Electrically switchable liquid crystal grating unit based on amplitude mask optical alignment

Abstract

A switchable liquid crystal grating cell and a method for manufacturing the same. The switchable liquid crystal grating unit comprises a pair of switchable liquid crystal grating components, and each switchable liquid crystal grating comprises a substrate; a conductive layer on the substrate; and an optical alignment layer having an alignment pattern over the conductive layer on the substrate, the alignment pattern having domains of a first type and domains of a second type, the domains of the first type having a first uniform alignment direction, the domains of the second type having a second uniform alignment direction, the second type of domain has a redirected second uniform alignment direction at a predetermined angle from the first uniform alignment direction, the predetermined angle being 85 DEG to 95 DEG, and a liquid crystal layer sandwiched between the pair of switchable liquid crystal grating components, wherein the alignment pattern on the top surface is arranged offset from another alignment pattern on the bottom surface.

Description

Technical Field

[0001] The present application relates to the design and manufacture of a liquid crystal grating unit, and in particular to an electrically switchable liquid crystal grating unit. Background Art

[0002] Switchable polarization gratings play a key role in various photonic devices, providing functions such as beam steering, beam shaping and optical communications due to their ability to manipulate light. A polarization grating is a grating that modulates the polarization state of incident light. This is achieved by spatially varying the local optical axis or alignment of the anisotropic material on the grating. Polarization gratings can selectively diffract different polarization states and are therefore useful in a variety of optical applications.

[0003] The basis for polarization gratings is usually a birefringent material, such as certain types of liquid crystal (LC). The structure of this material is such that the optical axis varies periodically across the grating, usually in a sinusoidal or sawtooth pattern. When unpolarized or linearly polarized light incident on the grating consists of two orthogonal polarization states, the grating effectively works by splitting the incident light into two beams, each with a different polarization state.

[0004] Liquid crystals have become the material of choice for light-modulating devices, primarily due to their unique ability to change the alignment of their molecular directors in response to an applied electric field. By developing switchable diffraction gratings that exploit the properties of liquid crystals, a high degree of control over light propagation can be achieved, facilitating the production of more adaptable and dynamic photonic devices.

[0005] Various methods have been proposed for making switchable liquid crystal gratings. Some techniques exploit intrinsic liquid crystal effects, such as the linear electro-optic effect found in chiral nematic phases. However, these methods that rely on intrinsic liquid crystal effects may be limited by the properties and state of the liquid crystal materials used, and therefore may lack sufficient flexibility for certain applications.

[0006] Another strategy employs a periodic electrode system to dictate the formation of the liquid crystal grating. This electric field driven technique can precisely control the liquid crystal alignment by adjusting the layout of the electrodes, thereby modulating the optical properties of the grating. However, the reliance on electrical systems introduces additional complexity and potential failure points, which may affect the reliability and stability of the resulting liquid crystal grating, thereby affecting its long-term performance. On the other hand, incorporating planar aligned polyimide (PI) via rubbing alignment introduces additional manufacturing difficulties that may affect the diffraction efficiency of the device.

[0007] Another method is to use a strong polarized ultraviolet (UV) laser beam to realign the liquid crystal molecules using a patterned mask to form a liquid crystal grating.

[0008] However, devices fabricated using this approach always face electrical stability issues. After a few cycles of electrical switching, the diffraction efficiency of the liquid crystal grating drops significantly. This is because this approach requires high azimuthal anchoring energy to return the liquid crystal director to its initial alignment state after switching. The photoalignment materials commonly used to construct polarization gratings provide 10 -5 J / m 2 The azimuthal anchoring energy in the range of 100 nm is not sufficient to maintain the alignment of the liquid crystal molecules. Therefore, other researchers have chosen to use grating structures on a single substrate, while the opposing substrate is bonded with planar-aligned polyimide (PI) via rubbing alignment. However, this approach may adversely affect the diffraction efficiency of the device and bring manufacturing difficulties associated with the rubbing alignment process.

[0009] Therefore, considerable obstacles still exist in these methods to fabricate efficient, low-cost and long-term stable switchable liquid crystal gratings. Summary of the invention

[0010] In order to overcome the deficiencies of the prior art, the present disclosure provides a highly efficient, low-cost and long-term stable switchable liquid crystal (LC) grating unit and a method for manufacturing the same.

[0011] The present application further provides a switchable liquid crystal grating unit, comprising: a pair of switchable liquid crystal grating components, each switchable liquid crystal grating component comprising: a substrate; a conductive layer on the substrate; and a light alignment layer having an alignment pattern above the conductive layer on the substrate, the alignment pattern having a first type of domain and a second type of domain: the first type of domain has a first uniform alignment direction, the second type of domain has a second uniform alignment direction redirected at a predetermined angle to the first uniform alignment direction, the predetermined angle being 85°-95°; and a liquid crystal layer sandwiched between the pair of switchable liquid crystal grating components; wherein the two light alignment layers in the pair of switchable liquid crystal grating components face each other, and for the pair of switchable liquid crystal grating components, the alignment pattern of the light alignment layer on the top surface of the lower switchable liquid crystal grating component is arranged to be offset by half a pitch to 45% of the pitch relative to another alignment pattern of the light alignment layer on the bottom surface of the upper switchable liquid crystal grating component.

[0012] In some embodiments, the predetermined angle is 90°.

[0013] In certain embodiments, the substrate is a transparent glass substrate, and the conductive layer includes a plurality of indium tin oxide (ITO) electrodes.

[0014] In some embodiments, the photo-alignment layer has a characteristic of periodically alternating alignment directions, resulting in periodic changes in the alignment directions.

[0015] In some embodiments, the switchable liquid crystal barrier element has a period in the range of 4-50 microns.

[0016] In some embodiments, the switchable liquid crystal barrier element has a period of 8 microns.

[0017] In some embodiments, the photo-alignment material of the photo-alignment layer is a sulfonic acid azo dye (SD1).

[0018] In some embodiments, when no electric field is applied, the liquid crystal molecules of the photo-alignment layer are periodically arranged along the plane of the substrate; and, when an electric field with an analog waveform such as a sine wave or a triangular wave is applied, the long axes of the liquid crystal molecules of the photo-alignment layer begin to align with the direction of the electric field, thereby becoming perpendicular to the plane of the substrate.

[0019] In some embodiments, there is a cell gap between the switchable liquid crystal barrier units.

[0020] In certain embodiments, the cell gap is in the range of about 1 μm-2 μm.

[0021] The present application further provides a method for manufacturing a switchable liquid crystal grating unit, the method comprising: coating a substrate with a conductive layer; spin coating the substrate with a photo-alignment material to establish a photo-alignment layer on the conductive layer; exposing the photo-alignment layer to a first polarized ultraviolet light polarized along a first direction to achieve uniform alignment of liquid crystal molecules in the photo-alignment layer; contacting an amplitude mask with the photo-alignment layer of the substrate to expose a non-masked area of ​​the photo-alignment layer; irradiating the photo-alignment layer with a second polarized ultraviolet light polarized along a second direction, the second direction being arranged at a predetermined angle to the first direction to redirect the non-masked area of ​​the photo-alignment layer, thereby generating a switchable liquid crystal grating component having an alignment pattern on the photo-alignment layer, the switchable liquid crystal grating The grating component has a first type of domain and a second type of domain, the first type of domain has a first uniform alignment direction, and the second type of domain has a redirected second uniform alignment direction at a predetermined angle to the first uniform alignment direction, wherein the predetermined angle is 85°-95°; a pair of switchable liquid crystal grating components are arranged, wherein two photo-alignment layers in a pair of switchable liquid crystal grating components face each other, and the alignment pattern of the photo-alignment layer on the top surface of the lower switchable liquid crystal grating component is offset by half a pitch or less than half a pitch relative to another alignment pattern of the photo-alignment layer on the bottom surface of the upper switchable liquid crystal grating component; and nematic liquid crystal is injected between the pair of switchable liquid crystal grating components to obtain a switchable liquid crystal grating unit.

[0022] In some embodiments, the predetermined angle is 90°.

[0023] In certain embodiments, the substrate is a transparent glass substrate, and the conductive layer includes a plurality of indium tin oxide (ITO) electrodes.

[0024] In some embodiments, the photo-alignment layer has a characteristic of periodically alternating alignment directions, resulting in periodic changes in the alignment directions.

[0025] In some embodiments, the switchable liquid crystal barrier element has a period in the range of 4-50 microns.

[0026] In some embodiments, the switchable liquid crystal barrier element has a period of 8 microns.

[0027] In certain embodiments, spin coating the substrate with the photoalignment material comprises: spin coating a solution comprising a sulfonic acid azo dye (SD1) dissolved in an N,N-dimethylformamide (DMF) solvent on the surface of the substrate; and removing the DMF solvent from the coated substrate.

[0028] In some embodiments, when the nematic liquid crystal is injected between the pair of photo-alignment layers, one or more optical spacers are arranged between the switchable liquid crystal grating units to space the switchable liquid crystal grating units in a cell gap.

[0029] In certain embodiments, the cell gap is in the range of about 1 μm-2 μm.

[0030] The present disclosure provides an electrically switchable liquid crystal grating unit that exhibits high efficiency, cost efficiency and long-term stability, addressing the limitations observed in existing liquid crystal grating technology. The liquid crystal grating unit produced by the present disclosure has a high diffraction efficiency and can be maintained for a long time. This allows the grating unit to have excellent performance and long-term reliability. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] The following describes exemplary embodiments of the present disclosure in conjunction with the accompanying drawings. The accompanying drawings and corresponding detailed descriptions are only for a better understanding of the present disclosure and do not constitute a limitation on the scope of the present disclosure as defined in the claims. Specifically,

[0032] Figure 1A , Figure 1B , Figure 1C , Figure 1D and Figure 1E An exemplary method for preparing a liquid crystal grating substrate having a switchable liquid crystal grating unit with a desired alignment direction is shown. Figure 1A Shown are steps for coating a conductive layer on a substrate according to certain embodiments of the present disclosure. Figure 1B A spin coating process according to certain embodiments of the present disclosure is shown. Figure 1Cshows the first step of achieving photo-alignment according to certain embodiments of the present disclosure, Figure 1D The second step to achieve photoalignment is shown. Figure 1E is a schematic diagram of a mask according to certain embodiments of the present disclosure.

[0033] Figure 2A and Figure 2B The arrangement of the switchable liquid crystal grating unit and the alignment vector of the liquid crystal in the unit according to some embodiments of the present disclosure is shown. Specifically, Figure 2A shows the arrangement of the switchable liquid crystal grating unit and the alignment vector of the liquid crystal in the unit in the absence of a driving electric field, Figure 2B The arrangement of the liquid crystal barrier elements and the alignment vectors of the liquid crystal are shown to be switchable in the presence of a driving electric field.

[0034] Figure 3 Microscope photographs of liquid crystal barrier cells fabricated under crossed polarizers according to certain embodiments of the present disclosure are presented.

[0035] Figure 4A shows an experimental setup for studying the diffraction and electro-optical properties of a switchable liquid crystal grating unit when no electric field is applied; Figure 4B The diffraction pattern of the outgoing beam is shown accordingly.

[0036] Figure 5A shows an experimental setup for studying the diffraction and electro-optical properties of a switchable liquid crystal grating unit when an electric field is applied; Figure 5B The diffraction pattern of the outgoing beam is shown accordingly.

[0037] Figure 6 is a graph showing the long-term diffraction efficiency of polarization grating units manufactured according to certain embodiments of the present disclosure and polarization grating units in the prior art. DETAILED DESCRIPTION

[0038] The present disclosure will be described more fully below with reference to the accompanying drawings. However, the present disclosure may be embodied in many different forms and should not be construed as being limited to the embodiments described herein.

[0039] The present disclosure provides a method for manufacturing an electrically switchable liquid crystal grating component and a switchable liquid crystal grating component. The present disclosure also provides a method for manufacturing an electrically switchable liquid crystal grating unit that can be controlled by applying an electric field, and a switchable liquid crystal grating unit. This property allows the diffraction angle to be effectively adjusted, which is crucial in many photonics applications.

[0040] Exemplary embodiments of the present disclosure provide a switchable liquid crystal grating component comprising a transparent glass substrate coated with, for example, an indium tin oxide electrode. The substrate is also provided with a patterned photo-alignment layer using a two-step patterned photo-alignment process using an amplitude mask. This process configures the substrate to provide multiple alignment regions within the gapped liquid crystal, thereby establishing a polarization grating profile.

[0041] Another exemplary embodiment of the present disclosure provides a switchable liquid crystal cell, which includes a pair of transparent glass substrates, each of which is coated with an indium tin oxide electrode and is respectively provided with two patterned photo-alignment layers. A liquid crystal layer is sandwiched between the two transparent substrates. The switchable liquid crystal cell has a high diffraction efficiency and can be maintained for a long time.

[0042] Figures 1A-1E An exemplary method for preparing a liquid crystal grating substrate having a switchable liquid crystal grating unit with a desired alignment direction is shown. Figure 1A As shown, a substrate 100 is provided, and the substrate can be transparent, such as a transparent glass substrate. Then, the method includes the step of coating a conductive layer. First, a conductive layer 101 is coated on the upper surface of the substrate 100. The conductive layer 101 includes a plurality of indium tin oxide (ITO) electrodes. Then, as shown in FIG. Figure 1B As shown, the upper surface of the substrate 100 is spin-coated to establish a photo-alignment layer 102 on the conductive layer 101. The photo-alignment material of the photo-alignment layer 102 can be any suitable photo-alignment material. The photo-alignment material used can be based on cross-linking or on a photodegradation process. In some embodiments, the photo-alignment material can be a sulfonic acid azo dye (SD1) or a commercial azo dye photo-alignment polymer, such as a product of Japan's DIC Corporation. This material can be redirected perpendicular to the polarization direction of the incident polarized ultraviolet light. Before the spin-coating process, SD1 has been dissolved in a solvent (e.g., N, N-dimethylformamide (DMF) solvent) to form a solution. In the spin-coating process, the solution is spin-coated on the substrate surface to form an SD1 film on the substrate. After the solution is spin-coated, the DMF solvent is removed from the coated substrate. Optionally, the substrate with the SD1 film may be baked at a temperature of 80-120° C. under an external heater 103, or baked at a temperature of 100° C. or higher or lower than 100° C. to remove the solvent of the SD1 film, thereby realizing the photo-alignment layer 102. Other suitable methods may also be used to remove the DMF solvent from the coated substrate.

[0043] The next step is to expose the photo-alignment layer to polarized UV light. The substrate coated with the photo-alignment material undergoes a multi-step patterned photo-alignment process using a shadow photomask to produce multiple alignment regions. The first step to achieve photo-alignment is as follows Figure 1CAs shown, the photo-alignment layer 102 is exposed to a first polarized ultraviolet light 104 polarized in a first direction to achieve uniform alignment of the liquid crystal molecules in the photo-alignment layer 102. Uniform alignment means that the liquid crystal molecules are uniformly aligned along a specific preferred alignment in the entire area of ​​the photo-alignment layer. The uniform alignment ensures that the deviation of the liquid crystal molecules from the desired alignment is minimal, thereby promoting efficient operation of the device. The second step of achieving photo-alignment is as follows Figure 1D As shown, the method includes placing a mask 106 in close contact with a substrate to expose a non-masked area of ​​the photo-alignment layer, and then irradiating the substrate with a second polarized ultraviolet light 105 polarized in a second direction, the second direction being arranged at a predetermined angle to the first direction to redirect the non-masked area of ​​the photo-alignment layer 102. The predetermined angle between the first direction and the second direction is between 85 degrees and 95 degrees, for example, 86 degrees, 87 degrees, 88 degrees, 89 degrees, 90 degrees, 91 degrees, 92 degrees, 93 degrees, 94 degrees. The above predetermined angle can achieve relatively acceptable efficiency. The above nearly orthogonal alignment between the LC molecules in adjacent domains can provide diffraction efficiency, which can vary according to the specific angle. Preferably, when the predetermined angle is 90 degrees, the orthogonal alignment between the LC molecules in adjacent domains ensures that the phase difference between the adjacent domains is maximized, thereby obtaining an optimal high diffraction efficiency. When the predetermined angle deviates from 90 degrees, the diffraction efficiency tends to decrease.

[0044] Mask 106 is a shadow photomask with specially designed opaque and transparent areas. Figure 1EAs shown, the mask 106 may be referred to as an amplitude mask, made of, for example, a chrome alloy, and having alternating light-transmitting and opaque stripes of a predetermined period, the predetermined period including but not limited to a period within about 2 μm-6 μm or about 3 μm-5 μm, such as about 3.5 μm, about 4 μm or about 4.5 μm. The mask 106 is used to manufacture an active liquid crystal grating component. The mask used in the present disclosure has an extremely small periodicity, so the method and the obtained liquid crystal polarization grating component have high efficiency. In the above steps, SD1 has excellent alignment ductility, allowing the alignment of the mask area of ​​the photo-alignment layer and the alignment of the non-mask area of ​​the photo-alignment layer by additional irradiation, while maintaining the same alignment quality. Therefore, the alignment pattern obtained on the photo-alignment layer has a first type and a second type of domains: the first type of domain has a first uniform alignment direction, and the second type of domain has a second uniform alignment direction redirected at a predetermined angle to the first uniform alignment direction. The predetermined angle of the first uniform alignment direction relative to the second uniform alignment direction is between 85 and 95 degrees, preferably about 90 degrees. This feature greatly improves the feasibility of the multi-domain alignment method. Therefore, in some embodiments, the second irradiation step induces SD1 in the non-mask area to be reoriented perpendicular to the light polarization direction, and therefore perpendicular to the molecular alignment of the mask area. These operations cause the liquid crystal grating component to have a periodic change in the alignment direction, thereby producing a patterned liquid crystal grating component. The above arrangement introduces a change in the refractive index of the liquid crystal, thereby producing a polarization grating profile.

[0045] The above provides a two-step photoalignment process using an amplitude mask according to certain embodiments of the present disclosure. Figure 1C and 1D The two-step photo-alignment process shown introduces multiple alignment regions within the liquid crystal on the substrate, thereby producing a polarizing liquid crystal grating profile.

[0046] The present application also provides a switchable liquid crystal grating unit including a pair of liquid crystal grating components and a manufacturing method thereof. Figure 2A As shown, after manufacturing two photo-alignment layers 102 with periodically alternating alignment directions, two substrates with two photo-alignment layers 102 are provided to form a liquid crystal cell. The two photo-alignment layers 102 are arranged on the two substrates as the top surface and the bottom surface, respectively, and undergo the same processing.

[0047] In certain embodiments, the method includes providing one or more pairs of switchable liquid crystal grating components manufactured according to the above method, wherein the two photo-alignment layers in each pair of switchable liquid crystal grating components face each other. The alignment pattern of the photo-alignment layer located on the top surface of the lower switchable liquid crystal grating component is offset by half a pitch or less than half a pitch relative to another alignment pattern of the photo-alignment layer located on the bottom surface of the upper switchable liquid crystal grating component. Taking into account the actual manufacturing situation, due to manufacturing limitations and other inherent process reasons, slight deviations may occur, that is, less than half a pitch of a liquid crystal grating component structure. "Less than half a pitch" refers to up to 45% of a liquid crystal grating component structure. For example Figure 2A As shown, the patterns on the top and bottom surfaces of the two substrates are offset by about half a pitch. Therefore, the alignment direction of the photo-alignment layer in the domain of the lower switchable liquid crystal grating component can be perpendicular to the alignment direction of the photo-alignment layer at the corresponding domain of the upper switchable liquid crystal grating component. Then, a nematic liquid crystal 201 is injected between the two photo-alignment layers of the pair of switchable liquid crystal grating components to obtain a switchable liquid crystal grating unit. The nematic liquid crystal can be any type of suitable nematic liquid crystal having dielectric and optical anisotropy, such as E7 (obtained from EM Industries). The effective area size of the switchable liquid crystal grating unit can be between 1.5cm x 1.5cm and 0.8cm x 0.8cm, for example 1.4cm x1.4cm, 1.3cm x 1.3cm, 1.2cm x 1.2cm, 1.1cm x 1.1cm, 1cm x 1cm, 0.9cm x 0.9cm. Typically, a unit may contain more than 100,000 orthogonal domains, for example, 105,000, 1,100,000, 115,000, 120,000, 125,000, 130,000, 135,000, or 140,000 orthogonal domains per unit. When forming the unit, a gap may be set between the units. For example, the unit gap may range from about 1-2 μm, about 1.3-1.8 μm, about 1.4 μm, about 1.5 μm, or about 1.6 μm. The length of the unit gap is substantially consistent within the region of each photo-alignment layer. The acceptable variation in the unit gap length over this region should be determined by a person skilled in the art based on the actual circumstances in which the unit is used. The difference in alignment direction in adjacent domains of the liquid crystal grating unit causes a change in the refractive index of the liquid crystal. This is caused by the varying distortion of the liquid crystal director within these domains. Thus, a Figure 2AThe liquid crystal is directed and aligned as shown. Nematic liquid crystal is aligned between two substrates (e.g., two ITO glass plates). The liquid crystal molecules are arranged periodically along the plane of the substrate. Within one period, the alignment of the liquid crystal molecules in one domain is rotated 180° relative to the adjacent domain. In this way, multiple alignment regions can be provided, thereby manufacturing a small-period switchable liquid crystal grating unit with satisfactory alignment quality.

[0048] Such a cell gap can be ensured through the use of photospacers. Photospacers can be introduced during the manufacturing process to ensure uniform cell thickness and maintain the proper distance between layers. Once the manufacturing process is complete, these spacers can be retained within the structure of the cell to continue to provide the desired spacing and maintain the structural integrity of the cell.

[0049] exist Figure 2A In the case shown, as shown in 202, the circuit connected to the liquid crystal cell is open, indicating that no voltage is applied thereto. Since the patterns on the top and bottom surfaces of the substrate of the liquid crystal grating unit are offset by half a pitch or less, and the liquid crystal molecules are rotated by an angle (such as about 180°), the liquid crystal cell presents a polarization grating profile. The liquid crystal cell provides an alignment layer with strong anchoring energy and ensures that the switchable liquid crystal cell can withstand multiple switching cycles. This arrangement can achieve high diffraction efficiency and stability.

[0050] Figure 2B The deformation of the liquid crystal director in the liquid crystal cell when an electric field 203 is applied to the liquid crystal cell is intuitively shown. By applying an AC voltage above a threshold, such as an AC triangular voltage or an AC sine wave, the positive liquid crystal between the liquid crystal grating components begins to redirect. Specifically, the long axes of the liquid crystal molecules begin to align with the direction of the electric field, thereby becoming perpendicular to the substrate plane. Ideally, the birefringence of the liquid crystal is close to zero, so that the diffraction effect disappears.

[0051] As mentioned above and Figure 2A The switchable liquid crystal cell shown can successfully realize a small-period switchable liquid crystal grating component with satisfactory alignment quality by implementing the method detailed in the present disclosure, which will also bring higher diffraction efficiency. Figure 3 Provides a direct schematic diagram of a switchable liquid crystal grating unit made by the process disclosed in the present disclosure as seen under a microscope. Figure 3As shown, the discernible pattern consists of equidistant light and dark stripes, indicating that the grating has a consistent period. Those skilled in the art can determine acceptable period length variations based on the actual conditions of unit use. The liquid crystal grating developed in certain embodiments has a smaller period than conventional gratings. Optionally, the switchable liquid crystal grating can have a period in the range of about 4-50 microns, such as about 7-40 microns, about 12-30 microns, about 14-28 microns, about 18-25 microns, about 20-23 microns, about 21-22 microns, about 5 microns, about 6 microns, about 8 microns, about 9 microns, about 10 microns, about 11 microns, about 13 microns, about 15 microns or about 16 microns. The above-mentioned small period enhances the ability of the grating to control light over a large range. The period of the grating directly corresponds to the period of the amplitude mask used in the above-mentioned process. Images taken under cross-polarized light show that the liquid crystal grating manufactured by certain embodiments has a well-structured grating pattern compared to the liquid crystal gratings in the prior art, which often have a period of more than 50 microns or even larger. In the field of photonics, it is crucial to fabricate gratings with periods less than 10 microns. The importance of gratings is emphasized by the fact that the period of a grating directly affects its diffraction angle, which in turn dictates the grating's ability to control light. The disclosed liquid crystal gratings can achieve smaller grating periods, resulting in larger diffraction angles, thereby improving the ability to control the direction of light over a larger range. This precise control of light directionality is critical to many photonics applications, including high-resolution imaging, laser beam steering, and optical switching.

[0052] like Figure 4A The experimental setup shown can demonstrate the performance of the above-mentioned liquid crystal cell. Figure 4A Figure 2 shows the experimental setup used to study the diffraction and electro-optical properties of liquid crystal gratings. Figure 4A In the embodiment, the He-Ne laser 401 emits a light beam vertically pointing to the liquid crystal grating unit 403, which is as shown in FIG. Figure 2AThe switchable liquid crystal cell shown. The incident light beam 402 is unpolarized and has a wavelength between about 600-700 nm, for example, between about 620-680 nm, such as about 625 nm, about 628.8 nm, about 632.8 nm, about 642.8 nm, about 650 nm, about 660 nm, or about 670 nm. The electrodes of the liquid crystal grating unit 403 are connected to the power supply 404, but the power supply 404 is in the off state and no voltage is applied. In the absence of voltage, the inherent change of the order of the liquid crystal director (the average direction of the long molecular axis of all molecules in the liquid crystal) in the liquid crystal grating causes the incident light beam to be diffracted into multiple orders. Once the unpolarized light beam is incident on the liquid crystal grating, the first-order diffracted light beams 405 and 406 from the liquid crystal grating unit are emitted along light paths in different directions (e.g., two directions) and appear as orthogonal circularly polarized light. The intensity of the first-order diffracted light beam can be captured by the photodetector 407.

[0053] Figure 4B shows the diffraction pattern of the outgoing beam in the absence of an electric field, which is obtained by using Figure 4A The experimental setup shown in Figure 1 was obtained. Ideally, the LC grating reaches its optimal thickness when the phase retardation of the LC layer is equal to half the target operating wavelength. At this point, in theory, the diffraction efficiency should reach 100%, meaning that the output light consists of only first-order (+1 and -1) diffracted beams, with no zero-order. However, due to inaccuracies in the manufacturing process, such as Figure 4B The particular liquid crystal grating example shown does not achieve 100% diffraction efficiency. A detailed discussion of diffraction efficiency will be given in Figure 6 Further provided in.

[0054] Figure 5A The switchable liquid crystal grating unit is shown when it is connected to a power source 404. If the applied electric field is above a predetermined threshold, the liquid crystal director orientation changes to a vertical direction. For example, when E7 is used as a nematic liquid crystal, the predetermined threshold of the electric field is in the range of 0.8V-1.2V, such as approximately 0.9V, 0.95V, 1V, 1.05V, 1.1V, 1.15V. Optionally, the possible range of the predetermined threshold can vary depending on the liquid crystal material used, the specific circumstances of the device configuration and the environmental conditions.

[0055] Therefore, the variation of the refractive index of adjacent regions of the liquid crystal grating unit is substantially eliminated, which means that the periodic phase distribution and grating structure disappear. Figure 5A As shown, there is only zero-order diffraction 501 in the output beam emitted in one direction. The polarization state of the zero-order diffracted light is the same as the polarization state of the incident light. Therefore, the electro-optical control of the alignment process of the liquid crystal grating unit of certain embodiments allows for efficient tuning of the diffraction angle in the liquid crystal grating unit. The diffraction pattern is as shown in FIG. Figure 5B As shown. In some embodiments, the photodetector 407 is also used to measure the light intensity of the zero-order diffraction spot. It can be seen that the switchable liquid crystal grating unit can achieve precise control of the grating on the directionality of light, thereby optimizing signal transmission, and is suitable for telecommunications companies and data centers.

[0056] In addition to the above embodiments, other embodiments may involve using left-handed or right-handed polarized light as the incident light. For example, in some embodiments, left-handed circularly polarized light can be used as the incident light. When the applied electric field is higher than a predetermined threshold value and the left-handed circularly polarized light is incident on the liquid crystal grating unit described in some embodiments, the change in refractive index between adjacent areas of the liquid crystal grating unit can be eliminated. In this way, only +1-order right-handed circularly polarized light can be generated as an output beam along one direction. In some embodiments, right-handed circularly polarized light can be used as the incident light. When the applied electric field is higher than a predetermined threshold value and the right-handed circularly polarized light is incident on the liquid crystal grating unit as described in some embodiments, the change in refractive index between adjacent areas can also be eliminated, and only -1-order right-handed circularly polarized light can be generated as an output beam along one direction.

[0057] Figure 6 The long-term diffraction efficiency of two types of liquid crystal polarization gratings (grating No. 1 and grating No. 2) is shown. Grating No. 1 represents a liquid crystal grating prepared by a conventional method, in which only one side of the substrate has a grating alignment. On the other hand, grating No. 2, which is manufactured using the method proposed in certain embodiments, has both an upper substrate and a lower substrate with periodically alternating alignment directions, in which the patterns on the top and bottom surfaces are offset by half a pitch or less than half a pitch. When a voltage of 10 V is applied to each liquid crystal grating, the transmittance of the 0th order diffraction spot has been monitored for an extended duration. As Figure 6 As shown, grating No. 2 maintains a stable high diffraction efficiency of about 70% for a long time. In contrast, the intensity of the 0th order diffraction spot of grating No. 1 drops rapidly. This drop is due to the electrical stability problems that are constantly encountered in devices manufactured using traditional methods. After several cycles of electrical switching, the diffraction efficiency of the liquid crystal grating decreases significantly. This reduction is mainly due to the high azimuthal anchoring energy required to redirect the liquid crystal director to its initial alignment after switching. The method proposed in certain embodiments results in a liquid crystal grating with a persistent and consistent high diffraction efficiency. Therefore, the proposed method for manufacturing an electrically switchable liquid crystal diffraction grating is not only capable of achieving high diffraction efficiency, but also capable of ensuring sustainable stability over a long period of time.

[0058] The methods provided in certain embodiments provide a reliable method to produce liquid crystal gratings and liquid crystal grating elements that exhibit both excellent performance and long-term reliability, making them suitable for various practical applications requiring stable and effective diffraction characteristics.

[0059] The liquid crystal grating unit proposed in the present disclosure can be used in a variety of applications, such as industrial manufacturing, medical surgery and military applications that utilize precise laser control of gratings, and can improve laser cutting, engraving and aiming systems. In addition, the provided liquid crystal grating unit can be used in lidar systems and optical communication systems for autonomous vehicles to increase data transmission rates. In addition, the liquid crystal grating unit can be used to improve the clarity and detail of imaging systems, such as microscopes, telescopes and cameras.

[0060] The invention provided in this disclosure offers an advanced solution in the field of switchable liquid crystal gratings by providing a high performance, low cost and long term stable device with precise control over the directionality of light, a key feature for many photonic applications.

[0061] High efficiency: The switchable liquid crystal gratings developed by the present invention show high diffraction efficiency. This is beneficial for applications that require efficient light management, such as high-resolution imaging and optical switching.

[0062] Low cost: Using cost-effective materials and implementing a simple manufacturing process that can be easily replicated reduces the overall cost of producing these gratings, making the solution more economically viable.

[0063] Long-term stability: Due to electrical stability issues, the diffraction efficiency of conventional liquid crystal gratings usually decreases over time. The present invention successfully solves this problem by introducing a liquid crystal grating with long-lasting and continuously high diffraction efficiency.

[0064] Small grating period: Many existing liquid crystal gratings have a period of more than 50 microns. However, the liquid crystal grating developed here has a smaller period of 8 microns, enhancing its ability to control the direction of light over a wider range. This is particularly important in the field of photonics, where controlling the directionality of light is crucial.

[0065] High performance and reliability: Gratings produced by this method not only exhibit excellent performance, but also reliably maintain this performance over a long period of time. This is ideal for practical applications that require stable and effective diffraction properties.

[0066] Improved fabrication method: A two-step photoalignment process using an amplitude mask enables the creation of multiple alignment regions, which is a significant improvement over conventional methods where only one side of the substrate has grating alignment.

[0067] The term "approximately" is used before a quantitative value, and the disclosure also includes the specific quantitative value itself unless otherwise specifically stated. As used herein, the term "about" refers to a variation of ±10%, ±7%, ±5%, ±3%, ±1%, or ±0% compared to the nominal value, unless otherwise stated or inferred.

[0068] The embodiments or elements shown in this disclosure, including the specific illustrations and materials used in the examples, are intended to be illustrative rather than limiting. They allow for extensive changes, adjustments or adaptations based on the basic concepts of this disclosure. It should be noted that all the diagrams described are for illustrative purposes only; they are neither drawn to scale nor accurate reproductions of actual devices.

[0069] In the case that it has not been explicitly described, as long as such combination or exchange is meaningful and consistent with the meaning of the present disclosure, the various embodiments or their various aspects and features related to the drawings can be combined or exchanged with each other without limiting or expanding the scope of the described disclosure. The advantages described for a specific embodiment or a specific figure of the present disclosure are also the advantages of other embodiments of the present disclosure, where applicable.

[0070] References

[0071] The following lists references that are occasionally cited in the specification. The disclosures of each of these references are incorporated herein by reference in their entirety.

[0072] [1] Hu W, Srivastava A, Xu F et al., “Liquid crystal gratings based on alternating TN and PA photoalignment” [J]. Optics Express, 2012, 20(5): 5384-5391.

[0073] [2] Guo Q, Srivastava AK, Chigrinov VG et al., “Polymer-azo dye composites: photoalignment layers for liquid crystals” [J]. Liquid Crystals, 2014, 41(10): 1465-1472.

[0074] [3] Srivastava AK, Wang XQ, Gong SQ, et al., “Micropatterned photoaligned ferroelectric liquid crystal Fresnel zone lenses” [J]. Optics Express, 2015, 40(8): 1643-1646.

[0075] [4] R, Kozanecka-Szmigiel A, Piecek W, et al., “Electro-optically tunable diffraction gratings with photoaligned liquid crystals” [J], Optical Communications, 201, 400: 144-149.

[0076] [5] Cheng Y, Sun Z, Yuan Z et al., P-12.10: “Fast switchable depth sounding system based on combination of liquid crystal polarization grating and ferroelectric liquid crystal Dammann grating” [C] / / SID Technical Papers Symposium Abstracts, 2022, 53(S1): 966-969.

[0077] [6] He Z, Nose T, Sato S, “Polarization characteristics of amplitude nematic liquid crystal gratings”[J], Optical Engineering, 1998, 37(11): 2885-2898.

[0078] [7] Fujida I. “Liquid crystal phase grating based on in-plane switching” [J]. Applied Optics, 2001, 40(34): 6252-6259.

[0079] [8] Ono H, Emoto A, Takahashi F et al., “Highly stable polarization gratings in photocrosslinked polymer liquid crystals” [J]. Chinese Journal of Applied Physics, 2003, 94(3): 1298-1303.

[0080] [9] Kapoustine V, Kazakevitch A, So V, et al., “A simple method to form switchable liquid crystal gratings by introducing periodic photoalignment patterns into liquid crystal cells” [J]. Optical Communications, 2006, 266(1): 1-5.

[0081]

[10] Wu WY, Fuh AY G, “Preparation of rewritable liquid crystal gratings using the photoalignment effect in dye-doped poly(vinyl alcohol) films” [J], Japanese Journal of Applied Physics, 2007, 46(10R): 6761.

Claims

1. A switchable liquid crystal grating unit, comprising: A pair of switchable liquid crystal grating components, each switchable liquid crystal grating component comprising: substrate; a conductive layer on the substrate; and a photo-alignment layer having an alignment pattern above the conductive layer on the substrate, the alignment pattern having a first type of domains and a second type of domains, the first type of domains having a first uniform alignment direction, the second type of domains having a redirected second uniform alignment direction at a predetermined angle to the first uniform alignment direction, wherein the predetermined angle is 85°-95°; a liquid crystal layer sandwiched between the pair of switchable liquid crystal grating components; In which, the two photo-alignment layers in the pair of switchable liquid crystal grating components face each other, and for the pair of switchable liquid crystal grating components, the alignment pattern of the photo-alignment layer on the top surface of the lower switchable liquid crystal grating component is arranged to be offset by half a pitch to 45% of the pitch relative to another alignment pattern of the photo-alignment layer on the bottom surface of the upper switchable liquid crystal grating component.

2. The switchable liquid crystal barrier unit according to claim 1, wherein: The predetermined angle is 90°.

3. The switchable liquid crystal barrier unit according to claim 1, wherein: The substrate is a transparent glass substrate, and the conductive layer includes a plurality of indium tin oxide (ITO) electrodes.

4. The switchable liquid crystal barrier unit according to claim 1, wherein: The photo-alignment layer has the characteristic of periodically alternating the alignment direction, resulting in periodic changes in the alignment direction.

5. The switchable liquid crystal barrier unit according to claim 1, wherein: The switchable liquid crystal barrier element has a period in the range of 4-50 microns.

6. The switchable liquid crystal barrier unit according to claim 1, wherein: The switchable liquid crystal grating unit has a period of 8 microns.

7. The switchable liquid crystal barrier unit according to claim 1, wherein: The photo-alignment material of the photo-alignment layer is sulfonic acid azo dye (SD1).

8. The switchable liquid crystal barrier unit according to claim 1, wherein: When no electric field is applied, the liquid crystal molecules of the photo-alignment layer are arranged periodically along the plane of the substrate; And, when an electric field is applied, the long axes of the liquid crystal molecules of the photo-alignment layer begin to align with the direction of the electric field, thereby becoming perpendicular to the plane of the substrate.

9. The switchable liquid crystal barrier unit according to claim 1, wherein: There are unit gaps between the switchable liquid crystal grating units.

10. The switchable liquid crystal barrier unit according to claim 9, wherein: The cell gap is in the range of 1 μm-2 μm.

11. A method for manufacturing a switchable liquid crystal barrier unit, the method comprising: coating the substrate with a conductive layer; spin coating the substrate with a photo-alignment material to establish a photo-alignment layer on the conductive layer; exposing the photo-alignment layer to first polarized ultraviolet light polarized along a first direction to achieve uniform alignment of liquid crystal molecules in the photo-alignment layer; placing an amplitude mask in contact with the photo-alignment layer of the substrate to expose non-masked areas of the photo-alignment layer; as well as irradiating the photo-alignment layer with second polarized ultraviolet light polarized in a second direction, the second direction being arranged at a predetermined angle to the first direction, to redirect the non-masked area of ​​the photo-alignment layer, thereby generating a switchable liquid crystal grating component having an alignment pattern on the photo-alignment layer, the alignment pattern having a first type of domain and a second type of domain, the first type of domain having a first uniform alignment direction, the second type of domain having a redirected second uniform alignment direction at a predetermined angle to the first uniform alignment direction, wherein the predetermined angle is 85°-95°; Arranging a pair of the switchable liquid crystal grating members, wherein two photo-alignment layers in the pair of switchable liquid crystal grating members face each other, and an alignment pattern of the photo-alignment layer on the top surface of the lower switchable liquid crystal grating member is offset by half a pitch or less than half a pitch with respect to another alignment pattern of the photo-alignment layer on the bottom surface of the upper switchable liquid crystal grating member; and Nematic liquid crystal is injected between the pair of switchable liquid crystal grating components to obtain a switchable liquid crystal grating unit.

12. The method according to claim 11, wherein: The predetermined angle is 90°.

13. The method according to claim 11, wherein: The substrate is a transparent glass substrate, and the conductive layer includes a plurality of indium tin oxide (ITO) electrodes.

14. The method according to claim 11, wherein: The photo-alignment layer has the characteristic of periodically alternating the alignment direction, resulting in periodic changes in the alignment direction.

15. The method according to claim 11, wherein: The switchable liquid crystal barrier element has a period in the range of 4-50 microns.

16. The method according to claim 11, wherein: The switchable liquid crystal grating unit has a period of 8 microns.

17. The method according to claim 11, wherein: Spin coating the substrate with the photo-alignment material comprises: Spin coating a solution comprising a sulfonic acid azo dye (SD1) dissolved in an N,N-dimethylformamide (DMF) solvent on the surface of the substrate; and The DMF solvent was removed from the coated substrate.

18. The method according to claim 11, wherein: When the nematic liquid crystal is injected between the pair of photo-alignment layers, one or more optical spacers are arranged between a corresponding pair of switchable liquid crystal grating units to space the switchable liquid crystal grating units in a cell gap.

19. The method according to claim 18, wherein: The cell gap is in the range of 1 μm-2 μm.

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