A terahertz dynamic beam steering coding metasurface based on vanadium dioxide phase transition

Abstract

This invention discloses a terahertz dynamic beam-tunable coded metasurface based on vanadium dioxide phase transition, relating to the fields of metasurface and terahertz wave manipulation technology. The coded metasurface consists of multiple periodically arranged units, each unit including a vanadium dioxide loop-shaped patch top layer, a silicon dioxide dielectric layer, and a copper reflective bottom layer. Localized photothermal effects are generated through external optical excitation, controlling the reversible phase transition of vanadium dioxide between an insulating state and a metallic state, corresponding to the coded "0" and "1" states, respectively. Within the 0.82–0.93 THz frequency band, the reflection amplitude of both coded states is greater than 0.8, and the reflection phase difference is stable within the range of 180°–20°, meeting the 1-bit encoding requirements. By changing the spatial arrangement of the coded sequence, dynamic deflection of the reflected terahertz beam is achieved in both one-dimensional and diagonal directions, with a relative bandwidth of 13.6%. This invention has advantages such as wide operating bandwidth, stable phase difference, and dynamic reconfigurability.

Description

Technical Field

[0001] This invention relates to the field of metasurface and terahertz wave modulation technology, and in particular to a terahertz dynamic beam modulation coded metasurface based on vanadium dioxide phase transition. Background Technology

[0002] Terahertz waves (0.1THz to 10THz) have significant application prospects in communication, imaging, and sensing. However, there is a lack of intrinsic materials in nature that are sensitive to terahertz waves, and traditional terahertz devices are large and expensive.

[0003] In recent years, electromagnetic metasurfaces have become a research hotspot due to their subwavelength structure and flexible control capabilities. In particular, coded metasurfaces can digitally manipulate electromagnetic waves by associating units with binary codes ("0" and "1"). However, once the structure of a traditional coded metasurface is fixed, its control function becomes fixed and it is difficult to achieve dynamic reconfiguration.

[0004] To overcome this bottleneck, researchers have attempted to introduce tunable materials, among which vanadium dioxide (VO2) can undergo a reversible phase transition from an insulating to a metallic state under temperature-driven conditions. Its electrical conductivity and dielectric constant change dramatically, making it suitable for constructing reconfigurable metasurfaces.

[0005] There are already some terahertz coded metasurfaces based on VO2, but they generally suffer from narrow effective operating bandwidth and poor phase modulation stability.

[0006] Therefore, there is an urgent need for a terahertz coded metasurface with wider bandwidth, stable phase difference, and the ability to achieve dynamic beam control. Summary of the Invention

[0007] To address the aforementioned issues, this application proposes a terahertz dynamic beam-controlled coded metasurface based on vanadium dioxide phase transition, which solves the problems of narrow operating bandwidth, poor phase stability, and insufficient dynamic control capability in the prior art.

[0008] Terahertz dynamic beam-controlled coded metasurfaces achieve dynamic switching between encoded "0" and "1" at different temperatures, including... The number of metasurface units is positive. It is a positive number; The metasurface unit comprises a top layer, a dielectric layer, and a bottom layer, from top to bottom. The top layer is a square-shaped patch of vanadium dioxide.

[0009] Preferably, the top layer undergoes a reversible transition between an insulating state and a metallic state under external temperature control, corresponding to codes "0" and "1" respectively.

[0010] The metasurface unit is in an insulating state at room temperature, defined as the coded "0" state; when the temperature rises above 68°C, VO2 transforms into a metallic state, defined as the coded "1" state; by generating a local photothermal effect through external optical excitation (such as an infrared laser beam shaped by a spatial light modulator), the temperature of each unit can be independently controlled, thereby forming arbitrary coded patterns on the array.

[0011] Preferably, the dielectric layer is a silicon dioxide substrate with a square structure; The bottom layer is a copper reflective substrate with a square structure.

[0012] The outer ring of the vanadium dioxide square patch has a length and width of 110 μm, the inner ring has a length and width of 56 μm, and a thickness of 2 μm. The silicon dioxide substrate has a length and width of 140 μm and a thickness of 40 μm; The copper reflective substrate has a length and width of 140 μm and a thickness of 2 μm.

[0013] The structural parameters of the unit are: period P = 140 μm, outer side length L1 = 110 μm, and inner side length L2 = 56 μm for the top VO2 patch.

[0014] VO2 has a relative permittivity of 9 and a conductivity of 200 S / m in its insulating state; in its metallic state, its permittivity is described by the Drude model. (1); in , Let be the angular frequency of the incident electromagnetic wave. Indicates the dielectric constant of the material. For plasma frequency, For collision frequency, Let be the conductivity of metallic VO2, in equation (1) And satisfy , , , .

[0015] The metasurface achieves the following beam modulation functions within the 0.82–0.93 THz frequency band by altering the spatial arrangement of the coded sequence: Preferably, at 0.82 Within the 0.93THz frequency band, the reflection amplitudes of both the coded "0" state and the coded "1" state are greater than 0.8, and the reflection phase difference is 180°. 20°.

[0016] When the encoded sequence is all "0"s or all "1"s, the reflected beam is along the normal direction of the metasurface ( =0°); When the coded sequence is arranged periodically along the x-axis or y-axis in the pattern "00110011...", two symmetrical reflected beams are generated, with a theoretical deflection angle of [value missing]. =36°, the actual simulation value is approximately 33°~36°; When the encoded sequence is arranged along the array diagonal (with "00110011..." introduced simultaneously in both x and y directions), a larger deflection angle is produced, theoretically... =56°, the simulated value is approximately 54°.

[0017] Within the operating frequency band, the reflection amplitudes of both the encoded "0" and "1" states are greater than 0.8, and the reflection phase difference remains stable at 180°. Within a 20° range, it meets the 1-bit encoding requirement.

[0018] Preferably, the relative bandwidth of the operating frequency band is 13.6%.

[0019] Application of a coded metasurface in terahertz communication, imaging, or radar beam scanning.

[0020] In summary, the terahertz dynamic beam-controlled coded metasurface based on vanadium dioxide phase transition of the present invention has the following advantages compared with traditional technologies: 1. A "U"-shaped patch unit was designed by utilizing the reversible phase transition of VO2 between the insulating and metallic states. This unit can achieve dynamic switching between encoding "0" and "1" at different temperatures, meeting the 1-bit encoding requirement. 2. Wide operating bandwidth: The reflection amplitude in both states is greater than 0.8 in the 0.82-0.93THz frequency band, with a relative bandwidth of 13.6%, which is much higher than most existing terahertz coded metasurfaces based on VO2 (usually point frequency or narrow band). 3. Stable phase difference: The reflection phase difference between the two encoding states is within 180°. Within 20°, coding accuracy and beamforming quality are ensured; 4. Dynamically reconfigurable: Utilizing the phase transition characteristics of VO2, the coding pattern can be changed in real time through external photothermal excitation, realizing dynamic control of the beam deflection angle without changing the physical structure; 5. Simple structure: Only three layers, easy to process, and compatible with standard photolithography and thin film deposition processes.

[0021] The technical method of the present invention will be further described in detail below with reference to the accompanying drawings and embodiments. Attached Figure Description

[0022] Figure 1 This is a schematic diagram of the structure of the coded metasurface of the present invention. Figure 1 (a) is a schematic diagram of the overall functions; Figure 1 (b) shows the structure of the encoding "0" unit; Figure 1 (c) represents the structure of the encoding "1" unit; Figure 2 This is the reflection characteristic curve of the unit of the present invention. Figure 2 (a) shows the reflection amplitude diagrams of the "0" and "1" cells of the metasurface; Figure 2 (b) represents the reflection phase; Figure 3 Three-dimensional far-field plots and radiation patterns encoded with all "0"s and all "1"s. Figure 3 Image (a) is a three-dimensional far-field image with all "1" encoding. Figure 3 Image (b) is a three-dimensional far-field image with all "0" encoding. Figure 3 (c) shows the far-field radiation pattern with all "0" encoding. Figure 3 (d) is the far-field radiation pattern of all-"1" encoding; Figure 4 Three-dimensional far-field plots and two-dimensional scattering plots for the "00110011" encoded sequence in different directions. Figure 4 (a) is a three-dimensional far-field plot of the "00110011" encoding in the x-direction. Figure 4 In the middle (d), the corresponding two-dimensional scattering map (x-direction encoding) is shown. Figure 4 Image (b) is a three-dimensional far-field image of the "00110011" encoding in the y-direction. Figure 4 In the middle (e), the corresponding two-dimensional scattering map (y-direction encoding) is shown. Figure 4 Image (c) is a 3D far-field image of the "00110011" encoding along the diagonal direction. Figure 4 In the middle (f) is the corresponding two-dimensional scattering map (diagonal direction encoding); Figure 5 The diagram shows the effect of modulation on the "001100" encoded sequence at different frequency points (0.82THz, 0.93THz). Figure 5 (a) is a three-dimensional far-field plot of the "001100" encoding in the x-direction. Figure 5 (b) shows the two-dimensional electric field diagram of the “001100” encoded sequence in the x-direction at 0.82 THz. Figure 5 Image (c) shows the two-dimensional electric field diagram of the "001100" encoded sequence in the x-direction at 0.93 THz. Figure 5 The middle (d) image is a three-dimensional far-field plot of the "001100" encoding in the y-direction. Figure 5 Image (e) shows the two-dimensional electric field diagram of the “001100” encoded sequence in the y-direction at 0.82 THz. Figure 5 (f) is a two-dimensional electric field diagram of the “001100” encoded sequence in the y direction at 0.93 THz. Detailed Implementation

[0023] The technical method of the present invention will be further described below with reference to the accompanying drawings and embodiments. It should be noted that, unless otherwise specifically stated, the relative arrangement, numerical expressions, and values ​​of the components and steps described in these embodiments do not limit the scope of this application.

[0024] The following description of at least one exemplary embodiment is merely illustrative and is in no way intended to limit the scope of this application and its application or use.

[0025] Techniques, systems, and equipment known to those skilled in the art may not be discussed in detail, but where appropriate, they should be considered part of the instruction manual.

[0026] In all the examples shown and discussed herein, any specific values ​​should be interpreted as merely exemplary and not as limitations. Therefore, other examples of exemplary embodiments may have different values.

[0027] Unless otherwise defined, the technical or scientific terms used in this invention shall have the ordinary meaning as understood by one of ordinary skill in the art to which this invention pertains.

[0028] Example 1 The terahertz coded metasurface designed in this application is as follows: Figure 1 As shown, Figure 1 (a) shows a schematic diagram of beam control functionality achieved by a 16×16 coded metasurface.

[0029] An infrared laser beam is shaped into a specific optical field distribution using external optical excitation and projected onto a phase change material metasurface. The photothermal effect induces a localized, patterned phase transition in the material. The conductivity of vanadium dioxide (VO2) is changed by controlling the temperature of the coded metasurface array units. By utilizing the reversible phase transition between the insulating and metallic states of VO2, different coded sequences are realized, thus completing the corresponding beam control function.

[0030] Figure 1 (b) and (c) represent the two states of the encoded metasurface unit structure: encoded "0" and encoded "1," corresponding to the vanadium dioxide insulating state and metallic state, respectively. The unit consists of a top layer of VO2, an intermediate dielectric layer of silicon dioxide, and a bottom layer of copper.

[0031] Its unit cell has a period of P=140μm and dimensions of L1=110μm and L2=56μm.

[0032] VO₂ exists in an insulating state at room temperature, with a relative permittivity of 9 and a conductivity of 200 S / m. When the temperature rises to 68℃, VO₂ transforms into a metallic state, and its permittivity can be described by the Drude model. (1); in , Let be the angular frequency of the incident electromagnetic wave. Indicates the dielectric constant of the material. For plasma frequency, For collision frequency, Let be the conductivity of metallic VO2, in equation (1) And satisfy , , , .

[0033] Theoretical analysis is conducted on the arrangement of metasurface coding sequences under different functions.

[0034] The functionality of a coded metasurface relies on the spatial arrangement of a coded sequence. By constructing a coded sequence with a specific phase difference, directional beam control can be achieved.

[0035] To achieve a along For beams to deflect in direction, a linear phase gradient needs to be introduced into the metasurface: (2); in , This is the phase gradient vector. And... , ,in is the free space wavenumber.

[0036] For a one-dimensional deflecting beam (along the x-direction), ), ,therefore, ,in, and Phase changes along the x and y directions, respectively. The length of the encoding cycle.

[0037] If two symmetrical deflection beams are generated and It is necessary to construct two metasurface phase distributions. , .

[0038] The result is obtained by superimposing the complex amplitudes of the two metasurfaces: (3); in, , , , The total phase distribution of the metasurface is: (4); This application uses a 1-bit metasurface to achieve dual-refraction beams. is the reflection phase of a single cell. For a 1-bit coded metasurface, there are only 0 or two phases (“0” and “ ” refers to the phase difference between two cells being ). For cell 0, ; for cell 1, .

[0039] For the actual phase distribution corresponding to the center of a certain cell of the metasurface the above continuous phase needs to be quantized and sampled: (5); where , .

[0040] (6); According to the above theoretical derivation, for the all-“0” coding sequence “00000000”, , , the pitch angle the theoretical value is 0°, that is, the reflected beam is along the z-axis direction.

[0041] For the one-dimensional coding sequence “00110011” that changes along the x direction, adjacent “0” and “1” cells form a phase change period. That is , , calculated , the pitch angle the theoretical value is a reflected beam of 36°.

[0042] Embodiment 2 In this embodiment, full-wave simulation is used to analyze and verify the cell performance and the beam steering effect of different coding sequences.

[0043] To verify the performance of the designed cell, Figure 2 (a) and (b) respectively show the reflection amplitude and phase of the “0” cell and the “1” cell. In the frequency band of 0.82 - 0.93 THz, the reflection amplitude of the “return” shaped patch cell in the coding “0” and coding “1” states is greater than 0.8, and the reflection phase difference is within the range of 180 ± 20°, meeting the 1-bit coding requirements and can be used for beamforming technology.

[0044] The designed coded metasurface was simulated and verified using a 16×16 array structure. First, the reflection characteristics of all-"0" encoding and all-"1" encoding were analyzed. Since the encoding periods in both the x and y directions tend to infinity in both encoding states, theoretically only a single reflected beam is generated along the z-axis. Figure 3 As shown in (a) and 3(b), the simulation results are consistent with the theoretical expectations. This indicates that the full coding states of both "0" and "1" cells have good directionality, laying the foundation for subsequent beam splitting coding.

[0045] Example 3 Based on the verification of the unit phase difference, the ability of one-dimensional coding to control the beam is further studied.

[0046] Arrange the "0" and "1" units along the x-direction in the sequence 00110011, and you will get the following: Figure 4 As shown in (a), it is clear that the incident terahertz wave is split into two symmetrical reflected waves. Its two-dimensional scattering diagram is as follows: Figure 4 As shown in (d), the azimuth angles of the two reflected beams are 90° and 270°, and the elevation angles are 33° and 328°. The deflection angle can be approximately calculated using a formula. The theoretical value is 36°, and the theoretical calculation and simulation results are approximately the same. Then, arranging the "0" and "1" elements along the y-direction in the sequence 00110011, we obtain the following... Figure 4 (b) shows the three-dimensional far-field plot. Its two-dimensional scattering plot is as follows: Figure 4 As shown in (e), the azimuth angles of the two reflected beams are 0° and 180°, and their elevation angles are 32° and 333°. The theoretical value of the elevation angle is 36°, and the simulation results are close to the theoretical calculations.

[0047] Example 4 The sequence “00110011…” is arranged simultaneously along the x and y directions, forming a checkerboard pattern. Simulation results show two symmetrical beams with azimuth angles of 45° and 225° and an elevation angle of 54° (theoretically 56°), achieving a larger deflection angle.

[0048] like Figure 4 (c) shows the three-dimensional far-field plot of the encoded metasurface. Due to the simultaneous increase in the encoding period in both the x and y directions, this encoding method achieves a larger beam deflection angle. The corresponding two-dimensional scattering plot is shown below. Figure 4 As shown in (f), the azimuth angles of the two reflected beams are 45° and 225°, their elevation angles are 54° and 306°, and their deflection angle is... The theoretical value is 56°, and the theoretical calculation and simulation results are approximately the same.

[0049] Example 5 Simulations were performed at 0.82 THz and 0.93 THz to verify the beam modulation effect of the coded metasurface at different frequency points. Figure 5 Image (a) is a three-dimensional far-field image of the "001100" encoding in the x-direction. Figure 5 (b) and (d) are two-dimensional electric field diagrams of the “001100” encoded sequence in the x-direction at 0.82THz and 0.93THz. Figure 5 (d) is the three-dimensional far-field plot of the "001100" encoding in the y direction. Figure 5 (e) and (f) are two-dimensional electric field diagrams of the “001100” encoded sequence in the y direction at 0.82THz and 0.93THz.

[0050] It can be seen that the beam modulation effects produced by the two coding sequences show approximately similar beam deflection angles at the two frequency points, indicating that the present invention has stable beam modulation performance within the operating frequency band. In the 0.82–0.93 THz frequency range, the relative bandwidth of this application reaches 13.6%. By changing the spatial arrangement sequence of the coded metasurface, flexible and controllable beam modulation (beam deflection) functions can be achieved. Compared with existing research, the coded metasurface proposed in this application has a significant advantage in terms of operating bandwidth.

[0051] In summary, this application designed and simulated a dynamically beam-tuned terahertz coded metasurface based on the VO2 phase transition characteristics, operating in the 0.82–0.93 THz frequency band. By controlling the coding sequence, the function of dynamically controlling the beam deflection angle was realized, verifying its ability to flexibly manipulate the wavefront over a wide bandwidth and demonstrating excellent broadband beamforming capabilities. Compared with existing coded metasurfaces, this design significantly expands the operating bandwidth while maintaining dynamic control capabilities, providing a new implementation path for applications such as terahertz communication, high-resolution imaging, and intelligent sensing.

[0052] Finally, it should be noted that the above embodiments are only used to illustrate the technical methods of the present invention and not to limit them. Although the present invention has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can still be made to the technical methods of the present invention, and these modifications or equivalent substitutions cannot cause the modified technical methods to deviate from the spirit and scope of the technical methods of the present invention.

Claims

1. A terahertz dynamic beam-tunable coded metasurface based on vanadium dioxide phase transition, characterized in that, The terahertz dynamic beam-controlled coded metasurface achieves dynamic switching between encoding "0" and "1" at different temperatures, including... The number of metasurface units is positive. It is a positive number; The metasurface unit comprises a top layer, a dielectric layer, and a bottom layer, from top to bottom. The top layer is a square-shaped patch of vanadium dioxide.

2. The terahertz dynamic beam-tuned coded metasurface based on vanadium dioxide phase transition according to claim 1, characterized in that, The top layer undergoes a reversible transition between an insulating state and a metallic state under external temperature control, corresponding to codes "0" and "1" respectively; By changing the spatial arrangement of the coding sequence, dynamic deflection of the reflected terahertz beam can be achieved in the 0.82~0.93THz frequency band.

3. The terahertz dynamic beam-tuned coded metasurface based on vanadium dioxide phase transition according to claim 1, characterized in that, At 0.82 Within the 0.93THz frequency band, the reflection amplitudes of both the coded "0" state and the coded "1" state are greater than 0.8, and the reflection phase difference is 180°. 20°.

4. The terahertz dynamic beam-tuned coded metasurface based on vanadium dioxide phase transition according to claim 1, characterized in that, When the encoded sequence is all "0"s or all "1"s, the reflected beam is along the normal direction of the metasurface.

5. The terahertz dynamic beam-tuned coded metasurface based on vanadium dioxide phase transition according to claim 1, characterized in that, When the coded sequence is arranged periodically along the x or y direction in the pattern "00110011...", two symmetrical reflection beams are generated with a deflection angle of 33°~36°. When the coded sequence is arranged in the order "00110011..." along both the x and y directions, a reflected beam with a deflection angle of 54° to 56° is generated.

6. The terahertz dynamic beam-tuned coded metasurface based on vanadium dioxide phase transition according to claim 1, characterized in that, The relative bandwidth of the operating frequency band is 12%~15%.

7. The terahertz dynamic beam-tuned coded metasurface based on vanadium dioxide phase transition according to claim 1, characterized in that, The dielectric layer is a silicon dioxide substrate with a square structure. The bottom layer is a copper reflective substrate with a square structure.

8. The terahertz dynamic beam-tuned coded metasurface based on vanadium dioxide phase transition according to claim 1, characterized in that, The outer ring of the vanadium dioxide square patch has a length and width of 110 μm, the inner ring has a length and width of 56 μm, and a thickness of 2 μm. The silicon dioxide substrate has a length and width of 140 μm and a thickness of 40 μm; The copper reflective substrate has a length and width of 140 μm and a thickness of 2 μm.

9. The application of a coded metasurface as described in any one of claims 1 to 7 in terahertz communication, imaging, or radar beam scanning.

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