Polarization holographic optical element and preparation method thereof

Abstract

The invention discloses a polarization holographic optical element and a preparation method thereof. The polarization holographic optical element comprises a substrate layer and an anisotropic dielectric layer, wherein the substrate layer comprises a substrate and a surface fluctuating microstructure layer, the surface fluctuating microstructure layer comprises fluctuating microstructures, the fluctuating microstructures are arranged on the substrate in an array mode, and the anisotropic dielectric layer is made of liquid crystal materials. According to the polarization holographic optical element and the preparation method thereof, the preparation difficulty of the polarization holographic optical element is reduced, the size and the angle of the surface microstructure are controllable, the design optimization dimension of a polarization holographic optical system is expanded, and possibility is provided for production and application of large-scale polarization holographic optical elements.

Description

Technical Field

[0001] The invention relates to a polarization holographic optical element and a preparation method thereof, belonging to the technical field of optical elements. Background Art

[0002] With the continuous development of optical technology, researchers have gradually explored the possibility of combining polarization optics and holographic optics to achieve more precise and versatile optical modulation. In 2016, researchers combined volume grating Bragg diffraction with Pancharatnam-Berry (PB) phase control mechanism, and proposed a polarization holographic grating by using polarized light orientation technology and liquid crystal self-assembly characteristics. Polarization holographic grating is a diffraction optical element formed by patterning thin film through anisotropy in optics. It has the characteristic of uniform anisotropy value, which leads to higher diffraction efficiency and single diffraction order compared with traditional holographic optical elements. Polarization holographic grating uses polymerizable liquid crystal materials to design optical elements into multiple liquid crystal sublayers with a single orientation layer. By adding chiral dopants to reactive liquid crystal molecules (also called low molecular weight polymeric liquid crystal, LCP), chiral distortion can be achieved in each layer. This chiral distortion 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 has excellent diffraction characteristics and can achieve more complex optical functions, the arrangement and solidification of the reactive liquid crystal molecules in this grating structure still make it impossible to avoid the preparation of the orientation layer and polarization holographic exposure in the grating preparation process. The orientation material requires a series of operations such as solution configuration and spin coating. The exposure light path has more stringent conditions than the ordinary light path, is more complicated to build, and has a narrow application field. It is often used in coupling devices in augmented reality.

[0004] Since the interaction between light and optical elements is subject to the joint constraints of material and geometric parameters, the design dimensions of the optical elements required for building the exposure light path are small, the process of preparing the orientation material solution required for the orientation layer preparation is complicated, and large-scale preparation cannot be initially achieved. In addition, the existing polarization holographic gratings have a single application field. Therefore, technical personnel in this field need to seek a new type of polarization holographic optical element. Summary of the invention

[0005] Purpose: In order to overcome the problems in the prior art of the high difficulty in the operation of the existing polarization holographic optical element preparation process, the complex exposure light path construction, the inability to initially realize large-scale preparation, and the single application field caused by the polarization holographic grating preparation process, the present invention provides a polarization holographic optical element and a preparation method thereof. The design and preparation method utilize the surface undulation microstructure to cause the surface of the substrate to be concave and convex and uneven, thereby overcoming the limitations of the orientation layer preparation materials and the polarization holographic exposure light path design dimensions in the existing polarization holographic optical element preparation process, reducing the difficulty of preparing the polarization holographic optical element, and the prepared polarization holographic optical element can be used for polarization optical lenses, polarization holographic modulators, etc.

[0006] Technical solution: To solve the above technical problems, the technical solution adopted by the present invention is:

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

[0008] The substrate layer includes a substrate and a surface undulating microstructure layer. The surface undulating microstructure layer includes undulating microstructures. The undulating microstructures are arranged in an array on the substrate. The anisotropic medium layer uses liquid crystal material.

[0009] Optionally, the liquid crystal material includes but is not limited to liquid crystal molecules arranged in a helical tilt.

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

[0011] Optionally, the undulating microstructure includes but is not limited to a triangular prism structure.

[0012] Optionally, the lateral period of the liquid crystal molecules in the anisotropic medium layer , the longitudinal period of liquid crystal molecules , Liquid crystal molecular period , a, b, c, , and The relationship between them is expressed as follows:

[0013]

[0014]

[0015]

[0016]

[0017]

[0018] Among them, a is the length of the left hypotenuse of the triangular face, b is the length of the right hypotenuse of the triangular face, and c is the length of the base of the triangular face. is the angle between the base and the left hypotenuse of the triangle face, is the angle between the base and the right hypotenuse of the triangle face, is the angle between the left hypotenuse and the right hypotenuse of the triangle face, is the Bragg wavelength in vacuum, P is the pitch of the anisotropic medium, is the effective refractive index of the anisotropic medium, and n is the multiple.

[0019] In a second aspect, a method for preparing a polarization holographic optical element specifically comprises:

[0020] Clean the substrate.

[0021] The undulating microstructures are arranged in an array on a substrate using a micro-nano manufacturing process, and the substrate is cleaned again.

[0022] The liquid crystal material solution is evenly coated through a coating process.

[0023] Determine the curing method according to the type of liquid crystal, and cure.

[0024] Optionally, the material of the substrate includes but is not limited to glass, quartz, plastic, resin, silicon dioxide or silicon nitride.

[0025] Optionally, the liquid crystal material solution includes a solute, a photoinitiator, a surfactant, a chiral agent and a solvent.

[0026] The solute is one or more liquid crystal monomers.

[0027] Photoinitiators employ one or more materials that excite a solution by ultraviolet light to form a polymer.

[0028] Surfactants employ one or more materials that smooth the surface of the finished product after coating.

[0029] The chiral agent is a material that allows the liquid crystal material to form a left-handed or right-handed optical state.

[0030] As the solvent, one or more organic solvents are used.

[0031] Optionally, the micro-nano fabrication process includes but is not limited to etching, nanoimprinting or photolithography.

[0032] Optionally, the curing method includes but is not limited to a curing process in an oxygen-free environment or a vacuum environment with a protective gas and an energy of not less than 3 J / cm 2 The sample was irradiated with ultraviolet light to form a photopolymer.

[0033] Beneficial effects: The present invention provides a polarization holographic optical element and a preparation method thereof, which can replace the orientation layer preparation and polarization holographic exposure process in the preparation of existing polarization holographic optical elements, reducing the difficulty of preparing polarization holographic optical elements, 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.

[0034] The present invention utilizes the surface concavity and unevenness caused by the surface microstructure to align the liquid crystal, and due to the self-assembly characteristics of the liquid crystal, a spiral longitudinal period is generated to form a three-dimensional period, which 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 production process, provides the possibility for large-scale preparation of polarization holographic optical elements, and expands the design optimization dimension of polarization holographic optical elements.

[0035] A preparation method for a polarization holographic optical element structure proposed in the present invention can adjust the structural parameters in real time through calculation based on actual needs, and can prepare a new type of polarization holographic optical element that can be used as an optical lens, a polarization holographic modulator, a vector light field generator, etc. BRIEF DESCRIPTION OF THE DRAWINGS

[0036] Figure 1 The figure is a schematic diagram of the overall structure of a polarization holographic optical element provided in the implementation of the present invention.

[0037] Figure 2 It is a schematic structural diagram of the surface relief microstructure layer and the anisotropic medium layer of the present invention.

[0038] Figure 3 It is a schematic structural diagram of the undulating microstructure of the present invention.

[0039] Figure 4 It is a schematic diagram of the relationship between the lateral period of the undulating microstructure of the present invention and the longitudinal period of the liquid crystal molecules in the anisotropic medium layer.

[0040] Figure 5 Schematic diagram of the simulation model of the polarization holographic optical element of the present invention, wherein: Figure 5 (a) is the simulation model after the light is incident normally. Figure 5 (b) is the simulation model after the light is incident obliquely. Figure 5 (c) is the simulation model of the undulating microstructure.

[0041] Figure 6 It is a schematic diagram of simulation results of the existing polarization volume holographic grating and the polarization holographic optical element mentioned in the present invention. Figure 6 (a) shows the relationship between the diffraction efficiency and diffraction angle of the existing polarization holographic grating. Figure 6(b) shows the relationship between the diffraction efficiency and the diffraction angle of the polarization holographic optical element of the present invention.

[0042] In the figure, 101, substrate, 102, surface relief microstructure layer, 103, anisotropic medium layer, 201, longitudinal period of liquid crystal molecules , 202, Liquid crystal molecular period , 203, lateral period of liquid crystal molecules , 204, left hypotenuse, 205, right hypotenuse, 206, bottom edge, 401, undulating microstructure. DETAILED DESCRIPTION

[0043] The following is a clear and complete description of the technical solutions in the examples of the present invention in conjunction with the accompanying drawings in the examples of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative work are within the protection scope of the present invention.

[0044] The present invention will be further described below in conjunction with specific embodiments.

[0045] Embodiment 1:

[0046] This embodiment introduces a polarization holographic optical element, such as Figure 1 As shown, it includes: a substrate layer and an anisotropic medium layer 103 .

[0047] The substrate layer includes a substrate 101 and a surface relief microstructure layer 102, wherein the surface relief microstructure layer includes relief microstructures arranged in an array on the substrate 101, and the anisotropic medium layer 103 is made of liquid crystal material. The liquid crystal material includes but is not limited to liquid crystal molecules arranged in a spiral tilt.

[0048] Furthermore, 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.

[0049] Furthermore, the material of the surface relief microstructure layer 102 should be the same as that of the substrate 101 .

[0050] Furthermore, the undulating microstructure includes but is not limited to Figure 3 The triangular prism structure shown.

[0051] Further, such as Figure 2 As shown, the lateral period of the liquid crystal molecules in the anisotropic medium layer 103 is 203 is set as the grating transverse period of the polarization holographic optical element, and the liquid crystal molecule longitudinal period of the anisotropic medium layer 103 is set as 201 is set as the longitudinal period of the grating of the polarization holographic optical element, and the period of the liquid crystal molecules in the anisotropic medium layer 103 is 202 is set as the Bragg period of the polarization holographic optical element.

[0052] Further, such as Figure 3 As shown, the variable of the length of the base 206 of the triangular face corresponding to the undulating microstructure 401 is c, the variable of the length of the left hypotenuse 204 is a, the variable of the length of the right hypotenuse 205 is b, and the angle between the base 206 and the left hypotenuse 204 is set to , the angle between the bottom edge 206 and the right hypotenuse 205 is set to , the angle between the left hypotenuse 204 and the right hypotenuse 205 is set to .

[0053] Furthermore, the parameters of the undulating microstructure can be adjusted in real time according to the grating period, tilt angle and direction of the optical element in the process of preparing the polarization holographic optical element, and the required optical performance. Therefore, the lateral period of the liquid crystal molecules in the anisotropic medium layer 103 is , the longitudinal period of liquid crystal molecules , Liquid crystal molecular period , a, b, c, , and The relationship between them is expressed as follows:

[0054]

[0055]

[0056]

[0057]

[0058]

[0059] in, is the Bragg wavelength in vacuum, P is the pitch of the anisotropic medium, is the effective refractive index of the anisotropic medium, and n is the multiple.

[0060] Furthermore, the liquid crystal material solution includes but is not limited to reactive liquid crystal, chiral material, photoinitiator and chemical solvent, wherein the chemical solvent is such as ethyl acetate.

[0061] Further, such as Figure 4As shown, the relationship between the right hypotenuse b of the undulating microstructure 401 and the longitudinal period 201 of the liquid crystal molecules in the anisotropic medium layer 103 is expressed as follows:

[0062]

[0063] Wherein, n is any positive integer.

[0064] Embodiment 2:

[0065] This embodiment introduces a method for preparing a polarization holographic optical element, which specifically includes:

[0066] Clean the substrate.

[0067] The undulating microstructures are arranged in an array on a substrate using a micro-nano manufacturing process, and the substrate is cleaned.

[0068] The liquid crystal material solution is evenly coated through a coating process.

[0069] Determine the curing method according to the type of liquid crystal, and cure.

[0070] Furthermore, the substrate is cleaned, including ultrasonic cleaning of the substrate with alcohol or other organic solvents, and vacuum cleaning of the substrate in a plasma cleaning machine.

[0071] Furthermore, the ultrasonic cleaning and plasma cleaning machine vacuum cleaning include but are not limited to other processes that can make the substrate hydrophobic and clean the surface.

[0072] Furthermore, the micro-nano manufacturing process includes but is not limited to etching, nanoimprinting or photolithography.

[0073] Furthermore, the undulating microstructure should be processed with the surface corresponding to the bottom edge as the bottom, and should be arranged in an array order on the interface.

[0074] Furthermore, the liquid crystal material solution includes a solute, a photoinitiator, a surfactant, a chiral agent and a solvent.

[0075] The solute is one or more liquid crystal monomers, such as RM101, RM257 and the like.

[0076] The photoinitiator uses one or more materials that can excite the solution to form a polymer through ultraviolet light, such as one or more materials such as irgacure651 and irgacure18.

[0077] The surfactant is one or more materials that make the surface of the finished product smooth after coating, such as one or more materials such as a leveling agent and a defoaming agent.

[0078] The chiral agent is a material that allows the liquid crystal material to form a left-handed or right-handed optically active state, such as R5011 / S5011, S811, etc.

[0079] The solvent adopts one or more organic solvents, such as one or more materials of ethyl acetate, toluene, butanone, etc.

[0080] Furthermore, the types of the liquid crystal material include but are not limited to active liquid crystal and passive liquid crystal.

[0081] Furthermore, the curing method includes but is not limited to an oxygen-free environment or a vacuum environment protected by a gas such as nitrogen, with an energy of not less than 3 J / cm 2 The sample is irradiated with ultraviolet light to form a photopolymer and the direction of the liquid crystal is controlled by electrodes.

[0082] Embodiment 3:

[0083] This embodiment introduces the working principle of a polarization holographic optical element of the present invention, and illustrates it by building a simulation model of the polarization holographic optical element. Figure 5 As shown, Figure 5 (a) is the simulation model after the light is incident normally. Figure 5 (b) is the simulation model after the light is incident obliquely. Figure 5 (c) is a simulation model of the surface undulation microstructure. Specifically includes:

[0084] Step 1: Calculate and build the simulation model parameters of the polarization holographic optical coupling element mentioned in the present invention, and build the simulation model according to the parameters.

[0085] Step 2: Set the light source to normal incidence and oblique incidence, perform simulations respectively, and observe the changes in the refractive index.

[0086] Step 3: simulate the surface relief microstructure and observe its diffraction efficiency.

[0087] Step 4: Analyze the diffraction characteristics of the polarization holographic optical coupling element mentioned in the present invention through simulation results, and adjust the simulation model in real time according to the simulation results as needed.

[0088] Furthermore, in the simulation process of this embodiment, a substrate layer and an anisotropic medium layer are provided, wherein the substrate layer includes a substrate and an undulating microstructure.

[0089] Furthermore, the anisotropic medium described in the model of this embodiment is a cholesteric liquid crystal whose molecular structure is helical in a liquid crystal state, and its liquid crystal materials are set to be RM257, S811, ethyl acetate, and a photoinitiator.

[0090] Furthermore, the undulating microstructure is represented by a triangular prism microstructure in this embodiment.

[0091] Furthermore, the substrate thickness in the substrate layer is 0.1-1 mm, and the refractive index is 1-2.5. The model of this embodiment sets the substrate thickness to 0.75 mm, and the refractive index to 1.6.

[0092] Furthermore, the inclination angle of the triangular prism microstructure is 10° to 40°. is 30°.

[0093] Furthermore, the substrate and the triangular prism microstructure described in the model of this embodiment are made of glass.

[0094] Furthermore, the lateral period of the liquid crystal molecules in the anisotropic medium layer is 380nm~780nm. The model of this embodiment sets the lateral period of the liquid crystal molecules to At 390nm, the effective refractive index is 1.67, the Bragg wavelength in vacuum is 530nm, and the calculation formulas for other structural parameters are:

[0095]

[0096]

[0097]

[0098]

[0099]

[0100] in is the lateral period of the liquid crystal molecules, is the longitudinal period of the liquid crystal molecules, is the Prague cycle, is the Bragg wavelength in vacuum, P is the pitch, is the effective refractive index, n is the multiple, and c is the length of the bottom side of the microstructure.

[0101] Furthermore, the parameters set in this embodiment can be obtained from material parameters during the actual preparation process.

[0102] Furthermore, the inclination angle of the obliquely incident light in this embodiment is 15°.

[0103] The polarization holographic optical element described in this embodiment can be directly filled with liquid crystal after actual preparation according to the structural parameters, including but not limited to the coating process, eliminating the process of tilting the liquid crystal molecules in the anisotropic medium layer in the traditional polarization holographic optical element preparation process, including but not limited to the polarization holographic exposure and orientation layer preparation process.

[0104] This embodiment simulates the existing polarization volume holographic grating and the structure mentioned in this article respectively, and analyzes their diffraction characteristics.

[0105] Step 1: Calculate the structure of the existing polarization holographic optical element and the structural parameters mentioned in this article.

[0106] Step 2: Build a simulation model of the existing polarization holographic optical element and a simulation model of the structure mentioned in this article respectively.

[0107] Step 3: record and analyze the diffraction characteristic curves of the two structural simulation models.

[0108] In this embodiment, the substrate thickness is 0.75 mm, the refractive index is 1.6, and the surface relief microstructure is represented by a triangular prism microstructure, the inclination angle of the triangular prism microstructure is 30°, and the lateral period of the existing polarization holographic grating is 390 nm. Figure 6 As shown, Figure 6 (a) shows the relationship between the diffraction efficiency and diffraction angle of the existing polarization holographic grating. Figure 6 (b) is the relationship between the diffraction efficiency and the diffraction angle of the structure mentioned in this article, where the horizontal axis represents the incident angle and the vertical axis represents the diffraction efficiency. The range of the incident angle is -25° to 25°. The peak diffraction efficiency of the existing polarizer holographic grating is close to 0.5 within the range of the incident angle. The peak diffraction efficiency of the structure mentioned in this article is close to 0.7 within the range of the incident angle. The structure mentioned in this article has higher diffraction efficiency, larger angular bandwidth and better diffraction characteristics after simulation.

[0109] The above is only a preferred embodiment of the present invention. It should be pointed out that for ordinary technicians in this technical field, several improvements and modifications can be made without departing from the principle of the present invention. These improvements and modifications should also be regarded as the scope of protection of the present invention.

Claims

1. A polarization holographic optical element, characterized in that: include: Substrate layer, anisotropic medium layer; The substrate layer includes a substrate and a surface undulating microstructure layer. The surface undulating microstructure layer includes undulating microstructures. The undulating microstructures are arranged in an array on the substrate. The anisotropic medium layer uses liquid crystal material.

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

3. The 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. The polarization holographic optical element according to claim 1, characterized in that: The undulating microstructure includes, but is not limited to, a triangular prism structure.

5. The polarization holographic optical element according to claim 4, characterized in that: The lateral period of liquid crystal molecules in the anisotropic medium layer , the longitudinal period of liquid crystal molecules , Liquid crystal molecular period , a, b, c, , and The relationship between them is expressed as follows: ； ； ； ； ； Among them, a is the length of the left hypotenuse of the triangular face, b is the length of the right hypotenuse of the triangular face, and c is the length of the base of the triangular face. is the angle between the base and the left hypotenuse of the triangle face, is the angle between the base and the right hypotenuse of the triangle face, is the angle between the left hypotenuse and the right hypotenuse of the triangle face, is the Bragg wavelength in vacuum, P is the pitch of the anisotropic medium, is the effective refractive index of the anisotropic medium, and n is the multiple.

6. The method for preparing a polarization holographic optical element according to any one of claims 1 to 5, characterized in that: Specifically include: Cleaning the substrate; The undulating microstructures are arranged in an array on the substrate using a micro-nano manufacturing process, and the substrate is cleaned again; uniformly coating the liquid crystal material solution through a coating process; Determine the curing method according to the type of liquid crystal, and cure.

7. The preparation method according to claim 6, characterized in that: The material of the substrate includes, but is not limited to, glass, quartz, plastic, resin, silicon dioxide or silicon nitride.

8. The preparation method according to claim 6, characterized in that: The liquid crystal material solution comprises a solute, a photoinitiator, a surfactant, a chiral agent and a solvent; Wherein, the solute is one or more liquid crystal monomers; The photoinitiator uses one or more materials that excite the solution by ultraviolet light to form a polymer; The surfactant uses one or more materials that make the surface of the finished product smooth after coating; The chiral agent is a material that makes the liquid crystal material form a left-handed or right-handed optical state; As the solvent, one or more organic solvents are used.

9. The preparation method according to claim 6, characterized in that: The micro-nano manufacturing process includes but is not limited to etching, nanoimprinting or lithography.

10. The preparation method according to claim 6, characterized in that: The curing method includes but is not limited to a curing process in an oxygen-free environment or a vacuum environment with a protective gas and an energy of not less than 3 J / cm 2 The sample was irradiated with ultraviolet light to form a photopolymer.

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