Transmissive liquid crystal terahertz spatial light modulator with large on-off ratio

Abstract

This invention discloses a transmissive liquid crystal terahertz spatial light modulator with a large on / off ratio. The device structure, from top to bottom, includes an upper quartz substrate, an upper metal structure, a liquid crystal layer, a lower metal structure, and another lower quartz substrate. The upper metal structure is adhered to the back surface of the upper quartz substrate and is a four-opening square ring-reflection structure array. The lower metal structure is adhered to the top surface of the lower quartz substrate. The liquid crystal layer fills the space between the upper and lower metal structures, forming a metal-dielectric-metal structure. This invention utilizes the electro-optic effect of liquid crystals to connect the programmable gate array (FPGA) and the corresponding electrodes of the metal structure. A voltage is applied between the upper and lower metal structures, and adjusting the voltage changes the equivalent dielectric constant of the liquid crystal, thereby achieving the on / off switching of the terahertz wave. The host computer commands control the FPGA output voltage to achieve programmable control of the device.

Description

Technical Field

[0001] This invention relates to the field of terahertz metasurface technology, and particularly to a liquid crystal-based transmissive terahertz spatial light modulator. Background Technology

[0002] Terahertz spatial light modulators based on metasurfaces dynamically control the amplitude of terahertz waves through artificial microstructures, overcoming the limitations of traditional terahertz devices in modulation depth, response speed, and operating frequency band. These modulators typically employ reconfigurable units and achieve precise control through pixelation design. At the application level, their development provides crucial support for the widespread application of terahertz technology in high-resolution imaging, wireless communication, spectral analysis, and security detection.

[0003] Currently, terahertz metasurface spatial light modulators mainly employ reflective structures to achieve dynamic control of terahertz waves. Liquid crystals, as common electro-optic materials, are also widely used in terahertz actively controlled metasurface devices. In 2022, a research team at Nanjing University developed a dual-color terahertz spatial light modulator using dual-frequency liquid crystals and successfully achieved compressed sensing imaging. In 2023, the team further developed a terahertz phase-type spatial modulator using a cross-switching structure, effectively reducing the fabrication complexity of large-scale array devices. However, the on / off ratio of existing terahertz spatial light modulators remains low, and the reflective operating mode leads to complex test optical paths, which is not conducive to integrated design. Therefore, developing terahertz transmissive spatial light modulators with programmability and pixel-level control has become a key research direction. Summary of the Invention

[0004] Objective of the Invention: This invention provides a transmissive liquid crystal terahertz spatial light modulator with a large on / off ratio, as well as its fabrication and testing methods. This modulator can be programmably controlled to achieve precise spatial modulation of the intensity of the transmitted terahertz wave.

[0005] Technical Solution: To achieve the above-mentioned objectives, the first part of the technical solution of this invention is a transmissive liquid crystal terahertz spatial light modulator with a large on / off ratio, comprising 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 fills the space between the upper and lower metal structures, forming a metal-dielectric-metal structure; the transmission modulation mechanism is as follows: when no voltage is applied between the upper and lower metal structures, 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 operating frequency is greater than 80%; when an external voltage is applied between the upper and lower metal structures, 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 the resonance of the metal structure shifts, resulting in a transmittance close to 0 at the operating frequency.

[0006] Furthermore, both the upper and lower metal structures are composed of periodically arranged four-opening square ring-shaped structures.

[0007] Furthermore, the liquid crystal layer is a nematic liquid crystal anchored to an alignment layer.

[0008] Furthermore, the lower metal structure consists of a unit array, electrodes, and feed lines, with each unit array and electrode connected by a feed line.

[0009] Furthermore, each unit array corresponds to one pixel, and the number of unit arrays corresponds to the array size 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.

[0010] The second part of the technical solution of the present invention is a method for fabricating a transmissive liquid crystal terahertz spatial light modulator with a large on / off ratio as described above, comprising the following steps:

[0011] (1) Substrate cleaning: Immerse both the upper and lower quartz substrates in cleaning solvent and place them in an ultrasonic cleaner to clean and dry; (2) Metal thin film deposition: Deposit a metal thin film on the quartz substrate after step (1) using a magnetron sputtering instrument; (3) Spin-coating photoresist: Spin-coat AZ1500 onto the quartz substrate and dry; (4) Photolithography and development: Place the mask on an ultraviolet exposure machine, align it with the target pattern mask and the quartz substrate, and expose it. 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 reverse structure on the upper and lower layers respectively; (6) Photoresist cleaning: After etching, clean the upper and lower quartz substrates with the metal structure. (7) Orientation: Spin-coating polyimide reagent onto the quartz substrate after step (6), drying it, and then placing it in a friction orientation machine for friction orientation; (8) Encapsulation: Stirring the spacer evenly in the frame adhesive, applying the frame adhesive to the non-patterned area at the edge of the lower quartz substrate, aligning the upper and lower metal structures under a microscope, and then bonding and curing; (9) Filling with liquid crystal: Placing the liquid crystal cell after step (8) on a hot stage, heating the liquid crystal to a clearing point, and then filling it between the upper and lower quartz substrates; (10) Wire bonding: Applying anisotropic conductive adhesive to the device electrodes, and bonding the flexible electrodes and device electrodes under a hot press.

[0012] The third part of the technical solution provided by this invention is a characterization method for a transmissive liquid crystal terahertz spatial light modulator with a large on / off ratio, comprising the following steps:

[0013] (1) Connecting to the test system: controlling the host computer control program to drive the FPGA output pin voltage; fixing the device on the circuit board; interconnecting the field-programmable gate array and the device through flexible electrodes;

[0014] (2) Terahertz spectral response test: Fix the circuit board in step (1) at the center of the two-dimensional displacement stage, and adjust the position of the transceiver module of the terahertz time-domain spectroscopy system so that the transmissive terahertz spatial light modulator sample is located at the focal point of the terahertz beam; use a signal generator to uniformly power the device electrodes, and control the signal as a 1kHz square wave, with the peak-to-peak value gradually increasing from 0V to 20V, and test the transmittance at the working frequency.

[0015] (3) Spatial coding pattern test of the transmissive terahertz spatial light modulator: After the pattern is encoded by the host computer, it is converted into control commands and transmitted to the field programmable gate array. The field programmable gate array controls the loading voltage of each pixel according to the commands. The host computer controls the two-dimensional displacement stage in step (2) to move point by point and collects the time-domain pulse signal of each point. The scanning area covers the effective area of ​​the device. According to the electric field intensity distribution of the transmissive terahertz wave obtained by scanning, the intensity distribution image is obtained.

[0016] Beneficial effects:

[0017] This invention utilizes the electro-optic effect of liquid crystals to connect the corresponding electrodes of an FPGA and a metal structure. A voltage is applied between the upper and lower metal structures, and the operating frequency of the resonant structure is changed by adjusting the voltage, thereby achieving a large on / off ratio for terahertz waves at the operating frequency. The host computer commands control the FPGA output voltage, enabling programmable control of the device. In computational imaging, it simplifies the imaging optical path and improves the quality of reconstructed images. In security information encryption and steganography, it dynamically controls the transmitted light field to encode information, achieving physical-level information encryption and stealth communication. In terahertz spectral analysis and control, it can be used in tunable terahertz filters to selectively transmit terahertz waves of specific frequencies, improving the resolution and flexibility of spectrometers. The transmission-type terahertz spatial light modulator with a large on / off ratio, with its flexible control capabilities, provides an innovative solution for the development of terahertz technology, promoting advancements in imaging, security, and materials science. Attached Figure Description

[0018] Figure 1 This is a cross-sectional view of the structure of the transmissive liquid crystal terahertz spatial light modulator with a large on / off ratio according to the present invention.

[0019] Figure 2 This is a schematic diagram of the unit structure of the transmissive liquid crystal terahertz spatial light modulator with a large on / off ratio according to the present invention.

[0020] Figure 3 This is a flowchart illustrating the fabrication process of a transmissive liquid crystal terahertz spatial light modulator with a large on / off ratio according to the present invention.

[0021] Figure 4 The figure shows the simulation results of the transmission coefficient of the transmissive liquid crystal terahertz spatial light modulator with a large on / off ratio according to the present invention.

[0022] Figure 5 This is the test control logic diagram for a transmissive liquid crystal terahertz spatial light modulator with a large on / off ratio according to the present invention.

[0023] Figure 6The figure shows the spatial coding test results of the present invention used in a transmissive liquid crystal terahertz spatial light modulator with a large on / off ratio. Detailed Implementation

[0024] The present invention will be further illustrated below with reference to the accompanying drawings and specific embodiments. It should be understood that these embodiments are for illustrative purposes only and are not intended to limit the scope of the invention. After reading this invention, any modifications of the invention in various equivalent forms by those skilled in the art will fall within the scope defined by the appended claims.

[0025] This invention discloses a transmissive terahertz spatial light modulation device using liquid crystal material as a tunable medium. The cross-sectional structure of the device is shown below. Figure 1 As shown, liquid crystal fills the space between an upper substrate and a lower quartz substrate, forming a metal-dielectric-metal structure with the upper and lower metal structures. Utilizing the electro-optic effect of liquid crystal, when a bias voltage is applied between the upper and lower metal structures, the rotation of the liquid crystal director causes a change in the equivalent dielectric constant of the liquid crystal layer perpendicular to the metal plane, thereby controlling the on / off state of the transmitted wave. This invention discloses a design scheme for a transmissive liquid crystal terahertz spatial light modulator with a large on / off ratio. This invention achieves pixel-level amplitude control of terahertz waves in a two-dimensional plane, with a simple structure and flexible control method. Details are described below.

[0026] I. Design Scheme of Transmissive Liquid Crystal Terahertz Spatial Light Modulator Unit with High On / Off Ratio

[0027] This invention provides a transmissive liquid crystal terahertz spatial light modulator array with a large on / off ratio, capable of pixel-level amplitude modulation. Its overall structure is as follows: Figure 2 As shown. The upper metal structure of the metasurface unit is a four-opening square ring-reflection structure. Before power is applied, 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 operating frequency. Changes in the applied voltage cause the director of the liquid crystal molecules to rotate. 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 operating frequency, thereby reducing the transmission coefficient at that point and thus achieving the turn-off of the terahertz wave. In order to design a liquid crystal spatial light modulator unit that meets the large switching ratio transmission amplitude modulation function, the optimized structural dimensions are: p = 300 μm, l = 250 μm, l r =35μm, g=8μm, t s =400μm, t lc =50μm.

[0028] II. Fabrication of Transmissive Liquid Crystal Terahertz Spatial Light Modulator Devices with High On / Off Ratio

[0029] Based on the optimized structural dimensions and array size derived from simulation, a mask is designed, and then device samples are fabricated using micro / nano fabrication techniques. 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, immerse them in the cleaning solvent (acetone, ethanol and deionized water in sequence), clean them in an ultrasonic cleaner and then blow them dry.

[0031] (2) Metal deposition: Using a magnetron sputtering instrument, a layer of copper metal is deposited on the quartz substrate after the operation in step (1);

[0032] (3) Photoresist spin coating: Spin-coating AZ1500 onto a quartz substrate and drying it;

[0033] (4) Photolithography and development: Place the target pattern mask on the photolithography machine, align the mask pattern and the quartz substrate, and expose it. Then place the exposed substrate in the positive photoresist developer for development.

[0034] (5) Etching: An inorganic solution etching process is used to form a four-opening square ring-reverse structure in the upper and lower layers respectively;

[0035] (6) Photoresist cleaning: After etching, the upper and lower quartz substrates with metal structures are immersed in cleaning solvent to remove the photoresist on the metal structures and then dried.

[0036] (7) Orientation: Spin-coat the substrate after the operation in step (6) with polyimide reagent, dry it and place it in a friction orientation machine for friction orientation;

[0037] (8) Encapsulation: The spacer is uniformly stirred in the frame adhesive, the frame adhesive is applied to the non-patterned area of ​​the lower quartz substrate, and the upper and lower metal patterns are aligned under a microscope before being bonded and cured.

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

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

[0040] Working principle of the device transmission switch:

[0041] The device operates through a combination of the substrate Fabry-Perot effect and the metal resonance effect. When no power is applied, the frequencies of the Fabry-Perot resonance and the metal resonance are close, resulting in a high transmission coefficient and enabling terahertz wave transmission. When a voltage is applied, the rotation of the liquid crystal director alters the equivalent dielectric constant of the liquid crystal layer perpendicular to the metal plane, causing a frequency shift in the metal resonance and separating the substrate Fabry-Perot resonance from the metal resonance. This allows for the terahertz wave cutoff at the operating frequency. Simulation results... Figure 4 As can be seen, with the switching of the power-on state, the transmitted wave at the operating frequency can achieve control of a large switching ratio.

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

[0043] Figure 5 A schematic diagram of a characterization method for a transmissive liquid crystal terahertz spatial light modulator with a large on / off ratio is presented. The steps include:

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

[0045] (2) Terahertz spectral characteristics test: Fix the PCB in (1) at the center of the two-dimensional displacement stage, and adjust the position of the transceiver module of the terahertz time-domain spectroscopy system so that the transmissive terahertz spatial light modulator sample is located at the focal point of the terahertz optical path. Use a signal generator to uniformly power the device electrodes, control the voltage signal to be 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) Spatial coding pattern test of the transmissive terahertz spatial light modulator: The test system built in (2) is used. The displacement stage is moved point by point in two dimensions at the control end of the computer. At each movement, the time-domain pulse signal at the current position is collected, and the terahertz spot scanning area is ensured to cover the effective area of ​​the device. The electric field distribution on the surface of the device at the working frequency is scanned. The device electrodes are independently powered by FPGA. After the patterned pre-encoding is performed on the host computer, it is converted into ASCII code and transmitted to the FPGA through the serial port. The FPGA addresses the corresponding unit according to the instruction and applies the voltage. The terahertz time-domain spectroscopy system scans the two-dimensional plane to obtain the transmissive coding image. The scanning test results of the 16×16 array device are as follows. Figure 6 As shown.

Claims

1. A transmissive liquid crystal terahertz spatial light modulator with a large on / off ratio, characterized in that, The system 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. Both the upper and lower metal structures are composed of periodically arranged four-opening square ring-reflection structures. The liquid crystal layer fills the space between the upper and lower metal structures, forming a metal-dielectric-metal structure. The transmission modulation mechanism is as follows: when no voltage is applied between the upper and lower metal structures, due to the combination of the Fabry-Perot effect of the substrate and the resonance characteristics of the metal structure, the system exhibits an electric field transmittance greater than 80% at the operating frequency. When an external voltage is applied between the upper and lower metal structures, the electric field perpendicular to the plane of the metal structure drives the liquid crystal director to rotate, causing a change in the equivalent dielectric constant of the liquid crystal layer. This shifts the resonance of the metal structure, resulting in a transmittance close to 0 at the operating frequency.

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

3. 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 consists of a unit array, electrodes, and feed lines, with each unit array and electrode connected by a feed line.

4. The transmissive liquid crystal terahertz spatial light modulator with a large on / off ratio according to claim 3, characterized in that, Each unit array corresponds to one pixel, and the number of unit arrays corresponds to the array size 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.

5. A method for preparing the transmissive liquid crystal terahertz spatial light modulator with a large on / off ratio as described in claim 1, characterized in that, The steps include: (1) Substrate cleaning: Immerse both the upper and lower quartz substrates in cleaning solvent and place them in an ultrasonic cleaner to clean and dry; (2) Metal thin film deposition: Deposit a metal thin film on the quartz substrate after step (1) using a magnetron sputtering instrument; (3) Spin-coating photoresist: Spin-coat AZ1500 onto the quartz substrate and dry it. (4) Photolithography and development: Place the mask on the UV exposure machine, align it with the target pattern mask and the quartz substrate, and expose it. Place the exposed sample in the positive photoresist developer for development. (5) Etching: An inorganic solution etching process is used to form a four-opening square ring-reverse structure in the upper and lower layers respectively; (6) Photoresist cleaning: After etching, the upper and lower quartz substrates with metal structures are immersed in cleaning solvent to remove the photoresist on the metal structures and then dried. (7) Orientation: Spin-coat polyimide reagent onto the quartz substrate after step (6), dry it, and place it in a friction orientation machine for friction orientation; (8) Encapsulation: Stir the spacer 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 and lower metal structures under a microscope, and then bond and cure; (9) Liquid crystal injection: Place the liquid crystal cell after step (8) on a hot stage, heat the liquid crystal to a clearing point, and then fill it between the upper and lower quartz substrates; (10) Wire bonding: Apply anisotropic conductive adhesive to the device electrodes, and bond the flexible electrodes and device electrodes under a hot press.

6. A test method for characterizing the transmissive liquid crystal terahertz spatial light modulator with a large on / off ratio as described in claim 1, characterized in that, Includes the following steps: (1) Connect to 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 spectral response test: Fix the circuit board in step (1) at the center of the two-dimensional displacement stage, and adjust the position of the transceiver module of the terahertz time-domain spectroscopy system so that the transmissive terahertz spatial light modulator sample is located at the focal point of the terahertz beam; use a signal generator to uniformly power the device electrodes, and control the signal as a 1 kHz square wave, with the peak-to-peak value gradually increasing from 0V to 20V, and test the transmittance at the working frequency; (3) Spatial coding pattern test of programmable transmission terahertz spatial light modulator: After the pattern is encoded by the host computer, 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 host computer controls the two-dimensional displacement stage in step (2) to move point by point and collects the time-domain pulse signal of each point. The scanning area covers the effective area of ​​the device. According to the electric field intensity distribution of the transmission terahertz wave obtained by scanning, the intensity distribution image is obtained.

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