A programmable transreflective terahertz metasurface based on vanadium dioxide

Abstract

The application provides a programmable transmissive and reflective terahertz metasurface based on vanadium dioxide, which comprises a plurality of metasurface units arranged in a periodic array, the metasurface unit comprises a first metal structure layer, a dielectric layer and a second metal structure layer stacked in sequence, the first metal structure layer comprises a first metal double-split ring and a first metal control element, the double-split ring is composed of two metal rings arranged coaxially and having symmetric notches, and the notches of the two rings are 90 degrees different; the control element is arranged in the notch of the inner ring and is in contact with the inner wall of the outer ring, the second metal structure layer comprises a second metal double-split ring and a second metal control element, and the phase change of vanadium dioxide is controlled through external heat or light excitation, so that the switching of the metasurface unit between the reflection mode and the transmission mode can be realized; in the reflection mode, the transmission phase coding is realized by adjusting the opening angle of the first metal ring and rotating the first metal structure layer; and in the transmission mode, the geometric phase coding is realized by synchronously rotating the angles of the first metal structure layer and the second metal double-split ring.

Description

Technical Field

[0001] This invention belongs to the field of terahertz device technology, specifically relating to a programmable transmissive and reflective terahertz metasurface based on vanadium dioxide. Background Technology

[0002] Terahertz waves, due to their unique spectral position, have enormous application potential in next-generation high-speed wireless communication, high-resolution imaging, and sensing. How to efficiently and flexibly control the terahertz wavefront is a core issue driving the development of related technologies. Metasurfaces, as two-dimensional planar materials composed of subwavelength artificial microstructure units, can precisely control the amplitude, phase, and polarization characteristics of electromagnetic waves, and have advantages such as low profile, low loss, and easy integration.

[0003] In recent years, the concept of digitally coded metasurfaces has opened up new avenues for the intelligent control of electromagnetic waves. To further enhance the functionality and flexibility of metasurfaces, researchers are attempting to introduce tunable materials (such as liquid crystals and graphene, phase-change materials) into metasurface design. Vanadium dioxide, as a typical material with strongly correlated electron systems, can undergo a reversible phase transition from an insulating to a metallic state under external light, heat, or electrical excitation, with its conductivity changing by up to four orders of magnitude. Utilizing the phase-change properties of vanadium dioxide, active metasurface devices with switchable functions can be designed.

[0004] However, most existing tunable metasurfaces are single-function, typically only capable of achieving a single wavefront control function (such as beam deflection or vortex beam generation) in either reflection or transmission modes, and it is difficult to achieve dynamic control of multiple modes and multiple degrees of freedom simultaneously in the same structure. Therefore, there is an urgent need for a programmable terahertz metasurface that can achieve transmission / reflection mode switching and integrate multiple wavefront control functions. Summary of the Invention

[0005] The embodiments of the present invention aim to at least solve one of the technical problems existing in the prior art, and provide a programmable transmissive and reflective terahertz metasurface based on vanadium dioxide.

[0006] An embodiment of the present invention provides a programmable transmissive and reflective terahertz metasurface based on vanadium dioxide, comprising a plurality of metasurface units arranged in a periodic array, wherein each metasurface unit comprises a first metal structure layer, a dielectric layer and a second metal structure layer stacked sequentially. The first metal structure layer includes a first metal double-split ring and a first metal adjustment device. The first metal double-split ring includes a first metal ring and a second metal ring arranged coaxially. The first metal ring is sleeved on the second metal ring. The first metal ring has two first notches that are symmetrical about its center. The second metal ring has two second notches that are symmetrical about its center. The angle between adjacent first notches and second notches is 90 degrees. The first metal adjustment device passes through the two second notches and is inserted into the second metal ring. The two ends of the first metal adjustment device are respectively in contact with the inner ring sidewall of the first metal ring. The second metal structure layer includes a second metal double-cracked ring and a second metal adjustment control. The second metal double-cracked ring is embedded in the second metal adjustment control. The second metal double-cracked ring has the same structure as the first metal double-cracked ring and is set accordingly. The material of the first metal control unit and the material of the second metal control unit are both vanadium dioxide; The phase transition state of vanadium dioxide is controlled by external thermal or optical excitation, enabling the metasurface unit to switch between reflection and transmission modes. In reflection mode, phase encoding is achieved by adjusting the opening angle of the first metal ring of the first metal structure layer and the overall rotation angle of the first metal structure layer. In transmission mode, geometric phase encoding is achieved by synchronously rotating the angles of the first metal structure layer and the second metal double-cracked ring.

[0007] In some embodiments of the present invention, the first metal adjustment device is a columnar structure, the second metal adjustment device is a cuboid structure, and the second metal adjustment device has a groove adapted to the shape of the second metal double-cracked ring.

[0008] In some embodiments of the present invention, the period of the metasurface unit is 200 μm, the thickness of the first metal structure layer is 2-6 μm, the thickness of the second metal structure layer is 2-6 μm, the thickness of the dielectric layer is 60-80 μm, the outer diameter of the first metal ring is 90 μm, the outer diameter of the second metal ring is 20 μm, and the width of both the first metal ring and the second metal ring is 10 μm.

[0009] In some embodiments of the present invention, the first metal double-cracked ring and the second metal double-cracked ring are both made of gold, and the dielectric layer is made of silicon dioxide.

[0010] In some embodiments of the present invention, the 3-bit encoding unit in reflection mode is implemented through the following parameter combination: The opening angles of the first metal ring are 14°, 58°, 81°, and 99°, respectively, and the rotation angles of the first metal structure layer are 45°, 45°, 45°, 45°, -45°, -45°, -45°, and -45°, respectively. This results in a phase difference of approximately 45° between adjacent states of cross-polarized reflection at a frequency of 0.7 THz, with a reflection amplitude greater than -1.8 dB.

[0011] In some embodiments of the present invention, the 3-bit encoding unit in the transmission mode is implemented using the following parameters: The opening angle of the first metal ring is fixed at 45°, and the rotation angles of the first metal structure layer are 90°, 78.75°, 67.5°, 56.25°, 45°, 33.75°, 22.5°, and 11.25° respectively. The first metal double-split ring and the second metal double-split ring have the same structure and the same rotation angle; This results in a phase difference of approximately 45° between adjacent states of cross-polarized transmission at a frequency of 0.63 THz, with a transmission amplitude greater than -1.8 dB.

[0012] In some embodiments of the present invention, wavefront modulation is achieved by constructing a coding sequence in the reflection mode: The 3-bit coding units are combined into a gradient coding sequence of "01234567" or "0246", and the anomalous reflection is achieved by using the generalized Snell's law to deflect the incident terahertz beam to 15° or 32°. Alternatively, the coding units can be rotated around the array center in a phase order from 0 to 2π or 0 to 4π to generate the topological load. =1 or =2 vortex beam.

[0013] In some embodiments of the present invention, a new coding sequence is generated by convolving the coding pattern used to generate the vortex beam with the gradient coding sequence used to generate anomalous reflection, thereby achieving anomalous reflection deflection of the vortex beam at a preset angle.

[0014] In some embodiments of the present invention, holographic imaging is achieved in the transmission mode by combining an improved Gerchberg-Saxton iterative algorithm: Using 3-bit discrete phase as a constraint, the transmission phase distribution of the metasurface array required for the target holographic image is calculated, and a holographic metasurface array is constructed.

[0015] In some embodiments of the present invention, the vanadium dioxide has a conductivity of 10 S / m when in an insulating state and a conductivity of [missing value] when in a metallic state. .

[0016] The vanadium dioxide-based programmable transmissive and reflective terahertz metasurface of this invention has the following beneficial effects: 1. This invention integrates vanadium dioxide phase change material in the same metasurface structure, achieving dynamic switching between reflection and transmission modes for the first time, which greatly expands the functional integration and application scenarios of metasurface devices.

[0017] 2. In reflection mode, this invention combines transmission phase and convolution operations to not only realize anomalous reflection of single beams and vortex beams with different topological charge numbers, but also anomalous reflection deflection of vortex beams, significantly improving the degree of freedom of beam manipulation.

[0018] 3. In transmission mode, this invention combines the Pancharatnam-Berry geometric phase with the improved Gerchberg-Saxton algorithm to successfully achieve high-quality holographic image reconstruction in the terahertz band, providing a new device solution for terahertz imaging technology.

[0019] 4. The metasurface unit structure designed in this invention is simple and easy to process, and can achieve "programmable" switching of multiple functions through external excitation, laying a solid foundation for the development of next-generation intelligent terahertz devices. Attached Figure Description

[0020] Figure 1 This is a schematic diagram of the structure of a metasurface unit provided in an embodiment of the present invention; wherein, Figure 1 Image (a) is a three-dimensional view of a metasurface unit. Figure 1 (b) is a side view of a metasurface unit. Figure 1 (c) is a front view of the metasurface unit. Figure 1 (d) is the back view of the metasurface unit.

[0021] Figure 2 This is a simulation diagram of the reflection performance of a metasurface unit under online polarized wave incidence, provided in an embodiment of the present invention; wherein, Figure 2 In the middle (a), the reflection amplitude curves of the metasurface unit under the incident online polarized wave are shown. Figure 2 (b) shows the reflection phase curve of the metasurface unit under the incident linearly polarized wave.

[0022] Figure 3 This is a simulation diagram of the transmission performance of the metasurface unit under circularly polarized wave incident light, provided in an embodiment of the present invention; wherein, Figure 3 In the middle (a), the transmission amplitude curve of the metasurface unit under circularly polarized wave incident light is shown. Figure 3 (b) shows the transmission phase curve of the metasurface unit under circularly polarized wave incident.

[0023] Figure 4This is a characteristic diagram of abnormal reflection and convolution operation in reflection mode provided by an embodiment of the present invention; wherein, Figure 4 (a) Figure 4 (b) Figure 4 (c) shows the far-field patterns corresponding to different coding sequences; Figure 4 (d) is Figure 4 The electric field distribution at the y=0 section corresponding to (a) is shown. Figure 4 (e) is Figure 4 The electric field distribution at the y=0 section corresponding to (b); Figure 4 (f) is Figure 4 Electric field distribution at the y=0 section corresponding to (c); Figure 4 (g) is Figure 4 A schematic diagram of the 32×32 metasurface array encoding corresponding to (a) is shown. Figure 4 (h) is Figure 4 Schematic diagram of the 32×32 metasurface array encoding corresponding to (b); Figure 4 (i) is Figure 4 A schematic diagram of the 32×32 metasurface array encoding corresponding to (c).

[0024] Figure 5 This is a diagram illustrating the characteristics of vortex beam generation in reflection mode provided in an embodiment of the present invention; wherein, Figure 5 (a) Figure 5 (b) Figure 5 (c) shows the 3D far-field plots of vortex beam generation under different topological charge numbers in reflection mode; Figure 5 (d) is Figure 5 The amplitude and phase distribution at the z=0 plane corresponding to (a); Figure 5 (e) is Figure 5 The magnitude and phase distribution at the z=0 plane corresponding to (b); Figure 5 (f) is Figure 5 The magnitude and phase distribution at the z=0 plane corresponding to (c); Figure 5 (g) is Figure 5 The electric field distribution at the y=0 plane corresponding to (a) is shown in the figure. Figure 5 (h) is Figure 5 The electric field distribution at the y=0 plane corresponding to (b); Figure 5 (i) is Figure 5 The electric field distribution at the y=0 plane corresponding to (c); Figure 5 (j) is Figure 5 A schematic diagram of the metasurface array encoding in (a); Figure 5 (k) is Figure 5 (b) Schematic diagram of metasurface array coding Figure 5 (l) is Figure 5A schematic diagram of the metasurface array encoding in (c).

[0025] Figure 6 This is a diagram illustrating the anomalous reflection characteristics of a vortex beam in reflection mode, provided in an embodiment of the present invention; wherein, Figure 6 (a) Figure 6 (b) is a 3D far-field image of anomalous reflection of vortex beams in reflection mode; Figure 6 (c) is Figure 6 The amplitude and phase distribution at the z=0 plane corresponding to (a); Figure 6 (d) is Figure 6 The magnitude and phase distribution at the z=0 plane corresponding to (b); Figure 6 (e) is Figure 6 The electric field distribution at the y=0 plane corresponding to (a) Figure 6 (f) is Figure 6 The electric field distribution at the y=0 plane corresponding to (b); Figure 6 (g) is Figure 6 The encoding diagram corresponding to (a) in the middle; Figure 6 (h) is Figure 6 The encoding diagram corresponding to (b) is shown in the figure.

[0026] Figure 7 This is a holographic imaging characteristic diagram in transmission mode provided by an embodiment of the present invention; wherein, Figure 7 (a) Figure 7 (b) Figure 7 (c) is the target image; Figure 7 (d) is Figure 7 The transmission phase distribution of the metasurface array after GS algorithm iteration is shown in (a). Figure 7 (e) is Figure 7 The transmission phase distribution of the metasurface array after GS algorithm iteration corresponding to (b) Figure 7 (f) is Figure 7 The transmission phase distribution of the metasurface array after GS algorithm iteration corresponds to (b); Figure 7 (g) is Figure 7 The simulated restored image corresponding to (a) Figure 7 (h) is Figure 7 The simulated restored image corresponding to (b) Figure 7 (i) is Figure 7 The simulated restored image corresponding to (c) in the middle; Figure 7 (j) is Figure 7 The holographic image 3D far-field map corresponding to (a) in the middle; Figure 7 (k) is Figure 7 The holographic image 3D far-field map corresponding to (b) in the middle; Figure 7 (l) Figure 7The holographic image 3D far-field map corresponding to (c); Figure 7 The middle (m) is Figure 7 The electric field intensity distribution at z=50mm corresponding to (a) is shown in Figure 1. Figure 7 The middle (n) is Figure 7 The electric field intensity distribution at z=50mm corresponding to (b) is shown in Figure 1. Figure 7 (o) is Figure 7 The electric field intensity distribution at z=50mm corresponding to (b) in the middle. Detailed Implementation

[0027] To enable those skilled in the art to better understand the technical solutions of the present invention, the present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments. It should be understood that the specific embodiments described herein are only for explaining the present invention and are not intended to limit disclosure. The described embodiments are some, but not all, of the embodiments of the present invention. Based on the described embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the protection scope of the present invention.

[0028] like Figure 1 As shown, an embodiment of the present invention provides a programmable transmissive and reflective terahertz metasurface based on vanadium dioxide, comprising a plurality of metasurface units arranged in a periodic array, wherein the metasurface unit comprises a first metal structure layer 10, a dielectric layer 20 and a second metal structure layer 30 stacked sequentially. The first metal structure layer 10 includes a first metal double-split ring 11 and a first metal adjustment device 12. The first metal double-split ring 11 includes a first metal ring 111 and a second metal ring 112 arranged coaxially. The first metal ring 111 is sleeved on the second metal ring 112. The first metal ring 111 has two first notches that are symmetrical about its center. The second metal ring 112 has two second notches that are symmetrical about its center. The angle between adjacent first notches and second notches is 90 degrees. The first metal adjustment device 12 passes through the two second notches and is inserted into the second metal ring 112. The two ends of the first metal adjustment device are in contact with the inner ring sidewall of the first metal ring 111, respectively. The second metal structure layer 30 includes a second metal double-split ring 31 and a second metal adjustment control 32. The second metal double-split ring 31 is embedded in the second metal adjustment control 32. The second metal double-split ring 31 has the same structure as the first metal double-split ring 11 and is set accordingly. The first metal control 12 and the second metal control 32 are both made of vanadium dioxide. The phase transition state of vanadium dioxide is controlled by external thermal or optical excitation to achieve the switching between reflection and transmission modes of the metasurface unit. In the reflection mode, the phase encoding is achieved by adjusting the opening angle of the first metal ring 111 of the first metal structure layer 10 and the overall rotation angle of the first metal structure layer 10. In the transmission mode, the geometric phase encoding is achieved by synchronously rotating the angle of the first metal structure layer 10 and the angle of the second metal double-cracked ring 31.

[0029] It should be noted that in this embodiment, the first metal ring 111 has a ring-shaped structure with two first notches. The two first notches are symmetrical about the center of the first metal ring 111. The opening angle of the first metal ring 111 is the angle formed by the line connecting the two ends of the first notches of the first metal ring 111 and the center of the first metal ring 111 on a cross section perpendicular to the thickness direction of the first metal structure layer 10. The rotation direction of the first metal structure layer 10 as a whole is around the central axis of the first metal double-split ring 11, and the rotation angle of the first metal structure layer 10 as a whole is the rotation offset angle on a cross section perpendicular to its thickness direction. In addition, the first metal double-split ring 11 and the second metal double-split ring 31 in this embodiment have the same structure, and the opening direction and opening angle of the first metal rings of the two metal double-split rings are consistent, as are the opening direction and opening angle of the second metal rings of the two metal double-split rings.

[0030] The vanadium dioxide-based programmable transmissive and transmissive terahertz metasurface of this invention achieves dynamic switching between reflection and transmission modes for the first time by integrating vanadium dioxide phase change material in the same metasurface structure, greatly expanding the functional integration and application scenarios of metasurface devices; multiple functions can be "programmable" switched through external excitation, laying a solid foundation for the development of next-generation intelligent terahertz devices.

[0031] In some embodiments of the present invention, the first metal adjustment control 12 is a columnar structure or a strip, with its two ends respectively connected to the inner sidewalls of the two arc-shaped parts of the first metal ring 111, and the first metal adjustment control 12 passing through the two notches of the second metal ring 112. The second metal adjustment control 32 is a cuboid structure, and has a groove adapted to the shape of the second metal double-lobed ring 31. In reflection mode, when the second metal double-lobed ring 31 rotates synchronously with the first metal structure layer 10, the groove adapted to the second metal double-lobed ring 31 rotates together.

[0032] In some embodiments of the present invention, the period of the metasurface unit is 200 μm, the thickness of the first metal structure layer 10 is 2-6 μm, the thickness of the second metal structure layer 30 is 2-6 μm, the thickness of the dielectric layer 20 is 60-80 μm, the outer diameter of the first metal ring 111 is 90 μm, the outer diameter of the second metal ring 112 is 20 μm, and the width of the first metal ring 111 and the second metal ring 112 is 10 μm.

[0033] In some embodiments of the present invention, the first metal double-splitter ring 11 is made of gold, the second metal double-splitter ring 31 is made of gold, and the dielectric layer 20 is made of silicon dioxide.

[0034] In this embodiment, the dielectric properties of vanadium dioxide are described using the Drude model: when vanadium dioxide is in an insulating state, its conductivity is set to... When vanadium dioxide is in the metallic state, its conductivity is set to... .

[0035] In some embodiments of the present invention, the 3-bit encoding unit in reflection mode is implemented through the following parameter combination: The opening angles of the first metal ring are 14°, 58°, 81°, and 99°, respectively, and the rotation angles of the first metal structure layer are 45°, 45°, 45°, 45°, -45°, -45°, -45°, and -45°, respectively. This results in a phase difference of approximately 45° between adjacent states of cross-polarized reflection at a frequency of 0.7 THz, with a reflection amplitude greater than -1.8 dB.

[0036] In some embodiments of the present invention, the 3-bit encoding unit in the transmission mode is implemented using the following parameters: The opening angle of the first metal ring is fixed at 45°, and the rotation angles of the first metal structure layer are 90°, 78.75°, 67.5°, 56.25°, 45°, 33.75°, 22.5°, and 11.25° respectively. The first metal double-cracked ring and the second metal double-cracked ring have the same structure and the same rotation angle. The rotation angle of the first metal structural layer is the same as the rotation angle of the first metal double-cracked ring.

[0037] This results in a phase difference of approximately 45° between adjacent states of cross-polarized transmission at a frequency of 0.63 THz, with a transmission amplitude greater than -1.8 dB.

[0038] In some embodiments of the present invention, wavefront modulation is achieved by constructing a coding sequence in the reflection mode: The 3-bit coding units are combined into a gradient coding sequence of "01234567" or "0246", and the anomalous reflection is achieved by using the generalized Snell's law to deflect the incident terahertz beam to 15° or 32°. Alternatively, the coding units can be rotated around the array center in a phase order from 0 to 2π or 0 to 4π to generate a vortex beam with a topological charge of l=1 or l=2.

[0039] In some embodiments of the present invention, a new coding sequence is generated by convolving the coding pattern used to generate the vortex beam with the gradient coding sequence used to generate anomalous reflection, thereby achieving anomalous reflection deflection of the vortex beam at a preset angle.

[0040] In some embodiments of the present invention, holographic imaging is achieved in the transmission mode by combining an improved Gerchberg-Saxton iterative algorithm: Using 3-bit discrete phase as a constraint, the transmission phase distribution of the metasurface array required for the target holographic image is calculated, and a holographic metasurface array is constructed.

[0041] Wavefront modulation in reflection mode (vanadium dioxide in metallic state): When vanadium dioxide is transformed into a metallic state through thermal excitation (temperature > 68℃) or other excitations, the second metallic structure layer acts as a metallic reflective surface, and the metasurface operates in reflection mode as a whole. The modulation of the propagation mode resulting from changes in the shape and size of the metasurface is reflected in the change in the equivalent refractive index *n* of light propagating through the metasurface. When the wavelength is... The electromagnetic wave travels a distance of in the medium. When, the amount of phase change during transmission It can be represented as: (1) Therefore, by changing the thickness of the metasurface unit structure or adjusting the microstructure of the transmission space, the equivalent refractive index n can be changed to achieve effective adjustment of the transmission phase.

[0042] like Figure 2 As shown, within the frequency range of 0.2 to 1 THz, the opening angle of the first metal ring is changed. and the rotation angle of the first metal structural layer This allows for a phase difference of approximately 45° between adjacent states of a unit under cross-polarized reflection, thus forming a 3-bit encoding unit (phase states "0" to "7"). When the LP wave is incident perpendicularly, the reflection amplitude of the terahertz wave is greater than -1.8dB, which meets the design requirements of the 3-bit encoded metasurface.

[0043] Anomalous reflection and convolution operations: Under plane wave excitation, the far-field scattering expression of the metasurface is: (2) In the formula, The scattering phase for each unit; The equivalent period of the lattice composed of metasurface units; and These are the elevation and azimuth angles of the metasurface scattering field, respectively. Let be the mode function of the lattice composed of metasurface units. According to equation (2), the elevation angle of the reflected beam can be controlled by specifying the metasurface encoding method. ) and azimuth ( ), which satisfies: (3) (4) In the formula: , Let be the periodicity of the metasurface unit lattice arrangement along the x and y directions. If the metasurface structure only exhibits periodic variations in the x or y directions, then equation (3) can be simplified to: (5) In the formula: The speed of light in a vacuum; The operating frequency of the metasurface; A positive integer indicates that the reflected beam from the metasurface may have higher-order modes, meaning that there may be small grating lobes with lower intensity than the main lobe next to the main lobe.

[0044] The aperture angle corresponding to the 3-bit encoding unit in reflection mode (vanadium dioxide in metallic state) The angles are, in order: 14°, 58°, 81°, 99°; further, the rotation angles... The values ​​are, in order: 45°, 45°, 45°, 45°, -45°, -45°, -45°, -45°, -45°.

[0045] like Figure 4 As shown, different encoding sequences are composed using 3-bit encoding units. Figure 4 The eight-cycle gradient coding sequence “01234567 / 01234567” shown in (g) corresponds to ,exist Anomalous reflection of 15° was achieved when the linearly polarized wave was incident perpendicularly (e.g., Figure 4 As shown in (a) and (d), it matches the calculated result of 15.54° according to the generalized Snell's law. Figure 4 The “0246 / 0246” four-period gradient coding sequence shown in (h) achieves an anomalous reflection of 32° (e.g., Figure 4 (As shown in (b) and (e)).

[0046] Furthermore, by introducing the concept of convolution operation of encoded metasurfaces, the incident terahertz beam can be reflected at any angle. The expression for the convolution operation is: (6) In formula (6), and The scattering angles of the two gradient-coded sequences corresponding to the convolution operation are respectively used. When these two sequence patterns are added or subtracted, a new anomalous reflected wave can be obtained. For example... Figure 4 As shown in (i), convolution (subtraction) is performed on two gradient coding sequences to generate a new coding sequence, the far-field result of which is ( Figure 4 (c) and (f) show that the reflected beam deflects to -15°, verifying the ability to flexibly adjust the abnormal reflection angle through convolution operation.

[0047] Vortex beam generation: When a plane wave is incident on a metasurface array, how can it be converted into... The phase distribution of a vortex beam on a metasurface can be expressed as: (7) In the formula: denoted as , where is the two-dimensional coordinate of the metasurface unit.

[0048] When an X-polarized terahertz wave is incident along the -Z direction, a topological charge can be generated. , vortex beams, such as Figure 5 As shown in (j) to (l), the 2-bit and 3-bit different coding units are arranged with the center of the metasurface array as the axis, according to the phase from 0 to 2. ( =1) or 0 to 4 ( By rotating the order of 2, a vortex beam carrying orbital angular momentum can be generated. Figure 5 3D far-field plots from (a) to (c) and Figure 5 The toroidal field distribution from (d) to (f) clearly demonstrates that the topological charge number was successfully generated. =1 and The vortex beams with a depth of 2 dB all have a central depression greater than 25 dB, indicating good quality.

[0049] Anomalous reflection of vortex beams: like Figure 6 As shown, two sets of vortex beam-coded metasurface arrays with anomalous reflection angles were designed. When an X-polarized terahertz wave source is incident along the -Z direction, the coded pattern used to generate the vortex beam is convolved with the gradient coded sequence used to generate anomalous reflections, resulting in a new coded metasurface such as... Figure 6 (g) and (h). Figure 6 Simulation results in (a) and (b) show that the generated vortex beams reflect abnormally at angles of approximately 15.6° and 8°, respectively, with the center concave and deviating from the Z-axis, thus successfully achieving vortex beam deflection control.

[0050] Wavefront modulation in transmission mode (vanadium dioxide in insulating state): When vanadium dioxide is in an insulating state, the second metallic structure layer acts as a dielectric, and the first and second metallic structure layers together form a transmissive metasurface. At this point, based on the Pancharatnam-Berry (PB) geometric phase principle, by synchronously rotating the first and second metallic double-cracked rings (rotation angle...), ), which can introduce 2 Phase delay.

[0051] The aperture angle corresponding to the 3-bit encoding unit in transmission mode (vanadium dioxide in the insulating state) For: 45°; further, rotation angle The angles are, in order: 90°, 78.75°, 67.5°, 56.25°, 45°, 33.75°, 22.5°, and 11.25°.

[0052] like Figure 3 As shown, under the perpendicular incidence of a circularly polarized wave at 0.63 THz, by setting... A linear variation from 90° to 11.25° (in 11.25° steps) allows for a phase difference of approximately 45° between adjacent states of the unit under cross-polarized transmission, thus enabling 3-bit encoding. The transmission amplitude of all encoding units is greater than -1.8 dB, meeting the design requirements.

[0053] Holographic imaging: like Figure 7 As shown in (a) to (c), the characters "V", "O", and "2" are selected as the target holographic images. Using an improved Gerchberg-Saxton (GS) iterative algorithm with a 3-bit discrete phase constraint, the required metasurface array transmission phase distribution for realizing these target images is calculated, as follows: Figure 7 As shown in (d) to (f), the calculated phase distribution was reconstructed using MATLAB software, resulting in a clear target image (e.g., ...). Figure 7 As shown in (g) to (i), the effectiveness of the algorithm is verified.

[0054] Finally, the 3-bit holographic metasurface array constructed based on the above phase distribution map was simulated using CST Microwave Studio software. Figure 7 3D far-field plots from (j) to (l) and Figure 7 The electric field intensity distribution at the z=50 mm plane from (m) to (o) clearly shows the images of the three characters "V", "O" and "2", proving the ability of the metasurface of the present invention to achieve high-quality terahertz holographic imaging in transmission mode.

[0055] In summary, the vanadium dioxide-based programmable transmissive and transmissive terahertz metasurface proposed in this invention successfully achieves switching between reflection and transmission modes by exciting the phase transition of vanadium dioxide. In each mode, different phase modulation mechanisms (transmission phase / PB phase) combined with advanced algorithms (convolution operation / improved GS algorithm) can be used to achieve multifunctional, high-degree-of-freedom dynamic control of the terahertz beam, including anomalous reflection, vortex beam generation, beam deflection, and holographic imaging.

[0056] The vanadium dioxide-based programmable transmissive and reflective terahertz metasurface of this invention has the following beneficial effects: 1. This invention integrates vanadium dioxide phase change material in the same metasurface structure, achieving dynamic switching between reflection and transmission modes for the first time, which greatly expands the functional integration and application scenarios of metasurface devices.

[0057] 2. In reflection mode, this invention combines transmission phase and convolution operations to not only realize anomalous reflection of single beams and vortex beams with different topological charge numbers, but also anomalous reflection deflection of vortex beams, significantly improving the degree of freedom of beam manipulation.

[0058] 3. In transmission mode, this invention combines the Pancharatnam-Berry geometric phase with the improved Gerchberg-Saxton algorithm to successfully achieve high-quality holographic image reconstruction in the terahertz band, providing a new device solution for terahertz imaging technology.

[0059] 4. The metasurface unit structure designed in this invention is simple and easy to process, and can achieve "programmable" switching of multiple functions through external excitation, laying a solid foundation for the development of next-generation intelligent terahertz devices.

[0060] It is understood that the above embodiments are merely exemplary implementations used to illustrate the principles of the present invention, and the present invention is not limited thereto. For those skilled in the art, various modifications and improvements can be made without departing from the spirit and essence of the present invention, and these modifications and improvements are also considered to be within the scope of protection of the present invention.

Claims

1. A programmable transmissive and reflective terahertz metasurface based on vanadium dioxide, characterized in that, It includes multiple metasurface units arranged in a periodic array, each metasurface unit comprising a first metal structure layer, a dielectric layer, and a second metal structure layer stacked sequentially. The first metal structure layer includes a first metal double-split ring and a first metal adjustment device. The first metal double-split ring includes a first metal ring and a second metal ring arranged coaxially. The first metal ring is sleeved on the second metal ring. The first metal ring has two first notches that are symmetrical about its center. The second metal ring has two second notches that are symmetrical about its center. The angle between adjacent first notches and second notches is 90 degrees. The first metal adjustment device passes through the two second notches and is inserted into the second metal ring. The two ends of the first metal adjustment device are respectively in contact with the inner ring sidewall of the first metal ring. The second metal structure layer includes a second metal double-cracked ring and a second metal adjustment control. The second metal double-cracked ring is embedded in the second metal adjustment control. The second metal double-cracked ring has the same structure as the first metal double-cracked ring and is set accordingly. The material of the first metal control unit and the material of the second metal control unit are both vanadium dioxide; The phase transition state of vanadium dioxide is controlled by external thermal or optical excitation, enabling the metasurface unit to switch between reflection and transmission modes. In reflection mode, phase encoding is transmitted by adjusting the opening angle of the first metal ring of the first metal structure layer and the overall rotation angle of the first metal structure layer. In transmission mode, geometric phase encoding is achieved by synchronously rotating the angles of the first metal structure layer and the second metal double-cracked ring.

2. The metasurface according to claim 1, characterized in that, The first metal adjustment device has a columnar structure, the second metal adjustment device has a cuboid structure, and the second metal adjustment device has a groove that matches the shape of the second metal double-cracked ring.

3. The metasurface according to claim 1, characterized in that, The metasurface unit has a period of 200 μm, the first metal structure layer has a thickness of 2-6 μm, the second metal structure layer has a thickness of 2-6 μm, the dielectric layer has a thickness of 60-80 μm, the first metal ring has an outer diameter of 90 μm, the second metal ring has an outer diameter of 20 μm, and the first metal ring has a width of 10 μm.

4. The metasurface according to claim 1, characterized in that, Both the first and second metal double-cracked rings are made of gold, and the dielectric layer is made of silicon dioxide.

5. The metasurface according to claim 1, characterized in that, The 3-bit encoding unit in the reflection mode is implemented through the following parameter combination: The opening angles of the first metal ring are 14°, 58°, 81°, and 99°, respectively, and the rotation angles of the first metal structure layer are 45°, 45°, 45°, 45°, -45°, -45°, -45°, and -45°, respectively. This results in a phase difference of approximately 45° between adjacent states of cross-polarized reflection at a frequency of 0.7 THz, with a reflection amplitude greater than -1.8 dB.

6. The metasurface according to claim 1, characterized in that, The 3-bit encoding unit in the transmission mode is implemented through the following parameters: The opening angle of the first metal ring is fixed at 45°, and the rotation angles of the first metal structure layer are 90°, 78.75°, 67.5°, 56.25°, 45°, 33.75°, 22.5°, and 11.25° respectively. The first metal double-split ring and the second metal double-split ring have the same structure and the same rotation angle; This results in a phase difference of approximately 45° between adjacent states of cross-polarized transmission at a frequency of 0.63 THz, with a transmission amplitude greater than -1.8 dB.

7. The metasurface according to claim 1, characterized in that, In the reflection mode, wavefront modulation is achieved by constructing a coding sequence: The 3-bit coding units are combined into a gradient coding sequence of "01234567" or "0246", and the anomalous reflection is achieved by using the generalized Snell's law to deflect the incident terahertz beam to 15° or 32°. Alternatively, the coding units can be rotated around the array center in a phase order from 0 to 2π or 0 to 4π to generate the topological load. =1 or =2 vortex beam.

8. The programmable transmissive and reflective terahertz metasurface according to claim 7, characterized in that: By convolving the coded pattern used to generate the vortex beam with the gradient coded sequence used to generate anomalous reflections, a new coded sequence is generated, thereby achieving anomalous reflection deflection of the vortex beam at a preset angle.

9. The programmable transmissive and reflective terahertz metasurface according to claim 1, characterized in that, Holographic imaging is achieved in the transmission mode by combining an improved Gerchberg-Saxton iterative algorithm. Using 3-bit discrete phase as a constraint, the transmission phase distribution of the metasurface array required for the target holographic image is calculated, and a holographic metasurface array is constructed.

10. The programmable transmissive and reflective terahertz metasurface according to claim 1, characterized in that, The vanadium dioxide has a conductivity of 10 S / m in the insulating state and a conductivity of [missing value] in the metallic state. .

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