Transmission type liquid crystal terahertz spatial light modulator with large switch ratio

Abstract

The invention discloses a transmission type liquid crystal terahertz spatial light modulator with a large switch ratio. The transmission type liquid crystal terahertz spatial light modulator structurally comprises an upper-layer quartz substrate, an upper-layer metal structure, a liquid crystal layer, a lower-layer metal structure and a lower-layer quartz substrate in sequence from top to bottom, the upper-layer metal structure is adhered to the back surface of the upper-layer quartz substrate and is a four-opening square ring reverse structure array, and the lower-layer metal structure is adhered to the top surface of the lower-layer quartz substrate; and the liquid crystal layer is filled between the upper-layer metal structure and the lower-layer metal structure to form a metal-medium-metal structure. The electro-optical effect of the liquid crystal is utilized, the FPGA is communicated with the electrodes corresponding to the metal structures, voltage is applied between the upper-layer metal structure and the lower-layer metal structure, the effective dielectric constant of the liquid crystal can be changed by adjusting the voltage, and therefore on-off of terahertz waves is achieved. And the upper computer controls the FPGA to output voltage through an instruction so as to realize programmable regulation and control of the device.

Description

Technical Field

[0001] The present invention relates to the technical field of terahertz metasurfaces, and particularly to a transmissive terahertz spatial light modulator based on liquid crystals. Background Art

[0002] The terahertz spatial light modulator based on metasurfaces dynamically regulates the amplitude of terahertz waves through artificial microstructures, breaking through the limitations of traditional terahertz devices in modulation depth, response speed, and operating frequency band. Such modulators usually adopt reconfigurable units and achieve precise regulation through pixelated design. At the application level, its development provides key support for the wide application of terahertz technology in high-resolution imaging, wireless communication, spectral analysis, security detection, and other fields.

[0003] Currently, terahertz metasurface spatial light modulators mainly adopt reflective structures to achieve dynamic regulation of terahertz waves. As a common electro-optical material, liquid crystals are also widely used in terahertz active regulation metasurface devices. In 2022, the research team at Nanjing University developed a dual-color terahertz spatial light modulator using dual-frequency liquid crystals and successfully achieved compressive sensing imaging. In 2023, the team further developed a terahertz phase-type spatial modulator using a cross-switch structure, effectively reducing the process complexity of large-scale array devices. However, the switching ratio of existing terahertz spatial light modulators is still low, and the reflective working mode leads to a complex test optical path, which is not conducive to integrated design. Therefore, developing a terahertz transmissive spatial light modulator with programmable capabilities and pixel-level regulation has become a key research direction. Summary of the Invention

[0004] Object of the Invention: The present invention provides a transmissive liquid crystal terahertz spatial light modulator with a large switching ratio and its preparation and testing methods. The modulator can be programmatically controlled to achieve precise spatial modulation of the intensity of transmitted terahertz waves.

[0005] Technical solution: To achieve the above-mentioned invention object, the first part of the technical solution of the present invention is a transmissive liquid crystal terahertz spatial light modulator with a large switching ratio, which includes an upper quartz substrate, an upper metal structure, a liquid crystal layer, a lower metal structure, and a lower quartz substrate; the lower metal structure is grown on the lower quartz substrate, and the upper metal structure is grown on the upper quartz substrate; the liquid crystal layer is filled between the upper metal structure and the lower metal structure to form a metal-dielectric-metal structure; the regulation mechanism of transmission is as follows: when no voltage is applied between the upper metal structure and the lower metal structure, due to the combination of the Fabry-Perot effect of the substrate and the resonance characteristics of the metal structure, the electric field transmittance at the working frequency point is greater than 80%; when an external voltage is applied between the upper metal structure and the lower metal structure, the electric field perpendicular to the plane of the metal structure drives the liquid crystal director to rotate, resulting in a change in the equivalent dielectric constant of the liquid crystal layer and a shift in the resonance of the metal structure, and the transmittance at the working frequency point is close to 0.

[0006] Further, both the upper metal structure and the lower metal structure are composed of a periodically arranged four-open square loop anti-structure.

[0007] Further, the liquid crystal layer is a nematic liquid crystal with orientation layer anchoring.

[0008] Furthermore, the lower metal structure is composed of a unit array, an electrode, and a feeder, and each unit array and the electrode are connected by the feeder.

[0009] Furthermore, each unit array corresponds to a pixel, and the number of unit arrays corresponds to the array scale of the device; the pixel is composed of the upper quartz substrate, the upper metal structure, the liquid crystal layer, the lower metal structure, and the lower quartz substrate within the area covered by the unit array.

[0010] The second part of the technical solution of the present invention is a preparation method of the transmissive liquid crystal terahertz spatial light modulator with a large switching ratio as described above, including the following steps:

[0011] (1) Substrate cleaning: Immerse both the upper quartz substrate and the lower quartz substrate in a cleaning solvent and place them in an ultrasonic cleaner, then wash and dry them; (2) Metal thin film deposition: Use a magnetron sputtering instrument to deposit a metal thin film on the quartz substrate after the operation in step (1); (3) Spin-coating photoresist: Spin-coat AZ1500 on the quartz substrate and dry it; (4) Lithography and development: Place the mask on an ultraviolet exposure machine, align the target pattern mask and the quartz substrate and then expose it, and place the exposed sample in a positive photoresist developer for development; (5) Etching: Use an inorganic solution etching process to form a four-opening square ring anti-structure for the upper and lower layers respectively; (6) Photoresist cleaning: After etching, immerse the upper quartz substrate and the lower quartz substrate with the metal structure in a cleaning solvent to remove the photoresist on the metal structure and dry it; (7) Alignment: Spin-coat a polyimide reagent on the quartz substrate after the operation in step (6), dry it and then place it on a rubbing alignment machine for rubbing alignment; (8) Encapsulation: Stir spacers evenly in the frame adhesive, apply the frame adhesive to the non-patterned area at the edge of the lower quartz substrate, align the upper metal structure and the lower metal structure under a microscope and then bond and cure them; (9) Liquid crystal filling: Place the liquid crystal cell after step (8) on a hot stage, heat the liquid crystal to the clearing point and then fill it between the upper quartz substrate and the lower quartz substrate; (10) Wire bonding: Apply anisotropic conductive adhesive to the device electrodes, and bond the flexible electrodes and the device electrodes under a hot press.

[0012] The third technical solution provided by the present invention is a characterization method for a transmissive liquid crystal terahertz spatial light modulator with a large switching ratio, including the following steps:

[0013] (1) Connect the test system: Control the upper computer control program to drive the voltage output of the FPGA pins; Fix the device on the circuit board; Interconnect the field programmable gate array and the device through flexible electrodes;

[0014] (2) Terahertz spectrum response test: Fix the circuit board in step (1) at the central position of the two-dimensional displacement stage, and adjust the positions of the transceiver modules of the terahertz time-domain spectroscopy system so that the transmissive liquid crystal terahertz spatial light modulator sample is located at the focus of the terahertz beam; Use a signal generator to supply power to the device electrodes uniformly, the control signal is a 1 kHz square wave, and the peak-to-peak value is gradually increased from 0 V to 20 V, and measure the transmittance at the working frequency;

[0015] (3) Transmission type terahertz spatial light modulator spatial encoding pattern test: After the upper computer encodes the pattern, it is converted into control instructions and transmitted to the field programmable gate array. The field programmable gate array controls the loading voltage of each pixel according to the instructions; the upper computer is used to control the two-dimensional displacement stage in step (2) to move point by point, and collect the time-domain pulse signal of each point. The scanning area covers the effective area of the device; according to the transmitted terahertz wave electric field intensity distribution obtained by scanning, an intensity distribution image is obtained.

[0016] Advantages:

[0017] The present invention utilizes the electro-optic effect of liquid crystal to connect the electrodes corresponding to the FPGA and the metal structure, apply a voltage between the upper metal structure and the lower metal structure, and change the operating frequency point of the resonant structure by adjusting the voltage, so as to realize the on-off of a large switching ratio of terahertz waves at the operating frequency. The upper computer instruction controls the FPGA to output voltage to realize the programmable regulation of the device. In the field of computational imaging, it can simplify the imaging optical path and improve the quality of the reconstructed image; in the fields of security information encryption and steganography, it realizes physical-level information encryption and invisible communication by dynamically controlling the encoded information of the transmitted light field. In the field of terahertz spectrum analysis and regulation, it can be used in tunable terahertz filters to selectively transmit terahertz waves of specific frequencies and improve the resolution and flexibility of spectrometers. The transmission type terahertz spatial light modulator with a large switching ratio provides an innovative solution for the development of terahertz technology with its flexible regulation ability, and promotes the development of fields such as imaging, security, and materials science. Description of the Drawings

[0018] Figure 1 It is a structural sectional view of the transmission type liquid crystal terahertz spatial light modulator with a large switching ratio of the present invention;

[0019] Figure 2 It is a schematic diagram of the unit structure of the transmission type liquid crystal terahertz spatial light modulator with a large switching ratio of the present invention;

[0020] Figure 3 It is a preparation flow chart of the transmission type liquid crystal terahertz spatial light modulator with a large switching ratio of the present invention;

[0021] Figure 4 It is a simulation result diagram of the transmission coefficient of the transmission type liquid crystal terahertz spatial light modulator with a large switching ratio of the present invention;

[0022] Figure 5 It is a test control logic diagram of the transmission type liquid crystal terahertz spatial light modulator with a large switching ratio of the present invention;

[0023] Figure 6This is the spatial encoding test result diagram of the transmission-type liquid crystal terahertz spatial light modulator with a large switching ratio for the present invention. Detailed implementation manners

[0024] The present invention will be further clarified below in conjunction with the accompanying drawings and specific embodiments. It should be understood that these embodiments are only used to illustrate the present invention and not to limit the scope of use of the present invention. After reading the present invention, those skilled in the art's various equivalent modifications of the present invention all fall within the scope defined by the appended claims of this application.

[0025] The present invention discloses a transmission-type terahertz spatial light modulation device with a liquid crystal material as a tunable medium. The cross-sectional structure of the device is as Figure 1 shown. The liquid crystal is filled between the upper adaptation substrate and the lower quartz substrate, and together with the upper metal structure and the lower metal structure, it forms a metal-dielectric-metal structure. Utilizing the electro-optic effect of the liquid crystal, when a bias voltage is applied between the upper metal structure and the lower metal structure, the rotation of the liquid crystal director causes the equivalent dielectric constant of the liquid crystal layer in the direction perpendicular to the metal plane to change, thereby regulating the on-off of the transmitted wave. The present invention discloses a design scheme of a transmission-type liquid crystal terahertz spatial light modulator with a large switching ratio. The present invention realizes the amplitude regulation at the pixel level in the two-dimensional plane of the terahertz wave, has a simple structure, and a flexible regulation method. The following is a detailed description.

[0026] I. Design scheme of a transmission-type liquid crystal terahertz spatial light modulator unit with a large switching ratio

[0027] The present invention provides that the transmission-type liquid crystal terahertz spatial light modulator array with a large switching ratio can realize the pixel-level amplitude modulation function, and its overall structure is as Figure 2 shown. The upper metal structure of the metasurface unit is a four-open square ring anti-structure. Before applying voltage, due to the Fabry-Perot effect of the substrate and the resonance effect of the metal structure, high transmission of terahertz waves is achieved at the working frequency point. The change in the applied voltage causes the rotation of the liquid crystal molecular director, and the change in the dielectric constant of the liquid crystal layer in the direction perpendicular to the metal plane causes the resonance of the metal structure to deviate from the working frequency point, thereby reducing the transmission coefficient at this point and realizing the turn-off of the terahertz wave. In order to design a liquid crystal spatial light modulator unit that meets the transmission amplitude modulation function with a large switching ratio, the optimized structural dimensions are specifically: p = 300 μm, l = 250 μm, l r = 35 μm, g = 8 μm, t s = 400 μm, t lc = 50 μm.

[0028] II. Fabrication of a transmission-type liquid crystal terahertz spatial light modulator with a large switching ratio

[0029] Design a mask according to the structural dimensions and array scale optimized by simulation, and then prepare a device sample according to the micro-nano processing technology, such as Figure 3 as shown. The specific steps are as follows:

[0030] (1) Substrate pretreatment: Place the upper quartz substrate and the lower quartz substrate in a beaker, soak them in cleaning solvents (acetone, ethanol, deionized water in sequence), and dry them after cleaning in an ultrasonic cleaner;

[0031] (2) Metal deposition: Use a magnetron sputtering instrument to deposit a layer of copper on the quartz substrate after the operation in step (1);

[0032] (3) Photoresist spin-coating: Spin-coat AZ1500 on the quartz substrate and dry it;

[0033] (4) Lithography and development: Place the target pattern mask on the lithography machine, align the mask pattern and the quartz substrate and then expose it, and place the exposed substrate in a positive photoresist developer for development;

[0034] (5) Etching: Use an inorganic solution etching process to form a four-opening square ring anti-structure for the upper and lower layers respectively;

[0035] (6) Photoresist cleaning: After etching, soak the upper quartz substrate and the lower quartz substrate with the metal structure in the cleaning solvent to remove the photoresist on the metal structure and dry it;

[0036] (7) Alignment: Spin-coat a polyimide reagent on the substrate after the operation in step (6), dry it and then place it on a rubbing aligner for rubbing alignment;

[0037] (8) Encapsulation: Stir the spacers evenly in the frame adhesive, apply the frame adhesive on the non-patterned area of the lower quartz substrate, and align and cure the upper and lower metal patterns under a microscope.

[0038] (9) Liquid crystal filling: Place the liquid crystal cell after step (8) on a hot stage, heat the liquid crystal to the clearing point and then fill it between the upper and lower quartz substrates

[0039] (10) Wire bonding: Apply an anisotropic conductive adhesive to the device electrodes, and connect the flexible electrodes and the device electrodes under a hot press.

[0040] Working principle of the device transmission switch:

[0041] The working mechanism of the device is the combination of the substrate Fabry - Perot effect and the metal resonance effect. When no voltage is applied, the resonance frequencies generated by the Fabry - Perot effect and the metal resonance are close. At this time, the transmission coefficient of the device is high, realizing the function of transmitting terahertz waves. When a voltage is applied, due to the rotation of the liquid crystal director, the equivalent dielectric constant of the liquid crystal layer perpendicular to the metal plane changes. At this time, the metal resonance frequency shifts, and the Fabry - Perot resonance of the substrate and the metal resonance are separated. At the working frequency point, the function of turning off terahertz waves is realized. From the simulation results Figure 4 It can be seen that with the switching of the powered - on state, the transmitted wave at the working frequency point can achieve control with a large switching ratio.

[0042] III. Test Scheme and Experimental Results of a Transmissive Liquid - Crystal Terahertz Spatial Light Modulator with a Large Switching Ratio

[0043] Figure 5 The schematic diagram of the characterization method of a transmissive liquid - crystal terahertz spatial light modulator with a large switching ratio is given. Its steps include:

[0044] (1) Connect the test system: Write a host - computer control program to drive the voltage distribution of the output pins on the FPGA board; fix the device on a printed circuit board (PCB); interconnect the FPGA or signal generator and the device through an FPC cable.

[0045] (2) Test the terahertz spectral characteristics: Fix the PCB in (1) at the center position of the two - dimensional displacement stage, and adjust the positions of the transmitting and receiving modules of the terahertz time - domain spectroscopy system so that the sample of the transmissive terahertz spatial light modulator is located at the focus of the terahertz optical path. Use a signal generator to supply power to the device electrodes uniformly, control the voltage signal as a 1kHz square wave, and gradually increase the peak - to - peak value from 0V to 20V, and measure the change in the transmitted electric - field intensity at the working frequency.

[0046] (3) Test the spatial encoding pattern of the transmissive terahertz spatial light modulator: Use the test system built in (2). Let the displacement stage move two - dimensionally point by point under the control of a computer. At each movement, collect the time - domain pulse signal at the current position, and ensure that the terahertz light - spot scanning area covers the effective area of the device, and scan the electric - field distribution on the device surface at the working frequency. Use the FPGA to supply power to the device electrodes independently. After pre - encoding the pattern on the host computer and converting it into ASCII code, transmit it to the FPGA through the serial port. The FPGA addresses the corresponding unit according to the instruction to load the voltage; the terahertz time - domain spectroscopy system scans the two - dimensional plane to obtain the transmitted encoded image. The scanning test results of a 16×16 array device are as Figure 6 shown.

Claims

1. A transmissive liquid crystal terahertz spatial light modulator with a large on / off ratio, characterized in that: The invention comprises an upper quartz substrate, an upper metal structure, a liquid crystal layer, a lower metal structure and a lower quartz substrate; the lower metal structure is grown on the lower quartz substrate, and the upper metal structure is grown on the upper quartz substrate; the liquid crystal layer is filled between the upper metal structure and the lower metal structure to form a metal-medium-metal structure; the transmission control mechanism is as follows: when no voltage is applied between the upper metal structure and the lower metal structure, due to the combination of the Fabry-Perot effect of the substrate and the resonance characteristics of the metal structure, the electric field transmittance is greater than 80% at the operating frequency; when an external voltage is applied between the upper metal structure and the lower metal structure, the electric field perpendicular to the plane of the metal structure drives the liquid crystal director to rotate, resulting in a change in the equivalent dielectric constant of the liquid crystal layer, a shift in the resonance of the metal structure, and a transmittance close to 0 at the operating frequency.

2. The transmissive liquid crystal terahertz spatial light modulator with a large switching ratio according to claim 1, characterized in that: The upper metal structure and the lower metal structure are both composed of periodically arranged four-opening square ring-inverted structures.

3. The transmissive liquid crystal terahertz spatial light modulator with a large switching ratio according to claim 1, characterized in that: The liquid crystal layer is a nematic liquid crystal anchored by an alignment layer.

4. The transmissive liquid crystal terahertz spatial light modulator with a large on / off ratio according to claim 1, characterized in that: The lower metal structure is composed of a unit array, electrodes and a feed line, and each unit array and electrode are connected by a feed line.

5. The transmissive liquid crystal terahertz spatial light modulator with a large on / off ratio according to claim 4, characterized in that: Each unit array corresponds to a pixel, and the number of unit arrays corresponds to the array scale of the device; the pixel is composed of an upper quartz substrate, an upper metal structure, a liquid crystal layer, a lower metal structure and a lower quartz substrate within the area covered by the unit array.

6. A method for preparing the transmissive liquid crystal terahertz spatial light modulator with a large on / off ratio as claimed in claim 1, characterized in that: The method comprises the following steps: (1) substrate cleaning: immersing the upper quartz substrate and the lower quartz substrate in a cleaning solvent and placing them in an ultrasonic cleaning machine to clean and blow dry; (2) metal film deposition: using a magnetron sputtering device to deposit a metal film on the quartz substrate after the operation in step (1); (3) spin coating photoresist: spin coating AZ1500 on the quartz substrate and drying; (4) Photolithography and development: Place the mask on a UV exposure machine, align the target pattern mask and the quartz substrate, and then expose. Place the exposed sample in a positive photoresist developer for development. (5) Etching: using an inorganic solution etching process to form a four-opening square ring-shaped anti-structure on the upper and lower layers respectively; (6) Photoresist cleaning: After etching, the upper quartz substrate with the metal structure and the lower quartz substrate are immersed in a cleaning solvent to remove the photoresist on the metal structure and blow dry; (7) Orientation: Spin-coat the quartz substrate after the operation in step (6) with a polyimide reagent, dry it, and place it in a friction orientation machine for friction orientation; (8) Packaging: Evenly stir the spacer in the frame glue, apply the frame glue to the non-graphic area at the edge of the lower quartz substrate, align the upper metal structure and the lower metal structure under a microscope, and then bond and solidify; (9) Liquid crystal injection: Place the liquid crystal box after step (8) on a hot table, heat the liquid crystal to a clear point, and then fill it between the upper quartz substrate and the lower quartz substrate; (10) Wire bonding: Apply anisotropic conductive glue to the device electrode, and bond the flexible electrode to the device electrode under a hot press.

7. A test method for characterizing the transmissive liquid crystal terahertz spatial light modulator with a large on / off ratio as claimed in claim 2, characterized in that: The steps include: (1) Connect the test system: control the host computer control program to drive the FPGA output pin voltage; fix the device on the circuit board; interconnect the field programmable gate array and the device through flexible electrodes; (2) Terahertz spectrum response test: fix the circuit board in step (1) at the center of the two-dimensional translation stage, and adjust the position of the transceiver module of the terahertz time-domain spectroscopy system so that the transmission-type terahertz spatial light modulator sample is located at the focus of the terahertz beam; use a signal generator to uniformly power the device electrodes, and the control signal is a 1kHz square wave, with the peak-to-peak value gradually increasing from 0V to 20V, and test the transmittance at the operating frequency; (3) Programmable terahertz transmission spatial light modulator spatial coding pattern test: The host computer performs pattern coding and converts it into control instructions and transmits it to the field programmable gate array, and the field programmable gate array controls the loading voltage of each pixel according to the instructions; the host computer controls the two-dimensional translation stage in step (2) to move point by point, and collects the time domain pulse signal of each point, and the scanning area covers the effective area of ​​the device; according to the electric field intensity distribution of the transmitted terahertz wave obtained by scanning, the intensity distribution image is obtained.

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