Photopolymer system, grating device containing chirped structure and method for manufacturing the same

Abstract

The application provides a photopolymer system, a grating device containing a chirp structure and a preparation method of the grating device, wherein a visible light and ultraviolet light dual initiator and an azobenzene polymerizable acrylate monomer are introduced into the photopolymer system, the acrylate group can be used as a writing monomer to participate in a copolymerization process, the azobenzene group can continuously photoisomerize during holographic exposure, the system is driven to react and crosslink by the visible light initiator and the ultraviolet light initiator in turn, the photopolymer system is recorded with interference fringes at the same time, the chirp structure is induced to be generated inside the formed volume holographic grating, the change of the grating period caused by material shrinkage is effectively compensated, the angle response bandwidth of the grating is significantly widened, the demand for the refractive index modulation of the photopolymer system is reduced, the selection range of the photopolymer system is widened, meanwhile, the aging resistance of the material system as a whole is improved, and the service life of the grating device is prolonged.

Description

Technical Field

[0001] This invention relates to the field of holographic projection display, and more specifically, to a photopolymer system, a chirped grating device, and a method for fabricating the same. Background Technology

[0002] With the rapid development of the intelligent vehicle and information security industries, photopolymers are ushering in new development opportunities. Leveraging their mature process systems and mass production stability, photopolymer materials have found wide application in scenarios such as automotive head-up display (HUD) systems and high-end anti-counterfeiting labels. In these applications, the optoelectronic functions are typically achieved through photopolymer holographic gratings. The fabrication of holographic gratings essentially involves the precise construction of micro- and nano-structures through laser interference. When two coherent laser beams with specific polarization states meet, periodic bright and dark interference fringes are formed in space. When using a photopolymer system to fabricate a holographic grating, the bright areas undergo monomer polymerization reactions triggered by illumination, while the dark areas remain unpolymerized. The resulting monomer concentration gradient drives the formation of a periodic refractive index distribution within the material. Therefore, the quality of the final phase-separated structure formed by the photopolymer system directly determines the diffraction performance of the holographic grating.

[0003] Specifically, the refractive index modulation of a holographic grating directly determines key parameters such as its diffraction efficiency. Optimizing the refractive index modulation relies primarily on the synergistic control of two key parameters: the inherent refractive index difference (Δn) between the writing monomer and the film-forming matrix, and the degree of phase separation during photopolymerization. In existing technologies, while using a high-refractive-index writing monomer paired with a low-refractive-index resin can enhance refractive index contrast to some extent, in actual material systems, the degree of polymerization, diffusion ability, and compatibility of the monomer with the resin all affect the final phase separation effect, making it difficult to achieve the ideal theoretical extreme value for refractive index modulation. Therefore, the main approach to improving the refractive index modulation focuses on controlling the refractive index matching and phase separation behavior between the writing monomer and the film-forming resin. Furthermore, volume holographic gratings often experience a certain degree of material shrinkage after exposure, causing changes in the grating period and affecting its Bragg condition, resulting in angular selectivity shifts. Traditionally, exposure angle compensation and other methods are used to alleviate this problem, but these methods cannot fundamentally expand the angular bandwidth of the device. Therefore, relying solely on adjusting phase separation or simply adjusting process parameters is still significantly insufficient to meet the demand for synergistic improvement of high refractive index modulation and wide-angle bandwidth.

[0004] Therefore, it is necessary to develop a photopolymer material system that can take into account both high refractive index modulation and wide angular response characteristics in order to prepare volume holographic gratings with better angular bandwidth and diffraction efficiency, so as to achieve a larger field of view and higher image quality in optical waveguide devices such as augmented reality and virtual reality. Summary of the Invention

[0005] The present invention aims to overcome at least one of the defects of the prior art and provides a photopolymer system, a chirped grating device and a method for fabricating the same.

[0006] The technical solution adopted by this invention firstly provides a photopolymer system, comprising, by mass percentage:

[0007] Solvent: 1%-5%,

[0008] Visible light photoinitiator: 0.01-1%,

[0009] UV initiator: 0.01-1%,

[0010] Co-initiator: 0.1-1%,

[0011] Writing individual units: 20-50%,

[0012] Azobenzene polymerizable acrylate monomers: 0.1-5%,

[0013] Film-forming resin: 20-30%,

[0014] Surfactant: 0-1%

[0015] Catalyst: 0.0001-0.01%,

[0016] The writing monomer is an acrylic writing monomer;

[0017] The azobenzene polymerizable acrylate monomers have both azophenyl groups and acrylate groups.

[0018] In this technical solution, a dual initiation system of visible light and ultraviolet light is introduced into the photopolymer system, and the synergistic ratio range of azobenzene polymerizable monomers, various acrylate writing monomers, and film-forming resins is precisely configured. The visible light initiator triggers the photoisomerization of azobenzene monomers and the selective polymerization of writing monomers during the holographic exposure stage to initially form a refractive index modulation and chirped gradient structure. Then, the deep curing reaction initiated by the ultraviolet light initiator further crosslinks and fixes the formed dynamic structure and unreacted components. Thus, while effectively constructing and stabilizing the chirped structure inside the grating, the final refractive index modulation of the system and the thermodynamic stability of the grating structure are significantly improved, and the angular response bandwidth and environmental tolerance of the volume holographic grating are further improved. Specifically, azobenzene polymerizable acrylate monomers with azophenyl and acrylate groups are introduced into the photopolymer system. Utilizing the properties of the acrylate groups in these monomers—which can act as writing monomers participating in the copolymerization process with the film-forming resin to form a polymer network, and whose azophenyl groups can undergo continuous photoisomerization during holographic exposure—the system's crosslinking process is driven sequentially by visible light photoinitiators and ultraviolet light photoinitiators. This allows the photopolymer system to record interference fringes while simultaneously experiencing reversible changes in the azobenzene molecular configuration and molecular redistribution due to local light intensity differences, thereby actively modulating the polymerization process. By combining dynamics and perturbing the diffusion process of monomers, a stable chirped structure is induced within the formed volumetric holographic grating. This effectively compensates for the grating period variation caused by material shrinkage simply by modifying the morphology of the Bragg grating, significantly broadening the angular response bandwidth of the grating. This is beneficial for fabricating large-field-of-view optical waveguide elements. Simultaneously, it avoids excessive pursuit of high-refractive-index material systems, reducing the need for refractive index modulation of photopolymer systems. This greatly expands the selection range of photopolymer systems, thus conveniently realizing high refractive index modulation and wide-angle response characteristics of grating devices based on photopolymer systems. Furthermore, by introducing azophenyl groups into the structure of the fabricated holographic grating device, the aging resistance of the material is improved, which is beneficial to the stability of the holographic grating device product and greatly increases its service life.

[0019] Furthermore, the writing monomer is selected from one or more of OPPEA, PETA, N-vinylcarbazole, PET4A, and TMPTA;

[0020] The film-forming resin is formed by cross-linking and polymerization of one or more of PPG-200, PPG-400, PPG-600, PPG-1000, PEG-200, PEG-400, and PEG-600, as well as one or more of N3390, XDI, N-3900, N3200, and N3600.

[0021] In this technical solution, one or more high-refractive-index monomers such as OPPEA, PETA, N-vinylcarbazole, PET4A, and TMPT are selected as writing monomers. These monomers are then combined with flexible polyols of various molecular weights, such as PPG-200, PPG-400, PPG-600, PPG-1000, PEG-200, PEG-400, and PEG-600, and film-forming resins of different functionalities and reactivity, such as N3390, XDI, N-3900, N3200, and N3600, to construct a copolymer system. By utilizing the complementary and adjustable functions of each component in the polymerization reaction, the final refractive index modulation of the system is significantly improved through the synergistic effect between the components. This also creates an ideal physicochemical environment for the formation of a stable chirped grating structure during exposure, synergistically optimizing the formation efficiency and quality of the chirped structure, thereby fundamentally improving the diffraction efficiency and angular bandwidth performance of the volume holographic grating. Specifically, a film-forming resin is obtained by cross-linking and polymerizing flexible polyols of various molecular weights, such as PPG-200, PPG-400, PPG-600, PPG-1000, PEG-200, PEG-400, and PEG-600, with isocyanates of different functionalities and reactivity, such as N3390, XDI, N-3900, N3200, and N3600, with OPPEA and PETA. The good compatibility of high-refractive-index writing monomers such as N-vinylcarbazole, PET4A, and TMPT with azobenzene monomers enables the formation of a stable and uniform polymer network during curing. This better utilizes the high-refractive-index monomers to provide a basic refractive index contrast and the multifunctional monomers to construct a high cross-linking density network to enhance the driving force for phase separation. On the one hand, it provides a suitable physicochemical environment for the full diffusion of monomers and phase separation, thereby improving the basic refractive index modulation of the photopolymer system. On the other hand, by using a flexible polyurethane network, it provides the necessary molecular motion freedom for the photoisomerization of azobenzene molecules. Its appropriate molecular chain segment freedom creates conditions for the continuous photoisomerization and rearrangement of azobenzene units during exposure, ensuring good processability of the material. Furthermore, it also synergistically promotes the spontaneous generation of Bragg planar chirped structures, ultimately achieving active modification of the internal structure of the volume holographic grating at the material level, effectively broadening its angular response bandwidth, and improving the uniformity and stability of grating forming.

[0022] Furthermore, in order to improve the control precision of the formation process of the chirped grating device, the visible light photoinitiator is selected from one or more of RB, SO, Acid Red 94, BTCP, BCIM, BDEA and DEAMC;

[0023] The UV initiator is selected from one or more of TPO, TMO, UV-1173, UV-184, BAPO, and UV-379;

[0024] By precisely selecting and combining the appropriate components, the visible light initiator is ensured to have a high efficiency response to the recording wavelength during the holographic exposure stage. This allows for the precise triggering of local polymerization and photoisomerization reactions of the writing monomer and azobenzene monomer, thereby simultaneously forming the basic refractive index modulation and inducing molecular migration of the chirped structure. Meanwhile, the ultraviolet light initiator used can induce deep, uniform, and comprehensive curing of the material system in subsequent stages, thoroughly crosslinking and fixing the dynamic grating structure formed in the previous step with the unreacted components. This ensures that the chirped structure can fully develop under optimized kinetic conditions and guarantees the complete curing and long-term stability of the final material system, thereby synergistically improving the structural integrity, diffraction efficiency, and stability of the fabricated grating device.

[0025] Furthermore, the co-initiator is selected from one or more of NPG, triethanolamine, HABI, EM, and Cl-MBT.

[0026] Furthermore, the solvent is selected from one or more of EM, EA, THF, DMF, DCM, NVP, and NMP.

[0027] Further, the surfactant is selected from one or more of Z-6040, KH-550, KH-560, KH-570, BYK-306, BYK-333, BYK-358N, and AF-104E.

[0028] Furthermore, the catalyst is one or more of dibutyltin laurylate, zinc isooctanoate, copper acetate, BTPT, DBU, and TEDA.

[0029] Another objective of this invention is to provide a method for fabricating a chirped grating device. The chirped grating device uses the photopolymer system provided in this technical solution. Specifically, the photopolymer system is weighed in proportion and mixed at room temperature. The mixed raw materials are then filled into a liquid crystal cell under vacuum in a dark room. After filling, the liquid crystal cell is sealed and transferred to stand. Subsequently, the reflective grating is exposed using a coherent beam with a dual-beam angle of 90°. After a period of dark reaction, it is irradiated with 395nm ultraviolet light to finally obtain the chirped grating device.

[0030] In this technical solution, a photopolymer system containing azobenzene polymerizable acrylate monomers is employed, combined with reflective exposure using a coherent beam with a 90° double-beam angle. The high spatial frequency interference field formed under this exposure configuration interacts with the photoresponse characteristics of the writing monomers in the azobenzene structure within the material. This allows the monomers to not only form a basic grating structure during polymerization but also spontaneously induce a longitudinal gradient distribution of refractive index modulation depth through the continuous photoisomerization of azobenzene molecules during exposure and the molecular relaxation process in the dark reaction stage. This constructs a chirped structure within the grating. Without adding additional complex processes, active control of the Bragg grating periodic distribution is achieved, significantly improving the device's angular bandwidth performance while ensuring good compatibility between the fabrication process and existing holographic grating technologies. Specifically, by employing a reflective holographic exposure optical path design, two coherent beams are incident from the same side of the substrate and interfere at a large angle within the material, forming a grating structure with a high spatial frequency that is approximately perpendicular to the substrate surface. On the one hand, this enables the formed bulk grating device to efficiently reflect incident light of a specific wavelength. On the other hand, the high sensitivity of the compact grating structure with high spatial frequency to changes within the material, combined with the azobenzene monomer in the formulation, creates a synergistic effect. That is, during exposure, the gradient of bright and dark fringes formed by the interference field couples with the photo-isomerization and migration behavior of azobenzene molecules, more effectively inducing a periodic chirped structure in the grating depth direction. Thus, without significantly increasing the traditional refractive index modulation, the spectral selectivity and angular response bandwidth of the final device are significantly improved. Furthermore, after the exposure process, the dark reaction stage provides a time window for the continued diffusion and polymerization of undepleted monomers within the system, as well as for the configurational relaxation of azobenzene molecules. This allows the refractive index modulation of the grating to be fully enhanced and preliminary structural relaxation to be formed. Subsequently, taking advantage of the efficient excitation capability of 395nm wavelength ultraviolet light on azobenzene molecules, specific and uniform light-driven light is applied to induce large-scale, cooperative cis-trans isomerization and migration to the non-illuminated region. This introduces periodic perturbations and longitudinal gradients into the already formed basic grating structure, thereby actively constructing a chirped structure. This effectively broadens the angular response bandwidth of the grating and improves the uniformity and stability of the final fabricated grating device structure.

[0031] Furthermore, the method for synthesizing the azobenzene-based polymerizable acrylate monomer is as follows:

[0032] S1. A benzene ring derivative containing amino and hydroxyl groups is reacted with HCl and NaNO2 to generate a diazonium salt; then, the diazonium salt is reacted with phenol under alkaline conditions to undergo electrophilic substitution to form an azo bond, thus obtaining an azobenzene compound containing dihydroxyl groups.

[0033] S2. The azobenzene compound is subjected to an esterification reaction with a methacrylate monomer to obtain the azobenzene polymerizable acrylate monomer.

[0034] Another object of the present invention is to provide a photopolymer holographic grating device with a chirped structure, which is prepared by the preparation method provided in this technical solution. Preferably, the diffraction efficiency of the holographic grating device is above 80%, the angular bandwidth is in the range of 10°, and the haze is less than 2%.

[0035] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0036] 1. A photopolymer system is provided, which introduces a dual initiation system of visible light and ultraviolet light into the photopolymer system and precisely configures the synergistic ratio range of azobenzene polymerizable monomer, various acrylate writing monomers and film-forming resin. The visible light initiator triggers the photoisomerization of azobenzene monomer and the selective polymerization of writing monomer during the holographic exposure stage to initially form a refractive index modulation and chirped gradient structure. Then, the deep curing reaction initiated by the ultraviolet light initiator further crosslinks and fixes the formed dynamic structure and unreacted components. Thus, while effectively constructing and stabilizing the chirped structure inside the grating, the final refractive index modulation of the system and the thermodynamic stability of the grating structure are significantly improved, and the angular response bandwidth and environmental tolerance of the volume holographic grating are further improved.

[0037] 2. By introducing azobenzene polymerizable acrylate monomers with azophenyl and acrylate groups into the photopolymer system, the acrylate groups in the azobenzene polymerizable acrylate monomers can both act as writing monomers to participate in the copolymerization process with the film-forming resin to form a polymer network, and their azophenyl groups can undergo continuous photoisomerization during holographic exposure. During the crosslinking process of the system reaction, driven successively by visible light photoinitiators and ultraviolet light photoinitiators, the photopolymer system records interference fringes while the local light intensity difference induces reversible changes in the configuration of azobenzene molecules and molecular redistribution, thereby actively modulating the polymerization. By combining dynamics and perturbing the diffusion process of monomers, a stable chirped structure is induced within the formed volumetric holographic grating. This effectively compensates for the grating period variation caused by material shrinkage simply by modifying the morphology of the Bragg grating, significantly broadening the angular response bandwidth of the grating. This is beneficial for fabricating large-field-of-view optical waveguide elements. Simultaneously, it avoids excessive pursuit of high-refractive-index material systems, reducing the need for refractive index modulation of photopolymer systems. This greatly expands the selection range of photopolymer systems, thus conveniently realizing high refractive index modulation and wide-angle response characteristics of grating devices based on photopolymer systems. Furthermore, by introducing azophenyl groups into the structure of the fabricated holographic grating device, the aging resistance of the material is improved, which is beneficial to the stability of the holographic grating device product and greatly increases its service life.

[0038] 3. A method for fabricating a chirped grating device is provided. This method employs a photopolymer system containing azobenzene-based polymerizable acrylate monomers, combined with reflective exposure using a coherent beam with a 90° dual-beam angle. Utilizing the interaction between the high spatial frequency interference field formed under this exposure configuration and the photoresponse characteristics of the azobenzene monomers in the material, the monomers not only form the basic grating structure during polymerization but also spontaneously induce a longitudinal gradient distribution of refractive index modulation depth through the continuous photoisomerization of azobenzene molecules during exposure and the molecular relaxation process in the dark reaction stage. This constructs a chirped structure within the grating. Without adding additional complex processes, this method achieves active control of the Bragg grating periodic distribution, significantly improving the device's angular bandwidth performance while ensuring good compatibility with existing holographic grating processes. Attached Figure Description

[0039] Figure 1 This is a schematic diagram illustrating the formation process of the chirped structure in the chirped grating device of the present invention.

[0040] Figure 2 This is a schematic diagram of the cis-trans isomerization process of azobenzene molecules in the fabrication method of a photopolymer system for forming a chirped grating device according to the present invention.

[0041] Figure 3 This is a schematic diagram of the synthesis process of azobenzene polymerizable acrylate monomers in Example 3 of the present invention. Detailed Implementation

[0042] It should be noted that the following detailed descriptions are illustrative and intended to provide further explanation of this application. Unless otherwise specified, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this application pertains.

[0043] It should be noted that the terminology used herein is for the purpose of describing particular embodiments only and is not intended to limit the exemplary embodiments according to this application. As used herein, the singular form is intended to include the plural form as well, unless the context clearly indicates otherwise. Furthermore, it should be understood that when the terms "comprising" and / or "including" are used in this specification, they indicate the presence of features, steps, operations, devices, components, and / or combinations thereof.

[0044] The present invention will now be further illustrated with specific examples. The following embodiments are only for explaining the present invention and do not constitute a limitation thereof. The test samples and test procedures used in the following embodiments include the following (if the specific experimental conditions are not specified in the embodiments, they are usually performed according to conventional conditions or the conditions recommended by the reagent company; the reagents, consumables, etc. used in the following embodiments can be obtained commercially unless otherwise specified).

[0045] Example 1

[0046] This embodiment provides a photopolymer system, characterized in that, by mass percentage, it comprises:

[0047] Solvent: 1%-5%,

[0048] Visible light photoinitiator: 0.01-1%,

[0049] Co-initiator: 0.1-1%,

[0050] UV initiator: 0.01-1%,

[0051] Writing individual units: 20-50%,

[0052] Azobenzene polymerizable acrylate monomers: 0.1-5%,

[0053] Film-forming resin: 20-30%,

[0054] Surfactant: 0-1%

[0055] Catalyst: 0.0001-0.01%,

[0056] The writing monomer is an acrylic writing monomer;

[0057] Azobenzene polymerizable acrylate monomers contain both azophenyl groups and acrylate groups.

[0058] Specifically, by introducing a dual visible and ultraviolet light initiator and azobenzene polymerizable acrylate monomers with azophenyl and acrylate groups into the photopolymer system, the acrylate groups in the azobenzene polymerizable acrylate monomers can both act as writing monomers to participate in the copolymerization process with the film-forming resin to form a polymer network, and their azophenyl groups can undergo continuous photoisomerization during holographic exposure. During the crosslinking process of the system reaction, the visible light photoinitiator and the ultraviolet light photoinitiator drive the process sequentially, causing the photopolymer system to record interference fringes while simultaneously inducing reversible changes in the molecular configuration and molecular remodeling of azobenzene due to local light intensity differences. The distribution of azophenyl groups actively modulates the polymerization kinetics and perturbs the monomer diffusion process, ultimately inducing a chirped structure within the formed volumetric holographic grating. This effectively compensates for the grating period variation caused by material shrinkage simply by modifying the morphology of the Bragg grating, significantly broadening the grating's angular response bandwidth. This is beneficial for fabricating large-field-of-view optical waveguide elements. Simultaneously, it avoids excessive pursuit of high-refractive-index material systems, reducing the need for refractive index modulation of the photopolymer system, thus greatly expanding the selection range of photopolymer systems. This allows for convenient realization of high refractive index modulation and wide-angle response characteristics in grating devices based on photopolymer systems. Furthermore, by introducing azophenyl groups into the structure of the fabricated holographic grating device, the material's aging resistance is improved, which is beneficial for the stability of the holographic grating device product and greatly increases its lifespan.

[0059] Furthermore, the writing monomer is selected from one or more of OPPEA, PETA, N-vinylcarbazole, PET4A, and TMPTA;

[0060] The film-forming resin is formed by crosslinking and polymerization of one or more of PPG-200, PPG-400, PPG-600, PPG-1000, PEG-200, PEG-400 and PEG-600 with one or more of N3390, XDI, N-3900, N3200 and N3600.

[0061] Specifically, this improves the efficiency and quality of the formation process, thereby fundamentally enhancing the diffraction efficiency and angular bandwidth performance of the volume holographic grating. Specifically, this is achieved by cross-linking and polymerizing flexible polyols of various molecular weights, such as PPG-200, PPG-400, PPG-600, PPG-1000, PEG-200, PEG-400, and PEG-600, with isocyanates of different functionalities and reactivity, including N3390, XDI, N-3900, N3200, and N3600, to form a film-forming resin, which is then combined with OPPEA and PETA. The good compatibility of high-refractive-index writing monomers such as N-vinylcarbazole, PET4A, and TMPT, as well as azobenzene monomers, enables the formation of a stable and uniform polymer network during curing. This better utilizes the high-refractive-index monomers to provide a basic refractive index contrast and the multifunctional monomers to construct a high-crosslink density network to enhance the driving force of phase separation. On the one hand, it provides a suitable physicochemical environment for sufficient monomer diffusion and phase separation, thereby improving the basic refractive index modulation of the photopolymer system. On the other hand, by employing a flexible polyurethane network, it provides the necessary molecular motion freedom for the photoisomerization of azobenzene molecules. Its appropriate molecular chain segment freedom creates conditions for the continuous photoisomerization and rearrangement of azobenzene units during exposure, ensuring good processability of the material. Furthermore, such as Figure 1 As shown, it also synergistically promotes the spontaneous generation of chirped structures in Bragg gratings, ultimately achieving active modification of the internal structure of volume holographic gratings at the material level, effectively broadening its angular response bandwidth, and improving the uniformity and stability of grating forming.

[0062] Furthermore, the visible light photoinitiator is selected from one or more of RB, SO, Acid Red 94, BTCP, BCIM, BDEA, and DEAMC;

[0063] The UV initiator is selected from one or more of TPO, TMO, UV-1173, UV-184, BAPO, and UV-379;

[0064] By precisely selecting and combining the appropriate components, the visible light initiator is ensured to have a high efficiency response to the recording wavelength during the holographic exposure stage. This allows for the precise triggering of local polymerization and photoisomerization reactions of the writing monomer and azobenzene monomer, thereby simultaneously forming the basic refractive index modulation and inducing molecular migration of the chirped structure. Meanwhile, the ultraviolet light initiator used can induce deep, uniform, and comprehensive curing of the material system in subsequent stages, thoroughly crosslinking and fixing the dynamic grating structure formed in the previous step with the unreacted components. This ensures that the chirped structure can fully develop under optimized kinetic conditions and guarantees the complete curing and long-term stability of the final material system, thereby synergistically improving the structural integrity, diffraction efficiency, and stability of the fabricated grating device.

[0065] Furthermore, the co-initiator is selected from one or more of NPG, triethanolamine, HABI, EM, and Cl-MBT.

[0066] Furthermore, the solvent is selected from one or more of EM, EA, THF, DMF, DCM, NVP, and NMP.

[0067] Furthermore, the surfactant is selected from one or more of Z-6040, KH-550, KH-560, KH-570, BYK-306, BYK-333, BYK-358N, and AF-104E.

[0068] Furthermore, the catalyst is one or more of the following: dibutyltin laurylate, zinc isooctanoate, copper acetate, BTPT, DBU, and TEDA.

[0069] Example 2

[0070] This embodiment provides a method for fabricating a chirped grating device using the photopolymer system as described in Example 1. The chirped grating device uses the photopolymer system provided in this technical solution. Specifically, the photopolymer system is weighed according to the proportion and mixed at room temperature. Then, the mixed raw material is filled into a liquid crystal cell under vacuum in a dark room. After filling, the liquid crystal cell is sealed and transferred to stand. Subsequently, the reflective grating is exposed using a coherent beam with a dual-beam angle of 90°. After a period of dark reaction, it is irradiated with 395nm ultraviolet light to finally obtain the chirped grating device.

[0071] Specifically, by employing a photopolymer system containing azobenzene polymerizable acrylate monomers and using a reflective holographic exposure optical path design, two coherent beams are incident from the same side of the substrate and interfere at a large angle within the material, forming a grating structure with a Bragg plane approximately perpendicular to the substrate surface and possessing a high spatial frequency. On the one hand, this enables the formed bulk grating device to efficiently reflect incident light of a specific wavelength. On the other hand, the high sensitivity of the compact grating structure with high spatial frequency to changes within the material, combined with the azobenzene monomers in the formulation, creates a synergistic effect. Specifically, during exposure, the gradient of bright and dark fringes formed by the interference field couples with the photo-isomerization and migration behavior of azobenzene molecules, more effectively inducing a periodic chirped structure in the grating depth direction. Thus, without significantly increasing the traditional refractive index modulation, the spectral selectivity and angular response bandwidth of the final device are significantly improved. Furthermore, after the exposure process, the dark reaction stage provides a time window for the continued diffusion and polymerization of undepleted monomers within the system, as well as for the configurational relaxation of azobenzene molecules. This allows for a significant enhancement of the grating's refractive index modulation and the formation of preliminary structural relaxation. Subsequently, utilizing the highly efficient excitation capability of 395nm wavelength ultraviolet light on azobenzene molecules, specific and uniform light-driven processes are employed to induce large-scale, cooperative cis-trans isomerization and migration to the non-illuminated region. The cis-trans isomerization process of azobenzene molecules is as follows: Figure 2 As shown, periodic perturbations and longitudinal gradients are introduced into the existing basic grating structure, thereby actively constructing a chirped structure, effectively broadening the angular response bandwidth of the grating, and improving the uniformity and stability of the final fabricated grating device structure.

[0072] Furthermore, the synthesis method of azobenzene polymerizable acrylate monomers is as follows:

[0073] S1. A benzene ring derivative containing amino and hydroxyl groups is reacted with HCl and NaNO2 to generate a diazonium salt; then, the diazonium salt is reacted with phenol under alkaline conditions to undergo electrophilic substitution to form an azo bond, thus obtaining an azobenzene compound containing dihydroxyl groups.

[0074] S2. The azobenzene compound is subjected to an esterification reaction with methacrylate monomers to obtain azobenzene polymerizable acrylate monomers.

[0075] Example 3

[0076] This embodiment also provides a method for fabricating a chirped grating device using the photopolymer system as described in Example 1, the difference being that, as in Example 2... Figure 3 As shown, the synthesis method of azobenzene polymerizable acrylate monomers is as follows:

[0077] S1. Take 0.1 mol of 4-amino-2-R1-6-R2-phenol, where R1 and R2 can be halogen atoms, or other heterocyclic structures that do not affect the diazotization reaction process or are structurally beneficial to increasing its refractive index. Gradually add the phenol to 250 ml of water at 0-5 °C, using hydrochloric acid added dropwise. Then add 0.012 mol of NaNO2 aqueous solution. After the solution changes color, continue stirring for 20 min. Next, add 0.11 mol of sodium phenolate solution to complete the coupling process. After the reaction, obtain the crude solid azobenzene compound using a vacuum filtration funnel, with a yield of 85.16%.

[0078] The crude solid azobenzene compound was separated and purified using a chromatographic column with a ratio of dichloromethane:ethanol = 60:20, and a reddish-brown powder was finally obtained. The reddish-brown powder was further baked in an oven for 24 hours to remove the water of crystallization and obtain a dark green powder crystal, which is the prepared azobenzene compound containing two hydroxyl groups, 4,4-dihydroxyazobenzene, abbreviated as ABP.

[0079] S2. ABP is reacted with (meth)acryloyl chloride to obtain a single-sided product M1 and / or a double-sided product M2. The ratio of the single-sided product M1 to the double-sided product M2 can be adjusted by controlling the amount of (meth)acryloyl chloride added. The single-sided product M1 and the double-sided product M2 can be separated and purified using a chromatographic column with ethyl acetate:ethanol = 20:80. The purified single-sided product M1 and the double-sided product M2 can both be used as azobenzene polymerizable acrylate monomers to participate in photopolymerization reactions. Since they have a certain color, the addition amount is usually kept below 5% to ensure the optical performance of the fabricated grating device.

[0080] Example 4

[0081] This embodiment provides a photopolymer holographic grating device with a chirped structure, wherein the azobenzene polymerizable acrylate monomer is prepared by the preparation method provided in any one of Examples 2-3.

[0082] Example 5

[0083] This embodiment also provides a method for fabricating a chirped grating device, which differs from embodiments 2 and 3 in that, as shown in... Figure 3 As shown, the azobenzene polymerizable acrylate monomers in the photopolymer system are M1 and / or M2 prepared as in Example 3.

[0084] Specifically, the photopolymer system was formulated according to the following proportions: RB 1%, NPG 2%, TPO 1%, OPPEA 14%, PETA 2%, N-vinylcarbazole 28%, M2 2%, PPG-400 20%, N3390 25%; dibutyltin laurylate 0.001%, EA 2%, DMF 3%;

[0085] Accurately weigh the raw materials of the above photopolymer system and stir them at room temperature for 6 hours with a magnetic stirrer with a speed of not less than 800 r / min. The mixing environment is best placed in a glove box to isolate the system from the interference of oxygen and water.

[0086] The mixed raw materials were filled into 10μm liquid crystal cells under vacuum in a darkroom. After filling, the liquid crystal cells were sealed and transferred to a 60℃ oven for post-processing for 30 minutes. Subsequently, the reflective grating was exposed using a coherent beam with a dual-beam angle of 90° at an exposure dose of 6mW / cm². 2 The exposure time was 20 seconds, followed by a 5-minute dark reaction; then, it was irradiated with 395nm ultraviolet light for 5 minutes to obtain a photopolymer holographic grating device with a chirped structure.

[0087] Example 6

[0088] This embodiment also provides a method for fabricating a chirped grating device, which differs from embodiments 2 and 3 in that, as shown in... Figure 3 As shown, the azobenzene polymerizable acrylate monomers in the photopolymer system are M1 and / or M2 prepared as in Example 3.

[0089] Specifically, the photopolymer system was formulated according to the following proportions: RB 1%, NPG 2%, TPO 1%, OPPEA 14%, PETA 2%, N-vinylcarbazole 28%, M1 2%, PPG-400 20%, N3390 25%; dibutyltin laurylate 0.001%, EA 2%, DMF 3%;

[0090] Accurately weigh the raw materials of the above photopolymer system and stir them at room temperature for 6 hours with a magnetic stirrer with a speed of not less than 800 r / min. The mixing environment is best placed in a glove box to isolate the system from the interference of oxygen and water.

[0091] The mixed raw materials were filled into 10μm liquid crystal cells under vacuum in a darkroom. After filling, the liquid crystal cells were sealed and transferred to a 60℃ oven for post-processing for 30 minutes. Subsequently, the reflective grating was exposed using a coherent beam with a dual-beam angle of 90° at an exposure dose of 6mW / cm². 2The exposure time was 20 seconds, followed by a 5-minute dark reaction; then, it was irradiated with 395nm ultraviolet light for 5 minutes to obtain a photopolymer holographic grating device with a chirped structure.

[0092] Example 7

[0093] This embodiment also provides a method for fabricating a chirped grating device, which differs from embodiments 2 and 3 in that, as shown in... Figure 3 As shown, the azobenzene polymerizable acrylate monomers in the photopolymer system are M1 and / or M2 prepared as in Example 3.

[0094] Specifically, the photopolymer system was formulated according to the following proportions: RB 1%, NPG 2%, TPO 1%, OPPEA 14%, PETA 2%, N-vinylcarbazole 28%, M2 1%, M1 1%, PPG-400 20%, N3390 25%; dibutyltin laurylate 0.001%, EA 2%, DMF 3%;

[0095] Accurately weigh the raw materials of the above photopolymer system and stir them at room temperature for 6 hours with a magnetic stirrer with a speed of not less than 800 r / min. The mixing environment is best placed in a glove box to isolate the system from the interference of oxygen and water.

[0096] The mixed raw materials were filled into 10μm liquid crystal cells under vacuum in a darkroom. After filling, the liquid crystal cells were sealed and transferred to a 60℃ oven for post-processing for 30 minutes. Subsequently, the reflective grating was exposed using a coherent beam with a dual-beam angle of 90° at an exposure dose of 6mW / cm². 2 The exposure time was 20 seconds, followed by a 5-minute dark reaction; then, it was irradiated with 395nm ultraviolet light for 5 minutes to obtain a photopolymer holographic grating device with a chirped structure.

[0097] Comparative Example 1

[0098] This embodiment also provides a method for fabricating a grating device. Specifically, the photopolymer system is prepared according to the following proportions: RB 1%, NPG 2%, TPO 1%, OPPEA 16%, PETA 2%, N-vinylcarbazole 28%, PPG-400 20%, N3390 25%; dibutyltin laurylate 0.001%, EA 2%, DMF 3%;

[0099] Accurately weigh the raw materials of the above photopolymer system and stir them at room temperature for 6 hours with a magnetic stirrer with a speed of not less than 800 r / min. The mixing environment is best placed in a glove box to isolate the system from the interference of oxygen and water.

[0100] The mixed raw materials were filled into 10μm liquid crystal cells under vacuum in a darkroom. After filling, the liquid crystal cells were sealed and transferred to a 60℃ oven for post-processing for 30 minutes. Subsequently, the reflective grating was exposed using a coherent beam with a dual-beam angle of 90° at an exposure dose of 6mW / cm². 2 The exposure time was 20 seconds, followed by a 5-minute dark reaction; then, it was irradiated with 395nm ultraviolet light for 5 minutes to obtain a photopolymer holographic grating device.

[0101] The diffraction efficiency of the photopolymer holographic grating devices prepared in Examples 5, 6, 7, and Comparative Example 1 was tested using a grating diffraction efficiency meter. Simultaneously, the haze of the obtained photopolymer holographic grating devices was tested using a haze meter at 23±2℃ according to the method shown in GB 2410-2008 "Test Methods for Transmittance and Haze of Transparent Plastics". The test results are shown in the table below:

[0102]

[0103] The test results show that, compared with the chirpless photopolymer holographic grating device prepared in Comparative Example 1, the haze of the chirpless photopolymer holographic grating devices prepared in Examples 5, 6 and 7 is slightly increased due to the addition of the color monomers azobenzene polymerizable acrylate monomers M1 and / or M2, but it is still within the limit range (≤2%) for high-precision optical applications. At the same time, the diffraction efficiency and angular bandwidth of the chirpless photopolymer holographic grating devices prepared in Examples 5, 6 and 7 are significantly improved, proving that the introduction of azobenzene polymerizable acrylate monomers effectively improves the diffraction efficiency and angular bandwidth of the volume holographic grating device.

[0104] Obviously, the above embodiments of the present invention are merely examples for clearly illustrating the technical solution of the present invention, and are not intended to limit the specific implementation of the present invention. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the claims of the present invention should be included within the protection scope of the claims of the present invention.

Claims

1. A photopolymer system, characterized in that, By weight percentage, including: Solvent: 1%-5%, Visible light photoinitiator: 0.01-1%, Co-initiator: 0.1-1%, UV initiator: 0.01-1%, Writing individual units: 20-60%, Azobenzene polymerizable acrylate monomers: 0.1-5%, Film-forming resin: 20-60%, Surfactant: 0-1% Catalyst: 0.0001-0.01%, The azobenzene polymerizable acrylate monomer has both an azophenyl group and an acrylate group; the writing monomer is selected from one or more of OPPEA, PETA, N-vinylcarbazole, PET4A, and TMPTA; The film-forming resin is formed by cross-linking and polymerization of one or more of PPG-200, PPG-400, PPG-600, PPG-1000, PEG-200, PEG-400, and PEG-600, as well as one or more of N3390, XDI, N-3900, N3200, and N3600.

2. The photopolymer system according to claim 1, characterized in that, The visible light photoinitiator is selected from one or more of RB, SO, Acid Red 94, BTCP, BCIM, BDEA, and DEAMC; The UV initiator is selected from one or more of TPO, TMO, UV-1173, UV-184, BAPO, and UV-379.

3. The photopolymer system according to claim 1, characterized in that, The co-initiator is selected from one or more of NPG, triethanolamine, HABI, EM and Cl-MBT.

4. The photopolymer system according to claim 1, characterized in that, The solvent is selected from one or more of EM, EA, THF, DMF, DCM, NVP, and NMP.

5. The photopolymer system according to claim 1, characterized in that, The surfactant is selected from one or more of Z-6040, KH-550, KH-560, KH-570, BYK-306, BYK-333, BYK-358N, and AF-104E.

6. The photopolymer system according to claim 1, characterized in that, The catalyst is one or more of dibutyltin laurylate, zinc isooctanoate, copper acetate, BTPT, DBU, and TEDA.

7. A method for fabricating a grating device with a chirped structure, characterized in that, The chirped grating device uses the photopolymer system as described in any one of claims 1-6. Specifically, the photopolymer system is weighed in proportion and mixed at room temperature. The mixed raw material is then filled into a liquid crystal cell under vacuum in a dark room. After filling, the liquid crystal cell is sealed and transferred to stand. Subsequently, the reflective grating is exposed using a coherent beam with a dual-beam angle of 90°. After a period of dark reaction, it is irradiated with 395nm ultraviolet light to finally obtain the chirped grating device.

8. The preparation method according to claim 7, characterized in that, The method for synthesizing the azobenzene polymerizable acrylate monomer is as follows: S1. A benzene ring derivative containing amino and hydroxyl groups is reacted with HCl and NaNO2 to generate a diazonium salt; then, the diazonium salt is reacted with phenol under alkaline conditions to undergo electrophilic substitution to form an azo bond, thus obtaining an azobenzene compound containing dihydroxyl groups. S2. The azobenzene compound is subjected to an esterification reaction with a methacrylate monomer to obtain the azobenzene polymerizable acrylate monomer.

9. A grating device with a chirped structure, characterized in that, It is prepared by the preparation method according to any one of claims 7-8.

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