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

Abstract

The application discloses a liquid crystal structural color holographic film and a preparation method thereof and a color holographic device. The liquid crystal structural color holographic film comprises a nematic liquid crystal layer and a cholesteric liquid crystal layer which are arranged in a stack, the total thickness of the nematic liquid crystal layer and the cholesteric 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 regions, the helical structure of the cholesteric liquid crystal layer has different pitches in different structural color regions to reflect light of different colors; and the cholesteric liquid crystal layer has a patterned surface orientation structure, the surface orientation structure is obtained by optimization according to a color holographic algorithm, and the orientation structure of different structural color regions independently encodes a phase hologram of a different wavelength channel. The liquid crystal structural color holographic film provided by the application can display a structural color pattern in a near field and display an independent color holographic image in a far field, realizes light regulation in combination of the near field and the far field and wavelengths, and improves the multiplexing dimension and functionality of a flexible optical element.

Description

Technical Field

[0001] This invention relates to the field of optical technology, and in particular to a liquid crystal structured color holographic thin film, its preparation method, and a color holographic device. Background Technology

[0002] In recent years, with the surge in data volume, the demands on data storage, transmission, and processing capacity have also increased significantly. Light, possessing numerous tunable dimensions such as wavelength, amplitude, phase, and polarization, and combining high speed and high parallelism, is therefore an ideal information carrier. To increase the information capacity of a single planar optical element, a series of optical field multiplexing technologies have emerged in recent years, promising applications in multiplexed optical communication, color display, multi-channel information storage, information encryption, and anti-counterfeiting. For example, wavelength multiplexing uses different wavelengths as independent channels for information; different wavelengths of light incident can yield different optical field information. Polarization multiplexing stores information in orthogonal / non-orthogonal polarization states; different incident polarization states will generate different optical fields. Near-field and far-field multiplexing utilize both near-field and far-field spaces simultaneously, enabling independent control of near-field display patterns and far-field diffraction light fields.

[0003] To meet the needs of big data, multidimensional optical field multiplexing, combining multiple dimensions, is a major development trend in the field of optical field manipulation, which is expected to further increase the number of multiplexing channels and broaden potential application scenarios. Based on rigid materials, metasurfaces can achieve multidimensional joint manipulation of optical fields by fabricating subwavelength microstructure arrays. For example, using metasurfaces that jointly multiplex polarization and wavelength, polarization-dependent multicolor holography can be realized; metasurfaces with reverse design can achieve hybrid multiplexing holography of polarization, wavelength, and longitudinal spatial position. These technologies can achieve optical field manipulation with large information capacity and high degree of freedom, but they still face challenges in terms of flexibility, fabrication difficulty, and processing cost. With the trend of integration in modern photonics technology, multidimensional, multi-channel, and multifunctional flexible optical elements are urgently needed.

[0004] Based on flexible materials, liquid crystal devices can control the spatial arrangement of anisotropic liquid crystal molecules to modulate dimensions such as the amplitude, phase, polarization, and orbital angular momentum of light, offering unique advantages such as high efficiency, low cost, and dynamic tunability. However, current liquid crystal-based light field multiplexing technologies still suffer from limitations such as a small number of channels and a single dimension, which restricts the development and application of multidimensional light field multiplexing technologies. Summary of the Invention

[0005] This invention provides a liquid crystal structural color holographic film, its preparation method, and a color holographic device. The liquid crystal structural color holographic film can display structural color patterns in the near field and independent color holographic images in the far field, realizing light modulation of near and far field and wavelength joint multiplexing, so as to improve the multiplexing dimension and functionality of flexible optical elements.

[0006] According to one aspect of the present invention, a liquid crystal structure color holographic film is provided, comprising a nematic liquid crystal layer and a cholesteric liquid crystal layer stacked thereon, 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.

[0007] The cholesteric liquid crystal layer includes multiple structural color regions. In different structural color regions, the helical structure of the cholesteric liquid crystal layer has different pitches to reflect different colors of light.

[0008] The cholesteric liquid crystal layer has a patterned surface orientation structure, which is optimized according to a color holographic algorithm. The orientation structure of different structural color regions independently encodes phase holograms of different wavelength channels.

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

[0010] Optionally, the nematic liquid crystal layer has a parallel structured liquid crystal polymer network, and the cholesteric liquid crystal layer has a periodic helical structured liquid crystal polymer network.

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

[0012] Optionally, within the plane where the liquid crystal structural color holographic film is located, there is no overlap between different structural color regions;

[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 has a spatially gradient surface orientation structure to control the initial orientation vector distribution of the liquid crystal molecules on the two surfaces of the cholesteric liquid crystal layer.

[0015] The surface orientation structure is obtained by optimizing the color holographic algorithm. First, multiple sets of near-field patterns and far-field holographic images are divided according to the set wavelength channels. Each set 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 correspondence between the geometric phase of the cholesteric liquid crystal and the surface orientation angle.

[0016] Optionally, the cholesteric liquid crystal layer has different Bragg reflection bands in different structural color regions. The structural color regions can reflect light with wavelengths located within the Bragg reflection bands and circular polarization rotation consistent with the helical structure of the cholesteric liquid crystal layer, and have a geometric phase modulation effect. 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 structured color region only takes effect when the incident wavelength matches the Bragg reflection band. Different structured color regions achieve 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 pitch of the 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 thin film is provided, for preparing the above-mentioned liquid crystal structural color holographic thin film, the preparation method comprising:

[0020] A first substrate and a second substrate are provided, a first alignment layer is formed on one side of the first substrate, and a second alignment layer doped with a photoinitiator is formed 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 provided between the first substrate and the second substrate to form an empty cell;

[0022] The empty cell is subjected to ultraviolet exposure with polarization direction varying with space to prepare a cholesteric liquid crystal polymer network scaffold with a patterned orientation structure between the first substrate and the second substrate.

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

[0024] The nematic liquid crystal layer and the cholesteric liquid crystal layer are completely polymerized using ultraviolet light;

[0025] The first substrate and the second substrate are peeled off 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 cell is subjected to ultraviolet exposure with polarization direction varying spatially to fabricate a cholesteric liquid crystal polymer network scaffold with a patterned orientation structure between the first substrate and the second substrate, comprising:

[0028] The empty cell composed of the first substrate, the second substrate and the spacer is subjected to ultraviolet light orientation so that the orientation directions of the first orientation layer and the second orientation layer are consistent with the orientation structure designed by the color holographic algorithm.

[0029] A mixture of cholesteric liquid crystal and polymerization monomers is filled between the first substrate and the second substrate, and surface-induced photopolymerization is performed by irradiating one side of the second substrate with ultraviolet light to form a cholesteric liquid crystal polymer network scaffold connected to the surface of the second substrate.

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

[0031] When the washed liquid crystal cell is removed from acetone, the cholesteric liquid crystal polymer network scaffold shrinks, but still retains the periodic helical structure and surface orientation structure of the cholesteric liquid crystal.

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

[0033] The shrunken cholesteric liquid crystal polymer network scaffold was subjected to different doses of ultraviolet light exposure, and different regions were exposed for different times to control the resilience of the polymer network scaffold.

[0034] When the mixture of polymerization monomers and photoinitiator is refilled into the liquid crystal cell, the cholesteric liquid crystal polymer network scaffold will spring back, forming the nematic liquid crystal layer and the cholesteric liquid crystal layer stacked together.

[0035] In particular, the longer the ultraviolet exposure time, the weaker the resilience of the cholesteric liquid crystal polymer network scaffold, the smaller the thickness of the formed cholesteric liquid crystal layer, the smaller the pitch, the shorter the Bragg reflection band wavelength, and the more bluish the structural color.

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

[0037] The multi-wavelength beams output by the supercontinuum laser are transmitted sequentially through the polarizer and the quarter-wave plate, and are converted into circularly polarized light with the same rotation as the cholesteric liquid crystal. The light is then incident on the liquid crystal structural color holographic film. The liquid crystal structural color holographic film independently modulates the geometric phase of the incident light of different wavelengths and reflects it. The color holographic image formed by the reflected light is received by the screen.

[0038] The liquid crystal structural color holographic film provided in this embodiment of the invention includes a nematic liquid crystal layer and a cholesteric liquid crystal layer stacked together. The total thickness of the nematic and cholesteric liquid crystal layers is fixed, and the nematic and cholesteric liquid crystal layers form a fully polymerized liquid crystal network. The cholesteric liquid crystal layer includes multiple structural color regions. In different structural color regions, the helical structure of the cholesteric liquid crystal layer has different pitches to reflect different colors of light. The cholesteric liquid crystal layer has a patterned surface orientation structure, which is optimized according to a color holographic algorithm. The orientation structure of different structural color regions independently encodes phase holograms of different wavelength channels. Multi-wavelength laser light is incident from one side of the cholesteric liquid crystal layer, and the reflected light is diffracted to reconstruct a wavelength-multiplexed color holographic image. The liquid crystal structural color holographic film provided in this embodiment of the invention uses a color holographic algorithm to design the surface orientation structure of the cholesteric liquid crystal layer, which can display structural color patterns in the near field and independent color holographic images in the far field, realizing light modulation with near-field and far-field, wavelength joint multiplexing. In addition, the fully polymerized film structure makes it flexible, self-supporting, and highly stable. The liquid crystal structured color holographic thin film provided in this invention integrates multiple independent wavelength channels in the near field and far field into a flexible optical element with a thickness of micrometers, effectively overcoming the limitations of traditional multidimensional light field multiplexing technology in terms of flexibility, fabrication difficulty, and processing cost.

[0039] It should be understood that the description in this section is not intended to identify key or essential features of the embodiments of the present invention, nor is it intended to limit the scope of the invention. Other features of the invention will become readily apparent from the following description. Attached Figure Description

[0040] To more clearly illustrate the technical solutions in the embodiments of the present invention, the accompanying drawings used in the description of the embodiments will be briefly introduced below. Obviously, the accompanying drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

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

[0042] Figure 2 This is a schematic diagram of an algorithm for the surface alignment structure of the cholesteric liquid crystal layer of a liquid crystal structure color holographic film provided in an embodiment of the present invention;

[0043] Figure 3 This is a schematic flowchart illustrating a method for preparing a liquid crystal structured color holographic thin film according to an embodiment of the present invention.

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

[0045] Figure 5 This is a graph showing the relationship between the center wavelength of the reflection band of the cholesteric liquid crystal layer of a liquid crystal structured color holographic film and the ultraviolet exposure time, provided in an embodiment of the present invention.

[0046] Figure 6 An experimental result diagram showing the display of near-field structural color patterns on a liquid crystal structural color holographic film under a microscope, provided in an embodiment of the present invention;

[0047] Figure 7 This invention provides circular polarization spectra of different structural color regions of a liquid crystal structural color holographic film, as shown in an embodiment of the invention.

[0048] Figure 8 This is a schematic diagram of the structure of a color holographic device provided in an embodiment of the present invention;

[0049] Figure 9 The image shows the far-field color holographic experimental results of a liquid crystal structured color holographic thin film provided in an embodiment of the present invention. Detailed Implementation

[0050] To enable those skilled in the art to better understand the present invention, the technical solutions of the present invention will be clearly and completely described below with reference to the accompanying drawings of the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort 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, claims, and accompanying drawings of this invention are used to distinguish similar objects and are not necessarily used to describe a specific order or sequence. It should be understood that such data can be interchanged where appropriate so that the embodiments of the invention described herein can be implemented in orders other than those illustrated or described herein. Furthermore, the terms "comprising" and "having," and any variations thereof, are intended to cover a non-exclusive inclusion; for example, a process, method, system, product, or apparatus that comprises a series of steps or units is not necessarily limited to those steps or units explicitly listed, but may include other steps or units not explicitly listed or inherent to such processes, methods, products, or apparatus.

[0052] Figure 1 This is a schematic diagram of the yz side structure of a liquid crystal structural color holographic film provided in an embodiment of the present invention, with reference to... Figure 1The liquid crystal structural color holographic film includes a nematic liquid crystal layer 10 and a cholesteric liquid crystal layer 20 stacked together. The total thickness (distance in the z-direction) of the nematic liquid crystal layer 10 and the cholesteric liquid crystal layer 20 is fixed. Both the nematic liquid crystal layer 10 and the cholesteric liquid crystal layer 20 include liquid crystal molecules 30, and the nematic liquid crystal layer 10 and the cholesteric liquid crystal layer 20 form a fully polymerized liquid crystal polymer network 40. The cholesteric liquid crystal layer 20 includes multiple structural color regions ( Figure 1 The diagram schematically shows three structural color regions 20a, 20b, and 20c, i.e., three regions with background colors of red, green, and blue (this is not a limitation of the invention). Within different structural color regions, the helical structure of the cholesteric liquid crystal layer 20 has different pitches to reflect different colors of light; the cholesteric liquid crystal layer 20 has a patterned surface alignment structure. Figure 1 (Not shown in the image), the surface orientation structure is optimized according to the color holographic algorithm, and the orientation structure of different structural color regions independently encodes the phase holograms of different wavelength channels; multi-wavelength lasers are incident from one side of the cholesteric liquid crystal layer 20, and the reflected light is diffracted to reproduce the wavelength-reused color holographic image.

[0053] In this design, the liquid crystal molecules 30 within both 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. Therefore, the liquid crystal structural color holographic film exhibits flexibility (bendability), self-support (no substrate required), and high stability. The structural color regions of the cholesteric liquid crystal layer 20 can be configured in any shape to meet different requirements. Optionally, within the plane (xy plane) of the liquid crystal structural color holographic film, different structural color regions do not overlap; within different structural color regions, the cholesteric liquid crystal layer 20 has different thicknesses and the same number of helices.

[0054] Within different structural color regions, the helical structure of the cholesteric liquid crystal layer 20 exhibits different pitches (i.e., periods), thus enabling the reflection of 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, the thinner the nematic liquid crystal layer 10. Furthermore, because the cholesteric liquid crystal layer 20 has a fixed number of pitches, the thicker structural color regions of the cholesteric liquid crystal layer 20 have larger pitches, resulting in longer Bragg reflection band wavelengths (a more reddish structural color). Conversely, the thinner the cholesteric liquid crystal layer 20, the shorter the Bragg reflection band wavelengths. The cholesteric liquid crystal layer 20 possesses a patterned surface alignment structure, optimized using a color holographic algorithm. The alignment structure of different structural color regions can independently encode phase holograms for different wavelength channels. After sufficient diffraction, the reflected light can reconstruct a wavelength-multiplexed color holographic image in the far field.

[0055] Both the nematic liquid crystal layer 10 and the cholesteric liquid crystal layer 20 are 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. The approximately parallel liquid crystal structure refers to the fact that the long axis of the liquid crystal molecules in the nematic liquid crystal layer 10 has a relatively small twist angle due to surface forces and other reasons in the cholesteric liquid crystal layer 20. Optionally, the nematic liquid crystal layer 10 can be uniformly oriented or non-uniformly oriented. The cholesteric liquid crystal layer 20 has a liquid crystal polymer network with a periodic helical structure, and its periodic helical structure has a consistent helix direction. Optionally, all of them can be left-handed helical structures or all of them can be right-handed helical structures, which can be selected according to the actual situation. The cholesteric liquid crystal layer 20 has different Bragg reflection bands in different structural color regions. Within its Bragg reflection band, it can reflect the circular polarization component with the same helix direction as the liquid crystal layer's helical structure 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 helix direction. The non-uniform structural color or Bragg reflection bands enable the cholesteric liquid crystal layer 20 to modulate the amplitude of incident light at specific wavelengths, allowing for simultaneous manipulation of amplitude and phase. For multi-wavelength incident light, the phase modulation function of the cholesteric liquid crystal only takes effect when the incident wavelength matches its Bragg reflection band, allowing different structural color regions of the cholesteric liquid crystal layer 20 to achieve independent phase modulation functions at different wavelengths, such as achieving wavelength multiplexing holography. Therefore, the technical solution of this embodiment can display structural color patterns in the near field while simultaneously displaying independent color holographic images in the far field, achieving light modulation with combined near-field and far-field wavelength multiplexing.

[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 pitch of the liquid crystal molecules in the cholesteric liquid crystal layer 20, so that the cholesteric liquid crystal can have high reflectivity when it undergoes circularly polarized selective Bragg reflection. It is understood that... Figure 1 The images shown are merely illustrative of the periodic helical structure in the cholesteric liquid crystal layer and do not represent the actual size and proportions.

[0057] For example, consider a liquid crystal structural color holographic film containing three independent wavelength channels: red, green, and blue. Figure 2 This is a schematic diagram of an algorithm for calculating the surface alignment structure of the cholesteric liquid crystal layer in a liquid crystal structured color holographic thin film, provided as an embodiment of the present invention. This algorithm can be used to calculate the alignment structure required to achieve color holography. (Reference) Figure 2 First, the target near-field structured color pattern ( Figure 2 (Example: a colored flower) and a target far-field color holographic image ( Figure 2The images (exemplified by a colored butterfly) are segmented according to wavelength channels, resulting in three sets of near-field and far-field patterns for red, green, and blue wavelength channels. The three near-field patterns correspond to the shapes of the three structural color regions of the cholesteric liquid crystal layer. Next, the Gerchberg-Saxton holographic algorithm is used to iteratively optimize the holographic phase corresponding to each wavelength channel. Specifically, the phase is iterated repeatedly between the near-field and far-field planes using Fourier transform (FFT) and inverse Fourier transform (IFFT) until the generated far-field holographic image is sufficiently close to the target image. Then, the optimized three-channel holographic phases are merged into a complete holographic phase distribution φ according to the shapes of the corresponding near-field patterns. Finally, based on the correspondence between the geometric phase of the cholesteric liquid crystal and its surface orientation angle, the surface orientation structure of the cholesteric liquid crystal layer is calculated. For example, Figure 2 The cholesteric liquid crystal used is dextrorotatory (not a limitation of the embodiments of the present invention), therefore the surface orientation angle of the cholesteric liquid crystal layer is equal to φ / 2. It is understood that the technical solution of this embodiment can independently modulate the geometric phase of incident light of different wavelengths, thus enabling the generation of color holographic images in the far field while displaying near-field structural color patterns.

[0058] It should be noted that, Figure 2 This is merely an illustrative calculation process for the surface alignment structure of a cholesteric liquid crystal layer, and is not intended to limit the invention. In other embodiments, other calculation processes can be used according to specific needs to obtain a cholesteric liquid crystal layer surface alignment structure with similar functions.

[0059] The liquid crystal structural color holographic film provided in this invention divides the cholesteric liquid crystal layer into multiple structural color regions of arbitrary shape and different reflection wavelengths, and endows the cholesteric liquid crystal layer with a patterned surface orientation structure. This enables independent phase modulation at different wavelengths, thereby achieving near-field and far-field multiplexed color image display. Furthermore, the fully polymerized film structure provides flexibility, self-support, and high stability. The liquid crystal structural color holographic film provided in this invention integrates multiple independent wavelength channels in the near and far fields into a flexible optical element with a thickness of micrometers. This allows for the display of near-field structural color patterns while simultaneously generating color holographic images in the far field, exhibiting high multiplexing dimensionality and high functionality. This liquid crystal structural color holographic film effectively overcomes the limitations of traditional multidimensional light field multiplexing technologies in terms of flexibility, fabrication difficulty, and processing cost.

[0060] Figure 3 This is a schematic flowchart illustrating a method for preparing a liquid crystal structural color holographic film according to an embodiment of the present invention, used to prepare the liquid crystal structural color holographic film provided in the above embodiment. Figure 4 This is a schematic diagram illustrating the process flow of a liquid crystal structural color holographic thin film according to an embodiment of the present invention. (Reference) Figure 3 The preparation method includes:

[0061] S110. A first substrate and a second substrate are provided, a first alignment layer is formed on one side of the first substrate, and a second alignment layer doped with a photoinitiator is formed on one side of the second substrate.

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

[0063] Optionally, the orientation materials of the first and second orientation layers can be at least one of photocrosslinking materials, photodegradable materials, and photoinduced cis-trans isomers, such as the photosensitive azo material SD1. These materials belong to photo-controlled orientation materials and can undergo physical or chemical reactions under irradiation with linearly polarized ultraviolet light to generate anisotropic surface forces, thereby inducing the oriented alignment of liquid crystal molecules. In addition to containing the orientation material, the second orientation layer also contains a uniformly mixed photoinitiator, such as benzophenone, which can induce the polymerization reaction to begin from the surface of the second orientation layer.

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

[0065] A solution containing an alignment material is spin-coated onto one side of a first substrate. After spin-coating, the first substrate is annealed to form a first alignment layer.

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

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

[0068] For example, the spin coating process may include: first, adjusting the rotation speed to 600–900 rpm and controlling the first-stage spin coating time to 5–10 s to ensure uniform material distribution on the substrate surface; then, adjusting the rotation speed to 2500–3500 rpm and controlling the second-stage spin coating time to 30–50 s to allow the material to be coated 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 30 nm–50 nm.

[0069] For example, the annealing process may include: annealing atmosphere is air, annealing temperature is 80℃~120℃, and annealing time is 8min~12min.

[0070] It should be noted that the spin coating solution composition, spin coating parameters, and annealing parameters described above are merely illustrative examples. In other embodiments, they can be adjusted according to actual needs.

[0071] S120. The first substrate and the second substrate are arranged opposite each other, and a spacer is provided between the first substrate and the second substrate to form an empty cell.

[0072] In this configuration, when the first substrate and the second substrate are facing each other, the first alignment layer and the second alignment layer are disposed opposite to each other, and the thickness between the first substrate and the second substrate is controlled by spacers. The spacers can be quartz microspheres or quartz pillars, and can be disposed at the boundary between the first substrate and the second substrate to support the first substrate and the second substrate, forming a liquid crystal filling space. Along a direction perpendicular to the first substrate and the second substrate, the extension length of the spacers must be greater than or equal to 20 times the maximum pitch of the liquid crystal molecules in the cholesteric liquid crystal layer.

[0073] S130. The empty cell is subjected to ultraviolet exposure with polarization direction varying in space to prepare a cholesteric liquid crystal polymer network scaffold with a patterned orientation structure between the first substrate and the second substrate.

[0074] Optionally, the empty cell is subjected to ultraviolet exposure with polarization direction varying spatially to fabricate a cholesteric liquid crystal polymer network scaffold with a patterned orientation structure between the first and second substrates, including:

[0075] The empty cell composed of the first substrate, the second substrate and the spacer is subjected to ultraviolet light orientation so that the orientation directions of the first orientation layer and the second orientation layer are consistent with the orientation structure designed by the color holographic algorithm.

[0076] A mixture of cholesteric liquid crystal and polymerization monomers is filled between the first substrate and the second substrate, and surface-induced photopolymerization is performed by irradiating the second substrate with ultraviolet light from one side to form a cholesteric liquid crystal polymer network scaffold connected to the surface of the second substrate.

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

[0078] When the cleaned liquid crystal cell is removed from acetone, the cholesteric liquid crystal polymer network scaffold shrinks, but still retains the periodic helical structure and surface orientation structure of the cholesteric liquid crystal.

[0079] The cholesteric phase liquid crystal polymer network scaffold was prepared by surface-induced photopolymerization and washing away unreacted molecules (the specific steps and corresponding structures can be found in [reference]). Figure 4 Its patterned orientation structure is composed of Figure 2 The color holographic algorithm shown is optimized. The color holographic algorithm divides multiple sets of near-field patterns and far-field holographic images according to the set wavelength channels. Each set 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 correspondence between the geometric phase of the cholesteric liquid crystal and its surface orientation angle.

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

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

[0082] For example, the cholesteric liquid crystal can be a mixture of nematic liquid crystal and a chiral agent. In this embodiment, the nematic liquid crystal is selected as E7 mixed crystal, and the chiral agent is selected as the dextrorotatory chiral agent R5011 (this is not a limitation on the embodiment of the present invention). In the above mixture of cholesteric liquid crystal and polymerization monomer, the polymerization monomer is selected as RM257, and the mass percentage of the polymerization monomer is selected as 20%.

[0083] S140. Using ultraviolet exposure with exposure time varying spatially, a nematic liquid crystal layer and a cholesteric liquid crystal layer are formed stacked between a first substrate and a second substrate.

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

[0085] The shrinkage cholesteric liquid crystal polymer network scaffold was subjected to different doses of ultraviolet exposure, and different regions were exposed for different times to control the resilience of the polymer network scaffold.

[0086] When a mixture of polymerization monomers and photoinitiators is refilled into the liquid crystal cell, the cholesteric liquid crystal polymer network scaffold will spring back, forming a stacked nematic liquid crystal layer and a cholesteric liquid crystal layer.

[0087] In particular, the longer the ultraviolet exposure time, the weaker the resilience of the cholesteric liquid crystal polymer network scaffold, the smaller the thickness of the formed cholesteric liquid crystal layer, the smaller the pitch, the shorter the Bragg reflection band wavelength, and the more bluish the structural color.

[0088] The stacked nematic liquid crystal layer and cholesteric liquid crystal layer were prepared by a wash-and-refill process (for specific steps and corresponding structures, please refer to [reference needed]). Figure 4 The duration of UV exposure applied to the shrunken cholesteric liquid crystal polymer network scaffold affects the final thickness and structural color of the cholesteric liquid crystal layer. Regions with longer UV exposure times result in thinner cholesteric liquid crystal layers and a more bluish structural color.

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

[0090] For example, in this embodiment, the polymerization monomer is RM257 and the photoinitiator is benzophenone, and the mass percentages of the polymerization monomer and the photoinitiator are 95% and 5%, respectively.

[0091] It should be noted that the above material composition and proportions are merely illustrative examples and not intended to limit the embodiments of the present invention. In other embodiments, other components and proportions may be used according to specific needs.

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

[0093] Optionally, the light source for ultraviolet light polymerization can also be a 365nm LED light source, but it does not require a projection system, and the polymerization light power can be 0.15mW / mm². 2 ~0.25mW / mm 2 The polymerization time can be 10 min to 15 min.

[0094] S160: Peel 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, while the total thickness of the nematic liquid crystal layer and the cholesteric liquid crystal layer is fixed.

[0096] Optionally, the first substrate and the second substrate are stripped, including:

[0097] Use a blade to pry open the liquid crystal cell, separating the first substrate and the second substrate. The liquid crystal structural color holographic film will be adsorbed onto the second substrate containing the photoinitiator.

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

[0099] Continue to refer to Figure 4 After the liquid crystal structural color holographic film is fully polymerized, the first and second substrates on its outer side are peeled off to obtain a self-supporting liquid crystal structural color holographic film. This film can realize near-field structural color and far-field color holography at the same time, and has flexibility and high stability.

[0100] For example, Figure 5 This diagram illustrates the relationship between the center wavelength of the reflection band of the cholesteric liquid crystal layer in a liquid crystal structured color holographic thin film and the ultraviolet exposure time, as provided in an embodiment of the present invention. (Reference) Figure 5 The horizontal axis represents the UV exposure time of the shrunken cholesteric liquid crystal polymer network scaffold, in seconds (s), and the vertical axis represents the center wavelength of the Bragg reflection band of the final cholesteric liquid crystal layer, in nanometers (nm). At a wavelength of 365 nm and a power of 1.58 mW / mm², [the following parameters were used]. 2 Under the specified exposure conditions, the measured relationship between the center wavelength of the reflective band of the cholesteric liquid crystal layer and the ultraviolet exposure time is marked with a dark red square and fitted with a solid red line. From Figure 5 It can be seen that the longer the exposure time, the shorter the center wavelength of the cholesteric liquid crystal layer's reflection band, and the more bluish the corresponding structural color. According to... Figure 5 The relationship curves shown correspond to three exposure times (i.e., 3s, 35s, and 150s) for the three structural colors of red, green, and blue. These were selected as experimental parameters in this embodiment to prepare liquid crystal structural color holographic films containing the three structural color regions of red, green, and blue.

[0101] It should be noted that, Figure 5 The provided diagram illustrates, by way of example, the relationship between the center wavelength of the reflective band of a cholesteric liquid crystal layer and ultraviolet exposure time, and is not intended to limit the embodiments of the present invention. In other embodiments, other relationship curves between the center wavelength of the reflective band and exposure time can be obtained by changing the material composition ratio, ultraviolet exposure power, and other conditions.

[0102] For example, Figure 6 This image shows experimental results of displaying near-field structural color patterns on a liquid crystal structural color holographic film under a microscope, as provided in an embodiment of the present invention. (Reference) Figure 6 Under a microscope, the microscopic color patterns of the liquid crystal structure color holographic film in this embodiment can be observed. Figure 6As can be seen, the three structural color regions—orange-red, green, and blue—represent the shapes of flowers, leaves, and background, respectively, together forming a colorful flower pattern (the colors and shapes are for illustrative purposes only and are not intended to limit the scope of this invention). All three structural color regions exhibit fine patterns and clear boundaries, with boundary widths up to 5 μm. Furthermore, there is no significant crosstalk between the different structural colors, demonstrating the excellent near-field color display function of the liquid crystal structural color holographic film.

[0103] For example, Figure 7 Circular polarization spectra of different structural color regions of a liquid crystal structural color holographic thin film provided in an embodiment of the present invention. (Reference) Figure 7 The horizontal axis represents the incident light wavelength in nanometers (nm), and the vertical axis represents the relative reflectance. The reflection spectrum of right-handed circularly polarized light is represented by a solid line, and the reflection spectrum of left-handed circularly polarized light is represented by a dashed line. Figure 6 The circularly polarized spectra corresponding to the three structural color regions of orange-red, green, and blue are represented by red, green, and blue curves, respectively. From Figure 7 It can be seen that the liquid crystal structural color holographic film in this embodiment, due to its right-hand helical structure, has high reflectivity for right-hand circularly polarized incident light within the Bragg band, and its reflectivity for left-hand circularly polarized light is close to 0. The circularly polarized selective Bragg bands corresponding to the three structural color regions respectively cover the red, green, and blue wavelengths, consistent with the design. Figure 7 The circularly polarized reflectance spectrum shown corresponds to three specific wavelengths of blue, green, and red, which were selected as experimental parameters for this embodiment: 485 nm, 560 nm, and 650 nm. These three wavelength channels have high reflectivity and low crosstalk, and can be used to realize high-quality three-channel wavelength multiplexed 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, depending on actual needs.

[0105] Figure 8 This is a schematic diagram of a color holographic device provided in an embodiment of the present invention. The color holographic device illustrates the optical path structure of a liquid crystal structural color holographic film used for far-field holographic display. (Reference) Figure 8The color holographic device includes 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 screen 5. The multi-wavelength beam output by the supercontinuum laser 1 is transmitted sequentially through the polarizer 2 and the quarter-wave plate 3, and is converted into circularly polarized light with the same rotation as the cholesteric liquid crystal. The light is then incident on the liquid crystal structural color holographic film 4. The liquid crystal structural color holographic film 4 performs independent geometric phase modulation on the incident light of different wavelengths and reflects it. The color holographic image formed by the reflected light is received by the screen 5.

[0106] Among them, the supercontinuum laser 1 is used to output multi-wavelength beams. Figure 8 The diagram schematically shows red, green, and blue lasers and is not intended to limit the embodiments of the present invention. The quarter-wave plate 3 is a broadband quarter-wave plate, and the polarizer 2 and quarter-wave plate 3 are used to change the polarization state of the incident light. If the incident light is tuned to circularly polarized light with the same rotation as the cholesteric liquid crystal, the liquid crystal structural color holographic film 4 can independently modulate and reflect the incident light of different wavelengths, ultimately displaying a color holographic image on the screen 5. However, when the incident light is circularly polarized light with the opposite rotation to the cholesteric liquid crystal, all wavelengths of incident light will directly pass through the liquid crystal structural color holographic film 4, and the holographic image cannot be observed on the screen 5.

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

[0108] For example, Figure 9 This image shows the far-field color holographic experimental results of a liquid crystal structured color holographic thin film provided in an embodiment of the present invention. (Reference) Figure 9 When the incident light is right-handed circularly polarized light at wavelengths of 485nm, 560nm, and 650nm respectively (the arrow in the lower right corner of the figure indicates the polarization state of the incident light), the liquid crystal structural color holographic film in this embodiment sequentially reproduces blue, green, and red butterfly images of different shapes on the far-field screen (the colors and shapes are for illustrative purposes only and are not intended to limit the scope of this invention). When right-handed circularly polarized light of wavelengths 485nm, 560nm, and 650nm is incident simultaneously, the image on the far-field screen is a superposition of the diffracted light fields of the three wavelength channels, i.e., a holographic image of a red, green, and blue butterfly. There is virtually no aliasing between the images of different wavelength channels, demonstrating the excellent far-field color holographic function of the liquid crystal structural color holographic film.

[0109] It should be noted that the embodiments of the present invention only exemplify the near-field and far-field multiplexing display effects of tricolor flowers and tricolor butterflies, and are not intended to limit the liquid crystal structural color holographic film provided by the present invention. In other embodiments, by adjusting the structural color region setting and surface orientation structure of the cholesteric liquid crystal layer according to actual needs, the liquid crystal structural color holographic film provided by the present invention can be made suitable for near-field displays with richer colors, as well as 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 in this invention divides the cholesteric liquid crystal layer into multiple structural color regions of arbitrary shape and different reflection wavelengths, and endows the cholesteric liquid crystal layer with a patterned surface orientation structure. This enables independent phase modulation at different wavelengths, thereby achieving near-field and far-field multiplexed color image display. Furthermore, the fully polymerized film structure provides flexibility, self-support, and high stability. The liquid crystal structural color holographic film provided in this invention integrates multiple independent wavelength channels in the near and far fields into a flexible optical element with a thickness of micrometers. It can display near-field structural color patterns while generating color holographic images in the far field, exhibiting high multiplexing dimensionality and high functionality. This effectively overcomes the limitations of traditional multidimensional light field multiplexing technologies in terms of flexibility, fabrication difficulty, and processing cost.

[0111] The specific embodiments described above do not constitute a limitation on the scope of protection of this invention. Those skilled in the art should understand that various modifications, combinations, sub-combinations, and substitutions can be made according to design requirements and other factors. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of this invention should be included within the scope of protection of this invention.

Claims

1. A liquid crystal structural color holographic film, characterized in that, The liquid crystal structure color holographic film comprises a nematic liquid crystal layer and a cholesteric liquid crystal layer arranged in a stack, 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 the helical structure of the cholesteric liquid crystal layer has different pitches in different structural color regions to reflect light of different colors; The cholesteric liquid crystal layer has a patterned surface orientation structure, and the surface orientation structure is obtained by optimization according to a color holographic algorithm, and the orientation structure of different structural color regions independently encodes the phase hologram of different wavelength channels; Multi-wavelength laser is incident from one side of the cholesteric liquid crystal layer, and the reflected light diffracts to reproduce a wavelength multiplexed color holographic image.

2. The liquid crystal structural color holographic film according to claim 1, wherein, The nematic liquid crystal layer has a parallel structure liquid crystal polymer network, and the cholesteric liquid crystal layer has a periodic helical structure liquid crystal polymer network; The periodic helical structure of all structural color regions in the cholesteric liquid crystal layer has a consistent handedness, presenting a left-handed helical structure or a right-handed helical structure.

3. The liquid crystal structural color holographic film of claim 1, wherein, In the plane in which the liquid crystal structure color holographic film is located, there is no overlap between different structural color regions; 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 of claim 1, wherein, The cholesteric liquid crystal layer is provided with a spatially gradually changing surface orientation structure for controlling the initial director distribution of liquid crystal molecules on two surfaces of the cholesteric liquid crystal layer; The surface orientation structure is obtained by optimization according to the color holographic algorithm, first, a plurality of near-field patterns and far-field holographic images are divided according to the set wavelength channels, each set of near-field patterns and far-field holographic images corresponds to an independent wavelength channel, then the holographic phase corresponding to each wavelength channel is iteratively optimized and combined by using the Gerchberg-Saxton holographic algorithm, and finally the surface orientation structure of the cholesteric liquid crystal layer is calculated according to the corresponding relationship between the cholesteric liquid crystal geometric phase and the surface orientation angle.

5. The liquid crystal structural color holographic film of claim 1, wherein, The cholesteric liquid crystal layer has different Bragg reflection bands in different structural color regions, the structural color region can reflect light with a wavelength located in the Bragg reflection band and a circular polarization handedness consistent with the helical structure of the cholesteric liquid crystal layer, and has a geometric phase modulation effect, the size 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 only takes effect 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 multiplexed holography.

6. The liquid crystal structural color holographic film of claim 1, wherein, 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 the liquid crystal molecules in the cholesteric liquid crystal layer.

7. A method for preparing a liquid crystal structural color holographic thin film, characterized in that, A method for preparing the liquid crystal structure color holographic film according to any one of claims 1-6, 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 provided between the first substrate and the second substrate to form an empty cell; The empty cell is subjected to ultraviolet exposure with polarization direction varying in space to prepare a cholesteric liquid crystal polymer network scaffold with a patterned orientation structure between the first substrate and the second substrate. By using ultraviolet exposure with exposure time varying spatially, a nematic liquid crystal layer and a cholesteric liquid crystal layer are formed stacked between the first substrate and the second substrate; The nematic liquid crystal layer and the cholesteric liquid crystal layer are completely polymerized using ultraviolet light; The first substrate and the second substrate are peeled off 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 of claim 7, wherein the method further comprises the step of applying a protective layer on the surface of the liquid crystal structure color holographic film. The empty cell is subjected to ultraviolet exposure with polarization direction varying spatially to fabricate a cholesteric liquid crystal polymer network scaffold with a patterned orientation structure between the first substrate and the second substrate, comprising: The empty cell composed of the first substrate, the second substrate and the spacer is subjected to ultraviolet light orientation so that the orientation directions of the first orientation layer and the second orientation layer are consistent with the orientation structure designed by the color holographic algorithm. A mixture of cholesteric liquid crystal and polymerization monomers is filled between the first substrate and the second substrate, and surface-induced photopolymerization is performed by irradiating one side of the second substrate with ultraviolet light to form a cholesteric liquid crystal polymer network scaffold connected to the surface of the second substrate. The polymerized liquid crystal cell is immersed in acetone to wash away molecules that have not undergone polymerization. When the washed liquid crystal cell is removed from acetone, the cholesteric liquid crystal polymer network scaffold shrinks, but still retains the periodic helical structure and surface orientation structure of the cholesteric liquid crystal.

9. The method of claim 8, wherein the method further comprises the step of applying a protective layer on the surface of the liquid crystal structure color holographic film. By utilizing ultraviolet exposure with exposure time varying spatially, a nematic liquid crystal layer and a cholesteric liquid crystal layer are formed stacked between the first substrate and the second substrate, including: The shrunken cholesteric liquid crystal polymer network scaffold was subjected to different doses of ultraviolet light exposure, and different regions were exposed for different times to control the resilience of the polymer network scaffold. When the mixture of polymerization monomers and photoinitiator is refilled into the liquid crystal cell, the cholesteric liquid crystal polymer network scaffold will spring back, forming the nematic liquid crystal layer and the cholesteric liquid crystal layer stacked together. In particular, the longer the ultraviolet exposure time, the weaker the resilience of the cholesteric liquid crystal polymer network scaffold, the smaller the thickness of the formed cholesteric liquid crystal layer, the smaller the pitch, the shorter the Bragg reflection band wavelength, and the more bluish the structural color.

10. A colour holographic device, characterised in that Includes a supercontinuum laser, a polarizer, a quarter-wave plate, a liquid crystal structured color holographic film as described in any one of claims 1 to 6, and a light screen; The multi-wavelength light beam output by the supercontinuum laser is transmitted through the polarizer and the quarter-wave plate in sequence, and is converted into circularly polarized light with a rotation direction consistent with the cholesteric liquid crystal, and is incident on the liquid crystal structural color holographic film, the liquid crystal structural color holographic film performs independent geometric phase modulation and reflection on incident light of different wavelengths, and a color holographic image formed by the reflected light is received by the light screen.

Images