A polarization holographic optical element and a preparation method thereof

Abstract

The present invention discloses a polarization holographic optical element and a preparation method thereof, comprising: a substrate layer and an anisotropic medium layer; wherein, the substrate layer includes a substrate and a surface relief microstructure layer, the surface relief microstructure layer includes relief microstructures, the relief microstructures are arranged in an array on the substrate, and the anisotropic medium layer uses a liquid crystal material. The polarization holographic optical element and the preparation method thereof provided by the present invention reduce the preparation difficulty of the polarization holographic optical element, and the size and angle controllability of the surface microstructure expand the design and optimization dimensions of the polarization holographic optical system, providing the possibility for the production and application of large-scale polarization holographic optical elements.

Description

Technical Field

[0001] The present 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 in order to achieve more precise and multifunctional optical modulation. In 2016, researchers combined volume grating Bragg diffraction with the Pancharatnam-Berry (PB) phase modulation mechanism, and proposed a polarization holographic grating by using the polarization light orientation technology and the liquid crystal self-assembly characteristics. The polarization holographic grating is a diffractive optical element formed by patterning a thin film through anisotropy in optics, and has the characteristic of a uniform anisotropy value, resulting in a relatively high diffraction efficiency, a single diffraction order for the polarization holographic grating compared with traditional holographic optical elements, and the polarization holographic grating uses a polymerizable liquid crystal material to design the optical element into multiple liquid crystal sub-layers with a single alignment layer. By adding a chiral dopant to the reactive liquid crystal molecules (also called low molecular weight polymerizable liquid crystals, LCPs), chiral twisting can be achieved in each layer. This chiral twisting helps to adjust and optimize the angular bandwidth and wavelength bandwidth at high diffraction efficiency.

[0003] However, although the structure of this polarization holographic grating makes its preparation process simpler than that of traditional holographic optical elements, and the grating has excellent diffraction characteristics and can achieve more complex optical functions, the arrangement and curing of the reactive liquid crystal molecules in this grating structure still make it impossible to avoid the preparation of the alignment layer and polarization holographic exposure during the grating preparation process. The alignment material requires a series of operations such as preparing a solution and spin coating. The exposure optical path requires more stringent conditions and is more complex to set up compared with ordinary optical paths, and the application field is narrow and is often used in coupling devices in augmented reality.

[0004] Since the interaction between light and optical elements needs to be jointly restricted by both material and geometric parameters, the design dimension of the optical elements required for setting up the exposure optical path is small, the process of preparing the alignment material for the alignment layer preparation is complex, large-scale preparation cannot be initially achieved, and the existing polarization holographic gratings have a single application field. Therefore, those skilled in the art need to seek a new type of polarization holographic optical element. Summary of the Invention

[0005] Objective: To overcome the problems in the prior art, such as high operation difficulty in the preparation process of existing polarization holographic optical elements, complex setup of the exposure optical path, inability to initially achieve large-scale preparation, and the single application field caused by the preparation process of polarization holographic gratings, the present invention provides a polarization holographic optical element and its preparation method. This design and preparation method utilize the surface undulating microstructure to cause the substrate surface to be concave and uneven, thereby overcoming the limitations of the alignment layer preparation material and the design dimension of the polarization holographic exposure optical path in the preparation process of existing polarization holographic optical elements, reducing the preparation difficulty of polarization holographic optical elements. The prepared polarization holographic optical elements can be used in 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 as follows:

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

[0008] Among them, 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, and the anisotropic medium layer uses liquid crystal materials.

[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 undulating 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 , the longitudinal period , the period , a, b, c, , , and The relationship expression between them is as follows:

[0013]

[0014]

[0015]

[0016]

[0017]

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

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

[0020] Clean the substrate.

[0021] Use a micro-nano fabrication process to arrange the undulating microstructures in an array on the substrate, and clean the substrate again.

[0022] Uniformly coat the liquid crystal material solution through a coating process.

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

[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] Among them, the solute uses one or more liquid crystal monomers.

[0027] The photoinitiator uses one or more materials that enable the solution to form a polymer through ultraviolet light excitation.

[0028] The surfactant uses one or more materials that make the surface of the coated product flat.

[0029] The chiral agent uses a material that enables the liquid crystal material to form a left-handed or right-handed optically active state.

[0030] The solvent uses one or more organic solvents.

[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, 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 of a protective gas or a vacuum environment.

[0033] Beneficial effects: A polarization holographic optical element and a preparation method thereof provided by the present invention can replace the alignment layer preparation and polarization holographic exposure processes in the preparation of existing polarization holographic optical elements, reduce the preparation difficulty of polarization holographic optical elements, and the size and angle of the surface microstructure are controllable, expanding the design and optimization dimensions of the polarization holographic optical system, and providing the possibility for the production and application of large-scale polarization holographic optical elements.

[0034] The present invention orients liquid crystals by means of the surface unevenness and irregularities caused by the surface microstructure, and forms a three-dimensional period due to the self-assembly characteristics of the liquid crystals to generate a helical longitudinal period, replacing the alignment layer preparation and polarization holographic exposure processes in the preparation process of polarization holographic optical elements, reducing the complexity of the manufacturing process, providing the possibility for the large-scale preparation of polarization holographic optical elements, and expanding the design and optimization dimensions of polarization holographic optical elements.

[0035] A preparation method based on the structure of the polarization holographic optical element proposed by the present invention can adjust the structural parameters in real time through calculation according to actual requirements, can prepare a new type of polarization holographic optical element, and 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 It 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 diagram of the surface undulating microstructure layer and the anisotropic medium layer of the present invention.

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

[0039] Figure 4 It is a schematic diagram of the relationship between the transverse 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 It is a schematic diagram of the simulation model of the polarization holographic optical element of the present invention, wherein, Figure 5 in (a) is the simulation model after the light is incident normally, Figure 5 in (b) is the simulation model after the light is incident obliquely, Figure 5 in (c) is the simulation model of the undulating microstructure.

[0041] Figure 6 It 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 invention. Figure 6 in (a) is the relationship between the diffraction efficiency and the diffraction angle of the existing polarization volume holographic grating, Figure 6In figure (b), the relationship between the diffraction efficiency and diffraction angle of the polarization holographic optical element of the present invention is shown.

[0042] In the figure, 101 is the substrate, 102 is the surface relief microstructure layer, 103 is the anisotropic medium layer, 201 is the longitudinal period of liquid crystal molecules , 202 is the period of liquid crystal molecules , 203 is the transverse period of liquid crystal molecules , 204 is the left hypotenuse, 205 is the right hypotenuse, 206 is the base, 401 is the relief microstructure. Detailed implementation mode

[0043] Next, in combination with the attached drawings in the embodiments of the present invention, the technical solutions in the embodiments of the present invention will be clearly and completely described. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative work belong to the protection scope of the present invention.

[0044] Next, the present invention will be further described in combination with specific embodiments.

[0045] Embodiment 1:

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

[0047] Among them, the substrate layer includes a substrate 101 and a surface relief microstructure layer 102. The surface relief microstructure layer includes relief microstructures, and the relief microstructures are arranged in an array on the substrate 101. The anisotropic medium layer 103 is made of liquid crystal material. The liquid crystal material includes but is not limited to being composed of liquid crystal molecules arranged in a helical 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 relief microstructure includes but is not limited to Figure 3 the triangular prism structure shown.

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

[0052] Furthermore, as Figure 3 shown, the variable of the length of the bottom side 206 of the triangular surface 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, the included angle between the bottom side 206 and the left hypotenuse 204 is set as , the included angle between the bottom side 206 and the right hypotenuse 205 is set as , and the included angle between the left hypotenuse 204 and the right hypotenuse 205 is set as .

[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 during the preparation of the polarization holographic optical element. Therefore, the lateral period of the liquid crystal molecules in the anisotropic dielectric layer 103 , the longitudinal period of the liquid crystal molecules , the period of the liquid crystal molecules , a, b, c, , and The relational expressions between are as follows:

[0054]

[0055]

[0056]

[0057]

[0058]

[0059] Among them, 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 solution of the liquid crystal material includes but is not limited to reactive liquid crystal, chiral material, photoinitiator and chemical solvent. Among them, the chemical solvent is ethyl acetate.

[0061] Furthermore, as Figure 4As shown, the relational expression between the right hypotenuse b of the undulating microstructure 401 and the longitudinal period 201 of the liquid crystal molecules in the anisotropic dielectric layer 103 is as follows:

[0062]

[0063] where n is any positive integer.

[0064] Example 2:

[0065] This example introduces a preparation method of a polarization holographic optical element, which specifically includes:

[0066] Clean the substrate.

[0067] Adopt micro-nano fabrication technology to arrange the undulating microstructures in an array on the substrate, and clean the substrate.

[0068] Uniformly coat the liquid crystal material solution through a coating process.

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

[0070] Furthermore, cleaning the substrate includes ultrasonic cleaning the substrate with alcohol or other organic solvents and putting it into a plasma cleaner for vacuum cleaning.

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

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

[0073] Furthermore, the undulating microstructures 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] Among them, the solute uses one or more liquid crystal monomers, such as one or more liquid crystal monomers such as RM101, RM257, etc.

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

[0077] The surfactant uses one or more materials that can make the surface of the coated finished product flat, such as one or more materials such as a leveling agent, an antifoaming agent, etc.

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

[0079] The solvent uses one or more organic solvents, such as ethyl acetate, toluene, methyl ethyl ketone, and other one or more materials.

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

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

[0082] Example 3:

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

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

[0085] Step 2: Set the light source to be normally incident and obliquely incident, perform simulations respectively, and observe the change in its 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 volume holographic optical coupling element mentioned in the present invention through the simulation results, and adjust the simulation model in real time according to the requirements according to the simulation results.

[0088] Furthermore, a substrate layer and an anisotropic medium layer are set in the simulation process of this example, where the substrate layer includes a substrate and a relief microstructure.

[0089] Furthermore, the anisotropic medium in the model of this example is a cholesteric liquid crystal with a helical molecular structure in the liquid crystal state, and its liquid crystal material is set as RM257, S811, ethyl acetate, and a photoinitiator.

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

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

[0092] Furthermore, the tilt angle of the triangular prism microstructures is 10° - 40°. In this embodiment, the included angle is set to 30°.

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

[0094] Furthermore, the lateral period of the liquid crystal molecules in the anisotropic dielectric layer is 380 nm - 780 nm. In this embodiment, the lateral period of the liquid crystal molecules of the model is set to 390 nm, the effective refractive index is 1.67, the Bragg wavelength in vacuum is 530 nm, and the calculation formulas for other structural parameters are as follows:

[0095]

[0096]

[0097]

[0098]

[0099]

[0100] where is the lateral period of the liquid crystal molecule arrangement, is the longitudinal period of the liquid crystal molecule arrangement, is the Bragg period, is the Bragg wavelength in vacuum, P is the pitch, is the effective refractive index, n is the multiple, and c is the bottom side length of the microstructures.

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

[0102] Furthermore, the tilt 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 dielectric layer during the preparation of traditional polarization holographic optical elements, including but not limited to the polarization holographic exposure and the preparation process of the alignment layer.

[0104] In this embodiment, simulations are respectively performed on the existing polarization volume holographic grating and the structure mentioned in this article, and their diffraction characteristics are analyzed.

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

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

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

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

[0109] The above are only the preferred embodiments of the present invention. It should be noted that for those of ordinary skill in the art, without departing from the principle of the present invention, several improvements and refinements can be made, and these improvements and refinements should also be regarded as the protection scope of the present invention.

Claims

1. A polarization holographic optical element, characterized in that: Comprising: A substrate layer and an anisotropic dielectric layer; Wherein, the substrate layer includes a substrate and a surface relief microstructure layer. The surface relief microstructure layer includes relief microstructures, and the relief microstructures are arranged in an array on the substrate. The anisotropic dielectric layer is made of a liquid crystal material; The relief microstructure includes a triangular prism structure; The lateral period Λ of liquid crystal molecules in the anisotropic medium layer x , the longitudinal period Λ of liquid crystal molecules y , the period Λ of liquid crystal molecules B , a, b, c, α, and the relationship expressions between θ are as follows: 2Λ y = P 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, c is the length of the base of the angular face, θ is the angle between the base of the triangular face and the left hypotenuse, is the angle between the base of the triangular face and the right hypotenuse, α is the angle between the left hypotenuse and the right hypotenuse of the triangular face, 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; The liquid crystal material includes liquid crystal molecules arranged in a helical tilt; The material of the surface relief microstructure layer should be the same as that of the substrate.

2. The preparation method of a polarization holographic optical element according to claim 1, characterized in that: Specifically including: Clean the substrate; Use a micro-nano fabrication process to arrange the relief microstructures in an array on the substrate, and clean the substrate again; Uniformly coat the liquid crystal material solution through a coating process; Determine the curing method according to the liquid crystal type and cure it.

3. The preparation method according to claim 2, characterized in that: The material of the substrate includes glass, quartz, plastic, resin, silicon dioxide or silicon nitride.

4. The preparation method according to claim 2, characterized in that: The liquid crystal material solution includes a solute, a photoinitiator, a surfactant, a chiral agent and a solvent; Wherein, the solute uses one or more liquid crystal monomers; The photoinitiator uses one or more materials that cause the solution to form a polymer by ultraviolet light excitation; The surfactant uses one or more materials that make the surface of the coated product flat; The chiral agent uses a material that causes the liquid crystal material to form a left-handed or right-handed optically active state; The solvent uses one or more organic solvents.

5. The preparation method according to claim 2, wherein: The micro-nano fabrication process includes photolithography.

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

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