Liquid crystal structural color holographic film, preparation method thereof and color holographic device

Abstract

The invention discloses a liquid crystal structural color holographic film, a preparation method thereof and a color holographic device. The liquid crystal structural color holographic film comprises a nematic phase liquid crystal layer and a cholesteric phase liquid crystal layer which are arranged in a stacked mode, the total thickness of the nematic phase liquid crystal layer and the cholesteric phase liquid crystal layer is fixed, and a completely polymerized liquid crystal polymer network is formed. The cholesteric liquid crystal layer comprises a plurality of structural color areas, and in the different structural color areas, the spiral structure of the cholesteric liquid crystal layer has different screw pitches so as to reflect light of different colors; the cholesteric liquid crystal layer is provided with a patterned surface orientation structure, the surface orientation structure is obtained through optimization according to a color holographic algorithm, and the orientation structures of different structural color areas independently encode phase holograms of different wavelength channels. The liquid crystal structural color holographic film provided by the invention can display a structural color pattern in a near field and an independent color holographic image in a far field, so that light regulation and control of near-far field and wavelength combined multiplexing are realized, and the multiplexing dimension and functionality of a flexible optical element are improved.

Description

Technical Field

[0001] The present invention relates to the field of optical technology, and in particular to a liquid crystal structural color holographic film and a preparation method thereof, and a color holographic device. Background Art

[0002] In recent years, with the surge in data volume, the requirements for data storage, transmission, and processing capacity are also increasing. Light has many adjustable dimensions such as wavelength, amplitude, phase, polarization, etc., and has both high speed and high parallelism, so it is an ideal information carrier. In order to increase the information capacity of a single planar optical element, a series of light field multiplexing technologies have been born in recent years, which are expected to be applied to multiplexed optical communications, color displays, multi-channel information storage, information encryption and anti-counterfeiting. For example, wavelength multiplexing uses different wavelengths as independent channels for information. When light of different wavelengths is incident, different light field information can be read; polarization multiplexing stores information in orthogonal / non-orthogonal polarization states, and different incident polarization states will produce different light fields; near-field and far-field multiplexing uses near-field and far-field space at the same time, and can independently control near-field display patterns and far-field diffraction light fields.

[0003] In order to meet the needs of big data, multi-dimensional light field multiplexing that combines multiple dimensions is a major development trend in the field of light field control, which is expected to further increase the number of multiplexing channels and broaden potential application scenarios. Based on rigid materials, metasurfaces can achieve multi-dimensional joint manipulation of light fields by processing sub-wavelength-sized microstructure arrays. For example, polarization-dependent multi-color holography can be achieved by using metasurfaces that jointly multiplex polarization and wavelength; mixed multiplexing holography of polarization, wavelength, and longitudinal spatial position can be achieved by using reverse-designed metasurfaces. These technologies can achieve large information capacity and high-degree-of-freedom light field control, but they still face challenges in terms of flexibility, preparation difficulty, and processing cost. With the integrated development trend of modern photonic technology, multi-dimensional, multi-channel, and multi-functional flexible optical components are urgently needed.

[0004] Based on flexible materials, liquid crystal devices can be used to control the amplitude, phase, polarization, orbital angular momentum and other dimensions of light by controlling the spatial arrangement of anisotropic liquid crystal molecules. They have unique advantages such as high efficiency, low cost and dynamic adjustability. However, the current liquid crystal-based light field multiplexing technology still has problems such as a small number of channels and a single dimension, which limits the development and application of multi-dimensional light field multiplexing technology. Summary of the invention

[0005] The embodiments of the present invention provide a liquid crystal structural color holographic film and a preparation method thereof, and a color holographic device. The liquid crystal structural color holographic film can display a structural color pattern in the near field and simultaneously display an independent color holographic image in the far field, thereby realizing light regulation of near-field and far-field and wavelength joint multiplexing to improve the multiplexing dimension and functionality of flexible optical elements.

[0006] According to one aspect of the present invention, there is provided a liquid crystal structure color holographic film, comprising a nematic liquid crystal layer and a cholesteric liquid crystal layer stacked, wherein the total thickness of the nematic liquid crystal layer and the cholesteric liquid crystal layer is fixed, and the nematic liquid crystal layer and the cholesteric liquid crystal layer form a completely polymerized liquid crystal polymer network;

[0007] The cholesteric liquid crystal layer comprises a plurality of structural color regions, and in different structural color regions, the spiral structure of the cholesteric liquid crystal layer has different pitches to reflect light of different colors;

[0008] The cholesteric liquid crystal layer has a patterned surface orientation structure, the surface orientation structure is optimized according to a color holographic algorithm, and the orientation structures of different structural color regions independently encode phase holograms of different wavelength channels;

[0009] The multi-wavelength laser is incident from one side of the cholesteric liquid crystal layer, and the reflected light is diffracted to reproduce a wavelength-multiplexed color holographic image.

[0010] Optionally, the nematic liquid crystal layer has a liquid crystal polymer network with a parallel structure, and the cholesteric liquid crystal layer has a liquid crystal polymer network with a periodic spiral structure;

[0011] The periodic spiral structures of all structural color regions in the cholesteric liquid crystal layer have a consistent hand direction, presenting a left-handed spiral structure or a right-handed spiral structure.

[0012] Optionally, in the plane where the liquid crystal structural color holographic film is located, different structural color regions do not overlap;

[0013] In different structural color regions, the cholesteric liquid crystal layer has different thicknesses and the same number of helices.

[0014] Optionally, the cholesteric liquid crystal layer is provided with a spatially gradient surface alignment structure for controlling the distribution of starting directors of liquid crystal molecules on two surfaces of the cholesteric liquid crystal layer;

[0015] The surface orientation structure is obtained by optimizing the color holographic algorithm. First, multiple groups of near-field patterns and far-field holographic images are divided according to the set wavelength channel, and each group of the near-field patterns and the far-field holographic images corresponds to an independent wavelength channel. Then, the Gerchberg-Saxton holographic algorithm is used to iteratively optimize and merge the holographic phases corresponding to each wavelength channel. Finally, based on the corresponding relationship between the geometric phase of the cholesteric liquid crystal and the surface orientation angle, the surface orientation structure of the cholesteric liquid crystal layer is calculated.

[0016] Optionally, the cholesteric liquid crystal layer has different Bragg reflection bands in different structural color regions, and the structural color region can reflect light with a wavelength within the Bragg reflection band and a circular polarization handedness consistent with the spiral structure of the cholesteric liquid crystal layer, and has a geometric phase modulation effect, and the magnitude of the geometric phase is ±2 times the surface orientation angle of the cholesteric liquid crystal layer;

[0017] For multi-wavelength incident light, the phase modulation function of the structural color region is effective only when the incident wavelength matches the Bragg reflection band, and different structural color regions realize independent phase modulation functions at different wavelengths, thereby realizing wavelength multiplexing holography.

[0018] Optionally, the total thickness of the nematic liquid crystal layer and the cholesteric liquid crystal layer is greater than or equal to 20 times the maximum value of the helical pitch of liquid crystal molecules in the cholesteric liquid crystal layer.

[0019] According to another aspect of the present invention, a method for preparing a liquid crystal structural color holographic film is provided, which is used to prepare the above-mentioned liquid crystal structural color holographic film, and the preparation method comprises:

[0020] Providing a first substrate and a second substrate, forming a first alignment layer on one side of the first substrate, and forming a second alignment layer doped with a photoinitiator on one side of the second substrate;

[0021] The first substrate and the second substrate are arranged opposite to each other, and a spacer is arranged between the first substrate and the second substrate to form an empty box;

[0022] The empty box is subjected to ultraviolet exposure with a polarization direction varying with space, so as to prepare a cholesteric liquid crystal polymer network support with a patterned orientation structure between the first substrate and the second substrate;

[0023] Using ultraviolet exposure with exposure time varying with space, a nematic liquid crystal layer and a cholesteric liquid crystal layer stacked between the first substrate and the second substrate are formed;

[0024] Using ultraviolet light to completely polymerize the nematic liquid crystal layer and the cholesteric liquid crystal layer;

[0025] Peeling off the first substrate and the second substrate to form the liquid crystal structural color holographic film;

[0026] The thickness and structural color of the cholesteric liquid crystal layer are related to the exposure time, and the total thickness of the nematic liquid crystal layer and the cholesteric liquid crystal layer is fixed.

[0027] Optionally, the empty box is subjected to ultraviolet exposure with a polarization direction varying with space, and a cholesteric liquid crystal polymer network support with a patterned orientation structure is prepared between the first substrate and the second substrate, comprising:

[0028] Performing ultraviolet light alignment on an empty box consisting of the first substrate, the second substrate and the spacer, so that the alignment directions of the first alignment layer and the second alignment layer are consistent with the alignment structure designed by the color holographic algorithm;

[0029] Filling a mixture of cholesteric liquid crystal and polymerization reaction monomers between the first substrate and the second substrate, irradiating the second substrate with ultraviolet light from one side to perform surface induced photopolymerization to form a cholesteric liquid crystal polymer network support connected to the surface of the second substrate;

[0030] The polymerized liquid crystal cell is immersed in acetone to wash away the molecules that have not undergone polymerization;

[0031] The washed liquid crystal box is taken out from acetone, and the cholesteric liquid crystal polymer network support shrinks, but the periodic spiral structure and surface orientation structure of the cholesteric liquid crystal are still retained.

[0032] Optionally, a nematic liquid crystal layer and a cholesteric liquid crystal layer are stacked between the first substrate and the second substrate by using ultraviolet exposure with exposure time varying with space, comprising:

[0033] The shrunken cholesteric liquid crystal polymer network stent is subjected to ultraviolet exposure of different doses, and different regions are exposed for different times to control the resilience of the polymer network stent;

[0034] Refilling the liquid crystal box with a mixture of polymerization monomers and photoinitiators, the cholesteric liquid crystal polymer network support will rebound to form the nematic liquid crystal layer and the cholesteric liquid crystal layer stacked;

[0035] Among them, in the area where the UV exposure time is longer, the resilience of the cholesteric liquid crystal polymer network support is weaker, the thickness of the formed cholesteric liquid crystal layer is smaller, the pitch is smaller, the wavelength of the Bragg reflection band is shorter, and the structural color is more blue.

[0036] According to another aspect of the present invention, there is provided a color holographic device, comprising a supercontinuum laser, a polarizer, a quarter wave plate, the above-mentioned liquid crystal structural color holographic film and a light screen;

[0037] The multi-wavelength light beam output by the supercontinuum laser is converted into circularly polarized light with a chirality consistent with that of the cholesteric phase liquid crystal after being transmitted through the polarizer and the quarter-wave plate in sequence, and is incident on the liquid crystal structural color holographic film. The liquid crystal structural color holographic film independently performs geometric phase modulation on the incident light of different wavelengths and reflects it, and the color holographic image formed by the reflected light is received by the light screen.

[0038] The liquid crystal structural color holographic film provided by the embodiment of the present invention comprises a nematic liquid crystal layer and a cholesteric liquid crystal layer which are stacked, wherein the total thickness of the nematic liquid crystal layer and the cholesteric liquid crystal layer is fixed, and the nematic liquid crystal layer and the cholesteric liquid crystal layer form a fully polymerized liquid crystal polymer network; the cholesteric liquid crystal layer comprises a plurality of structural color regions, and in different structural color regions, the spiral structure of the cholesteric liquid crystal layer has different pitches to reflect light of different colors; the cholesteric liquid crystal layer has a patterned surface orientation structure, which is obtained by optimizing the color holographic algorithm, and the orientation structures of different structural color regions independently encode phase holograms of different wavelength channels; a multi-wavelength laser is incident from one side of the cholesteric liquid crystal layer, and the reflected light is diffracted to reproduce a wavelength-multiplexed color holographic image. The liquid crystal structural color holographic film provided by the embodiment of the present invention adopts a color holographic algorithm to design the surface orientation structure of the cholesteric liquid crystal layer, and can display a structural color pattern in the near field, and display an independent color holographic image in the far field, thereby realizing light regulation of near-field and far-field and wavelength-joint multiplexing. In addition, the fully polymerized film structure makes it flexible, self-supporting and highly stable. The liquid crystal structural color holographic film provided by the embodiment of the present invention integrates multiple independent wavelength channels of the near field and the far field into a flexible optical element with a thickness of micrometer level, effectively overcoming the limitations of traditional multi-dimensional light field multiplexing technology in terms of flexibility, difficulty in preparation, and processing cost.

[0039] It should be understood that the contents described in this section are not intended to identify the key or important features of the embodiments of the present invention, nor are they intended to limit the scope of the present invention. Other features of the present invention will become easily understood through the following description. BRIEF DESCRIPTION OF THE DRAWINGS

[0040] In order to more clearly illustrate the technical solutions in the embodiments of the present invention, the following briefly introduces the drawings required for use in the description of the embodiments. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without creative work.

[0041] Figure 1 A schematic diagram of the yz side structure of a liquid crystal structural color holographic film provided by an embodiment of the present invention;

[0042] Figure 2 A schematic diagram of an algorithm for the surface orientation structure of a cholesteric liquid crystal layer of a liquid crystal structural color holographic film provided by an embodiment of the present invention;

[0043] Figure 3 A schematic flow chart of a method for preparing a liquid crystal structural color holographic film provided by an embodiment of the present invention;

[0044] Figure 4A schematic structural diagram corresponding to a process flow of a liquid crystal structural color holographic film provided by an embodiment of the present invention;

[0045] Figure 5 A graph showing the relationship between the central wavelength of the reflection band of the cholesteric liquid crystal layer of a liquid crystal structural color holographic film and the ultraviolet exposure time provided by an embodiment of the present invention;

[0046] Figure 6 A diagram showing experimental results of a liquid crystal structural color holographic film provided by an embodiment of the present invention displaying a near-field structural color pattern under a microscope;

[0047] Figure 7 A circular polarization spectrum diagram of different structural color regions of a liquid crystal structural color holographic film provided by an embodiment of the present invention;

[0048] Figure 8 A schematic structural diagram of a color holographic device provided by an embodiment of the present invention;

[0049] Fig. 9 This is a diagram of the far-field color holographic experimental results of a liquid crystal structural color holographic film provided in an embodiment of the present invention. DETAILED DESCRIPTION

[0050] In order to enable those skilled in the art to better understand the scheme of the present invention, the technical scheme in the embodiments of the present invention will be clearly and completely described below in conjunction with the drawings in the embodiments 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 ordinary technicians in this field without creative work should fall within the scope of protection of the present invention.

[0051] It should be noted that the terms "first", "second", etc. in the specification and claims of the present invention and the above-mentioned drawings are used to distinguish similar objects, and are not necessarily used to describe a specific order or sequence. It should be understood that the data used in this way can be interchanged where appropriate, so that the embodiments of the present invention described herein can be implemented in an order other than those illustrated or described herein. In addition, the terms "including" and "having" and any variations thereof are intended to cover non-exclusive inclusions, for example, a process, method, system, product or device that includes a series of steps or units is not necessarily limited to those steps or units that are clearly listed, but may include other steps or units that are not clearly listed or inherent to these processes, methods, products or devices.

[0052] Figure 1 A schematic diagram of the yz side structure of a liquid crystal structural color holographic film provided by an embodiment of the present invention, referring to Figure 1The liquid crystal structural color holographic film comprises a stacked nematic liquid crystal layer 10 and a cholesteric liquid crystal layer 20, the total thickness (distance in the z direction) of the nematic liquid crystal layer 10 and the cholesteric liquid crystal layer 20 is fixed, the nematic liquid crystal layer 10 and the cholesteric liquid crystal layer 20 both comprise liquid crystal molecules 30, and the nematic liquid crystal layer 10 and the cholesteric liquid crystal layer 20 form a completely polymerized liquid crystal polymer network 40; the cholesteric liquid crystal layer 20 comprises a plurality of structural color regions ( Figure 1 Schematically showing three structural color regions 20a, 20b and 20c, i.e., three regions with background colors of red, green and blue, which is not a limitation of the present invention), in different structural color regions, the helical structure of the cholesteric liquid crystal layer 20 has different pitches to reflect light of different colors; the cholesteric liquid crystal layer 20 has a patterned surface orientation structure ( Figure 1 (not shown), the surface orientation structure is optimized according to the color holographic algorithm, and the orientation structures of different structural color areas independently encode the phase holograms of different wavelength channels; the multi-wavelength laser is incident from one side of the cholesteric liquid crystal layer 20, and the reflected light is diffracted to reproduce the wavelength-multiplexed color holographic image.

[0053] Among them, the liquid crystal molecules 30 in the nematic liquid crystal layer 10 and the cholesteric liquid crystal layer 20 are cross-linked to form a fully polymerized liquid crystal polymer network, so the liquid crystal structural color holographic film is flexible (bendable), self-supporting (no substrate required) and highly stable. The structural color region of the cholesteric liquid crystal layer 20 can be set to any shape to meet different needs. Optionally, in the plane where the liquid crystal structural color holographic film is located (xy plane), there is no overlap between different structural color regions; in different structural color regions, the cholesteric liquid crystal layer 20 has different thicknesses and the same number of spirals.

[0054] In different structural color regions, the spiral structure of the cholesteric liquid crystal layer 20 has different pitches (i.e., periods), and thus can reflect light of different wavelengths / colors; since the total thickness of the liquid crystal structural color holographic film is fixed, the thicker the cholesteric liquid crystal layer 20 is, the thinner the nematic liquid crystal layer 10 is, and since the cholesteric liquid crystal layer 20 has a fixed number of pitches, the thicker the structural color region of the cholesteric liquid crystal layer 20 is, the larger the pitch is, and the longer the wavelength of the Bragg reflection band is (the structural color is more reddish); conversely, the thinner the cholesteric liquid crystal layer 20 is, the shorter the wavelength of the Bragg reflection band is. The cholesteric liquid crystal layer 20 has a patterned surface orientation structure, which is optimized according to the color holographic algorithm. The orientation structures in different structural color regions can independently encode phase holograms of different wavelength channels, and after the reflected light is diffracted over a sufficient distance, it can reproduce the wavelength-multiplexed color holographic image in the far field.

[0055] Wherein, the nematic liquid crystal layer 10 and the cholesteric liquid crystal layer 20 are both composed of fully polymerized liquid crystal molecules. The nematic liquid crystal layer 10 has a liquid crystal polymer network with an approximately parallel or completely parallel structure, and its orientation structure does not affect the function of the liquid crystal structural color holographic film. Wherein the approximately parallel liquid crystal structure refers to that due to the surface force of the cholesteric liquid crystal layer 20 and other reasons, the long axis of the liquid crystal molecules in the nematic liquid crystal layer 10 has a relatively small twist angle. Optionally, the nematic liquid crystal layer 10 can be either uniformly oriented or non-uniformly oriented. The cholesteric liquid crystal layer 20 has a liquid crystal polymer network with a periodic spiral structure, and its periodic spiral structure has a consistent hand direction. Optionally, it can be a left-handed spiral structure or a right-handed spiral structure, which can be selected according to actual conditions during specific implementation. The cholesteric liquid crystal layer 20 has different Bragg reflection bands in different structural color regions. In its Bragg reflection band, it can reflect the circular polarization component with a hand direction consistent with the spiral structure of the liquid crystal layer, and give it a geometric phase equal to ±2 times the surface orientation angle of the cholesteric liquid crystal layer 20, while transmitting the circular polarization component with the opposite hand direction. The non-uniform structural color or Bragg reflection band makes the cholesteric liquid crystal layer 20 have an amplitude modulation effect on incident light of a specific wavelength, and can realize the simultaneous control of amplitude and phase. For multi-wavelength incident light, the phase modulation function of the cholesteric liquid crystal will only take effect when the incident wavelength matches its Bragg reflection band, so that different structural color regions of the cholesteric liquid crystal layer 20 can realize independent phase modulation functions at different wavelengths, such as realizing wavelength multiplexing holography. Therefore, the technical solution of this embodiment can display structural color patterns in the near field and display independent color holographic images in the far field, realizing near-field and far-field, wavelength joint multiplexing light regulation.

[0056] Optionally, the thickness of the liquid crystal structural color holographic film (i.e., the total thickness of the nematic liquid crystal layer 10 and the cholesteric liquid crystal layer 20) is greater than or equal to 20 times the maximum value of the helical pitch of the liquid crystal molecules in the cholesteric liquid crystal layer 20, so that the cholesteric liquid crystal can have a high reflectivity when circularly polarized selective Bragg reflection occurs. It is understandable that Figure 1 The periodic spiral structure in the cholesteric liquid crystal layer is only shown by way of example, rather than in actual size and proportion.

[0057] For example, taking the case where the liquid crystal structural color holographic film includes three independent wavelength channels of red, green and blue, Figure 2 The algorithm diagram of the surface orientation structure of the cholesteric liquid crystal layer of a liquid crystal structure color holographic film provided by the embodiment of the present invention can be used to calculate the orientation structure required to realize color holography. Figure 2 First, the target near-field structural color pattern ( Figure 2 exemplified by colored flowers) and the target far-field color holographic image ( Figure 2The three near-field patterns are consistent with the shapes of the three structural color regions of the cholesteric liquid crystal layer mentioned above. Then, the Gerchberg-Saxton holographic algorithm is used to iteratively optimize the holographic phase corresponding to each wavelength channel, that is, the phase iteration is repeated between the near-field plane and the far-field plane using Fourier transform (FFT) and inverse Fourier transform (IFFT) until the generated far-field holographic image is close enough to the target image. Then, the optimized three-channel holographic phase is merged into a complete hologram phase distribution φ according to the shape of the near-field pattern of the corresponding wavelength channel. Finally, the surface orientation structure of the cholesteric liquid crystal layer is calculated based on the correspondence between the geometric phase of the cholesteric liquid crystal and its surface orientation angle. Exemplarily, Figure 2 The cholesteric liquid crystal used in the embodiment is right-handed (not limiting the embodiment of the present invention), so the surface orientation angle of the cholesteric liquid crystal layer is equal to φ / 2. It can be understood that the technical solution of this embodiment can independently perform geometric phase modulation on incident light of different wavelengths, so while displaying near-field structural color patterns, a color holographic image can also be generated in the far field.

[0058] It should be noted that Figure 2 The calculation process of the surface orientation structure of a cholesteric liquid crystal layer is merely provided as an example, and is not intended to limit the present invention. In other embodiments, other calculation processes may be used according to specific requirements to obtain the surface orientation structure of a cholesteric liquid crystal layer with similar functions.

[0059] The liquid crystal structural color holographic film provided by the embodiment of the present invention can realize independent phase modulation functions at different wavelengths by dividing the cholesteric liquid crystal layer into a plurality of structural color regions of arbitrary shapes and different reflection wavelengths, and giving the cholesteric liquid crystal layer a patterned surface orientation structure, thereby realizing near-field and far-field multiplexed color image display. In addition, the fully polymerized film structure makes it flexible, self-supporting and highly stable. The liquid crystal structural color holographic film provided by the embodiment of the present invention integrates multiple independent wavelength channels of the near field and far field into a flexible optical element with a thickness of micrometer level, and can generate a color holographic image in the far field while displaying the near-field structural color pattern, with high multiplexing dimension and high functionality. This liquid crystal structural color holographic film can effectively overcome the limitations of traditional multi-dimensional light field multiplexing technology in terms of flexibility, difficulty of preparation, processing cost, etc.

[0060] Figure 3 A schematic flow chart of a method for preparing a liquid crystal structural color holographic film provided in an embodiment of the present invention is used to prepare the liquid crystal structural color holographic film provided in the above embodiment. Figure 4 A schematic diagram of the structure corresponding to the process flow of a liquid crystal structural color holographic film provided by an embodiment of the present invention. Figure 3 , the preparation method comprises:

[0061] S110 , providing a first substrate and a second substrate, forming a first alignment layer on one side of the first substrate, and forming a second alignment layer doped with a photoinitiator on one side of the second substrate.

[0062] The first substrate and the second substrate can be flexible substrates or rigid substrates with high light transmittance (greater than or equal to 85%), the first substrate and the second substrate material can be ITO glass, and the thickness of the substrate can be 1mm to 2mm. The first alignment layer has no photoinitiator, and the second alignment layer contains a photoinitiator.

[0063] Optionally, the alignment materials of the first alignment layer and the second alignment layer may be at least one of a photo-crosslinking material, a photodegradable material, and a photoinduced cis-trans isomerization material, such as a photosensitive azo material SD1. These materials are photo-controlled alignment materials, which can undergo physical or chemical reactions under the irradiation of linear polarized ultraviolet light to generate anisotropic surface forces, thereby inducing the directional alignment of liquid crystal molecules. In addition to the alignment material, the second alignment layer also contains a uniformly mixed photoinitiator, such as benzophenone, which can induce a polymerization reaction to start from the surface of the second alignment layer.

[0064] Optionally, forming a first alignment layer on one side of the first substrate and forming a second alignment layer doped with a photoinitiator on one side of the second substrate includes:

[0065] Spin coating a solution containing an alignment material on one side of a first substrate, and after the spin coating is completed, annealing the first substrate to form a first alignment layer;

[0066] A uniform mixed solution containing an alignment material and a photoinitiator is spin-coated on one side of the second substrate. After the spin coating is completed, the second substrate is annealed to form a second alignment layer.

[0067] For example, in this embodiment, the orientation material is photosensitive azo material SD1, and the spin coating solution without photoinitiator contains 0.35% SD1 and 99.65% dimethylformamide; the photoinitiator is benzophenone, and the spin coating solution containing photoinitiator contains 0.35% SD1, 0.15% benzophenone and 99.5% dimethylformamide.

[0068] Exemplarily, the spin coating process may include: first adjusting the rotation speed to 600-900 rpm, controlling the first stage spin coating time to 5s-10s, so that the material is evenly distributed on the substrate surface; then adjusting the rotation speed to 2500-3500 rpm, controlling the second stage spin coating time to 30s-50s, so that the material is coated according to a specific thickness. Optionally, the thickness of the first alignment layer without a photoinitiator and the second alignment layer containing a photoinitiator can be 30nm-50nm.

[0069] Exemplarily, the annealing process may include: an annealing atmosphere of air, an annealing temperature of 80° C. to 120° C., and an annealing time of 8 min to 12 min.

[0070] It should be noted that the above-mentioned spin coating solution composition, spin coating parameters, and annealing parameters are only exemplary descriptions, and in other embodiments, they can be adjusted according to actual needs.

[0071] S120, placing the first substrate and the second substrate opposite to each other, and placing a spacer between the first substrate and the second substrate to form an empty box.

[0072] Wherein, when the first substrate and the second substrate are opposite to each other, the first alignment layer and the second alignment layer are arranged opposite to each other, and the thickness between the first substrate and the second substrate is controlled by a spacer. The spacer can be a quartz microsphere or a quartz column, and can be arranged at the boundary position of the first substrate and the second substrate to support the first substrate and the second substrate to form a liquid crystal filling space. In the direction perpendicular to the first substrate and the second substrate, the extension length of the spacer needs to be greater than or equal to 20 times the maximum value of the pitch of the liquid crystal molecules in the cholesteric liquid crystal layer.

[0073] S130, performing ultraviolet exposure with a polarization direction varying with space on the empty box, and preparing a cholesteric liquid crystal polymer network support with a patterned orientation structure between the first substrate and the second substrate.

[0074] Optionally, the empty box is subjected to ultraviolet exposure with a polarization direction varying with space, and a cholesteric liquid crystal polymer network support with a patterned orientation structure is prepared between the first substrate and the second substrate, comprising:

[0075] Performing ultraviolet light alignment on an empty box consisting of a first substrate, a second substrate and a spacer, so that the alignment directions of the first alignment layer and the second alignment layer are consistent with the alignment structure designed by the color holographic algorithm;

[0076] Filling a mixture of cholesteric liquid crystal and polymerization reaction monomers between the first substrate and the second substrate, irradiating the second substrate with ultraviolet light from one side to perform surface induced photopolymerization to form a cholesteric liquid crystal polymer network support connected to the surface of the second substrate;

[0077] The polymerized liquid crystal cell is immersed in acetone to wash away the molecules that have not undergone polymerization;

[0078] When the washed liquid crystal box is taken out from acetone, the cholesteric liquid crystal polymer network support shrinks, but the periodic spiral structure and surface orientation structure of the cholesteric liquid crystal are still retained.

[0079] Among them, the cholesteric liquid crystal polymer network scaffold is prepared by surface induced photopolymerization, washing away unreacted molecules and other processes (the specific steps and corresponding structures can be referred to Figure 4 ), whose patterned orientation structure consists of Figure 2 The color holographic algorithm shown in the figure is optimized. The color holographic algorithm divides multiple groups of near-field patterns and far-field holographic images according to the set wavelength channel. Each group of near-field patterns and far-field holographic images corresponds to an independent wavelength channel. Then, the Gerchberg-Saxton holographic algorithm is used to iteratively optimize and merge the holographic phase corresponding to each wavelength channel. Finally, the surface orientation structure of the cholesteric liquid crystal layer is calculated based on the corresponding relationship between the geometric phase of the cholesteric liquid crystal and its surface orientation angle.

[0080] Illustratively, in this embodiment, the light source for the differently polarized ultraviolet exposure is a 365nm LED light source, the exposure pattern is controlled by a projection system based on a digital micromirror device, and the linear polarization direction is controlled by a synchronously rotating polarizer to form a patterned orientation structure.

[0081] Optionally, the ultraviolet light orientation includes: using a projection system based on a digital micromirror device, synchronously controlling the exposure pattern and the angle of the polarizer according to an exposure order, and performing ultraviolet exposure treatment on the first orientation layer and the second orientation layer so that both the first orientation layer and the second orientation layer form a target orientation structure.

[0082] Exemplarily, the cholesteric liquid crystal can be a mixture of a nematic liquid crystal and a chiral agent. In this embodiment, the nematic liquid crystal is selected from mixed crystal E7, and the chiral agent is selected from right-handed chiral agent R5011 (not limiting the embodiment of the present invention). In the mixture of the cholesteric liquid crystal and the polymerization monomer, the polymerization monomer is selected from RM257, and the mass proportion of the polymerization monomer is selected to be 20%.

[0083] S140, forming a stacked nematic liquid crystal layer and a cholesteric liquid crystal layer between the first substrate and the second substrate by ultraviolet exposure with exposure time varying with space.

[0084] Optionally, a nematic liquid crystal layer and a cholesteric liquid crystal layer are formed between the first substrate and the second substrate by ultraviolet exposure with exposure time varying with space, including:

[0085] The shrunken cholesteric liquid crystal polymer network scaffold is exposed to different doses of ultraviolet light, and different areas are exposed for different times to control the resilience of the polymer network scaffold;

[0086] Refilling the liquid crystal box with a mixture of polymerization monomers and photoinitiators, the cholesteric liquid crystal polymer network support will rebound to form a stacked nematic liquid crystal layer and a cholesteric liquid crystal layer;

[0087] Among them, in areas where the UV exposure time is longer, the resilience of the cholesteric liquid crystal polymer network support is weaker, the thickness of the formed cholesteric liquid crystal layer is smaller, the pitch is smaller, the wavelength of the Bragg reflection band is shorter, and the structural color is bluer.

[0088] The stacked nematic liquid crystal layer and cholesteric liquid crystal layer are prepared by a wash-refill process (the specific steps and corresponding structures can be further referred to). Figure 4 ). Different UV exposure times act on the shrunken cholesteric liquid crystal polymer network support, which will affect the final thickness and structural color of the cholesteric liquid crystal layer. The longer the UV exposure time, the smaller the thickness of the cholesteric liquid crystal layer formed in the end, and the bluer the structural color.

[0089] Illustratively, in this embodiment, the light source for ultraviolet exposure at different times is a 365nm LED light source, and its exposure pattern is controlled by a projection system based on a digital micromirror device to achieve an exposure dose that varies with space.

[0090] Illustratively, in this embodiment, in the mixture of the polymerization reaction monomer and the photoinitiator, the polymerization reaction monomer is RM257, the photoinitiator is benzophenone, and the mass proportions of the polymerization reaction monomer and the photoinitiator are 95% and 5%, respectively.

[0091] It should be noted that the above material components and proportions are merely exemplary descriptions and are not limitations of the embodiments of the present invention. In other implementations, other components and proportions may be used according to specific needs.

[0092] S150, using ultraviolet light to completely polymerize the nematic liquid crystal layer and the cholesteric liquid crystal layer.

[0093] Optionally, the light source for UV polymerization can also be a 365nm LED light source, but without passing through a projection system, and the polymerization light power can be 0.15mW / mm 2 ~0.25mW / mm 2 The polymerization time can be 10min to 15min.

[0094] S160, peeling off the first substrate and the second substrate to form a liquid crystal structural color holographic film.

[0095] The thickness and structural color of the cholesteric liquid crystal layer are related to the exposure time, and the total thickness of the nematic liquid crystal layer and the cholesteric liquid crystal layer is fixed.

[0096] Optionally, peeling off the first substrate and the second substrate comprises:

[0097] Pry open the liquid crystal box with a blade to separate the first substrate from the second substrate, and the liquid crystal structural color holographic film will be adsorbed on the second substrate containing the photoinitiator;

[0098] Slowly insert a blade between the liquid crystal structural color holographic film and the second substrate, gently peel off the film, and carefully take it with tweezers.

[0099] Continue to refer Figure 4 After the liquid crystal structural color holographic film is completely polymerized, the first substrate and the second substrate on the outside are peeled off to obtain a self-supporting liquid crystal structural color holographic film, which can simultaneously realize near-field structural color and far-field color holography, and has flexibility and high stability.

[0100] For example, Figure 5 The relationship between the central wavelength of the reflection band of the cholesteric liquid crystal layer of a liquid crystal structure color holographic film and the ultraviolet exposure time provided by an embodiment of the present invention. Figure 5 The horizontal axis represents the UV exposure time of the shrunken cholesteric liquid crystal polymer network support, in seconds (s), and the vertical axis represents the central wavelength of the Bragg reflection band of the cholesteric liquid crystal layer finally formed, in nanometers (nm). 2 Under the exposure conditions of , the measured relationship between the center wavelength of the reflection band of the cholesteric liquid crystal layer and the UV exposure time is marked with a dark red square and fitted with a red solid line. Figure 5 It can be seen that the longer the exposure time, the shorter the central wavelength of the reflection band of the cholesteric liquid crystal layer, and the bluer the corresponding structural color. Figure 5 The relationship curve shown, corresponding to three exposure times of red, green and blue structural colors (i.e., 3s, 35s and 150s), was selected as the experimental parameters of this embodiment for preparing a liquid crystal structural color holographic film containing three structural color regions of red, green and blue.

[0101] It should be noted that Figure 5 The relationship curve between the central wavelength of the reflection band of the cholesteric liquid crystal layer and the ultraviolet exposure time is shown only as an example, and is not intended to limit the embodiments of the present invention. In other embodiments, other relationship curves between the central wavelength of the reflection band and the exposure time can be obtained by changing the material composition ratio, ultraviolet exposure power and other conditions.

[0102] For example, Figure 6 This is an experimental result diagram of a liquid crystal structural color holographic film provided by an embodiment of the present invention displaying a near-field structural color pattern under a microscope. Figure 6 Under a microscope, the microscopic color pattern of the liquid crystal structural color holographic film in this embodiment can be observed. Figure 6It can be seen that the three structural color areas of orange-red, green and blue are in the shape of flowers, leaves and background respectively, together forming a colorful flower pattern (the colors and shapes are schematic displays, not limitations of the embodiments of the present invention). The three structural color areas have fine patterns, clear boundaries, and the boundary width can be less than 5μm. There is no obvious crosstalk between different structural colors, which proves the excellent near-field color display function of the liquid crystal structural color holographic film.

[0103] For example, Figure 7 The circular polarization spectrum diagram of different structural color regions of a liquid crystal structural color holographic film provided by an embodiment of the present invention. Figure 7 , the horizontal axis represents the wavelength of the incident light, in nanometers (nm), and the vertical axis represents the relative reflectivity; the reflection spectrum corresponding to right-handed circularly polarized light is represented by a solid line, and the reflection spectrum corresponding to left-handed circularly polarized light is represented by a dotted line; Figure 6 The circular polarization spectra corresponding to the three structural color regions of orange-red, green, and blue are represented by red, green, and blue curves, respectively. Figure 7 It can be seen that the liquid crystal structural color holographic film in this embodiment has a right-handed spiral structure, so it has a high reflectivity for right-handed circularly polarized incident light in the Bragg reflection band, and a reflectivity close to 0 for left-handed circularly polarized light. The circularly polarized selective Bragg reflection bands corresponding to the three structural color regions cover the red, green, and blue bands, respectively, which is consistent with the design. Figure 7 The circularly polarized reflectance spectrum shown in the figure, corresponding to three special wavelengths of blue, green and red, are selected as the experimental parameters of this embodiment, namely 485nm, 560nm and 650nm. These three wavelength channels have high reflectivity and low crosstalk, and can be used to realize high-quality three-channel wavelength multiplexing holography.

[0104] It should be noted that Figure 7 The circular polarization spectra of three different structural color regions are shown only as examples, and are not intended to limit the embodiments of the present invention. In other embodiments, more or fewer structural color regions may be provided for the liquid crystal structural color holographic film, or other Bragg reflection band positions may be selected according to actual needs.

[0105] Figure 8 The schematic diagram of the structure of a color holographic device provided by an embodiment of the present invention shows the optical path structure of a liquid crystal structural color holographic film for far-field holographic display. Figure 8The color holographic device comprises a supercontinuum laser 1, a polarizer 2, a quarter wave plate 3, a liquid crystal structural color holographic film 4 provided in the above embodiment, and a light screen 5; the multi-wavelength light beam output by the supercontinuum laser 1 is successively transmitted through the polarizer 2 and the quarter wave plate 3, and then converted into circularly polarized light with a chirality consistent with that of the cholesteric phase liquid crystal, which is incident on the liquid crystal structural color holographic film 4. The liquid crystal structural color holographic film 4 independently performs geometric phase modulation on the incident light of different wavelengths and reflects it, and the color holographic image formed by the reflected light is received by the light screen 5.

[0106] Wherein, the supercontinuum laser 1 is used to output a multi-wavelength light beam. Figure 8 The red, green and blue lasers are schematically shown, which is not a limitation of the embodiments of the present invention. The quarter wave plate 3 is a broadband quarter wave plate, and the polarizer 2 and the quarter wave plate 3 are used to change the polarization state of the incident light. If the incident light is adjusted to circularly polarized light with a handedness consistent with that of the cholesteric phase liquid crystal, the liquid crystal structural color holographic film 4 can independently perform geometric phase modulation and reflection on the incident light of different wavelengths, and finally display a color holographic image on the light screen 5. When the incident light is circularly polarized light with a handedness opposite to that of the cholesteric phase liquid crystal, the incident light of each wavelength will directly pass through the liquid crystal structural color holographic film 4, and the holographic image cannot be observed on the light screen 5.

[0107] For example, this embodiment uses a supercontinuum laser as the light source of the multi-wavelength laser, and the incident angle of the laser incident on the liquid crystal structural color holographic film can be 0° to 15°. It should be noted that in other embodiments, the output light beams of multiple single-wavelength lasers can also be combined to serve as the multi-wavelength laser light source required for color holography.

[0108] For example, Fig. 9 This is a diagram showing the far-field color holographic experimental results of a liquid crystal structural color holographic film provided by an embodiment of the present invention. Fig. 9 , when the incident light is right-handed circularly polarized light of 485nm, 560nm and 650nm in sequence (the arrow in the lower right corner of the figure shows the polarization state of the incident light), the liquid crystal structural color holographic film in this embodiment reproduces blue, green and red butterfly images of different shapes in sequence on the far-field light screen (the colors and shapes are schematic displays, not limitations of the embodiments of the present invention). When right-handed circularly polarized light of three wavelengths of 485nm, 560nm and 650nm is incident at the same time, the image on the far-field light screen is the superposition of the diffraction light fields of the three wavelength channels, that is, the holographic image of the red, green and blue butterflies. There is basically no aliasing between the images of different wavelength channels, which proves the excellent far-field color holographic function of the liquid crystal structural color holographic film.

[0109] It should be noted that the embodiment of the present invention only exemplarily shows the near-field and far-field multiplexing display effects of three-color flowers and three-color butterflies, and is not a limitation of the liquid crystal structural color holographic film provided by the present invention. In other embodiments, according to actual needs, the structural color area setting and surface orientation structure of the cholesteric liquid crystal layer are adjusted, so that the liquid crystal structural color holographic film provided by the present invention can be suitable for near-field display with richer colors and wavelength multiplexing diffraction with more diverse functions. When observing the near-field structural color pattern of the liquid crystal structural color holographic film, its macroscopic color pattern can be observed with the naked eye, and its microscopic color pattern can be observed with a microscope.

[0110] The liquid crystal structural color holographic film provided by the embodiment of the present invention can realize independent phase modulation functions at different wavelengths by dividing the cholesteric liquid crystal layer into a plurality of structural color regions of arbitrary shapes and different reflection wavelengths, and giving the cholesteric liquid crystal layer a patterned surface orientation structure, thereby realizing near-field and far-field multiplexed color image display. In addition, the fully polymerized film structure makes it flexible, self-supporting and highly stable. The liquid crystal structural color holographic film provided by the embodiment of the present invention integrates multiple independent wavelength channels of the near field and far field into a flexible optical element with a thickness of micrometer level, and can generate a color holographic image in the far field while displaying the near-field structural color pattern. It has high multiplexing dimension and high functionality, and effectively overcomes the limitations of traditional multi-dimensional light field multiplexing technology in terms of flexibility, preparation difficulty, processing cost, etc.

[0111] The above specific implementations do not constitute a limitation on the protection scope of the present invention. It should be understood by those skilled in the art that various modifications, combinations, sub-combinations and substitutions can be made according to design requirements and other factors. Any modification, equivalent substitution and improvement made within the spirit and principle of the present invention should be included in the protection scope of the present invention.

Claims

1. A liquid crystal structural color holographic film, characterized in that: It comprises a nematic liquid crystal layer and a cholesteric liquid crystal layer which are stacked, wherein the total thickness of the nematic liquid crystal layer and the cholesteric liquid crystal layer is fixed, and the nematic liquid crystal layer and the cholesteric liquid crystal layer form a completely polymerized liquid crystal polymer network; The cholesteric liquid crystal layer comprises a plurality of structural color regions, and in different structural color regions, the spiral structure of the cholesteric liquid crystal layer has different pitches to reflect light of different colors; The cholesteric liquid crystal layer has a patterned surface orientation structure, the surface orientation structure is optimized according to a color holographic algorithm, and the orientation structures of different structural color regions independently encode phase holograms of different wavelength channels; The multi-wavelength laser is incident from one side of the cholesteric liquid crystal layer, and the reflected light is diffracted to reproduce a wavelength-multiplexed color holographic image.

2. The liquid crystal structural color holographic film according to claim 1, characterized in that: The nematic liquid crystal layer has a liquid crystal polymer network with a parallel structure, and the cholesteric liquid crystal layer has a liquid crystal polymer network with a periodic spiral structure; The periodic spiral structures of all structural color regions in the cholesteric liquid crystal layer have a consistent hand direction, presenting a left-handed spiral structure or a right-handed spiral structure.

3. The liquid crystal structural color holographic film according to claim 1, characterized in that: In the plane where the liquid crystal structural color holographic film is located, different structural color regions do not overlap; In different structural color regions, the cholesteric liquid crystal layer has different thicknesses and the same number of helices.

4. The liquid crystal structural color holographic film according to claim 1, characterized in that: The cholesteric liquid crystal layer is provided with a spatially gradient surface alignment structure for controlling the distribution of the starting directors of the liquid crystal molecules on two surfaces of the cholesteric liquid crystal layer; The surface orientation structure is obtained by optimizing the color holographic algorithm. First, multiple groups of near-field patterns and far-field holographic images are divided according to the set wavelength channel, and each group of the near-field patterns and the far-field holographic images corresponds to an independent wavelength channel. Then, the Gerchberg-Saxton holographic algorithm is used to iteratively optimize and merge the holographic phases corresponding to each wavelength channel. Finally, based on the corresponding relationship between the geometric phase of the cholesteric liquid crystal and the surface orientation angle, the surface orientation structure of the cholesteric liquid crystal layer is calculated.

5. The liquid crystal structural color holographic film according to claim 1, characterized in that: The cholesteric liquid crystal layer has different Bragg reflection bands in different structural color regions, and the structural color region can reflect light with a wavelength within the Bragg reflection band and a circular polarization handedness consistent with the spiral structure of the cholesteric liquid crystal layer, and has a geometric phase modulation effect, and the magnitude of the geometric phase is ±2 times the surface orientation angle of the cholesteric liquid crystal layer; For multi-wavelength incident light, the phase modulation function of the structural color region is effective only when the incident wavelength matches the Bragg reflection band, and different structural color regions realize independent phase modulation functions at different wavelengths, thereby realizing wavelength multiplexing holography.

6. The liquid crystal structural color holographic film according to claim 1, characterized in that: The total thickness of the nematic liquid crystal layer and the cholesteric liquid crystal layer is greater than or equal to 20 times the maximum value of the helical pitch of liquid crystal molecules in the cholesteric liquid crystal layer.

7. A method for preparing a liquid crystal structural color holographic film, characterized in that: Used for preparing the liquid crystal structural color holographic film according to any one of claims 1 to 6, the preparation method comprising: Providing a first substrate and a second substrate, forming a first alignment layer on one side of the first substrate, and forming a second alignment layer doped with a photoinitiator on one side of the second substrate; The first substrate and the second substrate are arranged opposite to each other, and a spacer is arranged between the first substrate and the second substrate to form an empty box; The empty box is subjected to ultraviolet exposure with a polarization direction varying with space, so as to prepare a cholesteric liquid crystal polymer network support with a patterned orientation structure between the first substrate and the second substrate; Using ultraviolet exposure with exposure time varying with space, a nematic liquid crystal layer and a cholesteric liquid crystal layer stacked between the first substrate and the second substrate are formed; Using ultraviolet light to completely polymerize the nematic liquid crystal layer and the cholesteric liquid crystal layer; Peeling off the first substrate and the second substrate to form the liquid crystal structural color holographic film; The thickness and structural color of the cholesteric liquid crystal layer are related to the exposure time, and the total thickness of the nematic liquid crystal layer and the cholesteric liquid crystal layer is fixed.

8. The method for preparing a liquid crystal structural color holographic film according to claim 7, characterized in that: The empty box is subjected to ultraviolet exposure with a polarization direction varying with space, and a cholesteric liquid crystal polymer network support with a patterned orientation structure is prepared between the first substrate and the second substrate, comprising: Performing ultraviolet light alignment on an empty box consisting of the first substrate, the second substrate and the spacer, so that the alignment directions of the first alignment layer and the second alignment layer are consistent with the alignment structure designed by the color holographic algorithm; Filling a mixture of cholesteric liquid crystal and polymerization reaction monomers between the first substrate and the second substrate, irradiating the second substrate with ultraviolet light from one side to perform surface induced photopolymerization to form a cholesteric liquid crystal polymer network support connected to the surface of the second substrate; The polymerized liquid crystal cell is immersed in acetone to wash away the molecules that have not undergone polymerization; The washed liquid crystal box is taken out from acetone, and the cholesteric liquid crystal polymer network support shrinks, but the periodic spiral structure and surface orientation structure of the cholesteric liquid crystal are still retained.

9. The method for preparing a liquid crystal structural color holographic film according to claim 8, characterized in that: Using ultraviolet exposure with exposure time varying with space, a nematic liquid crystal layer and a cholesteric liquid crystal layer stacked between the first substrate and the second substrate are formed, comprising: The shrunken cholesteric liquid crystal polymer network stent is subjected to ultraviolet exposure of different doses, and different regions are exposed for different times to control the resilience of the polymer network stent; Refilling the liquid crystal box with a mixture of polymerization monomers and photoinitiators, the cholesteric liquid crystal polymer network support will rebound to form the nematic liquid crystal layer and the cholesteric liquid crystal layer stacked; Among them, in the area where the UV exposure time is longer, the resilience of the cholesteric liquid crystal polymer network support is weaker, the thickness of the formed cholesteric liquid crystal layer is smaller, the pitch is smaller, the wavelength of the Bragg reflection band is shorter, and the structural color is more blue.

10. A color holographic device, characterized in that: It comprises a supercontinuum laser, a polarizer, a quarter wave plate, a liquid crystal structural color holographic film as claimed in any one of claims 1 to 6, and a light screen; The multi-wavelength light beam output by the supercontinuum laser is converted into circularly polarized light with a chirality consistent with that of the cholesteric phase liquid crystal after being transmitted through the polarizer and the quarter-wave plate in sequence, and is incident on the liquid crystal structural color holographic film. The liquid crystal structural color holographic film independently performs geometric phase modulation on the incident light of different wavelengths and reflects it, and the color holographic image formed by the reflected light is received by the light screen.

Images