Terahertz programmable metasurface with nonvolatile polarization regulation and control function

Abstract

The invention discloses a terahertz programmable metasurface with a non-volatile polarization regulation and control function. The metasurface is composed of a plurality of pixels, each pixel forms an adjustable half-wave plate, programmable polarization rotation can be achieved between different linear polarization states, and the metasurface keeps preset spatial polarization distribution under the action of no continuous external field by means of the phase change hysteresis characteristic of materials in the pixels. The metasurface comprises a sapphire substrate, a metal resonant structure and a vanadium dioxide microbridge which are formed on the upper surface of the substrate, and a metal reflecting layer on the lower surface of the substrate, and each pixel can realize independent electric addressing to form a programmable array. The metasurface has the advantages of low power consumption, high flexibility and the like, and can be widely applied to terahertz polarization imaging, communication and information processing.

Description

Technical Field

[0001] This invention belongs to the field of terahertz metasurface technology, specifically relating to a terahertz programmable metasurface based on vanadium dioxide, and more particularly to a terahertz programmable metasurface that utilizes vanadium dioxide microbridges as thermistor units to achieve non-volatile polarization control, as well as a method for preparing the terahertz programmable metasurface. Background Technology

[0002] Terahertz waves have important applications in wireless communication, imaging, radar, and sensing, and their polarization states carry rich information. By effectively controlling the polarization of terahertz waves, polarization imaging can be achieved to obtain structural and material information that traditional intensity imaging cannot provide, thereby improving imaging resolution and target recognition capabilities. In communication systems, polarization state manipulation can be used for multiplexing, enhancing channel capacity and anti-jamming capabilities; in radar and sensing applications, it can improve detection accuracy and enhance environmental adaptability. Therefore, efficient and controllable terahertz polarization manipulation is of great significance for the development of next-generation high-performance information systems.

[0003] Traditional terahertz polarization control devices typically rely on multilayer structures or natural birefringent materials, which suffer from fixed functions, large size, high losses, and difficulty in achieving dynamic tunability. Metasurfaces, with their powerful electromagnetic control capabilities, have been widely used in the development of terahertz polarization control devices in recent years. Early metasurface polarization converters mostly relied on planar asymmetric metal structures, achieving only single polarization conversion and lacking dynamic control capabilities. By introducing tunable materials to construct reconfigurable metasurfaces, devices can switch polarization states, but pixel-level independent control remains challenging. Currently, programmable metasurfaces with polarization control capabilities are still relatively scarce, and traditional programmable devices usually require continuous external excitation, leading to complex control circuits and limiting their application in large-scale arrays. Therefore, there is an urgent need for a terahertz polarization control device with non-volatile characteristics and a flexibly programmable spatial polarization distribution. Summary of the Invention

[0004] Technical problem: In view of the shortcomings of the prior art, the present invention proposes a terahertz programmable metasurface with non-volatile polarization control function. This metasurface can generate arbitrary terahertz spatial polarization patterns and maintain the set state without continuous external action, effectively solving the problem of existing devices having single function and difficulty in achieving pixel-level control.

[0005] Technical Solution: The first technical solution proposed in this invention is a terahertz programmable metasurface with non-volatile polarization control function. The terahertz programmable metasurface is composed of multiple independently controllable pixels, each pixel constituting an adjustable half-wave plate for programmable switching between different linear polarization states. Through the phase transition hysteresis characteristics of the material inside the pixel, the entire terahertz programmable metasurface maintains a preset spatial polarization distribution without the action of a continuous external field.

[0006] Furthermore, the terahertz programmable metasurface includes a sapphire substrate, a metal and vanadium dioxide microbridge structure formed on the upper surface of the sapphire substrate, and a metal reflective layer formed on the lower surface of the sapphire substrate.

[0007] Furthermore, a thermally conductive interface material is provided between the metal reflective layer and the sapphire substrate to enhance thermal coupling.

[0008] Furthermore, the metal and vanadium dioxide microbridge structure includes a metal structure forming a double-opening circular ring array and vanadium dioxide microbridges. The two openings of each ring are connected by vanadium dioxide microbridges, and the upper and lower metal plates of the rings are electrically connected to external metal electrodes for applying bias.

[0009] Furthermore, the conductivity of each vanadium dioxide microbridge can be adjusted by applying current or Joule heating generated by an external heat source, thereby achieving the switching of terahertz wave polarization states.

[0010] Furthermore, when a vanadium dioxide microbridge in a pixel is biased to the phase transition hysteresis region, it is written one by one by applying a current pulse to achieve independent control of the pixel; after the current pulse is removed, each pixel maintains the set polarization state, thereby forming the desired spatial polarization pattern.

[0011] The second technical solution provided by the present invention is a method for preparing a terahertz programmable metasurface as described above, comprising the following steps: (1) depositing a vanadium dioxide thin film on a sapphire substrate; (2) forming a periodic vanadium dioxide microbridge structure on the vanadium dioxide thin film; (3) preparing a metal resonant structure on the upper surface of the sapphire substrate and electrically connecting it with the vanadium dioxide microbridge, while forming control leads for each pixel; (4) forming a metal reflective layer on the lower surface of the sapphire substrate.

[0012] Beneficial effects: This invention achieves precise and programmable control of terahertz wave polarization state through pixel-level independent control. Each pixel maintains its set polarization state even after the external control signal is removed, exhibiting non-volatility. Simultaneously, the device has a compact structure and high integration, enabling complex spatial polarization distributions, providing a new technical means for terahertz communication, imaging, sensing, and other related applications. Attached Figure Description

[0013] Figure 1 This is a schematic cross-sectional view of the unit structure of the metasurface of the present invention;

[0014] Figure 2 This is a three-dimensional schematic diagram of the unit structure of the metasurface of the present invention;

[0015] Figure 3 This is a schematic diagram of the upper surface structure of a single pixel in the metasurface of the present invention;

[0016] Figure 4 A schematic diagram of the laboratory preparation process of the sample of this invention;

[0017] Figure 5 (a) in the figure is a schematic diagram of the polarization conversion rate test of the sample of the present invention at room temperature and high temperature. Figure 5 (b) in the figure represents the cross-polarization reflection coefficient curve of the sample of the present invention. Figure 5 (c) in the figure represents the hysteresis curve of the reflection coefficient during the heating and cooling process;

[0018] Figure 6 This is a schematic diagram of the test system for characterizing the metasurface polarization conversion performance of the present invention;

[0019] Figure 7 This is a schematic diagram of the polarization conversion performance test results of the sample of this invention. Detailed Implementation

[0020] 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.

[0021] I. Structural Design Scheme of Terahertz Programmable Metasurface with Non-Volatile Polarization Control Based on Vanadium Dioxide

[0022] This invention designs and fabricates a terahertz programmable metasurface with non-volatile polarization control based on the phase change material vanadium dioxide. The metasurface operates at a frequency in the terahertz band and consists of 8×8 pixels, with each pixel containing 12×12 units.

[0023] like Figure 1 As shown, the programmable metasurface mainly consists of a sapphire substrate 3, a metal 1 and vanadium dioxide microbridge 2 structure formed on the upper surface of the sapphire substrate 3, and a metal reflective layer 4 formed on the lower surface of the sapphire substrate 3.

[0024] like Figure 2As shown, the metal 1 structure employs a double-opening circular ring array made of gold, with the two openings of each ring connected by vanadium dioxide microbridges 2. The metal reflective layer 4 on the lower surface of the sapphire substrate 3 is also made of gold, effectively blocking terahertz wave transmission and enabling the metasurface to operate in a reflective manner. The metal ground layer on the lower surface of the sapphire substrate is also made of gold, effectively blocking terahertz wave transmission and making the metasurface a reflective device. The specific parameters of the unit are as follows: P = 120 μm, R1 = 56 μm, W = 10 μm, a = 15 μm, U = 10 μm, h1 = 0.2 μm, h2 = 150 μm, h3 = 0.2 μm.

[0025] like Figure 3 As shown, the present invention simultaneously uses series and parallel methods to connect the units in the pixel to each other, and applies a current pulse to the pixel to cause the pixel to generate a Joule heating-driven vanadium dioxide phase transition in the modulation unit, thereby changing the symmetry of the unit structure and achieving a non-volatile polarization conversion effect.

[0026] II. A method for preparing a terahertz programmable metasurface with non-volatile polarization control based on vanadium dioxide, wherein sample preparation and processing are carried out according to the following steps, such as... Figure 4 As shown:

[0027] (1) Vanadium dioxide thin film deposition: A vanadium dioxide thin film of about 200 nm thickness was grown on a sapphire substrate by magnetron sputtering.

[0028] (2) Formation of vanadium dioxide microbridge structure: AZ1500 photoresist was spin-coated onto the deposited vanadium dioxide film. The spin coater parameters were: pre-rotation speed 600 rpm for 6 seconds, stable rotation speed 4000 rpm for 40 seconds, and then baked at 90°C for 5 minutes on a heating stage. The sample was placed on the photolithography stage and exposed to ultraviolet light using a microbridge array mask for 7 seconds. The sample was then developed in a positive photoresist developer for 17 seconds to form a vanadium dioxide microbridge photoresist pattern. The vanadium dioxide film not covered by the photoresist was removed using reactive ion etching (RIE) with a CF4 flow rate of 40 sccm, a pressure of 3 Pa, an RF power of 100 W, and an etching time of 3 minutes. After etching, residual photoresist was removed with acetone, and then the film was cleaned with alcohol and deionized water and dried to obtain a periodic vanadium dioxide microbridge structure.

[0029] (3) Deposition and electrical connection of metal resonant structure: Two layers of photoresist were spin-coated on the substrate with vanadium dioxide microbridges: the first layer was LOR10B, with a pre-rotation speed of 600 rpm for 6 seconds and a stable rotation speed of 4000 rpm for 40 seconds, and baked at 150°C for 5 minutes; the second layer was AZ1500, with a pre-rotation speed of 600 rpm for 6 seconds and a stable rotation speed of 4000 rpm for 40 seconds, and baked at 90°C for 5 minutes; UV exposure was performed on the spin-coated substrate to align the photoresist pattern with the double-opening circular ring metal structure mask, and then developed to form a template for metal deposition; a 200 nm thick gold film was deposited on the photoresist template by magnetron sputtering to electrically connect the metal structure with the vanadium dioxide microbridges and form control leads for each pixel; the sample was then immersed in N-methylpyrrolidone for 36-48 hours, and then gently shaken for 3-5 minutes to peel off the photoresist and the attached gold film to obtain a complete metal double-opening circular ring array.

[0030] (4) Metal reflective layer formation: A metal thin film is deposited on the lower surface of the sapphire substrate as a ground layer to block the transmission of terahertz waves.

[0031] III. Working principle of terahertz programmable metasurface with non-volatile polarization control function

[0032] The terahertz programmable metasurface consists of a metal structure on the upper surface, vanadium dioxide microbridges, a sapphire substrate, and a metal reflective layer on the lower surface, forming a reflective resonant unit that can achieve polarization control of terahertz waves. When a linearly polarized wave is incident, due to the geometric anisotropy of the double-opening metal ring, the two orthogonal components experience different phase delays during reflection. By precisely designing the structural parameters, the phase difference between the two components can be made close to 180°, thereby achieving the rotation of the linearly polarized state of the reflected wave and completing the polarization conversion function.

[0033] IV. Characterization Methods for Terahertz Programmable Metasurfaces with Non-Vacuum Polarization Control Based on Vanadium Dioxide

[0034] like Figure 6 The diagram shows a schematic of the experimental system for the metasurface. The testing and characterization process includes the following four steps:

[0035] (1) Test system connection

[0036] The sample is fixed on the circuit board and connected to an external power supply and control module via flexible wires to achieve independent power control of each pixel electrode. The sample is placed on a controllable heating stage for precise temperature adjustment.

[0037] (2) Terahertz Spectral Response Test

[0038] The circuit board was fixed at the center of the two-dimensional displacement stage. The transmitter and receiver modules of the terahertz time-domain spectroscopy system were adjusted to position the sample at the focal point of the terahertz beam. The sample temperature was controlled by the heating stage, gradually increasing from 25°C to 95°C, and then gradually decreasing from 95°C to 25°C. Under x-polarized incident conditions, such as... Figure 5 As shown in (a), the polarization conversion efficiency of the device at different temperatures is calculated based on the measured same-polarization and reverse-polarization signals. Figure 5 As shown in (b), the same-polarization and cross-polarization reflection coefficients of the sample are measured at the operating frequency (e.g., 0.43 THz) to characterize the polarization modulation performance of the device. Figure 5 As shown in (c), since the conductivity of vanadium dioxide lags with temperature changes during heating and cooling, the device's reflection coefficient exhibits hysteresis characteristics as it changes with temperature. (3) Programmable polarization control

[0039] The sample is typically heated to a bias temperature (e.g., 65°C) to place the vanadium dioxide microbridges in the phase transition hysteresis region. According to a preset spatial polarization distribution pattern, a corresponding current pulse (approximately 20 mA, pulse duration 10 s) is applied to each pixel electrode to drive changes in the microbridge conductivity, thereby achieving the desired pixel-level polarization state adjustment and completing the programmable control of the spatial polarization distribution.

[0040] (4) Polarization conversion image measurement

[0041] The reflected terahertz light field was acquired point-by-point using a two-dimensional displacement stage, covering the effective area of ​​the sample, and the time-domain pulse signal at each location was recorded. The scan was performed twice: in the first scan, the polarization of the transmitting and receiving antennas remained consistent, with an angle of 45° to the axis of symmetry of the double-opening circular ring; in the second scan, the polarization of the transmitting antenna remained unchanged, while the polarization of the receiving antenna was rotated by 90°. Based on the data from the two scans, the polarization state of the reflected wave at each location was calculated, generating a programmable polarization conversion image, thereby verifying the device's spatial polarization control capability and non-volatility characteristics.

[0042] like Figure 7 The polarization conversion test results shown demonstrate the signal in different polarization directions at a bias temperature. Each pixel of the sample is independently powered according to the letter "J". A current pulse is applied to the required pixel for a certain period of time and then the power is removed to obtain complementary letter "J" shapes, which shows the polarization conversion effect and the non-volatile properties of vanadium dioxide.

Claims

1. A terahertz programmable metasurface with non-volatile polarization control function, characterized in that, The terahertz programmable metasurface is composed of a plurality of independently controllable pixels, each pixel constituting a tunable half-wave plate for realizing programmable conversion between different linear polarization states; through the phase transition hysteresis characteristics of the internal material of the pixel, the entire terahertz programmable metasurface maintains a preset spatial polarization distribution under the action of no continuous external field.

2. The THz programmable metasurface with non-volatile polarization control function according to claim 1, wherein, The terahertz programmable metasurface includes a sapphire substrate, a metal and vanadium dioxide micro-bridge structure formed on the upper surface of the sapphire substrate, and a metal reflection layer formed on the lower surface of the sapphire substrate.

3. The THz programmable metasurface with non-volatile polarization control function according to claim 1, wherein, A heat-conducting interface material is arranged between the metal reflection layer and the sapphire substrate for enhancing thermal coupling.

4. The THz programmable metasurface with non-volatile polarization control function according to claim 2, wherein, The metal and vanadium dioxide micro-bridge structure includes a metal structure forming a double-opened circular ring array and a vanadium dioxide micro-bridge, two openings of each circular ring are connected by a vanadium dioxide micro-bridge respectively, and the upper and lower metal sheets of the circular ring are electrically connected with external metal electrodes respectively for applying bias.

5. The THz programmable metasurface with non-volatile polarization control function according to claim 4, wherein, The conductivity of each vanadium dioxide micro-bridge can be adjusted by applying current or Joule heat generated by an external heat source, thereby realizing switching of the polarization state of terahertz waves.

6. The THz programmable metasurface with non-volatile polarization control function according to claim 4, wherein, When the vanadium dioxide micro-bridge in a certain pixel is biased to the phase transition hysteresis region, independent control of the pixel is realized by applying current pulses one by one; after the current pulse is removed, each pixel maintains the set polarization state, thereby forming a required spatial polarization pattern.

7. A method of fabricating a terahertz programmable metasurface as claimed in any one of claims 1 to 5, characterized in that, The method includes the following steps: (1) depositing a vanadium dioxide film on a sapphire substrate; (2) forming a periodic vanadium dioxide micro-bridge structure on the vanadium dioxide film; (3) preparing a metal resonant structure on the upper surface of the sapphire substrate and realizing electrical connection with the vanadium dioxide micro-bridge, while forming a control lead of each pixel; (4) forming a metal reflection layer on the lower surface of the sapphire substrate.

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