Polarization holographic optical element and preparation method and application thereof

Abstract

The invention relates to the technical field of optical elements, and particularly discloses a polarization holographic optical element and a preparation method and application thereof. The polarization holographic optical element provided by the invention is prepared from a base material, an orientation layer and a high-refractive-index liquid crystal layer, the high refractive index liquid crystal layer is prepared from RM257, RM82 liquid crystal, CB15 liquid crystal, LC242 liquid crystal, a photoinitiator, a chiral agent, a surfactant and PGMEA according to the weight ratio of (5 to 7): (3 to 4): (1 to 2): (1 to 2): (0.007 to 0.013): (0.01 to 0.03): (0.04 to 0.06): (80 to 86); the orientation layer is prepared from dye and DMF (Dimethyl Formamide) according to the weight ratio of (0.5-1.5): 100. According to the technical scheme, the problems of low liquid crystal solubility and difficult orientation are solved, and the polarization holographic optical element with high refractive index modulation degree, high wavelength bandwidth and high uniformity is prepared.

Description

Technical Field

[0001] This application relates to the technical field of optical elements, and particularly relates to a polarization holographic optical element, a preparation method thereof, and an application thereof. Background Art

[0002] The volume holographic grating waveguide solution uses an etched grating of a holographic optical element (HOE) to realize the guiding of virtual images. The HOE is manufactured based on holographic interference technology, and directly interferes to form a nano-scale grating structure within a micron-scale optical polymer film. Therefore, in terms of process, the holographic optical waveguide is more efficient, and the cost also has obvious advantages compared with other optical waveguide technologies. Moreover, it has more advantages in terms of color uniformity (no rainbow effect) and realizing a single-chip full-color waveguide. Additionally, it is worth mentioning that for AR displays, it is necessary to ensure the characteristics of the external real scene as much as possible, such as the clarity, distortion, and brightness of the outdoor scene are not affected by the waveguide. In this regard, due to the high transparency and high diffraction efficiency characteristics of the HOE, the holographic optical waveguide has obvious advantages over the array waveguide and is not prone to generating ghost images, thus attracting great interest from AR optical module manufacturers.

[0003] The polarization volume grating (PVG) is a new type of holographic grating that generates periodic refractive index changes by periodically rotating the optical axis of liquid crystal molecules. Compared with traditional volume gratings, the PVG has a higher refractive index modulation degree and a wider response bandwidth, solves the problem of limited FOV of traditional gratings, and is more suitable for the field of near-eye display optical waveguides. The PVG also has the polarization sensitivity that traditional gratings lack, ensuring high transmittance at a large field of view angle, and can be customized in small or large sizes according to the application field, expanding the application range of optical waveguide coupling elements. The PVG technology has made remarkable progress in recent research and development with its excellent large-angle Bragg diffraction characteristics and good polarization response characteristics. It not only provides a wider field of view display range for waveguide display systems, but also brings new design dimensions and application potential to the field of optical displays.

[0004] In the process of realizing large-angle FOV single-green and full-color display of PVG, liquid crystals with a high refractive index modulation are required to achieve high uniformity of brightness and color display under FOV. Therefore, it is necessary for the liquid crystal grating to have a wide enough spectrum to achieve a wide spectrum of the RGB segment within the same diffraction efficiency range. This stringent requirement places higher demands on the refractive index modulation of the material, and the liquid crystal material molecules need to provide corresponding refractive index modulation for different FOVs. Existing materials such as RM257 and LC86 can only provide a refractive index modulation of 0.17, resulting in the existing PVG two-dimensional waveguide achieving only about 30% uniformity under a 30° FOV. Therefore, developing liquid crystals with a refractive index modulation above 0.2 has very great potential advantages. Most current liquid crystals with a high refractive index modulation need to achieve a larger conjugated structure as the main chain in the molecular structure, such as phenylenediyne. However, an overly large main chain will bring two basic problems: one is a significant reduction in solubility; the other is that traditional alignment materials cannot smoothly align the liquid crystals, thus unable to achieve the final desired liquid crystal grating structure. Summary of the Invention

[0005] To solve the above technical problems, the present application provides a polarization holographic optical element, its preparation method and application.

[0006] In a first aspect, the present application provides a polarization holographic optical element, which is prepared from a substrate, an alignment layer, and a high-refractive-index liquid crystal layer; The material of the high-refractive-index liquid crystal layer is prepared from liquid crystal, photoinitiator, chiral agent, surfactant, and PGMEA with a weight ratio of 10 - 15: 0.007 - 0.013: 0.01 - 0.03: 0.04 - 0.06: 80 - 86; The liquid crystal is composed of a mixture of RM257 liquid crystal, RM82 liquid crystal, CB15 liquid crystal, and LC242 liquid crystal with a weight ratio of 5 - 7: 3 - 4: 1 - 2: 1 - 2; The surfactant is composed of a mixture of polyoxyethylene fluorinated alcohol, Tween 20, and n-butyl acetate with a weight ratio of 90 - 110: 20 - 30: 1 - 7; The alignment layer is prepared from a dye and DMF with a weight ratio of 0.5 - 1.5: 100.

[0007] The present application introduces liquid crystals with different ratios and different side chains into the liquid crystal formulation of PVG, and at the same time selects specific types of surfactants, solves the problems of low solubility and difficult alignment of liquid crystals with a high refractive index modulation, realizes ultra-high solubility and alignment ability, and further realizes a refractive index modulation far exceeding other solutions, thereby achieving a larger wavelength bandwidth and FOV uniformity.

[0008] Through multiple experiments, it is found in this application that using four kinds of liquid crystals, namely RM257 liquid crystal, RM82 liquid crystal, CB15 liquid crystal, and LC242 liquid crystal, to form a liquid crystal formulation according to a specific ratio has good compatibility and can cooperate with each other to make them play their optimal roles respectively. Among them, RM257 is a diacrylate liquid crystal monomer with the property of high birefringence (Δn ∼ 0.2 - 0.3), which can provide a rigid skeleton and polymerizability; RM82 is a monoacrylate liquid crystal monomer that can improve solubility, reduce the viscosity of the system, and enhance the compatibility with other components; CB15 is a chiral liquid crystal dopant used to introduce a helical structure (adjust the pitch), which can effectively broaden the wavelength response range; LC242 is a reactive mesogen used to stabilize the liquid crystal phase, enhance the orientation order, and improve the uniformity of Δn.

[0009] In the design of surfactants in polarized holographic optical elements, polyoxyethylene fluorinated alcohol, as a fluorine-based surfactant, can reduce the interfacial energy, promote the vertical / horizontal alignment of liquid crystals, and reduce phase separation defects; its fluorocarbon chain (-CF2-) has a weak interaction with the aromatic ring of liquid crystal molecules (such as RM257), avoiding aggregation. Tween 20, as a non-ionic surfactant, can effectively improve the liquid crystal / polymer compatibility, enhance solubility, and stabilize the microdomain structure; for example, the polyoxyethylene chain of Tween 20 wraps the liquid crystal molecules, enhancing the dispersibility in the polymer matrix. n-Butyl acetate, as an ester solvent, adjusts the evaporation rate of the system, improves the leveling property, assists the uniform dispersion of the surfactant, and at the same time improves the compatibility between the raw material components of liquid crystal, photoinitiator, chiral agent, PGMEA, and surfactant, further enhancing the performance of the polarized holographic optical element.

[0010] Preferably, the high refractive index liquid crystal layer material is prepared from liquid crystal, photoinitiator, chiral agent, surfactant, and PGMEA with a weight ratio of 11 - 14: 0.008 - 0.012: 0.015 - 0.025: 0.045 - 0.055: 81 - 85.

[0011] Preferably, the liquid crystal is composed of a mixture of RM257 liquid crystal, RM82 liquid crystal, CB15 liquid crystal, and LC242 liquid crystal with a weight ratio of 5.5 - 6.5: 3.2 - 3.8: 1.2 - 1.8: 1.2 - 1.8.

[0012] Further, the liquid crystal is composed of a mixture of RM257 liquid crystal, RM82 liquid crystal, CB15 liquid crystal, and LC242 liquid crystal with a weight ratio of 6: 3.5: 1.5: 1.5.

[0013] Preferably, the surfactant is composed of a mixture of polyoxyethylene fluorinated alcohol, Tween 20, and n-Butyl acetate with a weight ratio of 95 - 105: 23 - 27: 3 - 5.

[0014] Preferably, the surfactant is composed of a mixture of polyoxyethylene fluorohydric alcohol, Tween 20, and n-butyl acetate at a weight ratio of 100:25:4.

[0015] Preferably, the photoinitiator is TPO, and the chiral agent is R5011 / S5011.

[0016] Preferably, the alignment layer is prepared from a dye and DMF at a weight ratio of 0.7 - 1.2:100.

[0017] Preferably, in the alignment layer, the dye is selected from one or more of BY, SD1, and PI.

[0018] In a second aspect, the present application provides a method for preparing the above-mentioned polarization holographic optical element, specifically including the following steps in sequence: Under light-shielded conditions, weigh the liquid crystal, photoinitiator, chiral agent, PGMEA, and surfactant in corresponding weight ratios and mix them evenly. The solution after suction filtration is the high-refractive-index liquid crystal layer solution; Under light-shielded conditions, weigh the dye and DMF in corresponding weight ratios and mix them evenly. The solution after suction filtration is the alignment dye layer solution; Apply the alignment dye layer solution onto the substrate and perform exposure using a laser; Spin-coat the high-refractive-index liquid crystal layer solution onto the exposed substrate and perform ultraviolet curing in a nitrogen environment to obtain the polarization holographic optical element.

[0019] In a second aspect, the present application provides the application of the above-mentioned polarization holographic optical element in image display or projection.

[0020] In summary, the technical solution of the present application has the following effects: The present application introduces liquid crystals with different ratios and different side chains into the liquid crystal formulation of PVG, and at the same time selects specific types of surfactants, solving the problems of low liquid crystal solubility and difficult alignment, and preparing a polarization holographic optical element with high refractive index modulation, high wavelength bandwidth, and high uniformity. Specific Embodiments

[0021] The following further describes the present application in detail in combination with examples, comparative examples, and performance detection tests. These examples should not be construed as limiting the scope claimed by the present application. Examples

[0022] Examples 1 - 5 Examples 1 - 5 respectively provide a polarization holographic optical element and a method for preparing the same.

[0023] The differences between the above-mentioned examples are as follows: The types of liquid crystals in the high-refractive-index liquid crystal layer material are different, as shown in Table 1 specifically.

[0024] The preparation method of the polarization holographic optical element in the above embodiments is as follows: Preparation of the high refractive index liquid crystal layer solution: Under light-shielded conditions, weigh liquid crystals (composed of RM257 liquid crystal, RM82 liquid crystal, CB15 liquid crystal, and LC242 liquid crystal in corresponding weight ratios according to Table 1) with a weight ratio of 12.5:0.01:0.02:0.05:83, photoinitiator TPO, chiral agent R5011 / S5011, surfactant (the surfactant is composed of polyoxyethylene fluorinated alcohol Zonyl FSN-100, Tween 20, and n-butyl acetate in a weight ratio of 100:25:4), and PGMEA. After 30 minutes, filter through a PTFE sieve with a pore size of 0.2 microns, and then seal the resulting solution with tin foil and store it at room temperature.

[0025] Preparation of the alignment dye layer solution: Under light-shielded conditions, weigh the dye azobenzene derivative SD1 and DMF with a corresponding weight ratio of 1:100. After 30 minutes, filter through a PTFE sieve with a pore size of 0.2 microns, and then seal the resulting solution with tin foil and store it at room temperature.

[0026] Construction of the holographic exposure optical path: Coat 2 ml of the alignment dye layer solution onto a 6 cm × 6 cm ITO glass substrate. The laser emitted by a 365 nm ultraviolet or 450 nm blue laser passes through a mirror (beam expander system + 15-micron aperture to expand the laser beam and then pass it through a small hole filter to eliminate stray light and improve the beam quality; PBS selects linearly polarized light to ensure consistent polarization direction; collimating lens adjusts the beam to parallel light to ensure the uniformity of the exposure area), and the laser is directed onto the substrate coated with the alignment dye solution to achieve holographic recording, obtaining the exposed substrate.

[0027] Preparation of the polarization holographic optical element: Place the exposed substrate on a spin coater with a vacuum chuck. Using a rotation speed of 6000 rpm, uniformly drip and spin coat 7 ml of the high refractive index liquid crystal layer solution onto the exposed substrate to form a grating. Subsequently, in a nitrogen environment, cure it with 365 nm ultraviolet light at 20 mW / cm 2 to obtain the polarization holographic optical element.

[0028] Table 1 Weight ratios of each raw material component in the liquid crystal of the high refractive index liquid crystal layer material in Examples 1-5 Examples 6-9 Examples 6-9 respectively provide a polarization holographic optical element and its preparation method.

[0029] The differences between the above embodiments and Embodiment 3 are as follows: the types of surfactants in the high refractive index liquid crystal layer materials are different, as specifically shown below.

[0030] In Embodiment 6: the surfactant is composed of a mixture of polyoxyethylene fluorinated alcohol, Tween 20, and n-butyl acetate with a weight ratio of 90:30:1.

[0031] In Embodiment 7: the surfactant is composed of a mixture of polyoxyethylene fluorinated alcohol, Tween 20, and n-butyl acetate with a weight ratio of 110:20:7.

[0032] In Embodiment 8: the surfactant is composed of a mixture of polyoxyethylene fluorinated alcohol, Tween 20, and n-butyl acetate with a weight ratio of 95:27:3.

[0033] In Embodiment 9: the surfactant is composed of a mixture of polyoxyethylene fluorinated alcohol, Tween 20, and n-butyl acetate with a weight ratio of 105:23:5.

[0034] Other process parameters in the above embodiments are the same as those in Embodiment 3.

[0035] Comparative Examples Comparative Examples 1-4 Comparative Examples 1-4 respectively provide a polarization holographic optical element and a preparation method thereof.

[0036] The differences between the above comparative examples and Embodiment 3 are as follows: the types of liquid crystals in the high refractive index liquid crystal layer materials are different, as specifically shown below.

[0037] In Comparative Example 1: the liquid crystal is composed of a mixture of RM257 liquid crystal, RM82 liquid crystal, and CB15 liquid crystal with a weight ratio of 6:3.5:3.

[0038] In Comparative Example 2: the liquid crystal is composed of a mixture of RM257 liquid crystal, RM82 liquid crystal, S811 liquid crystal, and LC242 liquid crystal with a weight ratio of 6:3.5:1.5:1.5.

[0039] In Comparative Example 3: the liquid crystal is composed of a mixture of RM257 liquid crystal, RM82 liquid crystal, CB15 liquid crystal, and LC242 liquid crystal with a weight ratio of 3:7:0.5:3.

[0040] In Comparative Example 4: the liquid crystal is composed of a mixture of RM257 liquid crystal, RM82 liquid crystal, CB15 liquid crystal, and LC242 liquid crystal with a weight ratio of 10:1:3:0.5.

[0041] Other process parameters in the above comparative examples are the same as those in Embodiment 3.

[0042] Comparative Examples 5-8 Comparative Examples 5-8 respectively provided a polarization holographic optical element and a preparation method thereof.

[0043] The differences between the above comparative examples and Example 3 are as follows: the types of surfactants in the high-refractive-index liquid crystal layer materials are different, as specifically shown below.

[0044] In Comparative Example 5: the surfactant is composed of a mixture of polyether-modified silicone oil Dow Corning DC-193, polyethylene glycol octyl phenyl ether Triton X-100, and n-butyl acetate with a weight ratio of 100:25:4.

[0045] In Comparative Example 6: the surfactant is composed of a mixture of polyoxyethylene fluorinated alcohol, Tween 20, and n-propyl acetate with a weight ratio of 100:25:4.

[0046] In Comparative Example 7: the surfactant is composed of a mixture of polyoxyethylene fluorinated alcohol, Tween 20, and n-butyl acetate with a weight ratio of 70:50:0.05.

[0047] In Comparative Example 8: the surfactant is composed of a mixture of polyoxyethylene fluorinated alcohol, Tween 20, and n-butyl acetate with a weight ratio of 130:10:10.

[0048] Other process parameters in the above comparative examples are the same as those in Example 3.

[0049] Performance Detection Test Refractive Index Modulation: By measuring the diffraction efficiency of the grating (such as the ratio of the intensity of the first-order diffracted light to the intensity of the incident light), the refractive index modulation Δn is inversely deduced in combination with the coupled-wave theory (Kogelnik theory).

[0050] Among them, η: diffraction efficiency; Δn: refractive index modulation; d: grating thickness; λ: wavelength; θ: Bragg angle.

[0051] Wavelength Bandwidth: The incident light irradiates the grating at the Bragg angle, and the wavelength distribution of the diffracted light is directly measured with a spectrometer; then the full width at half maximum of the diffraction spectrum is analyzed (the spectral width of the light source itself needs to be deducted).

[0052] FOV Uniformity: Using a spectrophotometer, by measuring the spectral data (brightness, color coordinates, color temperature) at different field angles of view, analyzing its variation within the full field of view, and evaluating the uniformity.

[0053] Detection Results: As shown in Table 2.

[0054] Table 2 Performance Detection Results of Polarization Holographic Optical Elements in Examples and Comparative Examples Combined with the performance detection results of the polarization holographic optical element in the above table, it can be seen that the technical solution of the present application solves the problems of low solubility of liquid crystal and difficult orientation, and a polarization holographic optical element with high refractive index modulation, high wavelength bandwidth and high uniformity is prepared.

[0055] By comparing the detection results of Examples 1-5 and Comparative Examples 1-4, it can be seen that the present application uses liquid crystals of RM257 liquid crystal, RM82 liquid crystal, CB15 liquid crystal, and LC242 liquid crystal with a weight ratio of 5-7:3-4:1-2:1-2 to form the liquid crystal in the high-refractive-index liquid crystal layer material, and then prepares a polarization holographic optical element, effectively improving the performance of the finished product.

[0056] By comparing the detection results of Examples 1, 6-9 and Comparative Examples 5-8, it can be seen that the present application uses a surfactant composed of polyoxyethylene fluorinated alcohol, Tween 20, and n-butyl acetate with a weight ratio of 90-110:20-30:1-7 to form the surfactant in the high-refractive-index liquid crystal layer material, and then prepares a polarization holographic optical element, further improving the performance of the polarization holographic optical element.

[0057] Although the present invention has been described in detail with general descriptions and specific embodiments above, on the basis of the present invention, some modifications or improvements can be made, which are obvious to those skilled in the art. Therefore, these modifications or improvements made without departing from the spirit of the present invention all fall within the scope of the present invention claimed.

Claims

1. A polarization holographic optical element, characterized in that: It is prepared from a substrate, an orientation layer, and a high-refractive liquid crystal layer; The high-fold liquid crystal layer material is composed of a weight ratio of 10-15: 0.007-0.013:0.01-0.03:0.04-0.06:80-86 of liquid crystal, photoinitiator, chiral agent, surfactant, and PGMEA are prepared; The liquid crystal is composed of a mixture of RM257 liquid crystal, RM82 liquid crystal, CB15 liquid crystal and LC242 liquid crystal in a weight ratio of 5-7:3-4:1-2:1-2; The surfactant is composed of a mixture of polyoxyethylene fluoroalcohol, Tween 20 and n-butyl acetate in a weight ratio of 90-110:20-30:1-7; The alignment layer is prepared from dye and DMF in a weight ratio of 0.5-1.5:

100.

2. The polarization holographic optical element according to claim 1, characterized in that: The high-refractive liquid crystal layer material is prepared from liquid crystal, photoinitiator, chiral agent, surfactant and PGMEA in a weight ratio of 11-14: 0.008-0.012: 0.015-0.025: 0.045-0.055: 81-85.

3. The polarization holographic optical element according to claim 1, characterized in that: The liquid crystal is composed of a mixture of RM257 liquid crystal, RM82 liquid crystal, CB15 liquid crystal and LC242 liquid crystal in a weight ratio of 5.5-6.5:3.2-3.8:1.2-1.8:1.2-1.

8.

4. The polarization holographic optical element according to claim 1, characterized in that: The surfactant is composed of polyoxyethylene fluoroalcohol, Tween 20 and n-butyl acetate in a weight ratio of 95-105:23-27:3-5.

5. The polarization holographic optical element according to claim 1, characterized in that: The surfactant is composed of a mixture of polyoxyethylene fluoroalcohol, Tween 20 and n-butyl acetate in a weight ratio of 100:25:

4.

6. The polarization holographic optical element according to claim 1, characterized in that: The photoinitiator is TPO, and the chiral agent is R5011 / S5011.

7. The polarization holographic optical element according to claim 1, characterized in that: The alignment layer is prepared from dye and DMF in a weight ratio of 0.7-1.2:

100.

8. The polarization holographic optical element according to claim 1, characterized in that: In the alignment layer, the dye is selected from one or more of BY, SD1, and PI.

9. A method for preparing a polarization holographic optical element according to any one of claims 1 to 8, characterized in that: Specifically, the following steps are performed in sequence: Under light-proof conditions, weigh the corresponding weight ratio of liquid crystal, photoinitiator, chiral agent, PGMEA, and surfactant and mix them evenly. The solution after suction filtration is a high-refractive liquid crystal layer solution; Under light-proof conditions, weigh the dye and DMF in corresponding weight ratios and mix them evenly. The solution after suction filtration is the orientation dye layer solution; Applying the alignment dye layer solution onto a substrate and exposing it with a laser; The high-refractive liquid crystal layer solution is spin-coated onto the exposed substrate, and ultraviolet curing is performed in a nitrogen environment to obtain a polarization holographic optical element.

10. Use of the polarization holographic optical element according to any one of claims 1 to 8 in image display or projection.

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