Photopolymer material, holographic optical element and near-eye display system

Abstract

The invention relates to the technical field of optical devices, in particular to a photopolymer material, a holographic optical element and a near-to-eye display system. The nano-diamond is introduced, and the N-methyl pyrrolidone solvent is combined with an ultrasonic dispersion process, so that the nano-diamond is uniformly dispersed in the matrix, the problem of light scattering is avoided, the optical transparency is maintained while the material performance is optimized, and the holographic imaging definition is guaranteed. The rigid structure of the nano-diamond effectively inhibits volume shrinkage in the grating recording process, improves the grating period uniformity and enhances the Bragg diffraction performance, and the strong interface combination of the nano-diamond and the polymer matrix also significantly improves the thermal stability and environmental tolerance of the material, reduces grating relaxation and prolongs the service life of the element. The prepared holographic optical element can directionally diffract image source light to the retina of a human eye, stable projection is formed in an extended depth-of-field range, and a light-weight and high-brightness immersive display effect is achieved.

Description

Technical Field

[0001] The present invention relates to the technical field of optical devices, and in particular to a photopolymer material, a holographic optical element and a near-eye display system. Background Art

[0002] In recent years, with the rapid development of near-eye display technologies such as augmented reality (AR) and virtual reality (VR), the demand for high-resolution, large field of view, and lightweight optical elements has become increasingly urgent. Holographic optical elements (HOEs) are regarded as the core components of the next generation of near-eye display systems due to their unique wavefront modulation capabilities, lightweight characteristics, and integrability. Photopolymer materials have become the mainstream recording medium for holographic optical elements due to their high diffraction efficiency, fast response characteristics, and low cost advantages. However, traditional photopolymer-based holographic elements still face the following technical bottlenecks in practical applications and material performance limitations: the refractive index modulation degree (Δn) of existing photopolymers is low, resulting in limited grating diffraction efficiency and angle selectivity of holographic elements, making it difficult to meet the needs of large field of view and high brightness near-eye display; insufficient environmental stability: photopolymers are easily affected by environmental factors such as temperature and humidity, and grating structure relaxation will occur during long-term use, resulting in degradation of holographic image quality;

[0003] To address the above problems, academia and industry have attempted to prepare photopolymers by doping inorganic nanoparticles (such as TiO2, SiO2, etc.), but traditional nanofillers have problems such as poor dispersibility, limited improvement in refractive index, and introduction of light scattering losses. Summary of the Invention

[0004] Based on the above problems, the first technical solution of the present application discloses a photopolymer material, comprising:

[0005] Polymethyl methacrylate: obtained by polymerization of 94.966%-95.056% by mass of methyl methacrylate, 1.139%-1.140% of azobisisobutyronitrile, and 3.798%-3.802% of a cationic photoinitiator;

[0006] Nano-diamond mixed solution is obtained by dispersing 0.00009504%-0.0009496% nano-diamonds in 0.009409%-0.09401% N-methylpyrrolidone organic solvent.

[0007] Furthermore, the particle size of the nanodiamond is less than 10 nm.

[0008] The second technical solution of the present application discloses a method for preparing the above-mentioned photopolymer material, comprising the following steps:

[0009] S1. Methyl methacrylate, azobisisobutyronitrile, and a cationic photoinitiator are mixed and ultrasonically treated to obtain a first mixed solution;

[0010] S2. ultrasonically treating the nanodiamond mixture to obtain a dispersion;

[0011] S3. The dispersion was mixed with the first mixed solution and then ultrasonically treated to obtain a second mixed solution;

[0012] S4. The second mixed liquid is stirred and reacted at a constant temperature until it becomes viscous, and then injected into a glass mold for sealing and reaction to obtain a photopolymer.

[0013] Furthermore, the ultrasonic treatment time of S1 is 15 minutes, the ultrasonic treatment time of S2 is 3-5 hours, the ultrasonic treatment time of S3 is 20 minutes, and the ultrasonic treatment temperatures of S1, S2 and S3 are all 60°C.

[0014] Furthermore, the constant temperature reaction time in S4 is 40-120 minutes, and the sealing reaction time is 48-72 hours.

[0015] And, the photopolymer material prepared according to the above preparation method.

[0016] The third technical solution of the present application discloses a holographic optical element comprising the above-mentioned photopolymer material, in which the center line of the spherical wavefront formed by the object light is parallel to the center normal of the photopolymer and forms a 45° angle with the center normal of the reference light.

[0017] The fourth technical solution of the present application discloses a near-eye display system including the above-mentioned holographic optical element, in which a 45° angle is formed between the center line of the diffracted image light after passing through the 4-f optical system and the center normal of the holographic optical element.

[0018] Beneficial effects:

[0019] This application introduces nanodiamonds and utilizes their high refractive index characteristics (n≈2.4) to form a significant refractive index difference (Δn≈0.9) with the polymer matrix (n≈1.5). High-refractive-index microregions are constructed in the photopolymer, significantly improving the refractive index modulation and diffraction efficiency of the holographic optical element, breaking through the performance limits of traditional materials and achieving higher light energy utilization and angular selectivity. At the same time, N-methylpyrrolidone solvent is combined with ultrasonic dispersion technology to uniformly disperse the nanodiamonds in the matrix, avoiding light scattering problems, maintaining optical transparency while optimizing material performance, and ensuring holographic imaging clarity. The rigid structure of the nanodiamonds effectively suppresses volume shrinkage during grating recording, improves grating period uniformity, and enhances Bragg diffraction performance. Its strong interfacial bonding with the polymer matrix also significantly improves the thermal stability and environmental tolerance of the material, reduces grating relaxation, and extends the service life of the element. By optimizing the raw material ratio (such as the precise mass percentage of MMA, photoinitiator, and nanodiamonds) and step-by-step mixing, constant temperature ultrasonication and other processes, the preparation process is simplified and the cost is reduced, making it suitable for large-scale production. The fabricated holographic optical element can diffract image source light in a directionally directed manner onto the human retina, forming a stable projection within an extended depth of field, achieving a lightweight, high-brightness immersive display. This provides key technical support for diverse applications in fields such as augmented reality (AR) and virtual reality (VR). BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Figure 1 This is a light path diagram for preparing the photopolymer-based holographic optical element provided by the present invention;

[0021] Figure 2 This is a principle diagram of the optical path of the near-eye display system provided by the present invention;

[0022] Figure 3 This is a graph showing the nanoparticle size measurement of a nanodiamond / N-methylpyrrolidone organic solvent dispersion using the nanoparticle size analyzer provided by the present invention;

[0023] Figure 4 This is a diffraction efficiency curve of the nanodiamond-doped photopolymer provided by the present invention;

[0024] Figure 5 This is a diffraction efficiency curve of the undoped nanodiamond photopolymer provided by the present invention;

[0025] Figure 6 This is a rendering of the near-eye display system provided by the present invention without using holographic optical elements;

[0026] Figure 7 This is a rendering of a near-eye display system using a holographic optical element provided by the present invention;

[0027] Figure 8This is a scanning electron microscope (SEM) image of the nanodiamond provided by the present invention. DETAILED DESCRIPTION

[0028] In order to more clearly illustrate the purpose, research methods and advantages of the present invention, the present invention is further described in detail through the following examples.

[0029] The first technical solution of the present application discloses a photopolymer material, comprising

[0030] Polymethyl methacrylate: obtained by polymerizing 94.966%-95.056% by mass of methyl methacrylate, 1.139%-1.140% of azobisisobutyronitrile, and 3.798%-3.802% of a cationic photoinitiator; nanodiamond mixture; obtained by dispersing 0.00009504%-0.0009496% by mass of nanodiamonds in 0.009409%-0.09401% of an organic solvent of N-methylpyrrolidone; the nanodiamond particle size is less than 10nm.

[0031] Nanodiamonds, due to their ultra-high refractive index (n≈2.4), provide a solution for improving the diffraction efficiency of holographic gratings. The refractive index difference between nanodiamonds (n≈2.4) and polymer matrices (n≈1.5) is significant (Δn≈0.9), forming high-refractive-index microregions in the cross-linked network of photopolymers, which can significantly increase the refractive-index modulation of the holographic grating, thereby pushing the limits of the diffraction efficiency of traditional materials. The rigid structure of nanodiamonds can suppress volume shrinkage during grating recording, improve the uniformity of the grating period, and further enhance the angular selectivity and light energy utilization of Bragg diffraction. By using N-methylpyrrolidone as an organic solvent for dispersion and dissolution and oscillation in an ultrasonic water bath, nanodiamonds can be evenly dispersed in the polymer matrix, avoiding the light scattering problem caused by agglomeration of traditional nanoparticles, ensuring high diffraction efficiency while maintaining the transparency of the optical element.

[0032] In a further embodiment, a method for preparing the above-mentioned photopolymer is disclosed, comprising the following steps:

[0033] S1. Methyl methacrylate, azobisisobutyronitrile, and a cationic photoinitiator are mixed and ultrasonically treated to obtain a first mixed solution;

[0034] S2. ultrasonically treating the nanodiamond mixture to obtain a dispersion;

[0035] S3. The dispersion was mixed with the first mixed solution and then ultrasonically treated to obtain a second mixed solution;

[0036] S4. The second mixed liquid is stirred and reacted at a constant temperature until it becomes viscous, and then injected into a glass mold for sealing and reaction to obtain a photopolymer.

[0037] Furthermore, the ultrasonic treatment time of S1 is 15 minutes, the ultrasonic treatment time of S2 is 3-5 hours, the ultrasonic treatment time of S3 is 20 minutes, and the ultrasonic treatment temperatures of S1, S2 and S3 are all 60°C.

[0038] Furthermore, the constant temperature reaction time in S4 is 40-120 minutes, and the sealing reaction time is 48-72 hours.

[0039] In the above-mentioned embodiment, N-methylpyrrolidone solvent combined with an ultrasonic dispersion process is used to uniformly disperse the nanodiamonds in the matrix, avoiding light scattering issues. This optimizes material properties while maintaining optical transparency and ensuring holographic imaging clarity. The rigid structure of the nanodiamonds effectively suppresses volume shrinkage during grating recording, improves grating period uniformity, and enhances Bragg diffraction performance. Their strong interfacial bonding with the polymer matrix also significantly improves the material's thermal stability and environmental tolerance, reduces grating relaxation, and extends the component's service life.

[0040] The second embodiment of the present application discloses the application of the above-mentioned photopolymer material, because the near-eye display system has strict requirements on the volume, weight and light energy utilization rate of optical elements. The traditional catadioptric optical solution is difficult to meet the requirements due to its bulky size, while the near-eye display technology based on holographic optical elements can realize light modulation through holographic gratings, significantly improving the compactness of the system. The holographic optical element prepared by nanodiamond-doped photopolymer can effectively reduce the energy loss in the near-eye display system by virtue of its high diffraction efficiency characteristics, achieve a more uniform brightness distribution entering the eye, and provide key technical support for immersive near-eye display. The photopolymer material of the present application is used in holographic optical elements or near-eye display systems, and its high refractive index characteristics can form a significant refractive index difference with the polymer matrix, constructing a high refractive index micro-region in the inform polymer, thereby greatly improving the refractive index modulation and diffraction efficiency of the holographic optical element, breaking through the performance limits of traditional materials to achieve higher light energy utilization and angle selectivity.

[0041] In one embodiment, a method for preparing the above-mentioned photopolymer material into a holographic optical element is disclosed, such as Figure 1 As shown, an optical path is set, including a laser 1, an attenuation plate 2, a first reflector 3, a first electronic shutter 4, a first beam splitter prism 5, a first pinhole filter 6, a first lens 7, a second reflector 8, a second electronic shutter 9, a second lens 10, a third reflector 11, a second pinhole filter 12, a third lens 13, and a photopolymer 14.

[0042] The optical path of the above-mentioned device is as follows: the laser 1 serves as the laser light source. The laser light is attenuated by an attenuation plate 2. The first reflector 3 redirects the optical path. The first beam splitter prism 5 separates the laser light into object light and reference light. The object light passes through a first pinhole filter 6, a first lens 7 of the beam expansion and collimation system, and a second reflector 8, before being incident on a second lens 10, forming a spherical wavefront. The reference light passes through a third reflector 11 and a second pinhole filter 12, then is expanded by a third lens 13 and irradiated onto a photopolymer 14. The two beams interfere at the dry plate, forming the interference fringes required for a holographic lens. These fringes are recorded through this optical path, transforming the photopolymer 14 into a holographic optical element. The exposure time is precisely controlled by a first electronic shutter 4 and a second electronic shutter 9.

[0043] In this process, the center line of the spherical wavefront formed by the object light is parallel to the center normal line of the photopolymer 14 , and the angle between the center line of the spherical wavefront formed by the object light and the center normal line of the reference light is 45°.

[0044] Another embodiment of the present application further discloses a near-eye display system comprising the above-mentioned holographic optical element, including a second laser 15, a second attenuation plate 16, a fourth reflector 17, a third pinhole filter 18, a fourth lens 19, a second beam splitter prism 20, a spatial light modulator 21, a fifth lens 22, an aperture stop 23, a sixth lens 24, a holographic optical element 14 (i.e., the photopolymer 14 in the above-mentioned embodiment, which becomes a holographic optical element through optical path recording), a human eye 25, and a computer 26.

[0045] According to some preferred embodiments of the present invention, the optical path of the near-eye display system is described as follows: a laser 15 serves as a laser light source. The laser light is attenuated by an attenuation plate 16. The fourth reflector 3 redirects the optical path, filtering the laser light through a third pinhole filter 18. The fourth lens 19 expands the beam. A second beam splitter prism 20 splits the expanded plane wave. The plane wave illuminates a spatial light modulator 21, generating diffracted image light modulated by a computer 26. After passing through the second beam splitter prism 20, the diffracted image light reaches a 4-f optical system formed by a fifth lens 22, an aperture stop 23, and a sixth lens 24. After removing stray light, the diffracted image light is converged by a holographic optical element 14 and enters the human eye 25.

[0046] like Figure 6 This is a near-eye display system that does not use holographic optical elements. The virtual holographic image display surface of the "human face" is combined with the real scene of the "plaster figure". The image is recorded using a camera. In addition, the distance from the camera to the virtual holographic image display surface of the "human face" is 250 mm, and from the camera to the "plaster figure" is 500 mm. Figure 7 (a) It can be seen that when the focus is on the focal plane of the virtual holographic image display of the "human face", the "plaster portrait" behind it is out of focus. Figure 7(b) When focusing on the focal plane of the "plaster figure" behind, the virtual holographic image of the "human face" appears out of focus. If holographic optical elements are not used to achieve retinal projection in a near-eye display system, the eye will frequently adjust focus, causing discomfort and dizziness.

[0047] like Figure 7 This is a near-eye display system using the holographic optical element of the present application. The virtual holographic image of the "teapot" is displayed in front of the holographic optical element and is integrated with the real scene of the "plaster cat" and "plastic flower". The white round label paper is the marker on the HOE. The image is recorded using a camera. In addition, the distance from the camera to the plane of the holographic optical element is 250 mm, and the distance from the camera to the "plaster cat" and "plastic flower" is 350 mm and 650 mm respectively. Figure 7 As shown in (a) and 7(b), the camera focuses on the "plastic cat" and "plastic flower" respectively. Figure 7 In (a), the "plastic cat" in the near distance is clear, while the "plastic flower" in the far distance is blurry. Figure 7 In (b), the "plastic flower" is clear, while the "plastic cat" is blurry. However, the virtual "teapot" holographic image remains clear regardless of where the camera is focused. Therefore, the proposed holographic near-eye display experimental device can achieve retinal projection, effectively solve VAC, and realize perspective augmented reality.

[0048] The technical effect of the photopolymer described in this application will be demonstrated through a specific example below.

[0049] Example 1 Preparation of Nanodiamond-doped Photopolymer Materials

[0050] Step 1: Raw material ratio

[0051] The following raw materials were weighed by mass percentage: methyl methacrylate (MMA): 94.966%-95.056%, photoinitiator (Irgacure 784): 3.798%-3.802%, azobisisobutyronitrile (AIBN): 1.139%-1.140%, nanodiamond (particle size <10 nm): 0.00009504%-0.0009496%, and N-methylpyrrolidone (NMP): 0.009409%-0.09401%.

[0052] Step 2: Mixture Preparation

[0053] The molds and glass substrates required in the preparation process are cleaned by ultrasonic oscillation in anhydrous ethanol, then rinsed with deionized water, and placed in a constant temperature oven for drying.

[0054] Methyl methacrylate (MMA), a photoinitiator (Irgacure 784), and azobisisobutyronitrile (AIBN) were added to a reaction container, and ultrasonically oscillated in a 60° C. water bath for 15 minutes to obtain a first mixed solution.

[0055] Nanodiamonds were mixed with N-methylpyrrolidone (NMP) in a ratio of 1:99 and ultrasonically dispersed in a water bath at 60°C for 4 h to obtain a uniformly dispersed nanoparticle dispersion.

[0056] Step 3: Synthesis of composite materials

[0057] The nanoparticle dispersion was added to the first mixed solution, and ultrasonic oscillation was continued in a 60° C. water bath for 20 minutes to fully mix to form a second mixed solution.

[0058] The second mixed liquid was stirred and reacted at a constant temperature of 60° C. for 40 minutes to 2 hours until it became viscous, and then injected into a glass mold with a thickness of 1.5 mm.

[0059] The mold is sealed and cured at a constant temperature (60°C) for 60 hours until it is completely thermally polymerized into a solid block material. After removal from the mold, a transparent photopolymer film is obtained after demoulding. After cutting and polishing, it can be used for holographic recording.

[0060] Comparative Example 1 Preparation of undoped nanodiamond photopolymer

[0061] The preparation method is the same as in Example 1, except that the photopolymer material in Comparative Example 1 does not contain nanodiamonds.

[0062] Test Example 1 Physical Property Analysis

[0063] The nanodiamond / N-methylpyrrolidone organic solvent dispersion prepared in Example 1 was measured for nanoparticle size using a nanoparticle size analyzer, wherein the particle size of the nanodiamond dispersion is as follows: Figure 3 As shown: the average particle size is 41.91 nm and the dispersion is uniform (PDI = 0.117).

[0064] like Figure 8 As shown in the figure, 5-10nm spherical diamond grains are aggregated together to form submicron and micron-sized aggregates. When the nanodiamond sample is scanned under a scanning electron microscope (SEM), it can be clearly seen that the agglomeration size of the nanoparticles has reached the micron level, while the holographic grating period in the actual experiment is about 1μm. Therefore, nanodiamond particles cannot be directly doped into the photopolymer material system. However, the present application eliminates the agglomeration of nanodiamonds by mixing the nanodiamond particles into an N-methylpyrrolidone solution and performing ultrasonic oscillation for 2 hours before introducing it into the photopolymer material system. The agglomeration of nanodiamonds is reduced from micron level to nanometer level ( Figure 3 ).

[0065] Experimental Example 2 Diffraction Experiment

[0066] Preparation of holographic optical elements

[0067] Optical path configuration (reference Figure 1 ): A 532nm laser 1 is used as the light source. After the laser power is adjusted by the attenuator 2, it is split into object light and reference light by the beam splitter prism 5.

[0068] The object light path passes through the pinhole filter 6, the beam expander collimator lens 7, the reflector 8 in sequence, and forms a spherical wavefront through the lens 10, with the central wave vector parallel to the normal of the photopolymer 14.

[0069] Reference light path: The reference light passes through the reflector 11, the pinhole filter 12 and the beam expander lens 13 to form a collimated plane wave, which is incident on the photopolymer 14 at 45 degrees.

[0070] Interference exposure: Two beams of light form a volume holographic grating on the surface of the photopolymer, and the exposure time is controlled by the electronic shutters 4 and 9.

[0071] The diffraction efficiency of the photopolymer materials prepared in Example 1 and Comparative Example 1 was used as an indicator to judge the holographic performance. The calculation formula of the diffraction efficiency is:

[0072]

[0073] Among them is I +1 I0 is the first-order diffraction light intensity, and I0 is the 0th-order transmission light intensity.

[0074] The diffraction efficiency of the volume holographic grating of a holographic optical element can effectively characterize the speed and amplitude of grating formation as the exposure time increases during the grating formation process. Under the same recording conditions, the higher the diffraction efficiency of the material, the more information it contains. It is an important characteristic parameter for measuring holographic optical elements.

[0075] The diffraction efficiency curve of the holographic grating is as follows: Figure 4 、 Figure 5 Shown by: Figure 4 It can be seen that the diffraction efficiency of the photopolymer material prepared in Example 1 is 84.48% at 532nm; while the diffraction efficiency of the photopolymer material prepared in Comparative Example 1 is only 50.42% at 532nm ( Figure 5 ).

[0076] Example 2 Construction and Application of Near-Eye Display System

[0077] System Configuration (Ref. Figure 2 ):

[0078] Light source module: 532nm laser 15 outputs green light, which is adjusted by attenuation plate 16 and reflector 17 and then expanded into a plane wave by lens 19.

[0079] Image generation module: The plane wave is incident on the spatial light modulator 21 through the beam splitter prism 20 and loaded with the holographic image generated by the computer 26 .

[0080] 4-f filtering system: The diffracted light passes through the lens 22, the aperture stop 23, and the lens 24 in sequence, and after the stray light is filtered out, it is incident on the holographic optical element 14.

[0081] Retinal projection: The holographic element directionally diffracts the image light to the optical center of the human eye lens, forming a retinal projection with an extended depth of field.

[0082] Finally, it should be noted that the above embodiments are merely preferred embodiments of the present invention, and the scope of protection of the present invention is not limited to the above embodiments. All technical solutions that fall within the scope of protection of the present invention are within the scope of protection of the present invention. It should be noted that improvements and modifications that can be made by persons of ordinary skill in the art without departing from the principles of the present invention are also considered to be within the scope of protection of the present invention.

Claims

1. A photopolymer material, characterized in that: include: Polymethyl methacrylate: obtained by polymerization of 94.966%-95.056% by mass of methyl methacrylate, 1.139%-1.140% of azobisisobutyronitrile, and 3.798%-3.802% of a cationic photoinitiator; Nano-diamond mixed solution is obtained by dispersing 0.00009504%-0.0009496% nano-diamonds in 0.009409%-0.09401% N-methylpyrrolidone organic solvent.

2. The photopolymer material according to claim 1, characterized in that The particle size of the nano-diamond is less than 10 nm.

3. A method for preparing the photopolymer material according to claim 1 or 2, characterized in that: The steps include: S1. Methyl methacrylate, azobisisobutyronitrile, and a cationic photoinitiator are mixed and ultrasonically treated to obtain a first mixed solution; S2. ultrasonically treating the nanodiamond mixture to obtain a dispersion; S3. The dispersion was mixed with the first mixed solution and then ultrasonically treated to obtain a second mixed solution; S4. The second mixed liquid is stirred and reacted at a constant temperature until it becomes viscous, and then injected into a glass mold for sealing and reaction to obtain a photopolymer.

4. The preparation method according to claim 3, characterized in that The ultrasonic treatment time of S1 is 15 minutes, the ultrasonic treatment time of S2 is 3-5 hours, the ultrasonic treatment time of S3 is 20 minutes, and the ultrasonic treatment temperature of S1, S2 and S3 is 60°C.

5. The preparation method according to claim 3, characterized in that: The constant temperature reaction time in S4 is 40-120 minutes, and the sealing reaction time is 48-72 hours.

6. A photopolymer material prepared according to the preparation method according to any one of claims 3 to 5.

7. A method for preparing a holographic optical element, characterized in that: By setting an optical path, the center line of the spherical wavefront formed by the object light in the optical system is parallel to the center normal of the photopolymer described in claim 1, 2 or 6, and forms a 45° angle with the center normal of the reference light. The photopolymer is then recorded through this optical path to form a holographic optical element.

8. The holographic optical element obtained by the preparation method according to claim 7.

9. A near-eye display system comprising the holographic optical element according to claim 8, characterized in that: In this system, a 45° angle is formed between the center line of the diffraction image light after passing through the 4-f optical system and the center normal line of the holographic optical element.

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