A mask plate, liquid crystal polarization grating and a preparation method thereof

Abstract

The present application relates to the technical field of polarization element, and provide a mask plate, liquid crystal polarization grating and preparation method thereof, a preparation method of liquid crystal polarization grating, comprising: using the mask plate with gradually changing transmittance to expose liquid crystal polymer precursor, obtain liquid crystal polarization grating;Liquid crystal polymer precursor includes liquid crystal raw material, photoinitiator, ultraviolet absorber and chiral dopant;Chiral dopant accounts for 2.15% to 2.21% of the mass percentage content of liquid crystal polymer precursor;The change range of the gradually changing transmittance of mask plate is 7% to 37%. By applying the mask plate with gradually changing transmittance on the liquid crystal polymer precursor, the specific light intensity distribution is generated after the ultraviolet light passes through the mask plate, the periodic spiral structure is formed in the horizontal direction and the vertical direction of the film formed by the liquid crystal polymer respectively, and then the wavelength width of the liquid crystal polarization grating can be increased and the angle bandwidth of the entrance pupil grating of the liquid crystal polarization grating can be increased.

Description

Technical Field

[0001] This invention relates to the field of polarization element technology, and more specifically, to a photomask, a liquid crystal polarization grating, and a method for fabricating the same. Background Technology

[0002] Augmented reality (AR) and virtual reality (VR) offer new ways for people to perceive the world and are promising next-generation display technologies. In VR, all the visual information received comes from computer-generated virtual images; while AR overlays virtual images onto the real world, allowing observers to see not only the computer-generated virtual images but also the rich surrounding environment. Comparing the two, AR has a wider range of applications, a more comfortable immersive experience, and a higher level of optical design complexity.

[0003] Optical waveguide combiners, a key component of augmented reality near-eye displays, offer advantages in terms of size, weight, optical visibility, optical efficiency, and exit pupil size. Among optical waveguides, arrayed waveguides, surface-embossed gratings, volume holographic gratings, and liquid crystal polarization gratings each possess unique advantages and technological barriers. Compared to other waveguide coupling technologies, liquid crystal polarization gratings offer significant advantages in optical coupling efficiency, optical visibility, size, weight, and cost. Currently, liquid crystal polarization grating waveguide coupling technology suffers from a drawback: the entrance pupil grating has an excessively small angular bandwidth, resulting in a small field of view (FOV) in waveguide display systems employing liquid crystal polarization gratings. Summary of the Invention

[0004] To address the problems in the prior art, the present invention provides a photomask, a liquid crystal polarization grating, and a method for fabricating the same.

[0005] On one hand, the present invention provides a method for fabricating a liquid crystal polarization grating, comprising: exposing a liquid crystal polymer precursor using a mask having a gradient transmittance to obtain the liquid crystal polarization grating;

[0006] The liquid crystal polymer precursor includes liquid crystal raw materials, photoinitiators, ultraviolet absorbers, and chiral dopants;

[0007] The chiral dopant accounts for 2.15%-2.21% of the mass percentage of the liquid crystal polymer precursor;

[0008] The gradient transmittance of the photomask ranges from 7% to 37%.

[0009] This invention applies a mask with gradient transmittance to a liquid crystal polymer precursor. After ultraviolet light passes through the mask, a specific light intensity distribution is generated. After the liquid crystal polymer precursor is irradiated with ultraviolet light, a liquid crystal polymer with a periodic spiral structure is formed in the horizontal and vertical directions of the liquid crystal polymer film, which forms a gradient liquid crystal polarization grating. This increases the wavelength width of the liquid crystal polarization grating and the angular bandwidth of the entrance pupil grating.

[0010] Optionally, the liquid crystal raw material includes 1,4-bis-[4-(3-acryloyloxypropoxy)benzoyloxy]-2-methylbenzene and / or 1,4-bis-[4-(6-acryloyloxyhexyloxy)benzoyloxy]-2-methylbenzene.

[0011] Optionally, the photoinitiator includes α,α-dimethoxy-α-phenylacetophenone;

[0012] Optionally, the ultraviolet absorber includes any one of butyl methoxybenzoylmethane, 2-(2′-hydroxy-3′,5′-di-tert-butylphenyl) and 2-hydroxy-4-n-octyloxybenzophenone.

[0013] Optionally, the chiral dopant comprises (13BR)-5,6-dihydro-5-(trans-4-propylcyclohexyl)-4H-dinaphtho[2,1-F:1',2'-H][1,5]dioxane-nonatetraene or 5,6-dihydro-5-(trans-4-propylcyclohexyl)-4H-dinaphtho(2,1-F:1',2'-H)(1,5)dioxane-nonatetraene.

[0014] Optionally, the liquid crystal polymer precursor further includes a surfactant, including a fluorocarbon surfactant.

[0015] Optionally, the ultraviolet absorber accounts for 1.5%-1.9% of the mass percentage of the liquid crystal polymer precursor.

[0016] Optionally, the exposure dose is 2.147 J / cm². 2 -1.45J / cm 2 .

[0017] Optionally, the chiral dopant accounts for 2.19% of the mass percentage of the liquid crystal polymer precursor, and the ultraviolet absorber accounts for 1.7% of the mass percentage of the liquid crystal polymer precursor.

[0018] On the other hand, the present invention also provides a liquid crystal polarization grating prepared by the above-described method, comprising a photo-alignment layer and a liquid crystal layer covering the photo-alignment layer, wherein the liquid crystal layer comprises a liquid crystal polymer having a periodic helical structure with a gradient pitch length, the pitch of the helical structure being 184-199 nm. By giving the liquid crystal polymer in the liquid crystal polarization grating a periodic helical structure with a gradient pitch length, and the pitch of the helical structure being in the range of 184-199 nm, the angular bandwidth of the entrance pupil grating of the liquid crystal polarization grating is increased.

[0019] On the other hand, the present invention also provides a mask for preparing the above-mentioned liquid crystal polarization grating, which has a gradient transmittance, the gradient transmittance ranging from 7% to 37%.

[0020] Optionally, the gradient transmittance of the mask may vary between 9%-36%, 10%-30%, or 7%-31%.

[0021] Optionally, the mask template is provided with grid areas with different transmittances, and the number of grid areas is more than 5, such as 6, 9, 10, 13 or 16.

[0022] Optionally, the transmittance of the grid area in each row gradually decreases from left to right, and the transmittance of the rightmost grid area in each row is greater than the transmittance of the leftmost grid area in the next row.

[0023] Optionally, the transmittance of the grid area in each row gradually increases from left to right, and the transmittance of the rightmost grid area in each row is less than the transmittance of the leftmost grid area in the next row.

[0024] Optionally, the transmittance difference between adjacent grid areas is 0.5%-5%, for example, 2%, 3% or 4%.

[0025] Optionally, the mask template is any one of the following: a horizontal strip gradient mask template, a six-grid gradient mask template, a nine-grid gradient mask template, a sixteen-grid gradient mask template, a twenty-five-grid gradient mask template, a thirty-six-grid gradient mask template, and a forty-nine-grid gradient mask template.

[0026] Optionally, the shape of the mask template can be any one of a square, rectangle, rhombus, ellipse, and circle.

[0027] Compared with existing technologies, the present invention has the following advantages:

[0028] This invention uses a mask with gradient transmittance to prepare a liquid crystal polymer with a periodic spiral structure, thereby improving the wavelength width of the liquid crystal polarization grating and increasing the angular bandwidth of the entrance pupil grating. In some preferred embodiments, compared with the method without using a mask, the wavelength width of the prepared liquid crystal polarization grating increases from 53 nm to 84 nm, and the angular bandwidth of the entrance pupil grating with a diffraction efficiency greater than 80% increases from 38° to 61°. Attached Figure Description

[0029] Figure 1 A schematic diagram of the structure of the liquid crystal polarization grating provided in the embodiment;

[0030] Figure 2 This is a schematic diagram of the helical structure of the liquid crystal polymer in the XY plane in the embodiment;

[0031] Figure 3 This is a schematic cross-sectional view of the helical structure of the liquid crystal polymer in the XZ plane in the embodiment;

[0032] Figure 4 This is a schematic cross-sectional view of the helical structure of the liquid crystal polymer in the XY plane in the embodiment;

[0033] Figure 5 This is a schematic diagram of the sixteen-grid gradient mask used in Example 1;

[0034] Figure 6 The figure shows the curves of the center wavelength of liquid crystal polarization gratings with different concentrations of chiral dopants as a function of exposure dose in the examples.

[0035] Figure 7 The figure shows the curves of the center wavelength of the liquid crystal polarization grating with different concentrations of ultraviolet absorber as a function of exposure dose in the examples.

[0036] Figure 8 This is a schematic diagram of the nine-grid gradient mask used in Example 2;

[0037] Figure 9 This is a schematic diagram of the six-grid gradient mask used in Example 3;

[0038] Figure 10 This is a schematic diagram of the horizontal strip gradient mask used in Example 4;

[0039] Figure 11 This is a schematic diagram of the horizontal strip gradient mask used in Example 5;

[0040] Figure 12 The wavelength-transmission spectrum curves of the liquid crystal polarization gratings prepared in Example 1 and Comparative Example 1 are shown.

[0041] Figure 13 The graph shows the diffraction efficiency of the liquid crystal polarization gratings prepared in Example 1 and Comparative Example 1 as a function of the incident angle.

[0042] Figure 14 The graph shows the diffraction efficiency of the liquid crystal polarization grating prepared in Example 2 as a function of the incident angle.

[0043] Figure 15 The graph shows the diffraction efficiency of the liquid crystal polarization grating prepared in Example 3 as a function of the incident angle.

[0044] Figure 16 This is a graph showing the diffraction efficiency of the liquid crystal polarization grating prepared in Example 4 as a function of the incident angle.

[0045] Figure 17 This is a graph showing the diffraction efficiency of the liquid crystal polarization grating prepared in Example 5 as a function of the incident angle.

[0046] Explanation of reference numerals in the attached figures:

[0047] 1-Base;

[0048] 2-Optical alignment layer;

[0049] 3-Liquid crystal layer. Detailed Implementation

[0050] It should be noted that, unless otherwise specified, the embodiments and features described in the embodiments of this invention can be combined with each other. Furthermore, the terms "comprising," "containing," and "having" are non-limiting, meaning that other steps and components that do not affect the results can be added. Unless otherwise specified, all materials, equipment, and reagents are commercially available.

[0051] To better understand the invention and not to limit its scope, all figures indicating amounts, percentages, and other numerical values ​​used in this invention should, in all cases, be understood to be modified with "approximately." Therefore, unless specifically stated otherwise, the numerical parameters listed in the specification and claims are approximate values ​​and may vary depending on the desired properties being sought. Each numerical parameter should at least be considered as obtained based on reported significant figures and through conventional rounding methods.

[0052] To make the above-mentioned objects, features and advantages of the present invention more apparent and understandable, specific embodiments of the present invention will be described in detail below with reference to the accompanying drawings.

[0053] This invention provides a method for fabricating a liquid crystal polarization grating, comprising: exposing a liquid crystal polymer precursor using a mask with gradient transmittance to obtain the liquid crystal polarization grating;

[0054] The liquid crystal polymer precursor includes liquid crystal raw materials, photoinitiators, ultraviolet absorbers, and chiral dopants;

[0055] The chiral dopant accounts for 2.15%-2.21% of the mass percentage of the liquid crystal polymer precursor;

[0056] The gradient transmittance of the photomask ranges from 7% to 37%.

[0057] In the context of this invention, the term "liquid crystal polymer precursor" refers to a mixture that can undergo a polymerization reaction to form a liquid crystal polymer upon exposure to light.

[0058] In the context of this invention, the term "liquid crystal material" refers to a material that can undergo a polymerization reaction to form a liquid crystal polymer under photochemical irradiation, generally including monomers or oligomers.

[0059] In the context of this invention, the term "gradual transmittance" refers to transmittance that changes gradually in a certain order.

[0060] Existing technologies for fabricating liquid crystal polarization gratings offer significant advantages in terms of optical coupling efficiency, optical visibility, size, weight, and cost. However, these gratings suffer from a drawback: a small angular bandwidth at the entrance pupil, resulting in a limited field of view (FOV) for waveguide display systems. In this embodiment, a mask with gradient transmittance is applied to a liquid crystal polymer precursor. Ultraviolet light passing through the mask generates a specific intensity distribution, causing the precursor to form periodic helical structures in both the horizontal and vertical directions of the resulting film. This creates a gradient-type liquid crystal polarization grating, maximizing the range of wavelength shift at the center of the grating and thus increasing both the wavelength width and the angular bandwidth at the entrance pupil.

[0061] Specifically, the method for fabricating a liquid crystal polarization grating provided in this embodiment includes the following steps:

[0062] Step S1: Form an azobenzene photoalignment layer on a transparent substrate such as glass, and then form a liquid crystal polymer precursor layer on the other side of the azobenzene photoalignment layer;

[0063] The liquid crystal polymer precursor contains liquid crystal raw materials, photoinitiators, chiral dopants, and ultraviolet absorbers; the chiral dopants account for 2.15%-2.21% of the mass percentage of the liquid crystal polymer precursor.

[0064] Photoinitiators initiate polymerization during liquid crystal polymerization; UV absorbers absorb some UV light during polymerization, weakening the light intensity reaching the bottom of the polymer. This results in a gradient pitch structure perpendicular to the polymer film direction during photocuring, with a larger pitch near the UV light source and a smaller pitch away from the UV light source. Calculations show that controlling the mass percentage of chiral dopant to 2.15%-2.21% can fix the range of the pitch structure formed by the liquid crystal polymer within the green light range.

[0065] Step S2: Apply a mask to the liquid crystal polymer precursor described in step S1. The mask has a gradient transmittance, and the transmittance of the mask varies from 7% to 37%.

[0066] The transmittance of the mask ranges from 7% to 37%, determined based on the blue shift of the center wavelength of the liquid crystal polarization grating. First, the concentration of the chiral dopant was varied while the concentration of the ultraviolet absorber was kept constant. Then, based on the blue shift of the center wavelength of the liquid crystal polarization grating device under different ultraviolet exposure doses, the concentration of the chiral dopant with the maximum center wavelength blue shift was determined. The transmittance range of the mask, 7% to 37%, combined with the amounts of chiral dopant and ultraviolet absorber in the liquid crystal precursor, allows the prepared liquid crystal polarization grating to achieve a significant blue shift of the center wavelength, significantly increasing the wavelength width and the angular bandwidth of the entrance pupil grating.

[0067] Step S3: Expose the liquid crystal polymer precursor with ultraviolet light applied in step S2 to form a liquid crystal polymer with a periodic spiral structure, thereby obtaining the liquid crystal polarization grating.

[0068] This invention provides a method for preparing a liquid crystal polarization grating, wherein the liquid crystal raw material includes 1,4-bis-[4-(3-acryloyloxypropoxy)benzoyloxy]-2-methylbenzene (RM257) or 1,4-bis-[4-(6-acryloyloxyhexyloxy)benzoyloxy]-2-methylbenzene (RM82).

[0069] Optionally, the photoinitiator includes α,α-dimethoxy-α-phenylacetophenone.

[0070] Optionally, the photoinitiator is an ultraviolet absorber, which includes any one or more of butyl methoxybenzoylmethane (avobenzone), 2-(2′-hydroxy-3′,5′-di-tert-butylphenyl (UV327) and 2-hydroxy-4-n-octyloxybenzophenone (UV531).

[0071] Optionally, the chiral dopant comprises (13BR)-5,6-dihydro-5-(trans-4-propylcyclohexyl)-4H-dinaphtho[2,1-F:1',2'-H][1,5]dioxane (R5011) or 5,6-dihydro-5-(trans-4-propylcyclohexyl)-4H-dinaphtho(2,1-F:1',2'-H)(1,5)dioxane (S5011). Optionally.

[0072] Optionally, the ultraviolet absorber accounts for 1.5%-1.9% of the mass percentage of the liquid crystal polymer precursor.

[0073] Optionally, the exposure dose is 2.147 J / cm². 2 -1.45J / cm 2 .

[0074] In one embodiment of the present invention, the chiral dopant accounts for 2.19% of the mass percentage of the liquid crystal polymer precursor, and the ultraviolet absorber accounts for 1.7% of the mass percentage of the liquid crystal polymer precursor. When the mass percentage of the chiral dopant is controlled to be 2.19% and the mass percentage of the ultraviolet absorber is controlled to be 1.7%, the prepared liquid crystal polarization grating can have a large entrance pupil angular bandwidth.

[0075] This invention also provides a liquid crystal polarization grating, which is prepared by the above method. It comprises a substrate 1, a light-controlled alignment layer 2, and a liquid crystal layer 3 covering the light-controlled alignment layer 2. The liquid crystal layer 3 contains a liquid crystal polymer, which has a periodic helical structure. The pitch of the helical structure is gradient distributed on the horizontal and vertical planes of the film formed by the liquid crystal polymer. The pitch of the helical structure is 184-199 nm.

[0076] By giving the liquid crystal polymer in the liquid crystal polarization grating a periodic helical structure, and having the pitch of the helical structure be gradient-distributed in the range of 184-199nm, the angular bandwidth of the entrance pupil grating of the liquid crystal polarization grating is increased.

[0077] Specifically, such as Figure 1 As shown, the liquid crystal polarization grating provided in this embodiment of the invention includes, in sequence: a substrate 1, a light-controlled alignment layer 2, and a liquid crystal layer 3;

[0078] Substrate 1, used to support the light-controlled alignment layer 2 and the liquid crystal layer 3;

[0079] A light-alignment layer 2, covering the surface of the substrate 1, is used to provide a light alignment pattern;

[0080] Liquid crystal layer 3 covers the surface of light-controlled alignment layer 2. The liquid crystal polymer in liquid crystal layer 3 has a periodic helical structure, and the pitch of the helical structure is gradient-distributed in the range of 184-199nm.

[0081] The pattern in the light-controlled alignment layer 2 is formed by using the Mach-Zehnder optical path and two orthogonal circularly polarized light beams to interfere and expose, thus forming a PB phase grating pattern on the light-controlled alignment layer 2.

[0082] A schematic diagram of the spiral structure in the XY plane is shown below. Figure 2 As shown, the cross-sectional diagram in the XZ plane is as follows. Figure 3 As shown, the cross-sectional diagram in the XY plane is as follows. Figure 4 As shown in the figure, the optical axis of the liquid crystal molecules rotates along the x-axis around the z-axis at different twist angles α. This continuous change in angle produces the rotation period Λ. x Meanwhile, due to the self-assembling spiral structure of chiral liquid crystals, there is also optical axis rotation in the y-axis direction, generating a rotation period Λ. y The torsion angle α can be expressed by the following formula (1):

[0083]

[0084] The period in the x-axis direction is achieved by adjusting the angle between the two exposure beams of the Mach-Zehnder, while the period in the y-axis direction is achieved by adjusting the concentration of the chiral dopant in the liquid crystal. Figure 3 This indicates the gradual change in pitch gradient of the polymer along the thickness y-axis after the liquid crystal polymer has been exposed and cured using a photomask.

[0085] The present invention also provides a mask for preparing the above-mentioned liquid crystal polarization grating with a gradient transmittance, wherein the gradient transmittance varies in the range of 7%-37%.

[0086] Using a mask with a transmittance range of 7%-37%, which can be matched with the amount of chiral dopant and ultraviolet absorber in the liquid crystal precursor, can enable the prepared liquid crystal polarization grating to obtain the maximum blue shift of the center wavelength, significantly increase the wavelength width of the liquid crystal polarization grating, and thus increase the angular bandwidth of the entrance pupil grating of the liquid crystal polarization grating.

[0087] Specifically, the photomask has grid areas with different transmittances. The transmittance of the grid areas in each row gradually decreases from left to right, and the transmittance of the rightmost grid area in each row is greater than that of the leftmost grid area in the next row; or the transmittance of the grid areas in each row gradually increases from left to right, and the transmittance of the rightmost grid area in each row is less than that of the leftmost grid area in the next row; the transmittance difference between adjacent grid areas is 0.5%-5%, for example, 2%, 3%, or 4%. The number of grid areas can be at least 5, for example, 6, 9, 10, 13, 16, 25, 36, or 49. That is, the photomask can be any one of a six-grid gradient photomask, a nine-grid gradient photomask, a sixteen-grid gradient photomask, a twenty-five-grid gradient photomask, a thirty-six-grid photomask, and a forty-nine-grid photomask, or it can be a horizontal strip gradient photomask with ten or thirteen strips. The mask can be any shape selected from square, rectangle, rhombus, ellipse, and circle, but is generally square or rectangular. The range of gradient transmittance of the mask can also be 9%-36%, 10%-30%, or 7%-31%.

[0088] In one embodiment of this invention, the chiral dopant concentration in the polymer precursor layer is 2.19% by mass, and the ultraviolet absorber concentration is 1.7% by mass. This concentration was selected by controlling the variable method to change the concentration range of either the chiral dopant or the ultraviolet absorber, choosing the concentration of the chiral dopant or the ultraviolet absorber that corresponds to the largest shift in the center wavelength. The blue shift of the center wavelength of the liquid crystal polarization grating device under different ultraviolet exposure doses was investigated, with the ultraviolet absorber concentration fixed and the chiral dopant concentration changed. The different formulations of the liquid crystal polarization grating samples are shown in Table 1 below. The liquid crystal raw material is 1,4-bis-[4-(3-acryloyloxypropoxy)benzoyloxy]-2-methylbenzene (RM257), the photoinitiator is α,α-dimethoxy-α-phenylacetophenone (IR651), the UV absorber is butylmethoxydibenzoylmethane (avobenzone), and the chiral dopant is (13BR)-5,6-dihydro-5-(trans-4-propylcyclohexyl)-4H-dinaphtho[2,1-F:1',2'-H][1,5]dioxane-nonatetraene (R5011). The UV exposure dose ranges from 4.661 J / cm². 2 Reduced to 0.458 J / cm 2 The dosages differ by an order of magnitude, and the dosage variation range is shown in Table 2.

[0089] Table 1 Different proportions of liquid crystal polarization grating samples

[0090]

[0091]

[0092] Table 2. Different UV exposure doses for the samples

[0093]

[0094] Different samples (A1–A4) with chiral dopant concentrations ranging from 2.15 wt.% to 2.21 wt.% were prepared and cured under the same UV exposure dose range (C1–C6). The changes in the center wavelength of the different samples were measured, such as… Figure 6 As shown in the figure, at a specific concentration of UV absorber (1.5 wt.%), when the concentration of chiral dopant increases from 2.15 wt.% to 2.21 wt.%, the measured spectrum shows that as the UV exposure dose increases from 4.661 J / cm², the change in concentration with increasing UV exposure dose is observed. 2 It gradually decreased to 0.458 J / cm 2 Subsequently, the diffraction center wavelength shift range of the polarization grating corresponding to different concentrations of chiral dopant varied. When the chiral dopant concentration was 2.15 wt.%, the center wavelength blue-shifted by 2.5 nm; when the chiral dopant concentration was 2.17 wt.%, the center wavelength blue-shifted by 5.5 nm; when the chiral dopant concentration was 2.19 wt.%, the center wavelength blue-shifted by 9.5 nm; and when the chiral dopant concentration was 2.21 wt.%, the center wavelength blue-shifted by 5 nm. That is, as the chiral dopant concentration increased, the center wavelength shift range showed a trend of first increasing and then decreasing. There was a maximum center wavelength blue-shift range at a chiral dopant concentration of 2.19 wt.%. With increasing chiral dopant concentration, the center wavelength blue-shift range first increased and then decreased.

[0095] By varying the concentration of ultraviolet absorber in the samples (B1–B3), and preparing and curing them under the same ultraviolet exposure dose range (C1–C6), the change in center wavelength of different samples was measured. Figure 7 As shown in the figure, at a specific concentration of chiral dopant (2.19 wt.%), when the concentration of the UV absorber increases from 1.5 wt.% to 1.9 wt.%, the measured spectrum shows that as the UV exposure dose increases from 4.661 J / cm², the change in UV intensity is observed. 2 It gradually decreased to 0.458 J / cm 2Subsequently, the diffraction center wavelength shift range of the polarization grating corresponding to different concentrations of ultraviolet absorber varied. When the ultraviolet absorber concentration was 1.5 wt.%, the center wavelength blue-shifted by 9.5 nm; when the ultraviolet absorber concentration was 1.7 wt.%, the center wavelength blue-shifted by 11.5 nm; and when the chiral agent concentration was 1.9 wt.%, the center wavelength blue-shifted by 7 nm. That is, as the ultraviolet absorber concentration increases, the center wavelength shift range shows a trend of first increasing and then decreasing. There is a maximum center wavelength blue-shift range at an ultraviolet absorber concentration of 1.7 wt.%.

[0096] In this embodiment, the transmittance variation range of the mask was determined by using the sample ratio with the largest blue shift in the center wavelength, i.e., a chiral dopant concentration of 2.19 wt.% and a UV absorber concentration of 1.7 wt.%. Based on the UV exposure dose variation range mentioned above, the exposure mask was designed, and after normalization calculations, the gradient transmittances were obtained as 100%, 77%, 46%, 31%, 17%, and 10%. Further analysis... Figure 6 and Figure 7 The results show that, regardless of whether the spectral curves of different concentrations of chiral dopants change with UV exposure dose or the spectral curves of different concentrations of UV absorbers change with UV exposure dose, within the range (2.147 J / cm²), the results are consistent. 2 -0.458J / cm 2 The blue shift of the center wavelength will be more pronounced only under ultraviolet exposure doses within a certain range. Since the maximum dose may be selected for different samples, it is possible that it will be 2.147 J / cm². 2 -1.45J / cm 2 Therefore, based on the previous experiments, we adjusted the gradient transmittance distribution of the ultraviolet exposure mask to 7%-37%.

[0097] The present invention will be further illustrated below with reference to specific embodiments. It should be understood that these embodiments are for illustrative purposes only and are not intended to limit the scope of the invention. Experimental methods in the following embodiments, unless otherwise specified, are generally performed according to the conditions recommended by the manufacturer.

[0098] Example 1

[0099] The method for fabricating a liquid crystal polarization grating provided in this embodiment includes the following steps:

[0100] S1. An azobenzene photo-alignment layer is formed on a glass substrate, and a liquid crystal polymer precursor layer is formed on the other side of the azobenzene photo-alignment layer; the mass percentage of chiral dopant in the polymer precursor layer is 2.19%, the mass percentage of ultraviolet absorber is 1.7%, and the remainder is liquid crystal raw material.

[0101] S2. A sixteen-grid gradient mask is applied to the liquid crystal polymer precursor film described in step S1. The sixteen-grid gradient mask is as follows: Figure 5 As shown, the size of the sixteen-grid gradient mask is 120μm×120μm. The gradient transmittance of the sixteen-grid gradient mask ranges from 7% to 37%. The transmittance of adjacent grids differs by 2%. The numbers in the grids represent the transmittance of that grid.

[0102] S3. Expose the liquid crystal polymer precursor, for which a mask was applied in step S2, to ultraviolet light at a dose of 2.147 J / cm². 2 -1.45J / cm 2 The liquid crystal polymer precursor is used to form a liquid crystal polymer with a periodic helical structure to obtain the liquid crystal polarization grating.

[0103] Example 2

[0104] The method for fabricating a liquid crystal polarizing grating provided in this embodiment differs from that in Embodiment 1 in that a nine-grid gradient mask is used in step S2. This nine-grid gradient mask is as follows: Figure 8 As shown, the size of the nine-grid gradient mask is 90μm×90μm, and its gradient transmittance varies between 7% and 31%. The numbers in the grid represent the transmittance of that grid, and the transmittance of adjacent grids differs by 3%.

[0105] Example 3

[0106] The method for fabricating a liquid crystal polarizing grating provided in this embodiment differs from that in Embodiment 1 in that a six-grid gradient mask is used in step S2. This six-grid gradient mask is as follows: Figure 9 As shown, the size of the six-grid gradient mask is 60μm×40μm, and its gradient transmittance varies between 10% and 30%. The numbers in the grid represent the transmittance of that grid, and the transmittance of adjacent grids differs by 4%.

[0107] Example 4

[0108] The method for fabricating a liquid crystal polarizing grating provided in this embodiment differs from that in Embodiment 1 in that step S2 uses a transversely striped gradient mask with ten gradient stripes, such as... Figure 10 As shown, the dimensions of the horizontal strip gradient mask are 100μm×100μm, and its gradient transmittance varies between 9% and 36%. The numbers in the grid represent the transmittance of that grid, and the transmittance of adjacent grids differs by 3%.

[0109] Example 5

[0110] The method for fabricating a liquid crystal polarizing grating provided in this embodiment differs from that in Embodiment 1 in that step S2 uses a transversely striped gradient mask with 13 gradient stripes. Figure 11 As shown, the dimensions of the horizontal strip gradient mask are 130μm×130μm, and its gradient transmittance varies between 7% and 31%. The numbers in the grid represent the transmittance of that grid, and the transmittance of adjacent grids differs by 2%.

[0111] Comparative Example 1

[0112] The method for fabricating a liquid crystal polarization grating provided in this embodiment includes the following steps:

[0113] S1. An azobenzene photo-alignment layer is formed on a glass substrate, and a liquid crystal polymer precursor layer is formed on the other side of the azobenzene photo-alignment layer; the mass percentage of chiral dopant in the polymer precursor layer is 2.19%, the mass percentage of ultraviolet absorber is 1.7%, and the remainder is liquid crystal raw material.

[0114] S2. The liquid crystal polymer precursor is exposed to ultraviolet light at a dose of 2.147 J / cm². 2 -1.45J / cm 2 The liquid crystal polarization grating is obtained.

[0115] Effect Example

[0116] The liquid crystal polarization gratings prepared in Examples 1-5 and Comparative Example 1 were tested using the following methods:

[0117] The wavelength-transmittance test method is to perform a spectral scan using an ultraviolet spectrophotometer, place the sample at the test position, and set the test wavelength range to 400-700 nm for visible light.

[0118] The diffraction efficiency versus incident angle curve was obtained by placing the sample on a fixture and then immersing it in a refractive index matching solution with a refractive index of 1.56, consistent with the average refractive index of the liquid crystal polymer. Keeping the incident light position constant, the sample was rotated, and the initial light intensity and the diffracted light intensity were recorded using an optical power meter. The ratio of these two values ​​yielded the diffraction efficiency at a specific angle. This process was repeated throughout the rotation, and the diffraction efficiency versus incident angle curve was then calculated.

[0119] Figure 12 The wavelength-transmittance spectra of the liquid crystal polarization gratings prepared in Example 1 and Comparative Example 1 are shown below. Figure 12It can be seen that, compared with the liquid crystal polarization grating prepared without the gradient transmittance mask, the half-wavelength of the liquid crystal polarization grating prepared using the sixteen-grid gradient transmittance mask in Example 1 increased from 53° to 84°, which is about 37%.

[0120] Figure 13 The curves showing the diffraction efficiency of the liquid crystal polarization gratings prepared in Example 1 and Comparative Example 1 as a function of the angle of incidence are presented. Figure 13 It can be seen that, compared with the liquid crystal polarization grating prepared without the gradient transmittance mask, the angular bandwidth of the liquid crystal polarization grating prepared using the sixteen-grid gradient transmittance mask in Example 1 increased from 38° to 61°, which is about 38%.

[0121] Figure 14 This is a graph showing the diffraction efficiency of the liquid crystal polarization grating prepared in Example 2 as a function of the incident angle. Figure 14 It can be seen that the diffraction efficiency of the liquid crystal polarization grating prepared by using a nine-grid gradient mask in Example 2 is greater than 80%, and the angular bandwidth becomes 59°, which is much higher than the angular bandwidth of 38° of the liquid crystal polarization grating prepared by Comparative Example 1 without using a gradient transmittance mask.

[0122] Figure 15 This is a graph showing the diffraction efficiency of the liquid crystal polarization grating prepared in Example 3 as a function of the incident angle. Figure 15 It can be seen that the diffraction efficiency of the liquid crystal polarization grating prepared by using a six-grid gradient mask in Example 3 is greater than 80%, and the angular bandwidth becomes 59°, which is much higher than the angular bandwidth of 38° of the liquid crystal polarization grating prepared by Comparative Example 1 without using a gradient transmittance mask.

[0123] Figure 16 This is a graph showing the diffraction efficiency of the liquid crystal polarization grating prepared in Example 4 as a function of the incident angle. Figure 16 It can be seen that the diffraction efficiency of the liquid crystal polarization grating prepared by the transverse strip gradient mask used in Example 4 is greater than 80%, and the angular bandwidth is 56°, which is much higher than the angular bandwidth of 38° of the liquid crystal polarization grating prepared by Comparative Example 1 without the use of a gradient transmittance mask.

[0124] Figure 17 This is a graph showing the diffraction efficiency of the liquid crystal polarization grating prepared in Example 5 as a function of the incident angle. Figure 17 It can be seen that the diffraction efficiency of the liquid crystal polarization grating prepared by the transverse strip gradient mask used in Example 5 is greater than 80%, and the angular bandwidth becomes 59°, which is much higher than the angular bandwidth of 38° of the liquid crystal polarization grating prepared by Comparative Example 1 without the use of a gradient transmittance mask.

[0125] While the disclosure is as stated above, its scope of protection is not limited thereto. Those skilled in the art can make various changes and modifications without departing from the spirit and scope of this disclosure, and all such changes and modifications will fall within the protection scope of this invention.

Claims

1. A method for fabricating a liquid crystal polarization grating, characterized in that, include: Step S1: Form an azobenzene photoalignment layer on a transparent substrate, and then form a liquid crystal polymer precursor on the other side of the azobenzene photoalignment layer; Step S2: Apply a mask to the liquid crystal polymer precursor described in step S1. The mask has a gradient transmittance. The mask is a horizontal strip gradient mask or a grid gradient mask. The transmittance difference between adjacent grid areas of the gradient photomask is 0.5%-5%; Step S3: Expose the liquid crystal polymer precursor with ultraviolet light applied in step S2 to form a liquid crystal polymer with a periodic helical structure, thereby obtaining the liquid crystal polarization grating; the pitch of the helical structure is gradient distributed on the horizontal and vertical planes of the film formed by the liquid crystal polymer. The liquid crystal polymer precursor includes liquid crystal raw materials, photoinitiators, ultraviolet absorbers, and chiral dopants; The chiral dopant accounts for 2.15%-2.21% of the mass percentage of the liquid crystal polymer precursor; The ultraviolet absorber accounts for 1.5%-1.9% of the mass percentage of the liquid crystal polymer precursor; The gradient transmittance of the photomask ranges from 7% to 37%.

2. The method for fabricating a liquid crystal polarization grating according to claim 1, characterized in that, The liquid crystal raw material includes 1,4-bis-[4-(3-acryloyloxypropoxy)benzoyloxy]-2-methylbenzene and / or 1,4-bis-[4-(6-acryloyloxyhexyloxy)benzoyloxy]-2-methylbenzene.

3. The method for fabricating a liquid crystal polarization grating according to claim 1, characterized in that, The photoinitiator includes α,α-dimethoxy-α-phenylacetophenone.

4. The method for fabricating a liquid crystal polarization grating according to claim 1, characterized in that, The ultraviolet absorber includes any one of butyl methoxybenzoylmethane, 2-(2′-hydroxy-3′,5′-di-tert-butylphenyl) and 2-hydroxy-4-n-octyloxybenzophenone.

5. The method for fabricating a liquid crystal polarization grating according to claim 1, characterized in that, The chiral dopant includes (13BR)-5,6-dihydro-5-(trans-4-propylcyclohexyl)-4H-dinaphtho[2,1-F:1',2'-H][1,5]dioxane-nonatrane or 5,6-dihydro-5-(trans-4-propylcyclohexyl)-4H-dinaphtho(2,1-F:1',2'-H)(1,5)dioxane-nonatrane.

6. The method for fabricating a liquid crystal polarization grating according to claim 1, characterized in that, The liquid crystal polymer precursor also includes a surfactant, which includes a fluorocarbon surfactant.

7. The method for fabricating a liquid crystal polarization grating according to claim 1, characterized in that, The exposure dose is 1.45 J / cm². 2 - 2.147J / cm 2 .

8. The method for fabricating a liquid crystal polarization grating according to claim 1, characterized in that, The chiral dopant accounts for 2.19% of the mass of the liquid crystal polymer precursor, and the ultraviolet absorber accounts for 1.7% of the mass of the liquid crystal polymer precursor.

9. A liquid crystal polarization grating prepared by the method of preparing a liquid crystal polarization grating according to any one of claims 1-8, characterized in that, The helical structure has a gradient pitch length, with the pitch of the helical structure being 184-199 nm.

Images