Nonlinear gradient geometric phase polarization grating liquid crystal element and preparation method and application thereof

Abstract

The invention belongs to the field of nonlinear optics, and particularly relates to a nonlinear gradient geometric phase polarization grating liquid crystal element and a preparation method and application thereof. The liquid crystal element comprises a first substrate and a second substrate which are oppositely arranged. A first orientation layer and a second orientation layer are arranged on the inner sides of the first substrate and the second substrate; frame glue is arranged on the edges of the inner sides of the first substrate and the second substrate, after the first substrate and the second substrate are bonded, a liquid crystal box is formed among the first orientation layer, the second orientation layer and the frame glue, and after a ferroelectric nematic phase liquid crystal material is injected, a liquid crystal layer is formed; metal electrodes are oppositely arranged at the two ends of the inner side of the first substrate. The ferroelectric nematic phase liquid crystal material is an ion-doped ferroelectric nematic phase liquid crystal material. By utilizing the characteristic that the liquid crystal responds to external stimulation, the application of dynamically adjustable nonlinear liquid crystal light field regulation and control is realized in the field of nonlinear soft material photonics.

Description

Technical Field

[0001] The present invention belongs to the field of nonlinear optics, and particularly relates to a nonlinear gradient geometric phase polarization grating liquid crystal element, a preparation method thereof, and an application thereof. Background Art

[0002] Liquid crystal (LC) has unique anisotropic optical properties, self-assembly ability, and dynamic tunability, and has become an ideal choice for structured light field manipulation in the field of linear optics. A newly discovered type of liquid crystal in recent years, ferroelectric nematic liquid crystal, breaks the head-to-tail symmetry compared with traditional liquid crystals, has the characteristics of spontaneous polarization, large dielectric constant, and strong nonlinear optical response, and is very suitable for realizing programmable and dynamic structured light field manipulation in the field of nonlinear optics. So far, the research on ferroelectric nematic liquid crystals has focused on their ferroelectric properties, topological structures, formation mechanisms, and new phases, such as helical ferroelectric nematic phase, polar twisted bend nematic phase, and other new phases. Ferroelectric nematic liquid crystals combine the advantages of liquid crystals and ferroelectricity, and the second-order nonlinear susceptibility can reach 5.6 pm / V - 10 pm / V, comparable to solid-state nonlinear materials. Therefore, ferroelectric nematic liquid crystals are the most promising materials for preparing dynamically tunable nonlinear optical components.

[0003] Currently, planar optical elements with geometric phase gradients can continuously manipulate the amplitude, phase, and polarization of light fields, and have made exciting progress in related fields of scientific research and technological applications. However, geometric phase gradient optical elements prepared based on hard material are limited by complex processes and component fixation, and it is difficult to achieve real-time reconfigurable adjustment of nonlinear geometric phases. By using chiral liquid crystals, the fabrication of sub-wavelength gratings and orthogonal modulation of multiple degrees of freedom of light can be realized through a simple manufacturing method. In addition, with the engineering progress of nonlinear photonic crystals and nonlinear metasurfaces, the concept of optical geometric phase has also been extended from linear optics to nonlinear optics. Considering that the research on reconfigurable nonlinear photonic devices can greatly promote the development of advanced optical processing and quantum information technology, how to use dynamically tunable materials to achieve reconfigurable nonlinear gradient geometric phases and develop their optical applications is a current research hotspot. Summary of the Invention

[0004] The technical problem to be solved by the present invention is to overcome the deficiencies of the prior art, use the structured orientation of liquid crystals to achieve the regulation of linear and non-linear structured light fields, and use the characteristics of liquid crystals' response to thermal and electrical stimuli to achieve dynamic regulation of liquid crystal orientation, so as to realize the application of a dynamically tunable gradient geometric phase polarization grating liquid crystal element in the field of non-linear photonics. Specifically, a non-linear gradient geometric phase polarization grating liquid crystal element and its preparation method are provided. Through a photo-alignment and thermal annealing treatment scheme, a doped-ion ferroelectric nematic liquid crystal is finely patterned, enabling the geometric phase polarization grating liquid crystal element to convert the pumped fundamental Gaussian light into a spatially special structured light field while undergoing non-linear frequency conversion, and the sample responds to external stimuli such as thermal and electric fields, thus realizing its application in dynamically reconfigurable non-linear photonics.

[0005] To solve the above technical problems, the present invention discloses a non-linear gradient geometric phase polarization grating liquid crystal element, which includes a first substrate and a second substrate arranged opposite to each other; a first alignment layer and a second alignment layer are provided on the inner sides of the first substrate and the second substrate; frame sealants are provided at the inner edges of the first alignment layer and the second alignment layer. When the first substrate and the second substrate are bonded together, a liquid crystal cell is formed between the first alignment layer, the second alignment layer and the frame sealants. After injecting a ferroelectric nematic liquid crystal material, a liquid crystal layer is formed; metal electrodes are respectively provided at both ends between the first substrate and the first alignment layer.

[0006] Among them, the ferroelectric nematic liquid crystal material is a doped-ion ferroelectric nematic liquid crystal material.

[0007] Specifically, the liquid crystal layer material uses a doped-ion ferroelectric nematic liquid crystal to achieve thermally and electrically switchable dynamic non-linear optical responses. The ion-doped ferroelectric nematic liquid crystal exhibits different phases during temperature changes, and the non-linear optical response changes accordingly. In addition, the ion-doped ferroelectric nematic liquid crystal responds rapidly to an electric field, and applying an electric field will cause dynamic switching of the liquid crystal tilt angle and non-linear optical response.

[0008] Among them, the doped-ion ferroelectric nematic liquid crystal material is a ferroelectric nematic liquid crystal RM734 doped with 1-butyl-3-methylimidazolium hexafluorophosphate (BMIM-PF6), and in actual operation, it includes but is not limited to this material.

[0009] Preferably, the mass ratio of 1-butyl-3-methylimidazolium hexafluorophosphate to ferroelectric nematic liquid crystal RM734 is (0.1 - 1.5):(98.5 - 99.9).

[0010] More preferably, the mass ratio of 1-butyl-3-methylimidazolium hexafluorophosphate to ferroelectric nematic liquid crystal RM734 is 0.9:99.1.

[0011] Specifically, the preparation of the ferroelectric nematic liquid crystal RM734 doped with 1-butyl-3-methylimidazolium hexafluorophosphate includes the following steps: weighing the ionic reagent BMIM-PF6 and the RM734 liquid crystal according to a mass ratio of 0.9:99.1 and putting them into a brown bottle, adding a stirrer, and then adding dichloromethane to dissolve, sealing the bottle cap with tape, stirring on a stirrer for 5 minutes and then ultrasonicating for 5 minutes, and finally opening the bottle cap and stirring on a magnetic stirrer and evaporating the solvent, the magnetic stirrer is set to a temperature of 50°C and a stirring rate of about 200 r / min.

[0012] The present invention also provides a method for preparing the above nonlinear gradient geometric phase polarization grating liquid crystal element, the specific steps of which are:

[0013] Step 1: ultrasonically clean the glass substrate, dry it, and then clean it with ultraviolet ozone to obtain a first substrate and a second substrate;

[0014] Step 2: Plating two opposite metal electrodes on both ends of the inner side of the first substrate;

[0015] Step 3: Spin-coating a light-controlled alignment agent on the first substrate and the second substrate, and annealing after the spin coating to obtain a first alignment layer and a second alignment layer;

[0016] Step 4: Spin-coat the edges of the first alignment layer and the second alignment layer processed in step 3 with a frame glue, stagger the first substrate and the second substrate, and obtain a liquid crystal box after UV curing;

[0017] Step 5: performing polarized ultraviolet exposure alignment on the first alignment layer and the second alignment layer by a micro-projection exposure system;

[0018] Step 6: pouring the ion-doped ferroelectric nematic liquid crystal material into a liquid crystal box to prepare a liquid crystal layer, thereby obtaining the nonlinear gradient geometric phase polarization grating liquid crystal element.

[0019] Among them, in step 1, ultrasonic cleaning includes ultrasonic cleaning with washing liquid and ultrasonic cleaning with ultrapure water, and the specific steps are: ultrasonic cleaning the glass substrate with washing liquid for 30 minutes, and then ultrasonic cleaning with ultrapure water twice, each time for 10 minutes.

[0020] Wherein, in step 1, the specific conditions for the drying are: drying at 120° C. for 40 minutes.

[0021] Wherein, in step 1, the ultraviolet ozone cleaning time is 30 minutes.

[0022] Wherein, in step 2, the metal electrode is a copper electrode, but is not limited to this material in actual operation.

[0023] Among them, in step three, for the spin coating, the specific conditions are as follows: in the first step, spin coat at 800 revolutions per minute for 10 seconds; in the second step, spin coat at 3000 revolutions per minute for 40 seconds; in the third step, spin coat at 300 revolutions per minute for 1 second.

[0024] Among them, in step three, for the annealing, the specific conditions are as follows: maintain at 100 °C for 10 minutes.

[0025] Among them, in step three, the photoalignment agent includes one of a surfactant, a rubbing alignment agent, a photocrosslinking material, a photodegradable material, and a photoinduced cis-trans isomerism material;

[0026] Preferably, the photoalignment agent is one of the nitrogen-based photoalignment material SD1 and brilliant yellow. Under the irradiation of linearly polarized light, the molecules of the photoalignment material will align along the direction perpendicular to the direction of the linearly polarized light, and through the interaction with liquid crystal molecules, the liquid crystal molecules will form an ordered alignment, thereby realizing the control of the liquid crystal molecule alignment. In actual operation, it is not limited to this material;

[0027] Specifically, the azo-based photoalignment material SD1 and brilliant yellow are selected. Under the irradiation of linearly polarized light, the molecules of the photoalignment material will align along the direction perpendicular to the direction of the linearly polarized light, and through the interaction with liquid crystal molecules, the liquid crystal molecules will form a specifically designed ordered arrangement.

[0028] Among them, in step six, for the filling, the specific conditions are as follows: heat the ferroelectric nematic liquid crystal material doped with ions to 190 °C, and fill it into the liquid crystal cell through a capillary glass tube, and perform slow thermal annealing treatment.

[0029] Furthermore, the application of the above non-linear gradient geometric phase polarization grating liquid crystal element in realizing dynamically tunable non-linear liquid crystal light field regulation is also within the protection scope of the present invention;

[0030] Specifically, in the embodiment of the present invention, the prepared non-linear gradient geometric phase polarization grating liquid crystal element can, while performing non-linear frequency conversion on the pumped fundamental Gaussian light, convert the original Gaussian mode into a spatially special structured light field. For example, the polarization grating can convert the fundamental Gaussian light into a second harmonic light and the second harmonic light has different polarization components, and at the same time convert the fundamental linearly polarized light into a fundamental light with spin angular momentum. In addition, when an external variable electric field is introduced, the angular spread of the polarization grating changes accordingly, and the intensity and polarization state of the non-linear optical response also change correspondingly, so as to meet the application of the non-linear gradient geometric phase polarization grating liquid crystal element described in the present invention in the field of dynamically reconfigurable non-linear photonics.

[0031] Beneficial effects:

[0032] The present invention utilizes the property that liquid crystals respond to external stimuli to achieve the application of dynamically tunable nonlinear liquid crystal optical field regulation in the field of nonlinear soft matter photonics. The present invention realizes the suppression of the generation of intrinsic defect lines and the chiral structures that often appear in ferroelectric nematic liquid crystals by doping a small amount of ions BMIM-FP6 into the ferroelectric nematic liquid crystal RM734. This can enable the growth of a more complete patterned alignment texture in the ferroelectric nematic state, and further play a role in improving the quality of the output beam and the diffraction spot quality in the experiments of linear and nonlinear structured optical fields; under the conditions of changing the polarization state of the incident Gaussian light and applying an electric field to the prepared nonlinear gradient geometric phase polarization grating liquid crystal element, the phase distribution and intensity distribution of the output diffraction spot are more in line with the theoretical expectations. BRIEF DESCRIPTION OF THE DRAWINGS

[0033] The following further specifically describes the present invention in conjunction with the drawings and specific embodiments, and the above and / or other advantages of the present invention will become clearer.

[0034] Figure 1 Structural diagram of the liquid crystal element prepared according to the present invention;

[0035] Figure 2 Schematic diagram of the liquid crystal molecular orientation, linear diffraction and nonlinear diffraction of the nonlinear gradient geometric phase polarization grating liquid crystal element prepared according to the present invention;

[0036] Figure 3 Texture comparison diagram of the doped ion ferroelectric nematic nonlinear liquid crystal element and the pure ferroelectric nematic nonlinear liquid crystal element prepared according to the present invention.

[0037] Figure 4 Phase transition texture diagram of the nonlinear gradient geometric phase polarization grating liquid crystal element prepared according to the present invention;

[0038] Figure 5 Magnified texture diagram of the sample of the nonlinear gradient geometric phase polarization grating liquid crystal element prepared according to the present invention under a polarized light microscope;

[0039] Figure 6 Inspection result diagram of the achiral texture and texture result diagram of dynamically regulating the liquid crystal opening angle by applying an external electric field of the nonlinear gradient geometric phase polarization grating liquid crystal element prepared according to the present invention;

[0040] Figure 7 Schematic diagram of the orientation coordinates of the external electric field in the nonlinear gradient geometric phase polarization grating liquid crystal element prepared according to the present invention;

[0041] Figure 8 Variation result diagram of the liquid crystal opening angle when an external electric field is introduced into the nonlinear gradient geometric phase polarization grating liquid crystal element prepared according to the present invention;

[0042] Figure 9 Schematic diagram of a non - linear gradient geometric phase polarization grating liquid crystal element with a 180 - degree spreading angle prepared by the present invention, and graphs of the intensity changes of diffracted light of each order and the test results of polarization states;

[0043] Figure 10 Schematic diagram of a non - linear gradient geometric phase polarization grating liquid crystal element with a 120 - degree spreading angle prepared by the present invention, and graphs of the intensity changes of diffracted light of each order and the test results of polarization states. Detailed implementation manners

[0044] In the following embodiments, the experimental methods are conventional methods unless otherwise specified; the reagents and materials can be obtained from commercial sources unless otherwise specified.

[0045] Embodiment 1:

[0046] This embodiment provides a non - linear gradient geometric phase polarization grating liquid crystal element. Figure 1 It is a structural diagram of the liquid crystal element prepared by the present invention. Specifically, the liquid crystal element includes a first substrate and a second substrate arranged opposite to each other. A first alignment layer and a second alignment layer are respectively arranged on the inner sides of the first substrate and the second substrate. Frame sealants are arranged at the edges of the first alignment layer and the second alignment layer. When the first substrate and the second substrate are bonded, a liquid crystal cell is formed between the first alignment layer, the second alignment layer and the frame sealants. When a liquid crystal material is injected, a liquid crystal layer is formed; metal electrodes are respectively arranged at both ends between the first substrate and the first alignment layer.

[0047] The preparation steps of the above - mentioned liquid crystal element are as follows:

[0048] Step 1: The first substrate and the second substrate are made of glass substrates. First, the two glass substrates are ultrasonically cleaned with a cleaning solution for 30 minutes. Then, they are ultrasonically cleaned with ultrapure water twice repeatedly, with each cleaning time being 10 minutes. The cleaned substrates are placed in a drying oven, the temperature of the drying oven is adjusted to 120 °C, and the drying time is 40 minutes. Finally, the glass substrates are subjected to ultraviolet ozone cleaning for 30 minutes to obtain the first substrate and the second substrate;

[0049] Step 2: Two opposite copper electrodes are plated at both ends on the inner side of the first substrate. First, the first substrate is cleaned, the areas without copper plating are masked, then the first substrate is placed in a vacuum chamber, and then vacuum is pumped, copper is sputtered by magnetron, and finally cooling annealing, mask removal, and antioxidant treatment are carried out to obtain the metal electrodes;

[0050] Step 3: The first alignment layer and the second photo-alignment agent select the photo-alignment agent SD1, and the photo-alignment agent SD1 is spin-coated on the first substrate and the second substrate. The spin-coating method is as follows: First step, spin-coat at 800 revolutions per minute for 10 seconds; Second step, spin-coat at 3000 revolutions per minute for 40 seconds; Third step, spin-coat at 300 revolutions per minute for 1 second. The thickness of the finally formed photo-alignment agent layer is about 30 nanometers. After spin-coating the alignment agent, the first substrate and the second substrate spin-coated with the photo-alignment agent SD1 are annealed, the annealing temperature is 100 °C, and the time is 10 minutes;

[0051] Step 4: The sealant is evenly applied to the edges of the first substrate and the second substrate processed in Step 2. Subsequently, the first and second substrates are adhesively bonded in a staggered manner and placed under ultraviolet light until the sealant is cured to form a liquid crystal cell. The cell gap of the liquid crystal cell in the embodiment is measured to be about 1.0 micrometer by the interference method;

[0052] Step 5: In order to achieve a structure consistent with the alignment of the ferroelectric nematic liquid crystal molecules, a multi-step partial overlapping exposure is performed on the first alignment layer and the second alignment layer using a micro-projection exposure system based on a digital micromirror device (this system has been disclosed in CN106647045A). The light beams reflected by each micromirror in this system are given a specific polarization state after passing through an electro-optic polarizer and then irradiated on the surface of the liquid crystal cell. After a certain dose of light irradiation, the photo-alignment agent molecules in the exposed area will be patterned. By rotating the polarizer in the system, different positions of the exposure pattern can correspond to polarized light in different directions, so that the photo-alignment agent molecules have different alignments;

[0053] Step 6: After the first alignment layer and the second alignment layer are subjected to polarized ultraviolet exposure and alignment, a liquid crystal material is injected into the liquid crystal cell between the first substrate and the second substrate. The liquid crystal materials respectively select a ferroelectric nematic liquid crystal material RM734 doped with 0.9% wt BMIM-PF6 and an undoped pure ferroelectric nematic liquid crystal material RM734; the liquid crystal material is heated to 190 °C and injected into the liquid crystal cell through a capillary glass tube, and slowly heat-annealed to make the liquid crystal molecules align along the alignment directions of the first alignment layer and the second alignment layer;

[0054] Specifically, the preparation method of the ferroelectric nematic liquid crystal RM734 doped with the ionic reagent BMIM-PF6 is to fully mix the ferroelectric nematic liquid crystal RM734 and the ionic reagent BMIM-PF6 in a mass ratio of 99.1:0.9. The specific steps are as follows: Weigh 1.8 mg of the ionic reagent BMIM-PF6, then weigh 198.2 mg of the RM734 liquid crystal and put it into a brown bottle, add a magnetic stir bar, and then add dichloromethane to dissolve. Seal the bottle cap with tape, stir on a stirrer for 5 minutes and then ultrasonicate for 5 minutes. Finally, open the bottle cap and stir on a magnetic stirrer to evaporate the solvent. The temperature of the magnetic stirrer is set at 50 °C, and the stirring rate is about 200 r / min.

[0055] It should be noted that for the doped and undoped liquid crystals in this embodiment, the principles of molecular orientation direction, linear diffraction, and nonlinear diffraction are the same. Figure 2 This is a schematic diagram of the liquid crystal molecular orientation, linear diffraction, and nonlinear diffraction of the reconfigurable nonlinear gradient geometric phase liquid crystal element prepared by the present invention. The prepared reconfigurable gradient geometric phase nonlinear liquid crystal element is used to realize nonlinear Raman-Nath diffraction. In the nonlinear Raman-Nath diffraction mechanism, the transverse phase is matched through the periodic modulation of the nonlinear susceptibility. The diffraction angle of linear diffraction is twice that of nonlinear diffraction.

[0056] The pure ferroelectric nematic liquid crystal RM734 and RM734 doped with 0.9% wt BMIM-PF6 are respectively poured into a liquid crystal cell with a homogeneous orientation and a thickness of 1.0 micrometer. Figure 3 These are the texture diagrams of the nonlinear liquid crystal elements prepared for the two respectively. It can be seen from the figure that at 90 °C, both the pure ferroelectric nematic liquid crystal RM734 and RM734 doped with 0.9% wt BMIM-PF6 enter the ferroelectric nematic phase state. However, the pure ferroelectric nematic liquid crystal RM734 will grow defect lines, and the existence of these defect lines will destroy the original patterned orientation of the liquid crystal, thus affecting the regulation of the structured light field by the nonlinear gradient geometric phase polarization grating liquid crystal element prepared by the present invention.

[0057] Figure 4 This is the phase transition texture diagram of the nonlinear gradient geometric phase polarization grating liquid crystal element prepared by the present invention. During the cooling process, the ferroelectric nematic liquid crystal RM734 doped with 0.9% wt BMIM-PF6 undergoes the following phase state transitions: when the temperature is higher than 175 °C, it is in the isotropic state (Iso); when the temperature is 125 °C - 175 °C, it is in the nematic state (N); when the temperature is 105 °C - 125 °C, it is in the mesophase state (N x ); when the temperature is below 105 °C, it is in the ferroelectric nematic state (N F)。During the process of gradually cooling the ferroelectric nematic liquid crystal RM734 doped with 0.9% wt BMIM-PF6 from the isotropic phase to the ferroelectric nematic phase, the intrinsic defect lines that only appear in the pure ferroelectric nematic liquid crystal RM734 in the ferroelectric nematic phase were not observed.

[0058] Figure 5 This is the magnified texture diagram of the nonlinear gradient geometric phase polarization grating liquid crystal element sample prepared by the present invention under a polarized light microscope. The spreading angles of the liquid crystal polarization grating are 180 degrees and 120 degrees respectively. For the liquid crystal polarization grating with a spreading angle of 180 degrees, the arrangement mode is that the liquid crystal director gradually changes by 180 degrees within one period, and the director of adjacent liquid crystal molecules changes by 5 degrees. For the liquid crystal polarization grating with a spreading angle of 120 degrees, the arrangement mode is that the liquid crystal director gradually changes by 120 degrees within one period, and the director of adjacent liquid crystal molecules changes by 5 degrees.

[0059] Figure 6 This is the non-chiral texture inspection result diagram and the texture result diagram of dynamic regulation by an external electric field of the nonlinear gradient geometric phase polarization grating liquid crystal element prepared by the present invention. The prepared nonlinear gradient geometric phase polarization grating liquid crystal element was placed under a polarized light microscope, and the analyzer and the polarizer were at 75 degrees ( Figure 6 a) and 105 degrees ( Figure 6 b) to test the chirality of the liquid crystal element. By adjusting the angle between the analyzer and the polarizer of the polarized light microscope and observing, if a chiral structure appears, the colors of the chiral structure regions observed under the polarized light microscope when the analyzer and the polarizer are at 75 degrees and 105 degrees are exactly reversed. Figure 6 The results show that the nonlinear gradient geometric phase polarization grating liquid crystal element prepared by the present invention has no chirality. In the ferroelectric nematic phase state, an external electric field was applied to the prepared nonlinear gradient geometric phase polarization grating liquid crystal element. Under the action of an electric field of 1.0 Hz and 0.06 V / μm, the spreading angle of the liquid crystal changed. The schematic diagram of the electric field orientation coordinates based on the nonlinear gradient geometric phase polarization grating liquid crystal element is shown in Figure 7 When the electric field is E=(0,1,0) ( Figure 6 c), the extinction region shown under the polarized light microscope increases, indicating that the region aligned with the electric field direction has a larger spatial range and the spreading angle of the liquid crystal decreases. When the electric field is E=(0, -1,0) ( Figure 6 d), the extinction region shown under the polarized light microscope is smaller, indicating that the region aligned with the electric field obtains a smaller spatial range and the spreading angle of the liquid crystal increases.

[0060] Figure 8The dynamic tuning of non - linear diffraction of the non - linear gradient geometric - phase liquid - crystal polarization grating element prepared by the present invention when an external electric field is introduced. An alternating electric field is applied in the direction parallel to the non - linear geometric - phase liquid - crystal element. Under the action of the alternating electric field, the opening angle of the polar liquid - crystal grating changes from the equilibrium state (θ = π, flat angle) to an obtuse angle or an acute angle. When the electric - field strength changes from - 0.06 V / μm to + 0.06 V / μm, when the incident light is horizontally polarized light, the diffraction intensity of the 0 - order of non - linear diffraction shows an increasing trend, and the diffraction intensities of the ±1 - order and ±2 - order decrease, indicating that the liquid - crystal opening angle gradually decreases and the diffraction efficiency gradually decreases; when the incident light is vertically polarized light, the diffraction intensity of the 0 - order of non - linear diffraction shows an increasing trend, the change of the diffraction intensities of the ±1 - order and ±2 - order is small, but the diffraction intensity of the ±2 - order shows a decreasing trend. By applying a periodic electric field to the non - linear gradient geometric - phase liquid - crystal polarization grating element, the liquid - crystal opening angle of the polarization grating is periodically changed, so as to realize the periodic dynamic regulation of the intensity of the non - linear structured light field.

[0061] Figure 9 Schematic diagram of the non - linear gradient geometric - phase polarization grating liquid - crystal element with an opening angle of 180 degrees prepared by the present invention, the change of diffraction light intensity of each order and the test results of polarization state. The temperature of the liquid - crystal element is controlled at 140 °C, and the ferroelectric nematic liquid crystal doped with 0.9% wt BMIM - PF6 enters the nematic state. At this time, the diffraction is linear diffraction, and the diffraction orders are 0 - order and ±1 - order. The 0 - order diffraction spot depends on the half - wave condition, and the ±1 - order diffraction depends on the spin angular momentum of the incident light. When the incident light is right - hand circularly polarized, the - 1 - order diffraction spot is left - hand circularly polarized; when the incident light is left - hand circularly polarized, the + 1 - order diffraction spot is right - hand circularly polarized. As the temperature of the liquid - crystal element is reduced to 90 °C, the ferroelectric nematic liquid crystal doped with 0.9% wt BMIM - PF6 enters the ferroelectric nematic state. At this time, the diffraction is non - linear diffraction. In this case, by modulating the momentum compensation provided by the liquid - crystal structure grating vector, the transverse (perpendicular to the propagation direction) satisfies the phase - matching condition. When the incident light is in the horizontal polarization state, the ±2 - order is the cross - circular polarization component, and the linear polarization state appears at the 0 - order and ±1 - order. When the incident light is in the vertical polarization state, the intensity of the ±2 - order weakens, and the cross - circular polarization component appears at the ±1 - order. By changing the polarization or circular - polarization state of the incident fundamental - frequency light, the regulation of the frequency, intensity and phase of the second - harmonic light field of this element is realized.

[0062] Figure 10 Schematic diagram of the non - linear gradient geometric - phase polarization grating liquid - crystal element with an opening angle of 120 degrees prepared by the present invention, the change of diffraction light intensity of each order and the test results of polarization state. In non - linear diffraction, compared with the non - linear geometric - phase liquid - crystal element with an opening angle of 180 degrees prepared, the diffraction intensity of the ±2 - order is much weaker. By changing the polarization or circular - polarization state of the incident fundamental - frequency light, the regulation of the frequency, intensity and phase of the second - harmonic light field of this element is realized.

[0063] The present invention provides an idea for a non-linear gradient geometric phase polarization grating liquid crystal element, its preparation method and application. There are many methods and ways to specifically implement this technical solution. The above is only the preferred embodiment of the present invention. It should be noted that for those of ordinary skill in the art of this technology, without departing from the principle of the present invention, several improvements and refinements can be made, and these improvements and refinements should also be regarded as the protection scope of the present invention. Each component not clearly defined in this embodiment can be implemented by existing technologies.

Claims

1. A non-linear gradient geometric phase polarization grating liquid crystal element, characterized in that, The liquid crystal element comprises a first substrate and a second substrate arranged opposite to each other; a first orientation layer and a second orientation layer are arranged inside the first substrate and the second substrate; a sealant is arranged at the edge of the first orientation layer and the second orientation layer, and when the first substrate and the second substrate are bonded, a liquid crystal box is formed between the first orientation layer, the second orientation layer and the sealant, and a liquid crystal layer is formed when a ferroelectric nematic liquid crystal material is injected; metal electrodes are arranged at both ends between the first substrate and the first orientation layer; Wherein, the ferroelectric nematic liquid crystal material is an ion-doped ferroelectric nematic liquid crystal material.

2. The liquid crystal element according to claim 1, characterized in that, The ferroelectric nematic liquid crystal material doped with ions is ferroelectric nematic liquid crystal RM734 doped with 1-butyl-3-methylimidazolium hexafluorophosphate.

3. The liquid crystal element according to claim 2, wherein, The mass ratio of the 1-butyl-3-methylimidazolium hexafluorophosphate to the ferroelectric nematic liquid crystal RM734 is (0.1-1.5): (98.5-99.9).

4. The preparation method of the non-linear gradient geometric phase polarization grating liquid crystal element according to any one of claims 1 to 3, characterized in that, The specific steps are: Step 1: ultrasonically clean the glass substrate, dry it, and then clean it with ultraviolet ozone to obtain a first substrate and a second substrate; Step 2: Plating two opposite metal electrodes on both ends of the inner side of the first substrate; Step 3: Spin-coating a light-controlled alignment agent on the first substrate and the second substrate, and annealing after the spin coating to obtain a first alignment layer and a second alignment layer; Step 4: Spin-coat the first alignment layer and the second alignment edge treated in step 3 with a sealant, stagger the first substrate and the second substrate, and obtain a liquid crystal box after UV curing; Step 5: performing polarized ultraviolet exposure alignment on the first alignment layer and the second alignment layer by a micro-projection exposure system; Step 6: pouring the ion-doped ferroelectric nematic liquid crystal material into a liquid crystal box to prepare a liquid crystal layer, thereby obtaining the nonlinear gradient geometric phase polarization grating liquid crystal element.

5. The preparation method according to claim 4, characterized in that, In step 1, the drying is carried out under the following specific conditions: drying at 120° C. for 40 minutes.

6. The preparation method according to claim 4, characterized in that, In step three, the spin coating has the following specific conditions: the first step is to spin coat at 800 rpm for 10 seconds; the second step is to spin coat at 3000 rpm for 40 seconds; and the third step is to spin coat at 300 rpm for 1 second.

7. The preparation method according to claim 4, characterized in that, In step three, the annealing is carried out under the following specific conditions: maintaining the temperature at 100° C. for 10 minutes.

8. The preparation method according to claim 4, characterized in that, In step three, the photo-controlled alignment agent includes any one of a surfactant, a rubbing alignment agent, a photo-crosslinking material, a photo-degradable material, and a photoinduced cis-trans isomerization material.

9. The preparation method according to claim 4, wherein, In step six, the specific conditions for the injection are as follows: the ferroelectric nematic liquid crystal material doped with ions is heated to 190° C., injected into the liquid crystal box through a capillary glass tube, and slowly subjected to thermal annealing treatment.

10. Use of the nonlinear gradient geometric phase polarization grating liquid crystal element according to any one of claims 1 to 3 in realizing dynamically adjustable nonlinear liquid crystal light field control.

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