Liquid crystal grating, control method and preparation method thereof

Abstract

The application discloses a liquid crystal grating and a control method and a preparation method thereof. The liquid crystal grating comprises a substrate, an electrode layer, an orientation film and a liquid crystal layer. The electrode layer is arranged on one side of the substrate. The orientation film is arranged on the side of the electrode layer away from the substrate. The liquid crystal layer is arranged on the side of the orientation film away from the electrode layer. The electrode layer has positive and negative electrodes with a pattern in a set distribution, so as to apply a set electric field to the liquid crystal layer. The orientation film has a control pattern with a set distribution of molecular directors, so as to make the liquid crystal molecules in the liquid crystal layer self-assemble into a set lamellar liquid crystal oil streak period structure. The application has the advantages of simple preparation process, low cost, high polarization dependence and tunable grating parameters.

Description

TECHNICAL FIELD

[0001] The present application relates to an optical element and a control method and a preparation method thereof, in particular to a liquid crystal grating and a control method and a preparation method thereof. BACKGROUND

[0002] The grating is an optical element that makes the light beam produce dispersion deflection based on the periodic structure, and ordinary grating devices are mostly made of glass, metal and other materials, which cannot adjust the parameters such as grating period, grating orientation and refractive index distribution. The grating device based on liquid crystal material combines the characteristics of large birefringence, dielectric anisotropy and multiple phase states of the material, and has the ability of dynamic tuning due to the control of electric field, magnetic field, light field and thermal field. In addition, the liquid crystal grating has the advantages of light weight, high integration, large modulation range, low power consumption and high precision due to its material characteristics. Therefore, the liquid crystal grating device has a good application prospect in the fields of beam shaping, optoelectronic switch, spectrum analysis and image display.

[0003] The conventional liquid crystal grating is generally based on the fixed orientation of the grating structure, and is realized by geometric structure limitation or patterned electrode, but the grating function is relatively single, and the dynamic reconfiguration of the structure parameters cannot be realized. How to simply and efficiently realize the rapid preparation and multi-parameter control of the liquid crystal grating element needs further research. SUMMARY

[0004] The present application aims to provide a liquid crystal grating to realize multi-parameter control and high polarization correlation of the grating structure, a control method of the liquid crystal grating, and a preparation method of the liquid crystal grating to realize rapid preparation of the multi-parameter control liquid crystal grating, simplify the preparation process and reduce the cost.

[0005] The liquid crystal grating device of the present application comprises a substrate, an electrode layer, an orientation film and a liquid crystal layer, the electrode layer is arranged on one side of the substrate, the orientation film is arranged on the side of the electrode layer away from the substrate, and the liquid crystal layer is arranged on the side of the orientation film away from the electrode layer; the electrode layer has positive and negative electrodes with a pattern in a set distribution to apply a set electric field to the liquid crystal layer; the orientation film has a control pattern with a set distribution of molecular director to make the liquid crystal molecules in the liquid crystal layer self-assemble into a set liquid crystal oil streak period structure.

[0006] The control pattern on the alignment film can be obtained by rubbing alignment or photo-alignment, and the alignment film has an anchoring effect on the liquid crystal molecules in the liquid crystal layer, so that the molecular director arrangement of the liquid crystal molecules adjacent to the alignment film in the liquid crystal layer is the same as the molecular director arrangement of the alignment film. Under the anchoring effect of the alignment film on the liquid crystal molecules in the liquid crystal layer and the anchoring effect of air on the liquid crystal far away from the alignment film side of the liquid crystal layer, the liquid crystal molecules in the liquid crystal layer self-assemble into a liquid crystal oil ridge periodic structure with a set distribution.

[0007] The pattern of the electrode can be obtained by dry etching or wet etching, and the pattern of the electrode is well aligned with the control pattern of the alignment film, so that the electrode generates a set electric field distribution related to the control pattern of the alignment film. The liquid crystal molecules in the liquid crystal layer are rearranged under the action of the electric field generated by the electrode, so that the self-assembled liquid crystal oil ridge periodic structure in the liquid crystal layer is reconstructed.

[0008] Principle: The present application is a liquid crystal grating based on a self-assembled liquid crystal oil ridge structure. By controlling the molecular director distribution of the alignment film, the liquid crystal oil ridge periodic structure with a set distribution grown on the alignment film side has anisotropic optical properties. Combined with the set distribution of the electric field generated by the patterned electrode, the liquid crystal oil ridge periodic structure can change in size and orientation, and finally obtain a liquid crystal grating device with high polarization dependence and parameter tunable characteristics.

[0009] In the present application, smectic liquid crystal material is used, and the formation condition of the liquid crystal oil ridge structure is the anti-boundary orientation, which is reflected in the parallel orientation effect brought by the alignment film and the inherent vertical orientation effect at the liquid crystal / air interface; and the electrode layer generates electric fields with different orientations in the electrode coverage area and the electrode interval area, thereby changing the proportion of parallel orientation and vertical orientation of the liquid crystal layer, and further affecting the final structure. That is, without electric field, only the parallel anchoring force of the bottom surface, the surface tension, and the bulk elastic force in the liquid crystal layer; after applying an electric field, a vertical orientation force is applied in the electrode coverage area, and a horizontal orientation force is applied in the electrode interval area, and different voltages are applied. The different forces result in different assembled structures and different technical effects.

[0010] The molecular director distribution of the alignment film, the patterned structure of the electrode, and the grown liquid crystal oil ridge periodic structure have various setting modes; wherein the orientation of the generated liquid crystal oil ridge is always perpendicular to the direction of the molecular director of the alignment film.

[0011] Preferably, the control pattern is square, grid-shaped or disc-shaped; that is, the control pattern is selected from any one of the following:

[0012] The control pattern is square, and the molecular director in the control pattern is oriented in the same direction, and is uniformly oriented in a single direction; the orientation of the molecular director can be along the horizontal direction, the vertical direction, or along any angle, as long as the orientation of the molecular director is the same. The liquid crystal oil ridge periodic structure is a liquid crystal oil ridge array, and the alignment film has a square control pattern with the molecular director distributed in a single direction, so that the liquid crystal molecules in the liquid crystal layer self-assemble to form a liquid crystal oil ridge array.

[0013] Alternatively, the control pattern is in the form of a grid, and the control pattern includes a plurality of rectangular control pattern units arranged periodically, and each control pattern unit includes a plurality of columns of sub-control patterns, wherein the molecular directors in at least two columns of sub-control patterns are oriented in different directions. That is, the liquid crystal oil ridge periodic structure is a liquid crystal oil ridge array, and the alignment film has a control pattern in the form of a grid with the molecular director distributed in the form of a grid, so that the liquid crystal molecules in the liquid crystal layer self-assemble to form a liquid crystal oil ridge array.

[0014] Alternatively, the control pattern is in the form of a disc, and the molecular directors in the disc-shaped control pattern are radially distributed in a radial direction with respect to the center of the disc, so that the liquid crystal molecules in the liquid crystal layer self-assemble to form a concentric circular ring liquid crystal oil ridge periodic structure. That is, the alignment film has a control pattern in the form of a disc with the molecular director distributed in the form of a disc, so that the liquid crystal molecules in the liquid crystal layer self-assemble to form a concentric circular ring liquid crystal oil ridge periodic structure; wherein the molecular directors in the disc-shaped control pattern are radially distributed in a radial direction with respect to the center of the disc; and the concentric circular ring liquid crystal oil ridge periodic structure includes a plurality of annular liquid crystal oil ridge structures with varying radii along the radial direction; and the line width of the annular liquid crystal oil ridge structure along the radial direction is uniform.

[0015] Further, the control pattern is square, and the side length of the control pattern is d, and d≥10μm.

[0016] Further, the control pattern is in the form of a grid, and the angle between the molecular director of the alignment film in each sub-control pattern in the grid-shaped control pattern and the boundary of the column direction of the sub-control pattern is θ, and -90°<θ<90°.

[0017] Further, the control pattern is in the form of a grid, and the length of the sub-control pattern along the row direction is a, and a≥10μm; and the length of the sub-control pattern along the column direction is b, and b≥10μm.

[0018] Further, the control pattern is in the form of a disc, and the radius of the control pattern is r, and r≥30μm.

[0019] Preferably, the thickness of the liquid crystal layer is 0.71-1.95μm.

[0020] Preferably, the positive and negative electrodes of the electrode layer are in an interdigital arrangement, and the positive and negative electrodes respectively have rectangular electrode units arranged in a staggered manner, the electrode units have long sides and short sides, and are arranged along the short sides; the in-plane component direction of the electric field generated by the electrodes is parallel to the short side direction of the electrode units. The short side length of the electrode units is w1, the long side length of the electrode units is w2, and the pitch of adjacent electrode units along the short side direction is s; wherein w1≥10 μm, w2≥10*w1, and s≥10 μm. The control pattern is square, and the alignment film has a square control pattern with a single-direction molecular director, and the alignment direction of the square control pattern has an angle ξ with the long side direction of the electrode units, wherein 0°≤ξ<180°.

[0021] The application further provides a control method of the liquid crystal grating, comprising: applying voltages of different sizes on the electrode layer of the liquid crystal grating to tune the period size and orientation of the liquid crystal oil ripple structure in the liquid crystal grating, so as to realize multi-parameter control of the liquid crystal grating; and vertically irradiating the liquid crystal grating with linearly polarized light with an adjustable polarization direction to generate high polarization-dependent beam diffraction.

[0022] The preparation method of the liquid crystal grating in the application comprises:

[0023] (1) forming an electrode on one side of a substrate;

[0024] The substrate can be a flexible substrate or a rigid substrate with a light transmittance greater than or equal to 85%, and can be quartz glass or ordinary glass. The electrode is a transparent electrode, for example, an indium tin oxide film, and a structure electrode with a required pattern is obtained by wet etching.

[0025] (2) forming an alignment film on the side of the electrode away from the substrate;

[0026] Optionally, the alignment film is a photoalignment film. Before forming the photoalignment film, to increase the wettability and adhesion of the photoalignment film to the substrate, the substrate is ultrasonically cleaned with a cleaning solution (a mixed reagent of acetone, alcohol, etc.) for 30 minutes, and then ultrasonically cleaned with ultrapure water twice, each for 10 minutes. After drying in an oven at 120 ℃ for 40 minutes, the substrate is subjected to UVO (ultraviolet ozone) cleaning for 30 minutes.

[0027] Optionally, the material of the photoalignment film comprises at least one of a photocrosslinking material, a photodegradation material and a photoisomerization material.

[0028] Optionally, the photoalignment film is formed on one side of the substrate by the following method: spin coating the photoalignment material on one side of the substrate, with spin coating parameters of low-speed spin coating for 5 seconds at a speed of 800 rpm, high-speed spin coating for 40 seconds at a speed of 3000 rpm; annealing the substrate with the spin-coated photoalignment material for 10 minutes at an annealing temperature of 100 ℃ to form the photoalignment film, and the thickness of the photoalignment film can be 30 nm to 50 nm.

[0029] (3) Orienting the orientation film to form a control pattern in which the molecular directors are distributed in a set manner.

[0030] (4) forming a liquid crystal layer on the side of the alignment film away from the substrate, wherein the control pattern of the alignment film controls the liquid crystal molecules in the liquid crystal layer to self-assemble into a set periodic structure of liquid crystal oil patterns;

[0031] Optionally, the liquid crystal layer material is a smectic liquid crystal material, specifically 4'-n-octyl-4-cyanobiphenyl.

[0032] Beneficial Effects: The liquid crystal grating of the present invention offers the advantages of multi-parameter tunable grating structure, high polarization dependence, simple fabrication process, and low cost. Compared to existing liquid crystal gratings, the present invention is based on a self-assembled liquid crystal oil-streak structure. Due to the reconfigurable nature of this multi-layered structure under external field conditions, the liquid crystal oil-streak grating possesses flexible and adjustable capabilities, enabling periodic adjustment, orientation change, and on / off switching under electric field control. The inherent anisotropy of the oil-streak structure imparts high polarization dependence to the liquid crystal grating, resolving the limitations of existing liquid crystal grating devices, which suffer from limited functionality and adjustable parameters. Furthermore, the self-assembly fabrication process is easy to implement, simplifying the liquid crystal grating fabrication process and reducing the cost of the grating device. BRIEF DESCRIPTION OF THE DRAWINGS

[0033] Figure 1 is a schematic structural diagram of the liquid crystal grating device of the present invention;

[0034] Figure 2 is a top view of the molecular director distribution of an alignment film of the present invention;

[0035] Figure 3 It is a schematic diagram of the corresponding relationship between the thickness of the liquid crystal layer and the period of the correspondingly grown liquid crystal grating;

[0036] Figure 4 This is a cross-polarized microscope photograph of a liquid crystal grating device;

[0037] Figure 5 yes Figure 4 A polarized light diffraction pattern of the liquid crystal grating device shown;

[0038] Figure 6 It is a schematic diagram of the corresponding relationship between the thickness of the liquid crystal layer and the first-order diffraction efficiency of the correspondingly grown liquid crystal grating;

[0039] Figure 7 It is a schematic diagram of the corresponding relationship between the thickness of the liquid crystal layer and the first-order diffraction angle of the correspondingly grown liquid crystal grating;

[0040] Figure 8 It is a structural diagram of liquid crystal oil streaks;

[0041] Figure 9 is a top view of an interdigital electrode;

[0042] Figure 10 is Figure 9 is an enlarged view of A in FIG. 1;

[0043] Figure 11 is Figure 9 is a schematic diagram of an electric field distribution of the interdigital electrode shown in FIG. 1;

[0044] Figure 12 is Figure 10 is a polarized light microscope photograph of another liquid crystal grating device driven by the interdigital electrode of Example 1;

[0045] Figure 13 is Figure 10 is a polarized light microscope photograph of another liquid crystal grating device driven by the interdigital electrode of Example 2;

[0046] Figure 14 is a top view of a molecular director distribution of another alignment film of Example 3;

[0047] Figure 15 is a polarized light microscope photograph of another liquid crystal grating device;

[0048] Figure 16 is Figure 15 is a polarized light diffraction pattern of the liquid crystal grating device shown in FIG. 1;

[0049] Figure 17 is Figure 15 is another polarized light diffraction pattern of the liquid crystal grating device shown in FIG. 1;

[0050] Figure 18 is a top view of a molecular director distribution of yet another alignment film;

[0051] Figure 19 is a microscope photograph of yet another liquid crystal grating device;

[0052] Figure 20 is a flowchart of a method of manufacturing a liquid crystal grating device;

[0053] Figure 21 is a pattern of a mask;

[0054] Figure 22 is a pattern of another mask;

[0055] Figure 23 is a schematic diagram of an interdigital electrode of Example 3;

[0056] Figure 24is a schematic diagram of a ring electrode of Example 4. DETAILED DESCRIPTION

[0057] The application is further described in detail below with reference to examples.

[0058] The raw materials and reagents used in the following examples are commercially available.

[0059] Example 1

[0060] A schematic diagram of a structure of a liquid crystal grating provided by the present embodiment is shown in Figure 1 , which includes a substrate 40, an electrode 30, an alignment film 20, and a liquid crystal layer 10. The electrode 30 is disposed on one side of the substrate 40, the alignment film 20 is disposed on the side of the electrode 30 away from the substrate 40, and the liquid crystal layer 10 is disposed on the side of the alignment film 20 away from the electrode 30. The electrode 30 has positive and negative electrodes with a pattern in a set distribution to apply a set electric field to the liquid crystal layer 10. The alignment film 20 has a control pattern with a set distribution of molecular directors to cause the liquid crystal molecules in the liquid crystal layer 10 to self-assemble into a set liquid crystal oil-stripe periodic structure.

[0061] The control pattern on the alignment film 20 can be obtained by rubbing alignment or photo-alignment, and the alignment film 20 has an anchoring effect on the liquid crystal molecules in the liquid crystal layer 10, so that the molecular directors of the liquid crystal molecules adjacent to the alignment film 20 in the liquid crystal layer 10 are arranged in the same way as the molecular directors of the alignment film 20. Under the anchoring effect of the alignment film 20 on the liquid crystal molecules in the liquid crystal layer 10 and the anchoring effect of air on the liquid crystal on the side of the liquid crystal layer 10 away from the alignment film 20, the liquid crystal molecules in the liquid crystal layer 10 self-assemble into a set distribution of liquid crystal oil-stripe periodic structures. The pattern of the electrode 30 can be obtained by dry etching or wet etching, and the pattern of the electrode 30 is well aligned with the control pattern of the alignment film 20, so that the electrode 30 generates a set electric field distribution related to the control pattern of the alignment film 20. The liquid crystal molecules in the liquid crystal layer 10 are rearranged under the action of the electric field generated by the electrode 30, so that the self-assembled liquid crystal oil-stripe periodic structure in the liquid crystal layer 10 is restructured.

[0062] Figure 2 is a top view of a molecular director distribution of an alignment film provided by the present embodiment. Referring to Figure 2 , the molecular director distribution of the alignment film is a square control pattern in a single direction. The side length of the square control pattern is 200 μm, and the molecular director direction is the horizontal direction parallel to the side. Under the combined action of anchoring energy, liquid crystal body elasticity, and surface energy, the liquid crystal molecules in the liquid crystal film self-assemble into a linear liquid crystal oil-stripe structure. The direction of the generated liquid crystal oil-stripe is always perpendicular to the molecular director direction of the alignment film, i.e., the liquid crystal oil-stripe periodic structure is distributed along the vertical direction of the square control pattern.

[0063] Figure 3 is a schematic diagram of the correspondence between the thickness of the liquid crystal layer and the period of the corresponding grown liquid crystal grating. As shown in Figure 3 , when the thickness of the liquid crystal layer continuously changes from 0.71 μm to 1.95 μm, the period of the liquid crystal grating continuously changes from 1.45 μm to 4.25 μm. This range of thickness of the liquid crystal layer can ensure that the liquid crystal oil streak period structure has obvious refractive index periodic variation, so that the transmitted light beams at different positions have obvious phase difference, and at the same time, it can also avoid the liquid crystal oil streak period structure from being transformed into a focal conic domain structure due to the too large thickness of the liquid crystal layer during the growth process, thereby maintaining the linear feature of the liquid crystal oil streak.

[0064] Figure 4 is a polarizing microscope photograph of a liquid crystal grating device. Figure 4 The liquid crystal grating device in Figure 2 adopts a square control pattern of the molecular director distribution of the alignment film in a single direction (as shown in Figure 4 ), the molecular director of the alignment film is in the horizontal direction, and the orientation of the liquid crystal oil streak is in the vertical direction perpendicular to the direction of the molecular director. The thickness of the liquid crystal layer is 1.50 μm, and the period size of the corresponding grown liquid crystal grating is 3.10 μm, as shown in

[0065] Figure 5 is a polarizing light diffraction pattern of the liquid crystal grating device shown in Figure 4 . The incident light is linearly polarized light, the propagation direction of the incident light is along the stacking direction of the liquid crystal layer in the liquid crystal grating device, and the polarization direction of the incident light gradually changes from the horizontal direction to the vertical direction. Here, the horizontal direction refers to the direction perpendicular to the orientation of the liquid crystal oil streak, and the vertical direction refers to the direction parallel to the orientation of the liquid crystal oil streak. The diffraction plane detection position is located at a distance of 3.5 cm from the liquid crystal grating device. As shown in Figure 5 , when the linear polarization direction of the incident light beam is perpendicular to the orientation of the liquid crystal oil streak, the first-order diffraction spot of the liquid crystal grating device is the brightest, and the corresponding first-order diffraction efficiency is 9.5%; when the linear polarization direction of the incident light beam gradually deviates from the vertical direction of the orientation of the liquid crystal oil streak, the first-order diffraction spot of the liquid crystal grating device gradually weakens, and the diffraction efficiency correspondingly decreases; when the linear polarization direction of the incident light beam is parallel to the orientation of the liquid crystal oil streak, the first-order diffraction spot of the liquid crystal grating device disappears, and the first-order diffraction efficiency is 0%. It can be seen that the liquid crystal grating device exhibits high polarization correlation of high-order diffraction, and can realize differential diffraction related to the polarization direction of the incident light.

[0066] Figure 6is a corresponding relationship diagram of liquid crystal layer thickness and first order diffraction efficiency of the corresponding grown liquid crystal grating provided by the embodiment of the present application. When the liquid crystal layer thickness is reduced from 1.50 μm to 1.10 μm, the maximum diffraction efficiency of the first order diffraction of the liquid crystal grating is reduced from 9.5% to 5.1%, and when the liquid crystal layer thickness is continuously reduced to 0.89 μm, the maximum diffraction efficiency of the first order diffraction of the liquid crystal grating is reduced to 4.0%. It can be seen that the diffraction efficiency of the liquid crystal grating gradually reduces with the reduction of the liquid crystal layer thickness. The main reason is that when the liquid crystal layer thickness is small, the phase difference accumulation of the transmitted light beam is small when passing through different positions, so the diffraction efficiency is low. With the increase of the liquid crystal layer thickness, the phase difference gradually approaches the half-wave condition of diffraction, and the diffraction efficiency increases accordingly.

[0067] Figure 7 is a corresponding relationship diagram of liquid crystal layer thickness and first order diffraction angle of the corresponding grown liquid crystal grating provided by the embodiment of the present application. As shown in Figure 7 , when the liquid crystal layer thickness is increased from 0.89 μm to 1.10 μm, the diffraction angle of the first order diffraction of the liquid crystal grating is reduced from 14.1° to 12.4°, and when the liquid crystal layer thickness is continuously increased to 1.50 μm, the diffraction angle of the first order diffraction of the liquid crystal grating is reduced to 9.5°. It can be seen that the diffraction angle of the liquid crystal grating gradually reduces with the increase of the liquid crystal layer thickness, which is mainly caused by the corresponding increase of the period of the liquid crystal grating (as shown in Figure 3 ).

[0068] Figure 8 is a structure diagram of a liquid crystal oil film provided by the embodiment of the present application. The mechanism of the grating effect and high polarization correlation of the liquid crystal oil film period structure in the above technical solution will be further described below. Figure 8 As shown in Figure 8 , the liquid crystal oil film is composed of a curved deformation liquid crystal layer, wherein the liquid crystal molecules are arranged vertically to the liquid crystal layer, the liquid crystal oil film has a plurality of crystal boundaries, which are a rotation crystal boundary 113, a center crystal boundary 112 and a vertical crystal boundary 111. In the cross section 110 of the liquid crystal oil film perpendicular to the direction, the director of the liquid crystal is distributed in gradient change on both sides of the vertical crystal boundary 111, and the director of the liquid crystal is arranged vertically to the bottom surface in the middle area between two adjacent vertical crystal boundaries. When linearly polarized light is incident along the stacking direction of the liquid crystal layer, for linear polarization parallel to the cross section 110, it senses the continuously changing refractive index distribution between the ordinary refractive index n o and the extraordinary refractive index n e of the liquid crystal, so the liquid crystal oil film structure shows a periodical phase distribution, and has the function of phase grating. For linear polarization perpendicular to the cross section 110, it always senses the ordinary refractive index n o, so it does not have the grating effect. In the above case, the liquid crystal grating based on the liquid crystal oil ripple periodic structure shows obvious polarization dependence related to the polarization direction of the incident linearly polarized light.

[0069] wherein the electrode has positive and negative electrodes in an interdigital distribution, and the positive and negative electrodes have rectangular electrode units in an interlaced arrangement, respectively. The in-plane component direction of the electric field generated by the electrode is parallel to the short side direction of the electrode unit. The angle between the orientation direction of the square control pattern (as shown in Figure 2 ) with the single direction of the molecular director distribution and the long side direction of the electrode unit ranges from ξ, 0°≤ξ<180°. Such a setting can exert an electric field with a pre-set direction on the liquid crystal oil ripple periodic structure.

[0070] Figure 9 is a top view of an interdigital electrode provided by an embodiment of the present application, the electrode unit has an electrode width in the short side direction ranging from w1≥10μm, an electrode width in the long side direction ranging from w2≥10*w1, and an electrode spacing in the short side direction ranging from s≥10μm. Such a setting can realize the effect that the electrode coating area generates a vertical electric field distribution and the electrode spacing area generates a horizontal electric field distribution, and can ensure that the electrode coating area and the electrode spacing area both have sufficient space for growing the liquid crystal oil ripple periodic structure.

[0071] Figure 10 is an enlarged view of A in Figure 9 , the electrode width of the electrode unit in the short side direction is w1=12.0μm, the electrode width of the electrode unit in the long side direction is w2=1cm, and the electrode spacing of the electrode unit in the short side direction is s=15.5μm (as shown in Figure 9 ). Figure 10 shows a partial enlarged view of one period unit of the electrode unit in Figure 9 in the direction of the short side, wherein the upper electrode coating area 211 is the positive electrode, the lower electrode coating area 212 is the negative electrode, and the middle area is the electrode spacing area 210. Figure 11 is a schematic diagram of an electric field distribution of the interdigital electrode shown in Figure 9 . As shown in Figure 11 , the electric field lines are in an arch shape from the electrode coating area 211 to the electrode coating area 212 through the electrode spacing area 210, wherein the upper part of the central area of the electrode coating area 211 and the electrode coating area 212 is mainly vertical electric field, the upper part of the electrode spacing area 210 is mainly horizontal electric field, and the upper part of the electrode boundary is a continuously changing inclined electric field.

[0072] Figure 12 is a polarizing microscope photograph of a liquid crystal grating device driven by the interdigital electrode shown in Figure 10 . Figure 12The molecular director distribution of the alignment film of the liquid crystal grating device is a single-directional square control pattern (as shown in Figure 2 The direction of the molecular director distribution of the alignment film has an angle ξ with the long side direction of the electrode unit, and ξ = 90°, that is, the orientation of the grown liquid crystal oil streaks is parallel to the long side direction of the electrode unit. The liquid crystal oil streak structure is generated under the joint action of the horizontal anchoring of the alignment film and the vertical anchoring at the air interface, and the horizontal anchoring of the alignment film is the dominant factor. When a voltage of 20V is applied between the positive and negative electrodes, the vertical electric field generated in the electrode coating area can enhance the effect of the vertical anchoring, reduce the effective thickness of the liquid crystal layer, and further cause the reduction of the liquid crystal grating period. The liquid crystal grating period above the corresponding electrode coating area is reduced from 1.89 μm to 1.35 μm. The transverse electric field in the inter-electrode area has the same effect as the original horizontal anchoring effect on the liquid crystal layer, so it has no obvious effect on the liquid crystal oil streak structure. The liquid crystal grating period above the corresponding electrode coating area still remains 1.35 μm when the voltage between the positive and negative electrodes continues to increase to 80V, and the excessive transverse electric field causes the liquid crystal layer in the inter-electrode area to arrange along the direction of the electric field as a whole, that is, perpendicular to the bottom surface. The corresponding liquid crystal oil streak structure cannot be assembled, and the liquid crystal grating structure in the inter-electrode area disappears. It can be seen that the liquid crystal grating device shows the ability of switchable switching and adjustable grating period under the action of the interdigital electrode.

[0073] As shown in Figure 20 The flow chart of the liquid crystal grating preparation method in the embodiment is shown in the figure, and the method specifically includes the following steps.

[0074] S110, forming an electrode on one side of a substrate.

[0075] The substrate is quartz glass, the electrode is an indium tin oxide film, and a structure electrode with a required pattern is obtained by wet etching.

[0076] S120, forming an alignment film on the side of the electrode away from the substrate.

[0077] The alignment film is a photoalignment film, and the material of the photoalignment film is acidic azo dye 4,4'-di(4-hydroxy-3-carboxy-phenylazo) biphenylamine-2,2'-disulfonic acid. Before forming the photoalignment film, in order to increase the wettability and adhesion of the photoalignment film and the substrate, the substrate is ultrasonically cleaned with a cleaning solution (a mixed reagent of acetone, alcohol, etc.) for 30 minutes, and then ultrasonically cleaned with ultrapure water twice, each for 10 minutes. After drying in a 120℃ oven for 40 minutes, the substrate is subjected to UVO (ultraviolet ozone) cleaning for 30 minutes.

[0078] The photo-alignment film is formed on one side of the substrate, including: spin-coating the photo-alignment material on one side of the substrate, the spin-coating parameters are: low-speed spin-coating for 5 seconds, the rotation speed is 800 rpm, high-speed spin-coating for 40 seconds, the rotation speed is 3000 rpm; annealing the substrate with the spin-coated photo-alignment material for 10 minutes, the annealing temperature is 100℃, to form the photo-alignment film, the thickness can be 30nm-50nm.

[0079] S130, performing orientation treatment on the orientation film to form a control pattern with the molecular director in a set distribution.

[0080] S140, forming a liquid crystal layer on the side of the orientation film away from the substrate, the control pattern of the orientation film controls the self-assembly of the liquid crystal molecules in the liquid crystal layer into a set liquid crystal moiré period structure; wherein the liquid crystal layer material is a smectic phase liquid crystal material 4'-n-octyl-4-cyanobiphenyl.

[0081] Spin-coating the smectic phase liquid crystal material on the side of the substrate coated with the orientation film, using the spin-coating method, the spin-coating parameters are: temperature control at 80℃, low-speed spin-coating for 5 seconds, the rotation speed is 800 rpm, high-speed spin-coating for 30 seconds, the rotation speed varies in the range of 3000 rpm to 12000 rpm, to form liquid crystal layers with different thicknesses; slowly cooling the substrate with the spin-coated 4'-n-octyl-4-cyanobiphenyl from 38℃ to room temperature at a rate of 0.2℃ / min in the nematic phase state.

[0082] In step S130, the orientation film is subjected to orientation treatment to form a control pattern with the molecular director in a set distribution; the treatment process includes:

[0083] Performing one-time exposure on the orientation film; the exposure is without a mask, the polarization direction of the induced light determines the orientation direction, and the orientation film is uniformly oriented within the control pattern.

[0084] Example 2:

[0085] The structure of the liquid crystal grating in this example is basically the same as that in Example 1, the difference is that the direction of the molecular director distribution of the orientation film has an angle ξ with the long side direction of the electrode unit, ξ = 135°.

[0086] Figure 13 is Figure 10 is another orthogonal polarizing microscope photo of a liquid crystal grating device driven by the interdigital electrode shown in Figure 12The difference between the corresponding liquid crystal grating device is that the direction of the distribution of the molecular director of the alignment film has an angle ξ with the long side direction of the electrode unit, and ξ = 135°, that is, the orientation of the grown liquid crystal oil ridge has an angle of 45° with the long side direction of the electrode unit. A voltage of 20V is applied between the positive and negative electrodes, and for the same reason, the period of the liquid crystal grating above the electrode coating area is reduced from 1.49μm to 1.08μm, while the period of the liquid crystal grating above the inter-electrode area remains unchanged. Among them, the electrode coating area is only affected by the vertical electric field, and the orientation of the corresponding liquid crystal grating remains unchanged, while the orientation of the liquid crystal grating in the inter-electrode area rotates counterclockwise due to the inconsistency between the horizontal anchoring direction of the electrode and the horizontal anchoring direction of the alignment film, and at the same time to meet the continuity of the generated liquid crystal oil ridge structure, and the rotation angle is 19°. When the voltage between the positive and negative electrodes continues to increase to 90V, for the same reason, the liquid crystal grating structure in the inter-electrode area disappears. It can be seen that the liquid crystal grating device shows the ability of adjustable grating orientation under the action of the interdigital electrode.

[0087] Example 3:

[0088] The structure of the liquid crystal grating of this embodiment is basically the same as that of Example 1, and the difference is that the molecular director of the alignment film is in a grid shape.

[0089] Figure 14 is another top view of the distribution of the molecular director of the alignment film provided by this embodiment. Referring to Figure 14 , the alignment film has a control pattern with a grid-shaped distribution of molecular directors, and the grid-shaped control pattern includes a plurality of unit cells, that is, control pattern units, as shown by the B area indicated by the dashed box on the left side of Figure 14 , each control pattern unit includes two columns of sub-control patterns, that is, left and right orientation period sub-cell units within the B area; the molecular director of the alignment film in each unit cell is mirror symmetric about the center split line of the column direction of the unit cell, that is, there are two orientation period sub-cell units with the same size, and the angle between the molecular director and the center split line of the column direction of the unit cell is θ = 30°. The size of each orientation period sub-cell unit in the row direction is a = 39.4μm, and the size of each unit cell in the column direction is b = 200μm. Since there are two different orientations of liquid crystal oil ridge structures within each unit cell, and the liquid crystal oil ridge continuously transitions at the boundaries and center split lines of the unit cell, for linearly polarized light incident in the direction along the stacking direction of the liquid crystal layer in the liquid crystal grating device, it senses different refractive index distributions at the two orientation period sub-cell units and their boundaries, and its optical effect is equivalent to a phase grating with a period of 2a in the row direction of the unit cell. Among them, in each orientation period sub-cell unit, the liquid crystal oil ridge period structure still has the effect of a liquid crystal grating, so that the finally grown liquid crystal oil ridge array is a two-dimensional liquid crystal grating, which has two sets of periods related to the liquid crystal oil ridge self-assembly and the grid-shaped control pattern, respectively.

[0090] Figure 14 The size of the cell, the number of orientation periodic sub-cells, the size of the orientation periodic sub-cells, and the molecular director direction of the alignment film in the orientation periodic sub-cells are only illustrative and not limiting to the present application. Each column of the cell contains multiple columns of orientation periodic sub-cells, the molecular directors of at least two columns of the orientation periodic sub-cells are distributed towards different directions, the molecular director of the alignment film in each orientation periodic sub-cell forms an angle θ with the boundary of the column direction of the orientation periodic sub-cell, -90° < θ < 90°, the size of each orientation periodic sub-cell along the row direction ranges from a ≥ 10 μm, and the size of each cell along the column direction ranges from b ≥ 10 μm. Such arrangement ensures that the generated liquid crystal oil-rings array is a two-dimensional liquid crystal grating, and meanwhile, there is sufficient space for the growth of the liquid crystal oil-rings periodic structure in each orientation periodic sub-cell in each cell.

[0091] As shown in Figure 15 is a polarized light microscope photograph of another liquid crystal grating device provided by the embodiment of the present application. Figure 15 The liquid crystal grating device in adopts an alignment film with a control pattern of gatelike molecular director distribution (as shown in Figure 14 ), the control pattern unit is rectangular, each control pattern unit has left and right two columns of sub-control patterns, the sub-control patterns are parallel and side-by-side rectangular units, the orientation angle of the molecular director in the sub-control pattern is 30°, i.e., the angle between the molecular director and the center dividing line of each control pattern unit is 15°, so the liquid crystal oil-rings continuously transition and connect at the cell boundary and the dividing line with an angle of 150°. The periodic size of the liquid crystal oil-rings in the two-dimensional liquid crystal grating is 2.0 μm, and the periodic size of the gatelike structure is 39.4 μm, as shown in Figure 15 .

[0092] Figure 16 is a polarized light diffraction pattern of the liquid crystal grating device shown in Figure 15 . The polarization direction of the incident linearly polarized light is along the column direction of the gatelike control pattern, and the diffraction screen detection position is located 3.5 cm away from the light source on the side of the liquid crystal grating device. As shown in Figure 16 , the liquid crystal grating device has two sets of diffraction patterns with different diffraction angles, the two-dimensional beam deflection with a larger diffraction angle is caused by the local self-assembled liquid crystal oil-rings with a smaller period, and the one-dimensional beam deflection with a smaller diffraction angle is caused by the refractive index periodic distribution of the gatelike structure as a whole. Among them, since the orientations of the liquid crystal oil-rings in the two kinds of orientation periodic sub-cells are respectively ±75° away from the column direction, the two-dimensional diffraction spots are respectively ±15° away from the column direction.

[0093] Figure 17 is another polarized light diffraction pattern of the liquid crystal grating device shown in Figure 15 . Compared withFigure 16 The difference between the corresponding diffraction processes is only that the incident light polarization direction is along the row direction of the grating control pattern. As shown in Figure 17 , with the rotation of the incident polarization direction, the diffraction efficiency corresponding to each order of the diffraction pattern changes. Among them, compared with Figure 16 , on the one hand, because the polarization component of linearly polarized light along the orientation of the liquid crystal oil ridge increases, the diffraction efficiency of the two-dimensional diffraction corresponding to the oil ridge itself decreases (as shown in Figure 5 or Figure 6 ), the spot brightness is significantly weakened. On the other hand, because the phase difference between the orientation period sub-cell region and its boundary region is reduced, the diffraction efficiency of the one-dimensional diffraction corresponding to the orientation structure also decreases.

[0094] Referring to Figure 16 , Figure 17 , it can be concluded that Figure 15 the liquid crystal grating device shown in can generate two-dimensional diffraction with a specific diffraction angle, while exhibiting diffraction efficiency distribution related to the polarization direction of the incident light, and by arbitrarily setting the period size of the orientation structure, the period size of the liquid crystal oil ridge, and the orientation of the liquid crystal oil ridge, the diffraction angle and polarization dependent characteristics of the liquid crystal grating device can be modulated respectively.

[0095] The preparation method of the liquid crystal grating in this embodiment is basically the same as that in Embodiment 1, except that the orientation film is subjected to orientation treatment (i.e. step S130) to form a control pattern with a set distribution of molecular director; the processing process includes:

[0096] First exposure to the orientation film; the first exposure has no mask, and the polarization direction of the induced light is the first direction, and the orientation film is uniformly oriented;

[0097] Second exposure to the orientation film; the mask of the second exposure includes light shielding area and light transmission area; the light shielding area and the light transmission area form a grating array pattern; along the row direction, the light shielding area and the light transmission area are arranged at intervals; the polarization direction of the induced light is the second direction, and the orientation film is patterned to have a control pattern with a grating-shaped distribution of molecular director;

[0098] Among them, the angle between the first direction and the column direction is θ1, 0°< θ1< 90°; the angle between the second direction and the column direction is θ2, -90°< θ1< 0°; the first direction and the second direction are mirror symmetric about the column direction.

[0099] Figure 21 is a pattern of a mask provided in this embodiment. As shown in Figure 21The orientation film is subjected to orientation treatment to form a control pattern with grid-shaped molecular director. The first full-area exposure is set, and the polarization direction of the induced light is 75°. The polarization direction of 75° means that the vertical direction (column direction of the mask pattern) is rotated counterclockwise by 75°, and the light control orientation film is uniformly oriented. The second patterned exposure is set, and the polarization direction of the induced light is -75°. The mask shown in Figure 21 The white area represents a light-transmitting area, i.e., an exposure area, and the black area represents a light-blocking area, i.e., a non-exposure area. In the grid pattern, the length of any unit cell (each unit cell includes one black area and one white area) in the row direction is 39.4 μm, and the length in the column direction is 200 μm. After the orientation treatment of the orientation film, the molecular director distribution of the orientation film is as shown in Figure 14 The liquid crystal oil streak array formed by self-assembly of the liquid crystal molecules in the liquid crystal layer is as shown in Figure 15 The liquid crystal oil streaks used as the liquid crystal grating are arranged in multiple rows in multiple directions.

[0100] Figure 23 is a schematic diagram of an interdigital electrode provided by the present embodiment. The difference between the interdigital electrode shown in Figure 9 is that the electrode width of the electrode unit in the short edge direction is the same as the size of the orientation period subunit cell of the grid control pattern in the row direction, i.e., w1=a; the electrode width of the electrode unit in the long edge direction is greater than the size of the orientation period subunit cell of the grid control pattern in the column direction, i.e., w2>b; and the electrode spacing of the electrode unit in the short edge direction is the same as the size of the orientation period subunit cell of the grid control pattern in the row direction, i.e., s=a. In addition, the relative position distribution of the electrode structure and the grid control pattern satisfies that the electrode structure is accurately aligned with the orientation period subunit cell in the short edge direction, and covers the control pattern range in the long edge direction. Such a setting can achieve different modulation effects on the liquid crystal grating corresponding to different orientation period subunits in the grid control pattern. The oil streak structure corresponding to the electrode coating areas 311 and 312 is modulated by a vertical electric field, and the oil streak structure corresponding to the electrode spacing area 310 is modulated by a horizontal electric field.

[0101] Embodiment 4

[0102] The structure of the liquid crystal grating of the present embodiment is basically the same as that of Embodiment 1, and the difference lies in that the molecular director of the orientation film is disc-shaped.

[0103] Figure 18 is a top view of another molecular director distribution of an orientation film provided by the present embodiment. Referring to Figure 18The orientation film has a control pattern with disc-shaped distribution of molecular director, wherein the disc-shaped control pattern has radial distribution of molecular director about the disc center. Due to the radial arrangement of the disc-shaped control pattern, the liquid crystal molecules in the liquid crystal layer self-assemble to form a concentric ring liquid crystal oil streak periodic structure, which includes a plurality of annular liquid crystal oil streaks with varying radius of curvature in the radial direction. The radius of the disc-shaped control pattern in the radial direction is r≥30 μm, which ensures sufficient space for the growth of the concentric ring liquid crystal oil streak periodic structure in the disc-shaped control pattern.

[0104] Figure 19 is a microscope photograph of another liquid crystal grating device provided by the embodiment of the present application. Figure 19 The orientation film of the liquid crystal grating device in the embodiment has a control pattern with disc-shaped distribution of molecular director (as shown in Figure 18 ), and the molecular director has radial distribution about the disc center. The refractive index distribution of the liquid crystal grating device varies periodically in the radial direction and remains unchanged in the angular direction, which can achieve the effect of a general circular phase grating. It can be understood that the circular liquid crystal grating device still has polarization-dependent characteristics due to the inherent optical properties of the liquid crystal oil streak structure. The radius of the circular liquid crystal grating device in the radial direction is r=60 μm, and the average period size of the liquid crystal grating is 2.0 μm, as shown in Figure 19 .

[0105] The preparation method of the liquid crystal grating in the embodiment is basically the same as that in Embodiment 1, except that the orientation film is subjected to orientation treatment (i.e., step S130) to form a control pattern with a set distribution of molecular director; the treatment process includes:

[0106] According to the exposure sequence, the corresponding exposure pattern and the corresponding induced light polarization direction are selected, and multiple exposures are sequentially performed;

[0107] wherein the exposure regions of the exposure patterns of adjacent steps are partially overlapped, and the induced light polarization direction monotonically increases or monotonically decreases with the exposure sequence, so that the orientation film has a control pattern with disc-shaped distribution of molecular director.

[0108] Figure 22 is a pattern of a mask provided by the embodiment. As shown in Figure 22 , the specific process of orientation treatment of the orientation film to form a control pattern with disc-shaped distribution of molecular director. As shown in Figure 22As shown, 36 masks are contained, and the black area in each mask is the non-exposed area, and the white area is the exposed area. The first mask is selected, the polarization direction of the inducing light is set to 90°, and the polarization direction of 90° here means the direction corresponding to the horizontal direction 510 rotated counterclockwise by 90°, and the first exposure is performed; the second mask is selected, the polarization direction of the inducing light is set to 95°, and the second exposure is performed; then, the masks are selected in turn according to the direction indicated by the arrow, and the polarization direction of the inducing light is increased by 5° each time the mask is replaced, until the thirty-sixth mask is selected, and the polarization direction of the inducing light is set to 265°, and the thirty-sixth exposure is performed. After the above orientation treatment of the mask on the orientation film, the molecular director distribution of the orientation film presents a disc-shaped control pattern, and the molecular director in the control pattern presents a radial distribution about the center of the disc, so that the liquid crystal molecules in the liquid crystal layer self-assemble to form a concentric circular ring liquid crystal oil ridge periodic structure.

[0109] Figure 22 Only the orientation treatment of the orientation film through thirty-six exposures is shown by way of example, but it is not a limitation of the present application, and those skilled in the art can select the pattern of the mask and the number of exposures according to the actual situation, as long as the molecular director of the orientation film presents a concentric circular ring control pattern.

[0110] Figure 24 is a schematic view of a ring electrode provided by the present embodiment. As shown in Figure 24 The positive electrode 411 and the negative electrode 412 of the ring electrode are arranged alternately and nested, the electrode unit of the positive electrode 411 along the radial direction satisfies w3≥10μm, the electrode unit of the negative electrode 412 along the radial direction satisfies w4≥10μm, and the width of the electrode interval region 410 along the radial direction satisfies s1≥10μm. Such a setting can realize the effect that the electrode coating area generates a vertical electric field distribution, and the electrode interval region generates a horizontal electric field distribution along the radial direction, and can ensure that the electrode coating area and the electrode interval region have sufficient space to grow the liquid crystal oil ridge periodic structure.

Claims

1. A liquid crystal grating, characterized in that: The invention comprises a substrate, an electrode layer, an orientation film and a liquid crystal layer, wherein the electrode layer is arranged on one side of the substrate, the orientation film is arranged on the side of the electrode layer away from the substrate, and the liquid crystal layer is arranged on the side of the orientation film away from the electrode layer; the electrode layer has positive and negative electrodes with a set distribution pattern to apply a set electric field to the liquid crystal layer; the orientation film has a control pattern with a set distribution of molecular directors to make the liquid crystal molecules in the liquid crystal layer self-assemble into a layered liquid crystal oil pattern periodic structure; the orientation film is a light-controlled orientation film, and the liquid crystal layer material is a smectic phase liquid crystal material; the positive and negative electrodes of the electrode layer are distributed in an interdigitated shape, and the positive and negative electrodes respectively have rectangular electrode units arranged in a staggered manner, and the electrode units have long sides and short sides and are arranged along the short side direction; the short side length of the electrode unit is w 1. The long side length of the electrode unit is w 2. The spacing between adjacent electrode units along the short side is s; where w 1≥10μm, w 2≥10* w 1, s≥10μm; the control pattern is square, grating or disc-shaped; wherein the molecular directors in the square control pattern have the same orientation and are uniformly oriented in a single direction; the grating control pattern includes a plurality of periodically arranged rectangular control pattern units, each control pattern unit includes a plurality of columns of sub-control patterns, and the molecular directors in at least two columns of sub-control patterns have different orientations; the molecular directors in the disc-shaped control pattern are radially arranged about the center of the disc, so that the liquid crystal molecules in the liquid crystal layer self-assemble to form a periodic structure of concentric circular liquid crystal oil grains, and the thickness of the liquid crystal layer is 0.71-1.95μm; the control method of the liquid crystal grating comprises: applying voltages of different magnitudes to the electrode layers of the liquid crystal grating to tune the periodic size and orientation of the liquid crystal oil grain structure in the liquid crystal grating to achieve multi-parameter control of the liquid crystal grating; and vertically irradiating the liquid crystal grating with linearly polarized light with adjustable polarization direction to generate a highly polarization-dependent beam diffraction.

2. The liquid crystal grating according to claim 1, wherein: The control pattern is square, and the side length of the control pattern is d, where d≥10 μm.

3. The liquid crystal grating according to claim 1, wherein: The control pattern is in a grid shape, and the length of the sub-control pattern along the row direction is a, a≥10μm; the length of the sub-control pattern along the column direction is b, b≥10μm.

4. The liquid crystal grating according to claim 1, wherein: The control pattern is in a disk shape, and the radius of the control pattern is r, where r≥30 μm.

5. The liquid crystal grating according to claim 1, wherein: The control pattern is square, and the orientation film has a square control pattern with a molecular director in a single direction. The orientation direction has an angle ξ with the long side direction of the electrode unit, 0°≤ξ<180°.

6. A method for preparing a liquid crystal grating according to any one of claims 1 to 5, characterized in that The method comprises the following steps: (1) forming an electrode on one side of a substrate; (2) forming an orientation film on a side of the electrode away from the substrate; (3) performing an orientation treatment on the orientation film to form a control pattern in which the molecular directors are distributed in a set manner; and (4) forming a liquid crystal layer on a side of the orientation film away from the substrate, wherein the control pattern of the orientation film controls the liquid crystal molecules in the liquid crystal layer to self-assemble into a set periodic structure of liquid crystal oil patterns.

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