 20 to 50% based on the total weight of the composition.
[5] The composition according to any one of [1] to [4], further comprising a plasticizer.
[6] The composition according to any one of [1] to [5], wherein the photoinitiator system comprises a photosensitive dye compound and a co-initiator.
[7] Further, the present invention also provides a reflective diffraction grating comprising a resin film having a grating structure, the resin film being obtained by curing the composition according to any one of the above [1] to [6].
[8] In addition, the invention also provides a preparation method of the reflection type diffraction grating, which is characterized by comprising the following steps:
a mixing step of mixing the components of the composition according to any one of the above [1] to [6] to obtain a mixture;
a step of forming a grating structure by forming a film of the mixture and forming a grating structure on at least a part of the film,
wherein the step of forming a grating structure includes a step of exposing the film with coherent light.
[9] The method according to [8], wherein the step of forming the grating structure comprises a step of compounding with the mixture using a spacer.
[10] The method according to [8] or [9], wherein the coherent light is derived from visible light.
[11] Further, based on the above, the present invention also provides a holographic optical waveguide display element comprising the reflective diffraction grating according to [7] or the reflective diffraction grating obtained by the method according to any one of [8] to [10].
ADVANTAGEOUS EFFECTS OF INVENTION
Through the implementation of the technical scheme, the invention can obtain the following technical effects:
1) through the matching of the limited writing monomer and the matrix monomer, the generated phase separation is more obvious in the photocuring (exposure) process, so that the final refractive index modulation degree (delta n can reach more than 0.03) of the diffraction grating is improved;
2) in the formula, a cross-linking polymer matrix or a film-forming component is not used in the preparation of the reflective diffraction grating, and a liquid phase system is adopted before exposure, so that the system viscosity is low, and the monomer phase separation is facilitated, thereby improving the refractive index modulation degree delta n of the reflective grating prepared by the photopolymer composition;
3) the preparation method of the diffraction grating provided by the invention is simple and feasible, does not use expensive or toxic monomer substances, is easy for industrial large-scale production, and has strong controllability.
Drawings
FIG. 1: the grating forming mechanism of the invention is schematically shown
FIG. 2: exposure light path diagram of reflective diffraction grating in one embodiment of the present invention
FIG. 3: diffraction efficiency curves for reflective diffraction gratings in one embodiment of the present invention
Detailed Description
The present invention will be described in detail below. The technical features described below are explained based on typical embodiments and specific examples of the present invention, but the present invention is not limited to these embodiments and specific examples. It should be noted that:
in the present specification, the numerical range represented by "numerical value a to numerical value B" means a range including the end point numerical value A, B.
In the present specification, the numerical ranges indicated by "above" or "below" mean the numerical ranges including the numbers.
In this specification, the description will be made using "vicinity" to a certain wavelength of light, and it is understood that, for a specific wavelength, some error may occur from a theoretical value in use due to an instrument error or the like, and therefore, the use of "vicinity" indicates that various types of wavelengths defined in the present invention include an instrument error or the like.
In the present specification, the term "acrylate" includes the meanings of "acrylate" and "(meth) acrylate"; the term "acrylic" as used includes the meaning of "acrylic" as well as "(meth) acrylic".
In the present specification, "plural" in "plural", and the like means a numerical value of 2 or more unless otherwise specified.
In the present specification, the meaning of "may" includes both the meaning of performing a certain process and the meaning of not performing a certain process.
As used herein, the term "optional" or "optional" is used to indicate that certain substances, components, performance steps, application conditions, and the like are used or not used.
In the present specification, the unit names used are all international standard unit names, and the "%" used means weight or mass% content, if not specifically stated.
In the present specification, the term "particle diameter" as used herein means an "average particle diameter" unless otherwise specified, and can be measured by a commercial particle sizer.
In the present specification, reference to "some particular/preferred embodiments," "other particular/preferred embodiments," "embodiments," and the like, means that a particular element (e.g., feature, structure, property, and/or characteristic) described in connection with the embodiment is included in at least one embodiment described herein, and may or may not be present in other embodiments. In addition, it is to be understood that the described elements may be combined in any suitable manner in the various embodiments.
< first aspect >
In a first aspect of the invention, there is provided a photopolymer composition for use in a diffraction grating, particularly a reflective diffraction grating. The composition includes a writing monomer, a matrix monomer, and a photoinitiator system.
The writing monomer comprises an acrylate monomer and/or an epoxy compound with the refractive index of more than 1.55, and the matrix monomer comprises a fluorine-containing acrylate monomer with the refractive index of less than 1.5 and/or substituted or unsubstituted fatty acid vinyl ester.
In the invention, the writing monomer and the matrix monomer (under the condition of coherent light irradiation) are mixed and exposed, so that the phase separation is generated in a bright area and a dark area, and further, the refractive indexes of the bright area and the dark area generate periodic difference, namely, the refractive index modulation degree delta n of the grating is generated. In some embodiments of the present invention, the writing monomers are enriched in the bright region by irradiation of coherent light to polymerize/cure, while the matrix monomers migrate to the dark region to enrich, after exposure, the bright region obtains a higher refractive index and the dark region has a relatively lower refractive index, thereby giving the final grating a higher refractive index modulation Δ n.
Writing unit
In the present invention, the writing monomer is a monomer having a polymerization reaction activity, and the writing monomer suitable for use in the present invention has a refractive index of 1.55 or more, preferably 1.57 or more, and more preferably 1.58 or more.
In some embodiments of the present invention, the writing monomer may include an acrylate monomer, an epoxy compound, or a mixture thereof, and preferably, such writing monomer includes at least an acrylate monomer.
The acrylic monomer used may be an acrylic monomer having an aromatic group. The present invention recognizes that the aromatic group in the acrylate monomer is advantageous for increasing the refractive index, and in some preferred embodiments, the aromatic group is selected from one or more of phenyl, biphenyl, naphthyl, or fluorenyl.
Further, in some specific embodiments, among the acrylate monomers, the acrylate monomer having an aromatic group may be selected from the group consisting of: biphenyl-containing acrylates such as [1, 1-biphenyl ] -4, 4-diylbis (2-methacrylate), 4' -biphenyldiacrylate and the like; naphthalene-containing acrylates such as 1-naphthalene methacrylate, 2 '-bis (2-acryloyloxy) -1, 1' -thiobinaphthalene, 2 '-bis [2- (2-acryloyloxyethoxy) -1, 1' -binaphthalene, 2 '-bis [ 2-acryloyloxyethoxy) -1, 1' -thiobinaphthalene and the like.
In addition to having an aromatic group, the halogen may optionally be substituted with a halogen, including fluorine, chlorine or bromine, preferably bromine. Such acrylate monomers as p-chlorophenyl acrylate, p-bromophenyl acrylate, pentachlorophenyl acrylate, pentabromophenyl acrylate, 2,4, 6-tribromophenyl acrylate, 2,4, 6-trichlorophenyl acrylate and the like can be exemplified.
In addition, in some preferred embodiments of the present invention, the acrylate monomer suitable for use in the present invention may have a structure of the following general formula (I-1) or (I-2):
Ar-L-(X-O)n-C(O)-CH＝C(R1)2 (I-1)
Ar-L-(X-O)n-C(O)-C(CH3)＝C(R1)2 (I-2)
wherein Ar represents a group having one or more aromatic groups, preferably having 1 to 3 benzene rings, more preferably a phenyl groupNaphthyl or biphenyl, optionally these phenyl rings may be substituted or unsubstituted; l represents an oxygen atom or a sulfur atom; x represents a linear or branched alkyl group having 1 to 6 carbon atoms, preferably a linear or branched alkyl group having 2 to 3 carbon atoms, which may be optionally substituted; n represents an integer of 1 to 5, preferably an integer of 1 to 3; r1Each occurrence, which is the same or different, independently represents a hydrogen or halogen atom including a fluorine atom, a chlorine atom or a bromine atom.
In addition to the acrylic monomers having one polymerizable group disclosed above, in some other preferred embodiments of the present invention, acrylic monomers having two functional groups may be used, and these monomers may have the following general formula (II-1) or (II-2):
wherein R is1L, X is as defined in (I-1) and (I-2), n represents an integer of 1 to 5, preferably an integer of 1 to 3, Z represents a group containing one or more aromatic groups, preferably Z represents a substituted or unsubstituted phenyl or biphenyl group, the substitution may be of a halogen including fluorine, chlorine or bromine.
As a preferable mode for the respective embodiments, the acrylate monomer preferably used in the present invention may be selected from 9, 9-bis [4- (2-acryloyloxyethoxy) biphenyl ] fluorene, 9-bis [4- (2-hydroxy-3-acryloyloxypropyloxy) phenyl ] fluorene, 9-bis [4- (2-mercapto-3-acryloyloxypropyloxy) phenyl ] fluorene, [1, 1-biphenyl ] -4, 4-diylbis (2-methacrylate), 4 '-biphenyldiacrylate, 1-naphthalene methacrylate, 2' -bis (2-acryloyloxy) -1,1 '-thiobinaphthyl, 2' -bis [2- (2-acryloyloxyethoxy) -1, one or more of 1 ' -binaphthalene, 2 ' -bis [ 2-acryloyloxyethoxy) -1,1 ' -thiobinaphthalene, 2,4, 6-tribromophenyl acrylate and pentabromophenyl acrylate.
As the epoxy compound monomer suitable for the present invention, those having a relatively high refractive index (1.55 or more) are preferable. The use of such epoxy compounds is advantageous for mitigating the effects of dimensional shrinkage in the fabrication of gratings.
In the present invention, the epoxy compound that can be used may have a structure of the following general formula (III):
wherein E represents an epoxy group-containing group. In some specific embodiments, each E group may contain 1 or 2 epoxy groups. Further, from the viewpoint of suppressing the dimensional shrinkage after film formation, in a preferred embodiment, each E group contains 1 epoxy group at the time of its occurrence.
The structure of the epoxy group is not particularly limited, and the epoxy group is preferably present as an aliphatic epoxy group. In other embodiments, the epoxy group or epoxy structure of the E group is bonded to Ar as described above through an ether group1The groups are linked. The ether group may be a sulfide group or an oxygen ether group, and is preferably an oxygen ether group from the viewpoint of suppressing the dimensional shrinkage after film formation.
In the general formula (III), n representing the number of E groups is an integer of 0 to 4, and each E group is the same or different. It goes without saying that the total number of n in the present invention is not 0. In some preferred embodiments, each occurrence of n is 1.
In the above general formula (III), each Ar1The same or different, independently represent an aryl-containing group. In some preferred embodiments of the invention, Ar1Represents a group having 1 or two substituted or unsubstituted benzene rings, typically Ar1May be selected from the following structures:
wherein X in the formula (b) is selected from a single bond, O or S atom.
In the above general formula (III), -CR3R4-formation of a carbonyl group, or, R3、R4The same or different, each occurrence independently represents a hydrogen atom, an alkyl group having 1 to 10 carbon atoms, an alkoxy group or an aryl group having 6 to 30 carbon atoms, and R3、R4May be connected via a single bond; preferably an alkyl group, an alkoxy group or a phenyl group having 1 to 3 carbon atoms.
In some preferred embodiments of the present invention, the epoxy compound suitable for use in the present invention, for example, 9-bis (4-epoxypropyloxyphenyl) fluorene or has a structure represented by the following general formula (IV):
wherein R is3And R4Have the same definition as in formula (III).
R5Each occurrence is the same or different and is independently selected from hydrogen, halogen and alkyl with 1-5 carbon atoms; preferably 1 to 3 alkyl groups, and x is 0 to 4, preferably an integer of 0 or 1. The halogen may be F, Cl or a Br atom.
In a further preferred embodiment, the epoxy compound suitable for use in the present invention has a structure represented by the following general formulae (IV-1) to (IV-3):
in the present invention, one kind of the epoxy compound may be used, or a mixture of two or more kinds of the epoxy compounds may be used.
The epoxy compounds suitable for use in the present invention can be obtained by a preparation method generally used in the art, and in a typical embodiment, can be carried out using a coupling reaction of epichlorohydrin with a phenolic compound:
in some preferred embodiments of the present invention, it is also advantageous to use an acrylate monomer having a plurality of (three or more) functional groups in addition to the acrylate monomer having a high refractive index and the epoxy compound monomer to increase the crosslinking density at the time of exposure/curing. Generally, such monomers may have a refractive index of 1.42 to 1.5. Further, such acrylate monomers may be exemplified by one or more of pentaerythritol triacrylate, pentaerythritol tetraacrylate, dipentaerythritol penta/hexa-acrylate, and polyester acrylate oligomers.
In some embodiments of the present invention, the acrylate monomer having three or more functional groups is used in an amount of 50% or less, preferably 10 to 45%, and more preferably 20 to 40%, based on the total weight of the writing monomers.
In the present invention, the writing monomer may be used in an amount of 30 to 60%, preferably 35 to 55%, and more preferably 40 to 50%, based on the total weight of the photopolymer composition of the present invention. In addition, in the present invention, the writing monomer may contain a polymerization-active monomer having one, two or more (three or more) functional groups as described above, and the average functional group number (the number of functional groups per molecule on average) of the writing monomer of the present invention is controlled to be more than 2, for example, 2.1 or more, 2.3 or more, 2.5 or more, and the like, from the viewpoint of producing the reflection type diffraction grating.
Matrix monomer
In the present invention, the matrix monomer that can be used is selected from monomers having a low refractive index. In some specific embodiments, the refractive index of such a matrix monomer is 1.50 or less, preferably 1.48 or less, more preferably 1.47, and even more preferably 1.45 or less.
In some preferred embodiments, the matrix monomers suitable for use in the present invention may include fluoroacrylate monomers, and/or substituted or unsubstituted vinyl esters of fatty acids, from the standpoint of facilitating phase separation from the writing monomer.
As the fluorine-containing acrylic ester monomer, one or more of C1 to C10 alkyl esters having fluorine-substituted acrylic acid, preferably one or more of C1 to C6 alkyl esters having fluorine-substituted acrylic acid, may be cited.
In some preferred embodiments, the fluorine-containing acrylate monomer may be an alkyl acrylate having a perfluoro substitution. Examples of such monomers are 1,1,1,3,3, 3-hexafluoroisopropyl acrylate (n ═ 1.319), octafluoropentyl acrylate (n ═ 1.349), 1H, 2H-perfluorooctanol acrylate (n ═ 1.338), 2,3,3, 3-pentafluoropropyl acrylate (n ═ 1.336), hexafluorobutyl methacrylate (n ═ 1.361), hexafluorobutyl acrylate (n ═ 1.352), 2,3,3,4,4, 4-heptafluoro-butyl methacrylate (n ═ 1.341), hexafluoroisopropyl methacrylate (n ═ 1.331) or heptafluorobutyl acrylate (n ═ 1.331).
For substituted or unsubstituted fatty acid vinyl esters, fatty acid vinyl ester monomers with or without halogen substitution can be used. In some specific embodiments, the number of carbon atoms of the fatty acid moiety is 2 to 25, preferably 4 to 17. Examples of such monomers include vinyl acetate (n ═ 1.395), vinyl propionate (n ═ 1.403), vinyl n-butyrate (n ═ 1.410), vinyl valerate (n ═ 1.417), vinyl n-hexanoate (n ═ 1.421), vinyl 2-ethylhexanoate (n ═ 1.426), vinyl octanoate (n ═ 1.429), vinyl neononanoate (n ═ 1.441), vinyl decanoate (n ═ 1.435), vinyl neodecanoate (n ═ 1.436), vinyl laurate (11C chain, n ═ 1.441), vinyl myristate (13C chain, n ═ 1.443-445), vinyl palmitate (15C chain) and vinyl stearate (17C chain, n ═ 1.442).
In the present invention, the content of each of the fluorine-containing acrylate monomer and the substituted or unsubstituted fatty acid vinyl ester in the matrix monomer is not particularly limited, and in some specific embodiments, the content of the substituted or unsubstituted fatty acid vinyl ester is 50 to 65% by mass of the total mass of the matrix monomer.
In the present invention, the total amount of the matrix monomer may be 20 to 50%, preferably 30 to 48%, and more preferably 35 to 45%, based on the total weight of the photopolymer composition of the present invention.
Other polymerizable Components
In the present invention, other optional polymerizable components may be used in the photopolymer composition in addition to the writing monomers and the matrix monomers described above without affecting the technical effects of the present invention.
These other optional polymerizable ingredients may include mono-and difunctional acrylates, mono-and difunctional urethane acrylates, specifically:
other acrylates that may be used are, for example, methyl acrylate, methyl methacrylate, ethyl acrylate, ethyl methacrylate, ethoxyethyl acrylate, ethoxyethyl methacrylate, N-butyl acrylate, N-butyl methacrylate, t-butyl acrylate, t-butyl methacrylate, hexyl acrylate, hexyl methacrylate, 2-ethylhexyl acrylate, 2-ethylhexyl methacrylate, butoxyethyl acrylate, butoxyethyl methacrylate, dodecyl acrylate, dodecyl methacrylate, isobornyl acrylate, isobornyl methacrylate, phenyl acrylate, N-carbazole acrylate, and the like.
Other urethane acrylates which may be used are understood to mean compounds having at least one acrylate group which have at least one urethane bond. Such compounds are known to be obtainable by reacting hydroxy-functional acrylates with isocyanate-functional compounds.
For this purpose, isocyanate-functional compounds such as aromatic, araliphatic, aliphatic and cycloaliphatic diisocyanates can be used. Mixtures of such diisocyanates may also be used. Suitable di-, tri-or polyisocyanates are, for example, butylidene isocyanate, Hexamethylene Diisocyanate (HDI), isophorone diisocyanate (IPDI), 2, 4-and/or 2,4, 4-trimethylhexamethylene diisocyanate, bis (4,4' -isocyanatocyclohexyl) methane isomers and mixtures thereof having any desired isomer content, isocyanatomethyl-1, 8-octane diisocyanate, 1, 4-cyclohexyl diisocyanate, cyclohexanedimethylene diisocyanate isomers, 1, 4-phenylene diisocyanate, 2, 4-and/or 2, 6-toluene diisocyanate, 1, 5-naphthalene diisocyanate, 2,4' -or 4,4' -diphenylmethane diisocyanate, 1, 5-naphthalene diisocyanate, m-methylthiophenyl isocyanate or derivatives thereof having a urethane, urea, carbodiimide, acylurea, isocyanurate, allophanate, biuret, oxadiazinetrione, uretdione or iminooxadiazinedione structure and mixtures thereof. Aromatic or araliphatic diisocyanates are preferred.
Hydroxy-functional acrylates or methacrylates suitable for preparing the abovementioned urethane acrylates are the following compounds: 2-hydroxyethyl (meth) acrylate, polyethylene oxide mono (meth) acrylate, polypropylene oxide mono (meth) acrylate, polyhexamethylene oxide mono (meth) acrylate, poly (. epsilon. -caprolactone) mono (meth) acrylate, for exampleM100(Dow, Schwalbach, Germany), 2-hydroxypropyl (meth) acrylate, 4-hydroxybutyl (meth) acrylate, 3-hydroxy-2, 2-dimethylpropyl (meth) acrylate, hydroxypropyl (meth) acrylate, 2-hydroxy-3-phenoxypropyl acrylate, hydroxy-functional mono-, di-or tetraacrylates of polyols such as trimethylolpropane, glycerol, pentaerythritol, dipentaerythritol, ethoxylated, propoxylated or alkoxylated trimethylolpropane, glycerol, pentaerythritol, dipentaerythritol or industrial mixtures thereof. 2-hydroxyethyl acrylate, hydroxypropyl acrylate, 4-hydroxybutyl acrylate and poly (. epsilon. -caprolactone) mono (meth) acrylate are preferred.
These other polymerizable components are contained in an amount of 15% or less, preferably 10% or less, and more preferably 5% or less, based on the total weight of the photopolymer composition of the present invention.
Photoinitiator system
In the present invention, the photo-initiation system includes a photosensitive dye compound and a co-initiator, and thus, the photo-initiator system may be a two-component system or a three-component system. The two-component system is a combined system of a dye compound and a hydrogen donor coinitiator, and the three-component system is a combined system of the dye compound, the hydrogen donor coinitiator and a hydrogen acceptor coinitiator.
The photosensitive dye compound is a dye compound having an excitation activity in the visible light range, and suitable dyes are, for example, Irgacure 784, new methylene blue, thionine, basic red 2, basic red 94, basic yellow, basic violet 4, pinacyanol chloride, rhodamine B, betacyanine, ethyl violet, victoria blue R, azurite blue, quinaldine red, crystal violet, brilliant green, basic orange 21, dalong (darow red), pyronine Y, rose bengal, potato red Y, mikrolone, 3.3' -carbonylbis (7-diethylaminocoumarin), diiodofluorescein, anthocyanin and methylene blue, tiana, crystal violet (leuconitrile), malachite green (leuconitrile), or the like.
Preferred hydrogen donor coinitiators are at least one selected from the group consisting of N-phenylglycine, 2, 6-diisopropyl-N, N-dimethylaniline, 2- (4-methoxyphenyl) -4, 6-bis (trichloromethyl) -S-triazine.
A preferred hydrogen acceptor co-initiator is bis (4-tert-butylphenyl) iodonium hexafluorophosphate.
In the present invention, the content of the photo-initiation system component is 0.1 to 3%, preferably 0.5 to 2%, based on the total weight of the photo-induced polymer composition.
Plasticizer
In the present invention, a plasticizer is used to increase the flexibility of the photopolymer composition and to alleviate the degree of dimensional shrinkage that occurs after film formation and curing.
In some particular embodiments, plasticizers suitable for use in the present invention are polymeric materials with good compatibility/dissolution characteristics, low volatility, and high boiling point. Typically, these polymeric materials may be polymeric polyols or glycidyl ethers of polymeric polyols. From the viewpoint of suppressing dimensional shrinkage, in a preferred embodiment of the present invention, the polymeric polyol may be polyethylene glycol, polypropylene glycol, or the like; the glycidyl ether of the polyhydric alcohol can be polyethylene glycol diglycidyl ether and polypropylene glycol diglycidyl ether.
For the plasticizer of the present invention, one kind or a combination of two or more kinds may be used.
In the present invention, the plasticizer is contained in an amount of 0.5 to 20%, preferably 1 to 15%, and more preferably 2 to 10% based on the total weight of the photopolymer composition.
Other ingredients
In the present invention, other components commonly used in the art may be used according to actual production needs as long as the technical effects of the present invention are not affected, and the components include: solvents, levelling agents, wetting agents, defoamers or tackifiers, as well as polyurethanes, thermoplastic polymers, oligomers, compounds having additional functional groups (e.g. acetals, epoxides, oxetanes, oxazolines, dioxolanes) and/or compounds having hydrophilic groups (e.g. salts and/or polyethylene oxides), can be used as additional auxiliaries and additives.
In some embodiments of the invention, the optional solvent is a volatile solvent that has good compatibility with the components of the invention, such as ethyl acetate, butyl acetate, and/or acetone. It is noted that the use of a solvent is generally believed to result in a significant dimensional shrinking effect, and therefore, in a preferred embodiment of the invention, no solvent is used.
In addition, some of the reported prior arts believe that the use of a film-forming component is advantageous for improving the diffraction efficiency of the grating and the degree of modulation of the refractive index. Unlike the prior art, the present invention is believed that the photopolymer composition does not contain a film-forming component, thereby minimizing the effect on the movement of writing monomers and substrate monomers during exposure and providing the resulting grating with a high refractive index modulation. It goes without saying that, for any processing requirements, small amounts (less than 15%, preferably less than 10%, more preferably less than 5% of film-forming components, based on the total weight of the photoresist composition) of film-forming agents may also be used under certain conditions, provided that the achievement of the desired high refractive index modulation according to the invention is not impaired.
These film-forming components, which may be used, may be generally selected from polymers or resinous materials having a molecular weight of 1000 or more with some adhesion. Preferably, these materials have a low refractive index, and in particular embodiments, the refractive index of these materials is 1.480 or less, preferably 1.475 or less, and more preferably 1.470 or less.
Specifically, suitable film-forming components include:
homopolymers of vinyl acetate or copolymers of vinyl acetate with acrylates, ethylene, styrene, etc.;
cellulose esters such as cellulose acetate, cellulose acetate-butyrate;
cellulose ethers such as methyl cellulose, ethyl cellulose, and benzyl cellulose, and the like;
polyvinyl alcohol;
polyurethanes, generally obtained by reacting polyols such as polytetrahydrofuran, polyethylene glycol, polypropylene glycol, castor oil, and isocyanates such as hexamethylene-1, 6-diisocyanate, 1, 4-cyclohexane diisocyanate, methyl-2, 4-diisocyanate;
polyvinyl acetals such as polyvinyl butyral, polyvinyl formal and the like;
styrene/butadiene-based block copolymers;
polyvinylpyrrolidone, and the like.
The preferred film-forming component of the present invention is selected from at least one of cellulose acetate butyrate, polyvinylpyrrolidone, polyvinyl alcohol, and polyvinyl acetate from the viewpoint of suppressing dimensional shrinkage of the final grating product, and improving diffraction efficiency and refractive modulation.
In some embodiments, where a film-forming component is used,advantageously (e.g., increased diffraction efficiency) the difference in refractive index (n) between the writing monomer and the film-forming component of the present inventionPolymerizing reactive monomers-nFilm-forming component) The value is 0.075 or more, preferably 0.078 or more, further preferably 0.080 or more, for example 0.085 or more, 0.090 or more, 0.100 or more.
< second aspect >
In a second aspect of the present invention, there is provided a reflective diffraction grating based on the photopolymer composition according to the above < first aspect >, and a method for preparing the same.
The grating includes a carrier layer and a polymer film layer. The carrier substrate used may preferably be a layer of a material or a composite of materials that is transparent in the visible spectrum (greater than 85% transmittance in the wavelength range 400-780 nm).
Preferred materials or material composites for the carrier substrate are based on Polycarbonate (PC), polyethylene terephthalate (PET), polybutylene terephthalate, polyethylene, polypropylene, cellulose acetate, cellulose hydrate, cellulose nitrate, cycloolefin polymers, polystyrene, polyepoxides, polysulfones, Cellulose Triacetate (CTA), polyamides, polymethyl methacrylate, polyvinyl chloride, polyvinyl butyral or polydicyclopentadiene or mixtures thereof. They are more preferably based on PC, PET and CTA. The material composite may be a foil laminate or a coextrusion. Preferred material composites are double or triple foils constructed according to one of the embodiments A/B, A/B/A or A/B/C. PC/PET, PET/PC/PET and PC/TPU (TPU ═ thermoplastic polyurethane) are particularly preferred.
As an alternative to the aforementioned carrier substrates, it is also possible to use flat glass plates, which are used in particular for the precise imagewise exposure of large areas, for example for holographic lithography (holographic interference lithography for integrated optics, IEEE Transactions on Electron Devices (1978), ED-25(10), 1193-.
Additionally, in some embodiments of the invention, the material or material composite of the carrier substrate may have a release, antistatic, hydrophobic or hydrophilic finish on one or both sides. The modification mentioned serves the purpose of enabling the non-destructive removal of the photopolymer from the carrier substrate on the side in contact with the photopolymer composition. The modification of the side of the carrier substrate facing away from the photopolymer composition serves to ensure that the media according to the invention meet the specific mechanical requirements, for example in the case of processing in a roll laminator, in particular in the roll-to-roll process. The carrier substrate may have a coating on one side or a coating on both sides.
The thickness of the carrier substrate suitable for use in the present invention may be 1.5mm or less, preferably 20 μm to 1mm, and more preferably 100 μm to 900 μm.
In some specific embodiments of the present invention, the grating may be a laminate of a film formed of a photopolymer composition and the carrier, i.e., the film is formed on the carrier, or the film is sandwiched between two carriers. Therefore, the film formed of the photopolymer in the present invention has a grating structure by exposure, bleaching, or the like, and it may be present as a hologram recording medium on the support or sandwiched between the supports. In other cases, the grating may additionally comprise a cover layer and/or other functional layers, each optionally at least partially associated with the film.
In the present invention, the method for preparing a grating by using the photopolymer composition, the carrier, and the like may include the steps of:
(i) a mixing step, namely mixing all the components of the photopolymer composition to obtain a mixture;
(ii) a step of forming a grating structure by forming a film of the mixture and forming a grating structure on at least a part of the film,
wherein, in the step of forming the grating, a step of exposing the film with coherent light is included, and in some preferred embodiments of the present invention, the coherent light is coherent light having a wavelength around 532 nm.
(i) Step (ii) of
In the present invention, a mixture is obtained by mixing the components of the photopolymer composition.
The compositions are mixed in proportions in a suitable container and, if desired, mechanically agitated or the like to provide uniform mixing. The temperature of mixing is not particularly limited, and in general, mixing under ambient or heated ambient conditions can be selected (in particular, if a crosslinkable polymeric matrix or film-forming ingredient is used, heating can be used to cause the photocurable composition to form a liquid or liquid).
In some preferred embodiments of the invention, the blend of the components of the photopolymer composition of the invention takes on a liquid form (e.g., the liquid matrix monomer acts in part as a solvent) which is advantageous for the migration of the writing monomer and the matrix monomer during exposure. The resulting liquid mixture can be used immediately or stored briefly at the processing temperature for use.
(ii) Step (ii) of
In this step, a film is formed on a support by using the liquid mixture obtained above, and subjected to an exposure process to obtain a polymer film having a grating structure. In some specific embodiments of the present invention, the polymer film has a thickness of 15 μm or more, preferably 20 μm or more, and further, the polymer film has a thickness of 50 μm or less, preferably 40 μm or less. The above-mentioned thickness of the polymer film can be coordinated or matched in practice with the use of spacers, for example as described below.
As for the material of the support, the same definition as < first aspect > described above, and in a preferred embodiment, glass may be used as the support. Optionally, the carrier glass sheet is cleaned, dried, etc. prior to use.
In the present invention, the grating structure is formed on at least a portion of the photopolymer film by exposure to light, during which exposure to coherent light can be used to control the microstructure.
In addition, in a preferred embodiment of the present invention, the use of spacers in the polymer film is advantageous for process control from the viewpoint of controlling the thickness of the polymer film, suppressing the dimensional shrinkage of the grating, and maintaining high diffraction efficiency, particularly in the case where two carrier layers sandwich one polymer film.
For spacers, in some particular embodiments of the invention, particles that are substantially opaque to visible light may be used. These particles may be inorganic particles, organic particles or metallic particles. The inorganic particles are preferably used in the present invention from the viewpoint of suppressing the dimensional shrinkage of the grating and the production cost.
The kind of the inorganic particles is not particularly limited, and for example, silica, titania, or the like can be used. In some specific embodiments, the inorganic particles have a substantially spherical steric shape; in other specific embodiments, the inorganic particles have an average particle size of 2 to 50 μm, preferably 3 to 40 μm, and the particle size of the spacer may be coordinated, selected or determined with the thickness of the photopolymer film being formed.
As for the method of using the spacer, in the present invention, the spacer may be previously formed on the surface of the support, and this process may be carried out by a method of coating a dispersion system containing the spacer. In some embodiments, the spacer may be dispersed in a hydrocarbon, alcohol, or ketone solvent, for example, to form a dispersion. For these solvents, it is preferable to use a substance having a relatively low boiling point, and examples of the solvent include one or more of benzene, toluene, cyclohexane, pentane, ethanol, isopropanol, acetone, methyl ethyl ketone, and the like. The dried spacer particles (powder) may be directly dispersed in these solvents, or a sol-like substance formed by the spacers may be dispersed in these solvents.
The concentration of the spacer-containing dispersion system may be 0.1 to 3mg/mL, preferably 0.1 to 0.3mg/mL in some specific embodiments of the present invention, and too high a concentration deteriorates the uniformity of dispersion, resulting in a decrease in diffraction efficiency of the grating.
In the present invention, the spacer can be uniformly applied to the surface of the support by a coating method, and the coating method is not particularly limited, and can be performed by a spray coating or spin coating method. After the spacer is formed on the surface of the support by a coating method, the solvent may be removed by heating, blowing, or the like.
Further, the liquid mixture obtained in the step (i) is formed into a film on the surface of the support having the spacer. For example, flat, onto a carrier substrate, in which case, for example, devices known to the person skilled in the art can be used, such as knife coating devices (doctor blades, knife rolls, curved bars (Commabar), etc.) or slit nozzles, etc. Optionally, if desired, a degassing step is carried out after the coating of the film, in order to eliminate possible bubbles in the film. After coating, a photopolymer film can be obtained by cooling or the like.
In the present invention, the above-described photopolymer film that can be used as a holographic medium can be processed into holograms by suitable exposure operations for various optical applications. Visual holograms include all holograms which can be recorded by methods known to those skilled in the art.
In some preferred embodiments of the present invention, the exposure treatment of the photopolymer film can be performed with two beams of coherent light. The source of the coherent light is not particularly limited, and in some specific embodiments of the present invention, the green (around 532 nm) laser beam may be split into two coherent light beams of the same or different intensities by an optical element, and the resulting photopolymer film may be simultaneously exposed.
By using coherent light exposure, it is possible to present spaced bright and dark regions in the photopolymer film (the two beams of coherent light produce alternating bright and dark stripes in the photopolymer film). The writing monomer migrates and enriches to the bright areas and polymerizes under the action of the initiator, while the matrix monomer migrates to the dark areas. Therefore, a refractive index difference Δ n (refractive index modulation degree) is formed between the bright area and the dark area. With respect to the exposure intensity, it may be 0.1 to 30mJ/cm in some embodiments of the present invention2It can be seen that the exposure sensitivity is high due to the present invention.
In some embodiments of the present invention, two beams of coherent light can be exposed simultaneously from both sides of the polymer film (reflective diffraction grating), and the grating period can be adjusted according to the incident angle of the two beams of coherent light (i.e. the angle between the incident light and the normal direction of the polymer film). The incident angle is not particularly limited, and may be adjusted within a range of 0 to 90 °, and in a preferred embodiment, the incident angles of the two coherent light beams are kept the same.
After exposure, refractive index distribution which is distributed in a sine function is formed in the photopolymer film, and the diffraction grating is obtained. The difference between the sinusoidal peaks, i.e., Δ n (degree of refractive index modulation). In some specific embodiments of the invention, Δ n can be 0.030 or more, such as 0.035 or more, 0.040 or more 0.045 or more, 0.050 or more, 0.055 or more, 0.060 or more 0.065 or more, and the like.
The grating prepared according to the present invention has a diffraction efficiency of 70% or more, preferably 80% or more, and more preferably 90% or more.
For example, in fig. 2, a typical exposure light path (recording light path) of the present invention is shown. The visible light laser is divided into two laser beams with the same or different intensities after beam splitting, and the two laser beams are respectively reflected and converged on the photopolymer film through a reflector (the incidence angles are respectively alpha and beta) to generate interference fringes.
After exposure, a holographic diffraction spectrum is formed in the photopolymer film, and then the color of the unexposed areas is removed after illumination by, for example, an LED lamp, to obtain the final reflective diffraction grating comprising the photopolymer film.
In addition, the grating obtained by the invention can be a plane grating or a curved grating with a certain curvature. The method for producing the curved grating is not particularly limited, and in some specific embodiments, a film may be formed on a substrate having a curvature by using the substrate and exposing the substrate. In other embodiments, a planar substrate may be used, and the coating film may be processed into a curved grating with a certain curvature after exposure.
< third aspect >
In a third aspect of the invention, the use of the grating obtained as described above in the invention is disclosed. Without limitation, the above-described gratings comprising photopolymer films of the present invention can be used in a variety of holographic display systems in the art, and can be used alone or in combination with other optical elements.
Further, the present invention provides a grating element for a holographic optical waveguide display system. The element comprises a carrier layer and a photopolymer film layer comprising spacers. The carrier layer, the photopolymer film layer and the spacer are as described or defined in < first aspect > and < second aspect > of the present invention above.
In some preferred embodiments, the grating element is formed by sandwiching a layer of photopolymer film between two carrier layers.
Typically, the grating elements have a regular shape to facilitate use and installation, and may be in the shape of a strip, a square, or a circular plate.
In some preferred embodiments, the grating element of the present invention has an elliptical or elongated sheet shape, and has exposure regions subjected to exposure or the like at both end regions in the length direction, and a grating (holographic recording) structure is formed in each exposure region. And the two exposure areas are physically unconnected. Typically, one exposure area may be used as an incoupling grating area, and the other exposure area may be used as an outcoupling grating area.
The grating element of the present invention can be used in a holographic optical waveguide display device, and is particularly suitable for Augmented Reality (AR) head display devices such as AR display glasses devices and the like.
Examples
Hereinafter, the present invention will be described by way of specific examples.
Example 1
Adding tribromophenyl 2,4, 6-acrylate into a mixed solution of hexafluoroisopropyl methacrylate, vinyl octanoate, pentaerythritol triacrylate and N-vinyl pyrrolidone, stirring uniformly, adding diiodofluorescein and 2, 6-diisopropyl-N, N-dimethylaniline, stirring magnetically for 5min, performing ultrasonic treatment for 30min to uniformly mix the components, defoaming in vacuum, and storing in the dark for later use. The contents of the components are shown in the following table 1:
table 1:
components
Weight (D)
Hexafluoroisopropyl methacrylate
0.20g
Vinyl octanoate ester
0.25g
9, 9-bis [4- (2-acryloyloxyethoxy) biphenyl]Fluorene compounds
0.35g
Pentaerythritol tetraacrylate
0.10g
N-vinyl pyrrolidone
0.05g
Diiodofluorescein
0.002g
2, 6-diisopropyl-N, N-dimethylaniline
0.005g
Injecting the mixture into the gap between the upper and lower sheets of glass, wherein the gap has a thickness of 10 μmAnd controlling the silica microspheres to obtain a photopolymer sample. The sample was placed in the light path shown in FIG. 2 at a laser wavelength of 532nm and an exposure intensity of 10mW/cm2The incident light is of a symmetrical reflection type, and α is 26.5 °, β is 26.5 °, and the exposure time is 30 s. And after exposure, placing the sample under an LED lamp for irradiating for 5min to obtain the reflective holographic volume grating. The diffraction efficiency of the grating can reach 97% at most according to the characteristic curve of the change of the Bragg deviation angle shown in figure 3, the angle selectivity is +/-12 degrees, and the delta n is 0.07.
Example 2
Adding pentabromophenyl acrylate into a mixed solution of hexafluoroisopropyl methacrylate, vinyl caprylate, dipentaerythritol penta-/hexyl-acrylic acid and N-vinyl pyrrolidone, uniformly stirring, adding diiodofluorescein and 2, 6-diisopropyl-N, N-dimethylaniline, magnetically stirring for 5min, performing ultrasonic treatment for 30min to uniformly mix the components, defoaming in vacuum, and storing in the dark for later use. The contents of the components are shown in the following table 2:
TABLE 2
Components
Weight (D)
Hexafluoroisopropyl methacrylate
0.21g
Vinyl octanoate ester
0.28g
9, 9-bis [4- (2-mercapto-3-acryloyloxypropyl) phenyl]Fluorene compounds
0.35g
Dipentaerythritol penta/hexa-acrylic acid
0.15g
N-vinyl pyrrolidone
0.05g
Diiodofluorescein
0.002g
2, 6-diisopropyl-N, N-dimethylaniline
0.005g
And (3) injecting the mixed solution into a gap between the upper glass sheet and the lower glass sheet, controlling the thickness of the gap by using silicon dioxide microspheres with the diameter of 10 mu m, and then sealing the periphery by using AB glue to obtain a photopolymer sample. The sample was placed in the light path shown in FIG. 1 at a laser wavelength of 532nm and an exposure intensity of 10mW/cm2The incident light is of a symmetrical reflection type, and α is 26.5 °, β is 26.5 °, and the exposure time is 30 s. And after exposure, placing the sample under an LED lamp for irradiating for 5min to obtain the reflective holographic volume grating, wherein delta n is 0.06.
It should be noted that, although the technical solutions of the present invention are described by specific examples, those skilled in the art can understand that the present disclosure should not be limited thereto.
Having described embodiments of the present disclosure, the foregoing description is intended to be exemplary, not exhaustive, and not limited to the disclosed embodiments. Many modifications and variations will be apparent to those of ordinary skill in the art without departing from the scope and spirit of the described embodiments. The terminology used herein is chosen in order to best explain the principles of the embodiments, the practical application, or improvements made to the technology in the marketplace, or to enable others of ordinary skill in the art to understand the embodiments disclosed herein.
Industrial applicability
The photopolymer composition of the present invention can be used industrially to produce reflective diffraction gratings.