Polarization holographic optical element and preparation method therefor

Abstract

Disclosed in the present invention are a polarization holographic optical element and a preparation method therefor. The polarization holographic optical element comprises: a substrate layer and an anisotropic medium layer, wherein the substrate layer comprises a substrate and a surface relief microstructure layer, the surface relief microstructure layer comprising relief microstructures, and the relief microstructures being arranged on the substrate in an array; and the anisotropic medium layer is made of a liquid crystal material. By means of the polarization holographic optical element and the preparation method therefor provided by the present invention, the preparation difficulty of the polarization holographic optical element is reduced, and the controllability of the size and angle of surface microstructures expands the design optimization dimensions of a polarization holographic optical system, making it possible for the large-scale production and application of the polarization holographic optical element.

Description

Polarization holographic optical element and preparation method thereof TECHNICAL FIELD

[0001] The present application relates to a polarization holographic optical element and a preparation method thereof, and belongs to the technical field of optical elements. BACKGROUND

[0002] With the continuous development of optical technology, researchers gradually explore the possibility of combining polarization optics and holographic optics in order to achieve more accurate and multifunctional optical modulation. In 2016, researchers combined volume Bragg diffraction with Pancharatnam-Berry (PB) phase control mechanism, and used polarization light orientation technology and liquid crystal self-assembly characteristics to propose a polarization holographic grating. The polarization holographic grating is a diffractive optical element formed by patterning a thin film through anisotropy in optics. It has the characteristic of uniform anisotropy value, which leads to higher diffraction efficiency, single diffraction order of the polarization holographic grating compared with traditional holographic optical elements, and the polarization holographic grating uses polymerizable liquid crystal material to design the optical element into multiple liquid crystal sub-layers with single directional layer. By adding chiral dopants to the reactive liquid crystal molecules (also called low molecular weight polymerizable liquid crystal, LCP), chiral twist can be achieved in each layer. This chiral twist helps to adjust and optimize the angular bandwidth and wavelength bandwidth under high diffraction efficiency.

[0003] However, although the structure of this polarization holographic grating makes its preparation process simpler than the traditional holographic optical element manufacturing process, and the grating diffraction characteristics are excellent, it can realize more complex optical functions, but the arrangement and curing of the reactive liquid crystal molecules in this grating structure still cannot avoid the preparation of the orientation layer and the polarization holographic exposure in the grating preparation process. The orientation material needs to be configured with a solution, spin-coated and a series of operations, and the required conditions for the exposure light path are more stringent than those for the ordinary light path, and the setup is more complex, and the application field is narrow, commonly used for coupling devices in augmented reality.

[0004] Because the interaction of light and optical elements needs to be limited by both material and geometric parameters, the design dimension of the optical elements required for the setup of the exposure light path is small, the process of preparing the orientation layer requires complex alignment of the orientation material solution, and it is not possible to initially achieve large-scale preparation, and the existing application field of the polarization holographic grating is single, therefore, the person skilled in the art needs to seek a new type of polarization holographic optical element. SUMMARY

[0005] Objective: In order to overcome the existing polarization holographic optical element preparation process in the prior art, the operation difficulty is high, the exposure light path is complex, the large-scale preparation cannot be initially realized, and the application field is single due to the preparation process of the polarization holographic grating, the present application provides a polarization holographic optical element and a preparation method thereof, which utilizes the concave-convex and uneven surface of the substrate caused by the surface relief microstructure, thereby overcoming the limitation of the orientation layer preparation material and the polarization holographic exposure light path design dimension in the existing polarization holographic optical element preparation process, reducing the preparation difficulty of the polarization holographic optical element, and the prepared polarization holographic optical element can be used for polarization optical lens, polarization holographic modulator and the like.

[0006] Technical scheme: In order to solve the above technical problems, the technical scheme adopted by the present application is:

[0007] In a first aspect, a polarization holographic optical element comprises: a substrate layer, an anisotropic medium layer.

[0008] The substrate layer comprises a substrate and a surface relief microstructure layer, the surface relief microstructure layer comprises relief microstructures arranged in an array on the substrate, and the anisotropic medium layer adopts liquid crystal material.

[0009] Optionally, the liquid crystal material comprises but is not limited to liquid crystal molecules arranged by helical tilt.

[0010] Optionally, the material of the surface relief microstructure layer and the material of the substrate should be the same.

[0011] Optionally, the relief microstructure comprises but is not limited to a triangular column structure.

[0012] Optionally, the liquid crystal molecules in the anisotropic medium layer have a transverse period Λ x , a longitudinal period Λ y , and a period Λ B , and the relationship between a, b, c, α, and θ is expressed as follows: 2Λ y =P

[0013] Wherein, a is the length of the left oblique side of the triangle, b is the length of the right oblique side of the triangle, c is the length of the base of the triangle, θ is the angle between the base and the left oblique side of the triangle, is the angle between the base and the right oblique side of the triangle, α is the angle between the left oblique side and the right oblique side of the triangle, λ B is the Bragg wavelength in vacuum, P is the pitch of the anisotropic medium, n eff is the effective refractive index of the anisotropic medium, and n is a multiple.

[0014] In a second aspect, a method for preparing a polarization holographic optical element, specifically comprising:

[0015] Cleaning the substrate.

[0016] Using a micro-nano fabrication process to arrange the relief microstructure in an array on the substrate, and cleaning the substrate again.

[0017] Uniformly coating the liquid crystal material solution by a coating process.

[0018] Determining the curing method according to the type of liquid crystal, and curing.

[0019] Optionally, the material of the substrate includes but is not limited to glass, quartz, plastic, resin, silicon dioxide or silicon nitride.

[0020] Optionally, the liquid crystal material solution includes solute, photoinitiator, surfactant, chiral agent and solvent.

[0021] The solute uses one or more liquid crystal monomers.

[0022] The photoinitiator uses one or more materials that can form a polymer by exciting the solution with ultraviolet light.

[0023] The surfactant uses one or more materials that can make the surface of the finished product after coating flat.

[0024] The chiral agent uses a material that can form a levorotatory or dextrorotatory optically active state of the liquid crystal material.

[0025] The solvent uses one or more organic solvents.

[0026] Optionally, the micro-nano fabrication process includes but is not limited to etching, nano-imprinting or photolithography.

[0027] Optionally, the curing method includes but is not limited to irradiating the sample with ultraviolet light with an energy of not less than 3 J / cm 2 to form a photopolymer in an oxygen-free environment or a vacuum environment under a protective gas.

[0028] Beneficial effects: The polarization holographic optical element and the preparation method thereof provided by the present application can replace the orientation layer preparation and polarization holographic exposure process in the preparation of the existing polarization holographic optical element, reduce the difficulty of preparing the polarization holographic optical element, and the size and angle of the surface microstructure are controllable, which expands the design optimization dimension of the polarization holographic optical system, and provides the possibility for realizing the production and application of large-scale polarization holographic optical elements.

[0029] The present application utilizes the surface concave-convex and unevenness caused by the surface microstructure to the liquid crystal orientation, and generates the helical longitudinal period due to the self-assembly characteristics of the liquid crystal, forms a three-dimensional period, replaces the orientation layer preparation and polarization holographic exposure process in the preparation process of the polarization holographic optical element, reduces the complexity of the manufacturing process, provides the possibility for large-scale preparation of the polarization holographic optical element, and expands the design optimization dimension of the polarization holographic optical element.

[0030] A preparation method of the polarization holographic optical element structure provided in the present application can adjust the structure parameters in real time through calculation based on the actual needs, can prepare a novel polarization holographic optical element, and can be used as an optical lens, a polarization holographic modulator, a vector light field generator and the like. BRIEF DESCRIPTION OF DRAWINGS

[0031] Fig. 1 is a schematic diagram of the overall structure of a polarization holographic optical element provided in the embodiment of the present application.

[0032] Fig. 2 is a schematic diagram of the structure of the surface relief microstructure layer and the anisotropic medium layer of the present application.

[0033] Fig. 3 is a schematic diagram of the structure of the relief microstructure of the present application.

[0034] Fig. 4 is a schematic diagram of the relationship between the lateral period of the relief microstructure and the longitudinal period of the liquid crystal molecules of the anisotropic medium layer of the present application.

[0035] Fig. 5 is a schematic diagram of the simulation model of the polarization holographic optical element of the present application, wherein Fig. 5(a) is a simulation model after normal incidence of light, Fig. 5(b) is a simulation model after oblique incidence of light, and Fig. 5(c) is a simulation model of the relief microstructure.

[0036] Fig. 6 is a schematic diagram of the simulation results of the existing polarization volume holographic grating and the polarization holographic optical element mentioned in the present application. Fig. 6(a) is the relationship between the diffraction efficiency and the diffraction angle of the existing polarization volume holographic grating, and Fig. 6(b) is the relationship between the diffraction efficiency and the diffraction angle of the polarization holographic optical element of the present application.

[0037] In the figure, 101 is a substrate, 102 is a surface relief microstructure layer, 103 is an anisotropic medium layer, 201 is a liquid crystal molecule longitudinal period Λ y , 202 is a liquid crystal molecule period Λ B , 203 is a liquid crystal molecule lateral period Λ x , 204 is a left oblique side, 205 is a right oblique side, 206 is a bottom side, and 401 is a relief microstructure. DETAILED DESCRIPTION

[0038] The technical solutions in the embodiments of the present application will be clearly and completely described below with reference to the drawings in the embodiments of the present application. Obviously, the described embodiments are only part of the embodiments of the present application, rather than all the embodiments of the present application. Based on the embodiments of the present application, all other embodiments obtained by a person of ordinary skill in the art without creative effort belong to the protection scope of the present application.

[0039] The present application will be further described below with reference to specific embodiments.

[0040] Embodiment 1

[0041] This embodiment introduces a polarization holographic optical element, as shown in FIG. 1, which comprises a substrate layer and an anisotropic medium layer 103.

[0042] The substrate layer comprises a substrate 101 and a surface relief microstructure layer 102, the surface relief microstructure layer comprises relief microstructures arranged in an array on the substrate 101, and the anisotropic medium layer 103 adopts liquid crystal material. The liquid crystal material includes but is not limited to liquid crystal molecules arranged by helical tilt.

[0043] Further, the material of the substrate 101 includes but is not limited to glass, quartz, plastic, resin, silicon dioxide or silicon nitride, etc., and the refractive index of the substrate 101 is variable.

[0044] Further, the material of the surface relief microstructure layer 102 should be the same as that of the substrate 101.

[0045] Further, the relief microstructure includes but is not limited to a triangular prism structure as shown in FIG. 3.

[0046] Further, as shown in FIG. 2, the lateral period Λ x 203 of the liquid crystal molecules in the anisotropic medium layer 103 is set as the grating lateral period of the polarization holographic optical element, and the longitudinal period Λ y 201 of the liquid crystal molecules in the anisotropic medium layer 103 is set as the grating longitudinal period of the polarization holographic optical element. B 202 is set as the Bragg period of the polarization holographic optical element.

[0047] Further, as shown in FIG. 3, the variable of the length of the base 206 of the triangular face corresponding to the relief microstructure 401 is c, the variable of the length of the left oblique side 204 is a, the variable of the length of the right oblique side 205 is b, the angle between the base 206 and the left oblique side 204 is set as θ, the angle between the base 206 and the right oblique side 205 is set as the angle between the left oblique side 204 and the right oblique side 205 is set as α.

[0048] Further, the parameters of the relief microstructure can be adjusted in real time according to the grating period, tilt angle and direction of the optical element during the preparation of the polarization holographic optical element, so that the liquid crystal molecules of the anisotropic medium layer 103 have a transverse period Λ x , a longitudinal period Λ y of the liquid crystal molecules, a period Λ B of the liquid crystal molecules, a, b, c, α, , and θ are as follows: 2Λ y = P

[0049] where λ B is the Bragg wavelength in vacuum, P is the pitch of the anisotropic medium, n eff is the effective refractive index of the anisotropic medium, and n is a multiple.

[0050] Further, the solution of the liquid crystal material includes but is not limited to reactive liquid crystals, chiral materials, photoinitiators, and chemical solvents. Among them, the chemical solvent is ethyl acetate.

[0051] Further, as shown in FIG. 4, the relationship between the right oblique edge b of the relief microstructure 401 and the longitudinal period 201 of the liquid crystal molecules of the anisotropic medium layer 103 is as follows:

[0052] where n is any positive integer.

[0053] Example 2:

[0054] This embodiment introduces a preparation method of a polarization holographic optical element, which specifically includes:

[0055] Cleaning the substrate.

[0056] Using a micro-nano fabrication process to arrange the relief microstructure in an array on the substrate, and cleaning the substrate.

[0057] Uniformly coating the solution of the liquid crystal material by a coating process.

[0058] Determining the curing method according to the type of the liquid crystal, and curing.

[0059] Further, the substrate is cleaned, including ultrasonic cleaning the substrate with alcohol or other organic solvents, and vacuum cleaning in a plasma cleaning machine.

[0060] Further, the ultrasonic cleaning and the vacuum cleaning in the plasma cleaning machine include but are not limited to other processes that can make the substrate hydrophobic and clean the surface.

[0061] Further, the micro-nano fabrication process includes, but is not limited to, etching, nano-imprinting, or photolithography, etc.

[0062] Further, the relief microstructure should be processed with the bottom corresponding to the surface, and should be arranged in array order on the interface.

[0063] Further, the liquid crystal material solution includes solute, photoinitiator, surfactant, chiral agent, and solvent.

[0064] The solute adopts one or more liquid crystal monomers, such as one or more liquid crystal monomers of RM101, RM257, etc.

[0065] The photoinitiator adopts one or more materials that can form a polymer by exciting the solution with ultraviolet light, such as one or more materials of irgacure 651, irgacure 18, etc.

[0066] The surfactant adopts one or more materials that can make the surface of the finished product after coating flat, such as one or more materials of leveling agent, defoaming agent, etc.

[0067] The chiral agent adopts a material that can form a left-handed or right-handed optically active state of the liquid crystal material, such as R5011 / S5011, S811, etc.

[0068] The solvent adopts one or more organic solvents, such as one or more materials of ethyl acetate, toluene, butanone, etc.

[0069] Further, the type of the liquid crystal material includes, but is not limited to, active liquid crystal and passive liquid crystal, etc.

[0070] Further, the solidification method includes, but is not limited to, under the protection of nitrogen or other gas in an oxygen-free environment or a vacuum environment, ultraviolet light with energy not less than 3 J / cm 2 irradiating the sample to form a photopolymer, and controlling the direction of the liquid crystal through an electrode, etc.

[0071] Embodiment 3:

[0072] This embodiment introduces the working principle of the polarization holographic optical element, and the simulation model of the polarization holographic optical element is built to illustrate, as shown in FIG. 5, (a) is the simulation model after the light is normally incident, (b) is the simulation model after the light is obliquely incident, and (c) is the simulation model of the surface relief microstructure. Specifically, it includes:

[0073] Step 1, calculate the simulation model parameters of the polarization holographic optical coupling element mentioned in the application, and build the simulation model according to the parameters.

[0074] Step 2, set the light source to normal incidence and oblique incidence, respectively, simulate and observe the refractive index change.

[0075] Step 3, simulate the surface relief microstructure and observe its diffraction efficiency.

[0076] Step 4, analyze the diffraction characteristics of the polarization volume holographic optical coupling element mentioned in the application through simulation results, and adjust the simulation model in real time according to the simulation results according to the needs.

[0077] Further, in the simulation process of the embodiment, a substrate layer and an anisotropic medium layer are set, wherein the substrate layer includes a substrate and a relief microstructure.

[0078] Further, the anisotropic medium of the model in the embodiment is a cholesteric liquid crystal with a helical molecular structure in a liquid crystal state, and the liquid crystal material is set as RM257, S811, ethyl acetate and a photoinitiator.

[0079] Further, the relief microstructure is represented by a triangular prism microstructure in the embodiment.

[0080] Further, the thickness of the substrate in the substrate layer is 0.1-1mm, and the refractive index is 1-2.5. The model in the embodiment is set to have a substrate thickness of 0.75mm and a refractive index of 1.6.

[0081] Further, the inclination angle of the triangular prism microstructure is 10°-40°, and the model in the embodiment is set to have an included angle of 30°.

[0082] Further, the substrate and the triangular prism microstructure of the model in the embodiment are set to be made of glass.

[0083] Further, the lateral period of the liquid crystal molecules in the anisotropic medium layer is 380nm-780nm, and the model in the embodiment is set to have a lateral period of liquid crystal molecules Λ x 390nm, an effective refractive index n eff 1.67, a Bragg wavelength in vacuum of 530nm, and the calculation formula of other structure parameters is: 2Λ y =P

[0084] Wherein Λ x is the lateral period of the liquid crystal molecule arrangement, Λ y is the longitudinal period of the liquid crystal molecule arrangement, Λ B is the Bragg period, λ B is the Bragg wavelength in vacuum, P is the pitch, n eff is the effective refractive index, n is the multiple, and c is the length of the bottom side of the microstructure. ​

[0085] Further, the parameters set in the embodiment can be obtained from the material parameters in the actual preparation process.

[0086] Further, the light oblique angle of the light oblique incidence in the embodiment is 15°.

[0087] The polarization holographic optical element in the embodiment can be directly filled with liquid crystal after actual preparation according to the structural parameters, including but not limited to coating process, which eliminates the process of tilting liquid crystal molecules in the anisotropic medium layer in the traditional polarization holographic optical element preparation process, including but not limited to polarization holographic exposure and orientation layer preparation process.

[0088] The existing polarization volume holographic grating and the structure mentioned in the embodiment are simulated respectively, and the diffraction characteristics are analyzed.

[0089] Step 1, calculate the structure of the existing polarization holographic optical element and the structural parameters mentioned in the embodiment.

[0090] Step 2, build the simulation model of the existing polarization holographic optical element and the simulation model of the structure mentioned in the embodiment respectively.

[0091] Step 3, record and analyze the diffraction characteristic curves of the two structure simulation models.

[0092] In the embodiment, the substrate thickness is 0.75 mm, the refractive index is 1.6, the surface relief microstructure is represented by a triangular column microstructure, the triangular column microstructure oblique angle is 30°, and the transverse period of the existing polarization volume holographic grating is 390 nm. As shown in FIG. 6, (a) in FIG. 6 is the relationship between the diffraction efficiency and the diffraction angle of the existing polarization volume holographic grating, (b) in FIG. 6 is the relationship between the diffraction efficiency and the diffraction angle of the structure mentioned in the embodiment, wherein the abscissa represents the incident angle, the ordinate represents the diffraction efficiency, the range of the incident angle is-25°-25°, the diffraction efficiency peak of the existing polarization volume holographic grating is close to 0.5 in the range of the incident angle, the diffraction efficiency peak of the structure mentioned in the embodiment is close to 0.7 in the range of the incident angle, the diffraction efficiency of the structure mentioned in the embodiment is higher, the angular bandwidth is larger, and the diffraction characteristics are more excellent.

[0093] The above only describes the preferred embodiments of the present application, and it should be noted that for ordinary skilled in the art, without departing from the principles of the present application, a number of improvements and refinements can be made, which should also be considered as the protection scope of the present application.

Claims

1. A polarization holographic optical element, characterized by: Comprise: a substrate layer, an anisotropic medium layer; wherein the substrate layer comprises a substrate, a surface relief microstructure layer, the surface relief microstructure layer comprises relief microstructures arranged in an array on the substrate, and the anisotropic medium layer adopts liquid crystal material.

2. A polarization holographic optical element according to claim 1, characterized in that: The liquid crystal material comprises but is not limited to liquid crystal molecules arranged in a helical tilt.

3. A polarization holographic optical element according to claim 1, characterized in that: The material of the surface relief microstructure layer should be the same as that of the substrate.

4. A polarization holographic optical element according to claim 1, characterized in that: The relief microstructure comprises but is not limited to a triangular column structure.

5. A polarization holographic optical element according to claim 4, characterized in that: The liquid crystal molecules of the anisotropic medium layer have a lateral period Λ x , a longitudinal period Λ y of the liquid crystal molecules B , a, b, c, α, , and θ are expressed as follows: 2Λ y = P wherein a is the length of the left oblique side of the triangular face, b is the length of the right oblique side of the triangular face, c is the length of the base of the triangular face, and θ is the angle between the base and the left oblique side of the triangular face, is the included angle between the base and the right hypotenuse of the triangular face, a is the included angle between the left hypotenuse and the right hypotenuse of the triangular face, l B is the Bragg wavelength in vacuum, P is the pitch of the anisotropic medium, n eff is the effective refractive index of the anisotropic medium, n is the multiple.

6. A method of making a polarization holographic optical element according to any one of claims 1 to 5, characterized in that: Specifically comprising: cleaning the substrate; using a micro-nano fabrication process to arrange the relief microstructures in an array on the substrate, and cleaning the substrate again; uniformly coating a liquid crystal material solution by a coating process; determining the solidification method according to the type of liquid crystal, and solidifying.

7. The method of claim 6, wherein: The material of the substrate comprises but is not limited to glass, quartz, plastic, resin, silicon dioxide or silicon nitride.

8. The method of claim 6, wherein: The liquid crystal material solution comprises solute, photoinitiator, surfactant, chiral agent and solvent; wherein the solute adopts one or more liquid crystal monomers; the photoinitiator adopts one or more materials that can form polymers by exciting the solution with ultraviolet light; the surfactant adopts one or more materials that can make the surface of the finished product after coating flat; the chiral agent adopts a material that can make the liquid crystal material form a left-handed or right-handed optically active state; the solvent adopts one or more organic solvents.

9. The method of claim 6, wherein: The micro-nano fabrication process comprises but is not limited to etching, nano-imprinting or photolithography.

10. The method of claim 6, wherein: The curing method, including but not limited to, in the protection of gas in an oxygen-free environment or vacuum environment, energy is not less than 3J / cm 2 UV light irradiation sample forming photopolymer.

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