Vanadium dioxide ultra-wideband / polarization switchable terahertz absorber and method

Abstract

This invention discloses a vanadium dioxide-based ultra-wideband / polarization-switching terahertz absorber, which is composed of M×N multilayered ultra-wideband absorption-polarization conversion switching units arranged in a periodic array. The switching units are distributed in an M×N two-dimensional pattern, where M and N are both positive integers. Each switching unit comprises, from bottom to top, a bottom metal reflective layer, a lower dielectric layer, an intermediate gold polarization conversion layer, a vanadium dioxide thin film layer, an upper dielectric layer, and a top vanadium dioxide resonant layer. This terahertz absorber can dynamically switch between "ultra-wideband cross-polarization conversion" and "ultra-wideband perfect absorption" functions in a single device structure, overcoming the limitation of traditional terahertz devices having only one function. A method for fabricating the vanadium dioxide-based ultra-wideband / polarization-switching terahertz absorber is also disclosed.

Description

Technical Field

[0001] This invention belongs to the field of terahertz wave technology, specifically relating to a terahertz absorber based on vanadium dioxide ultrawideband / polarization switchable, and also to a method for preparing the terahertz absorber based on vanadium dioxide ultrawideband / polarization switchable. Background Technology

[0002] Terahertz (THz) waves typically refer to electromagnetic waves in the frequency band between microwaves and infrared light. This frequency band possesses characteristics such as strong penetration, low photon energy, and a rich spectrum, making it a characteristic absorption band for many biological macromolecules and chemical substances. It has significant application value in fields such as non-destructive testing, biosensing, and imaging. However, the development of high-performance control devices (such as absorbers and filters) faces challenges. Due to the lack of natural materials capable of achieving efficient electromagnetic coupling with this frequency band, traditional terahertz devices are mostly based on metal-dielectric periodic structures, achieving efficient control at specific frequencies through geometric resonance. However, due to their fixed operating frequency and limited functionality, they are difficult to adapt to the demands of dynamic electromagnetic environments.

[0003] Metamaterials are artificial composite structures that, through flexible design of subwavelength units, can flexibly control the amplitude, phase, and polarization of electromagnetic waves, effectively overcoming the functional limitations of natural materials in the terahertz band. In 2008, Landy et al. first proposed a perfect absorber based on metamaterials, sparking a research boom in terahertz metamaterial absorbers. Subsequently, researchers have successively achieved single-frequency, multi-frequency, and broadband absorption characteristics. However, once traditional metamaterial absorbers are fabricated, their structures are fixed, resulting in an inability to dynamically adjust absorption performance, severely limiting their applicability in flexible application scenarios.

[0004] In recent years, with the introduction of phase change materials (such as vanadium dioxide VO2) and graphene, significant progress has been made in the research of tunable terahertz metamaterial devices. Vanadium dioxide undergoes an insulator-metal phase transition at approximately 340 K, and its conductivity can change abruptly by up to five orders of magnitude within the picosecond range, providing an ideal mechanism for dynamically controlling terahertz waves. Based on this characteristic, several studies have successively realized tunable absorbers with varying performance: In 2020, Song et al. from Xiamen University proposed a broadband-to-narrowband switchable terahertz absorber based on the phase change characteristics of vanadium dioxide. This device is composed of multiple layers of stacked materials. The combination of two rings significantly expands the operating bandwidth of the device. In 2022, Zhang et al. achieved tuning of the polarization converter's operating bandwidth by adjusting the voltage to change the Fermi level of graphene; Yu et al. and Xiong et al. utilized the temperature control characteristics of vanadium dioxide to respectively achieve adjustment of polarization conversion bandwidth and modulation of multi-band absorption peaks. Despite some progress in tunable terahertz absorption, challenges remain, including limited absorption bandwidth and relatively limited modulation modes. In particular, achieving flexible and reliable switching between ultra-wideband absorption and multi-band absorption in a single device remains a critical challenge in terahertz absorber design. Furthermore, given the broader demand for functional integration, realizing the switching of drastically different electromagnetic functions—from "absorption stealth" to "polarization conversion detection / communication"—within a single device structure is crucial for improving the environmental adaptability of terahertz systems and promoting their applications in industry and military fields. Summary of the Invention

[0005] The purpose of this invention is to provide a vanadium dioxide-based ultrawideband / polarization-switching terahertz absorber that can dynamically switch between "ultrawideband cross-polarization conversion" and "ultrawideband perfect absorption" in a single device structure, overcoming the limitation of traditional terahertz devices having only one function.

[0006] The first technical solution adopted in this invention is a vanadium dioxide ultrawideband / polarization switchable terahertz absorber, which is composed of M×N ultrawideband absorption-polarization conversion function switchable units with multi-layer structure arranged in a periodic array. The ultrawideband absorption-polarization conversion function switchable units are distributed in an M×N two-dimensional pattern, where M and N are both positive integers. The ultra-wideband absorption-polarization conversion switchable unit includes, from bottom to top, a bottom metal reflective layer, a lower dielectric layer, an intermediate gold polarization conversion layer, a vanadium dioxide thin film layer, an upper dielectric layer, and a top vanadium dioxide resonant layer.

[0007] The invention is further characterized in that: The bottom metallic reflective layer is a continuous thin metal film; the material of the bottom metallic reflective layer is gold, with a conductivity of 4.56 × 10⁻⁶.7 The thickness t1 is 0.18µm-0.22µm. The bottom metal reflective layer is a continuous metal thin film with a length and width equal to the cell period p of the ultra-wideband absorption-polarization conversion function switchable unit cell, where p is 19µm-21µm.

[0008] Both the lower and upper dielectric layers are made of PTFE with a relative permittivity of 2.1. The thickness h1 of the lower dielectric layer is 3.8µm-4.2µm, and the thickness h2 of the upper dielectric layer is 5.3µm-5.7µm. The length and width of both the lower and upper dielectric layers are equal to the cell period p.

[0009] The intermediate gold polarization conversion layer is made of gold with an electrical conductivity of 4.56 × 10⁻⁶. 7 S / m, thickness t2 is 1.4µm-1.6µm, the intermediate gold polarization conversion layer is composed of two parallel "I" shaped gold patches arranged along a 45° diagonal, wherein the distance l1 between the two "I" shaped gold patches is 8.8µm-9.2µm, and the "I" shaped gold patch is composed of a first straight arm patch and second straight arm patches respectively set at both ends of the first straight arm patch; The length l2 of the first straight arm patch is 9.8µm-10.2µm, the length l3 of the second straight arm patch is 4.8µm-5.2µm, the width w1 of the first straight arm patch is 1.1µm-1.3µm, and the width w2 of the second straight arm patch is 1.1µm-1.3µm.

[0010] The vanadium dioxide thin film layer is made of vanadium dioxide, with a thickness d1 of 0.18µm-0.22µm. The vanadium dioxide thin film layer is a continuous, unpatterned film that covers the entire unit cell region, and its length and width are both equal to the unit cell period p.

[0011] The thickness d2 of the top vanadium dioxide resonant layer is 1.4µm-1.6µm. The top vanadium dioxide resonant layer is a square vanadium dioxide thin film with a side length a of 9.8µm-10.2µm. Two mutually perpendicular "I"-shaped grooves are opened in the middle of the square vanadium dioxide thin film. The "I"-shaped groove is composed of a first straight arm groove and a second straight arm groove respectively set at both ends of the first straight arm groove. The first straight arm groove has an arm length b of 5.8µm-6.2µm and an arm width s1 of 0.9µm-1.1µm. The second straight arm groove has an arm length c of 3.8µm-4.2µm and an arm width s2 of 0.9µm-1.1µm.

[0012] The second technical solution adopted in this invention is a method for preparing a vanadium dioxide ultrawideband / polarization-switchable terahertz absorber, specifically including the following steps: Step 1: Preparation of the bottom metal reflective layer; Step 2: Deposition of the lower dielectric layer; Step 3: Preparation of the intermediate gold polarization conversion layer; Step 4: Preparation of vanadium dioxide thin film layer; Step 5: Deposition of the upper dielectric layer; Step 6: Preparation of the top vanadium dioxide resonant layer.

[0013] The invention is further characterized in that: Step 1 is as follows: Step 1.1, Substrate cleaning: High-purity silicon is selected as the substrate, and ultrasonically cleaned with acetone, ethanol and deionized water for 10 min-15 min in sequence to remove surface contaminants; after drying with nitrogen, it is dried in an oven at 110℃-120℃ for 30 min-40 min. Step 1.2, Metal Thin Film Deposition: Gold thin films are deposited using electron beam evaporation. Step 1.3, Annealing: Anneal at 300℃-350℃ for 30min-40min to improve the crystal quality and conductivity of the gold film and obtain the bottom metal reflective layer; Step 2 is as follows: Step 2.1, Spin coating of PTFE dielectric layer: Spin coat the PTFE precursor solution onto the bottom metal reflective layer. By precisely controlling the spin coating speed and time, the required dielectric layer thickness is obtained. Step 2.2, Curing treatment: Curing at 150℃-160℃ for 1h-2h to ensure the formation of a stable PTFE film, resulting in a lower dielectric layer on the bottom metal reflective layer; Step 3 specifically involves: Step 3.1, Photolithography Patterning: Using electron beam lithography, a structural pattern is defined by two parallel "I"-shaped gold patches arranged diagonally at 45°. Step 3.2, Gold Thin Film Deposition: Gold thin films are deposited using electron beam evaporation; Step 3.3, Lifting and Shaping: Use the lift-off method to remove the photoresist and the excess gold film on it to obtain the intermediate gold polarization conversion layer; Step 4 is as follows: Step 4.1, Mask preparation: Based on the designed periodic array pattern, prepare a mask for vanadium dioxide thin film deposition to ensure that the deposition area covers the entire unit cell and forms a continuous thin film layer; Step 4.2, Vanadium dioxide thin film deposition: A continuous vanadium dioxide thin film is deposited on the surface of the intermediate gold polarization conversion layer using pulsed laser deposition or magnetron sputtering. Step 4.3, Annealing and Crystallization Treatment: Annealing is performed at 400℃-450℃ for 20-30 minutes in an oxygen atmosphere to obtain a vanadium dioxide thin film layer with uniform thickness; Step 5 specifically involves: Step 5.1, Spin coating of PTFE dielectric layer: Spin coating of PTFE precursor solution onto vanadium dioxide thin film layer, and obtaining the required dielectric layer thickness by precisely controlling the spin coating speed and time; Step 5.2, Curing treatment: Curing at 150℃-160℃ for 1h-2h to ensure the formation of a stable PTFE film, and obtaining the upper dielectric layer on the vanadium dioxide film layer; Step 6 specifically involves: Step 6.1, Photolithography defines the vanadium dioxide region: The square vanadium dioxide region and its internal "I"-shaped groove pattern region are defined at the specified location by photolithography; Step 6.2, Vanadium dioxide thin film deposition: Deposit a vanadium dioxide thin film in the designated area using pulsed laser deposition; Step 6.3, Lifting and Shaping: Use the lift-off method to remove the photoresist and the excess vanadium dioxide film on it to obtain the top vanadium dioxide resonant layer.

[0014] The beneficial effects of this invention are: (1) The present invention is based on a vanadium dioxide ultrawideband / polarization switchable terahertz absorber. By adjusting the phase state of vanadium dioxide by temperature, the dynamic switching between the two functions of "ultrawideband cross polarization conversion" and "ultrawideband perfect absorption" in the same device is realized, overcoming the limitation of the single function of traditional terahertz devices.

[0015] (2) When vanadium dioxide is in the insulating state, the device operates in polarization conversion mode; when vanadium dioxide is in the metallic state, the device operates in absorption mode and has polarization insensitivity and wide incident angle absorption characteristics. (3) The terahertz absorber of the present invention has the advantages of simple structure, convenient preparation, flexible switching, wide bandwidth, high efficiency and insensitivity to incident angle, and has important application prospects in the fields of terahertz intelligent stealth, broadband communication, imaging and detection.

[0016] (4) The preparation method of the present invention is simple and easy to operate. Attached Figure Description

[0017] Figure 1 This is a three-dimensional structural schematic diagram of the vanadium dioxide ultrawideband / polarization-switching terahertz absorber based on the present invention. Figure 2 This is a schematic diagram of the unit structure of the vanadium dioxide ultrawideband / polarization-switching terahertz absorber of the present invention; Figure 3 This is a top view of the top vanadium dioxide resonant layer in the unit structure of the absorber of the present invention; Figure 4 This is a top view of the intermediate gold polarization conversion layer in the unit structure of the absorber of the present invention; Figure 5 This refers to the cross-polarization reflection coefficient and co-polarization reflection coefficient in the vanadium dioxide insulating state of Embodiment 1 of the present invention. Figure 6 This is a PCR curve of polarization conversion efficiency in the vanadium dioxide insulating state of Example 1 of the present invention; Figure 7 This is a graph showing the elliptic angle and polarization rotation azimuth angle of the reflected wave in polarization conversion mode according to Embodiment 1 of the present invention. Figure 8 This is an electric field decomposition diagram of the incident and reflected waves of the absorber in Embodiment 1 of the present invention; Figure 9 This is a diagram showing the reflection coefficients and phase differences of u-polarized and v-polarized electromagnetic waves incident on the absorber in Embodiment 1 of the present invention. Figure 10 This is a surface current distribution diagram of the gold polarization unit layer and the gold substrate at 4.88 THz in the absorber of Embodiment 1 of the present invention; Figure 11 This is a surface current distribution diagram of the gold polarization unit layer and the gold substrate at 8.20 THz in the absorber of Embodiment 1 of the present invention; Figure 12 This is a graph showing the effect of different polarization angles of the absorber on polarization conversion efficiency (PCR) in Embodiment 1 of the present invention; Figure 13 This is a graph showing the effect of different incident angles of the absorber on polarization conversion efficiency (PCR) in Embodiment 1 of the present invention; Figure 14 The absorption rate A, reflectivity R, and transmittance T of the vanadium dioxide in the metallic state in the absorber of Embodiment 1 of the present invention; Figure 15 These are the real and imaginary parts of the normalized equivalent impedance Zr of the vanadium dioxide in the metallic state in the absorber of Embodiment 1 of the present invention; Figure 16 These are the real and imaginary parts of the equivalent dielectric constant ε of the vanadium dioxide device in the metallic state in Embodiment 1 of the present invention; Figure 17 The real and imaginary parts of the equivalent permeability μ of vanadium dioxide in the metallic state in the absorber of Embodiment 1 of the present invention are shown. Figure 18 This is a top view of the electric field distribution in the absorber of Embodiment 1 of the present invention when vanadium dioxide is in the metallic state; Figure 19 This is a side view of the electric field distribution of vanadium dioxide in the absorber of Embodiment 1 of the present invention when it is in the metallic state; Figure 20 is a graph showing the effect of polarization angle on absorption rate in the absorption mode of Embodiment 1 of the present invention; Figure 21 is a diagram showing the influence of the incident angle on the absorption rate in the absorption mode of Embodiment 1 of the present invention.

[0018] In the figure, 1. bottom metal reflection layer, 2. lower dielectric layer, 3. intermediate gold polarization conversion layer, 4. vanadium dioxide thin film layer, 5. upper dielectric layer, 6. top vanadium dioxide resonance layer; 3-1. First "I"-shaped metal patch; 6-1. First "I"-shaped groove; 3-1-1. First straight arm patch, 3-1-2. Second straight arm patch; 6-1-1. First straight arm groove, 6-1-2. Second straight arm groove. Detailed implementation manners

[0019] The present invention will be described in detail below with reference to the accompanying drawings and specific implementation manners.

[0020] The present invention provides a vanadium dioxide-based ultra-wideband / polarization-switchable terahertz absorber, as Figure 1-4 shown, which is composed of a periodic array of M×N ultra-wideband absorption-polarization conversion function switchable units with a multi-layer structure. The ultra-wideband absorption-polarization conversion function switchable units are distributed in a two-dimensional M×N pattern, where both M and N are positive integers; The ultra-wideband absorption-polarization conversion function switchable unit includes a bottom metal reflection layer 1, a lower dielectric layer 2, an intermediate gold polarization conversion layer 3, a vanadium dioxide thin film layer 4, an upper dielectric layer 5, and a top vanadium dioxide resonance layer 6 arranged in sequence from bottom to top.

[0021] The bottom metal reflection layer 1 is a continuous metal thin film; the material of the bottom metal reflection layer 1 is gold, and the conductivity is 4.56×10 7 S / m, the thickness t1 is 0.18 µm - 0.22 µm. The bottom metal reflection layer 1 is a continuous metal thin film, and its length and width are both equal to the unit cell period p of the ultra-wideband absorption-polarization conversion function switchable unit. p is 19 µm - 21 µm. The thickness t1 is much larger than the skin depth of gold in the terahertz frequency band to ensure that the transmittance T of the absorber is 0.

[0022] The materials of the lower dielectric layer 2 and the upper dielectric layer 5 are both PTFE, and the relative dielectric constant is 2.1. The thickness h1 of the lower dielectric layer 2 is 3.8 µm - 4.2 µm, and the thickness h2 of the upper dielectric layer 5 is 5.3 µm - 5.7 µm. The length and width of the lower dielectric layer 2 and the upper dielectric layer 5 are both equal to the unit cell period p.

[0023] The material of the intermediate gold polarization conversion layer 3 is gold, and the conductivity is 4.56×10 7S / m, thickness t2 is 1.4µm-1.6µm, the intermediate gold polarization conversion layer 3 is composed of two parallel "I" shaped gold patches 3-1 arranged along a 45° diagonal, wherein the distance l1 between the two "I" shaped gold patches 3-1 is 8.8µm-9.2µm, and the "I" shaped gold patch 3-1 is composed of a first straight arm patch 3-1-1 and second straight arm patches 3-1-2 respectively set at both ends of the first straight arm patch 3-1-1; The length l2 of the first straight arm patch 3-1-1 is 9.8µm-10.2µm, the length l3 of the second straight arm patch 3-1-2 is 4.8µm-5.2µm, the width w1 of the first straight arm patch 3-1-1 is 1.1µm-1.3µm, and the width w2 of the second straight arm patch 3-1-2 is 1.1µm-1.3µm.

[0024] The vanadium dioxide thin film layer 4 is made of vanadium dioxide, with a thickness d1 of 0.18µm-0.22µm. The vanadium dioxide thin film layer 4 is a continuous, unpatterned thin film that covers the entire unit cell region, and its length and width are both equal to the unit cell period p.

[0025] The thickness d2 of the top vanadium dioxide resonant layer 6 is 1.4µm-1.6µm. The top vanadium dioxide resonant layer 6 is a square vanadium dioxide thin film with a side length a of 9.8µm-10.2µm. Two mutually perpendicular "I"-shaped grooves 6-1 are opened in the middle of the square vanadium dioxide thin film. The "I"-shaped groove 6-1 is composed of a first straight arm groove 6-1-1 and a second straight arm groove 6-1-2 respectively set at both ends of the first straight arm groove 6-1-1. The first straight arm groove has an arm length b of 5.8µm-6.2µm and an arm width s1 of 0.9µm-1.1µm. The second straight arm groove 6-1-2 has an arm length c of 3.8µm-4.2µm and an arm width s2 of 0.9µm-1.1µm.

[0026] When the ambient temperature is below 340K, vanadium dioxide is in the insulating phase with a conductivity σ of 200S / m, and the terahertz absorber operates in broadband cross-polarization conversion mode; when the ambient temperature is above 340K, vanadium dioxide is in the metallic phase with a conductivity σ of 200000S / m, and the terahertz absorber operates in ultra-wideband absorption mode.

[0027] The fabrication method of a vanadium dioxide-based ultrawideband / polarization-switchable terahertz absorber specifically includes the following steps: Step 1: Preparation of the bottom metal reflective layer 1; Step 1 is as follows: Step 1.1, Substrate cleaning: High-purity silicon is selected as the substrate, and ultrasonically cleaned with acetone, ethanol and deionized water for 10 min-15 min in sequence to remove surface contaminants; after drying with nitrogen, it is dried in an oven at 110℃-120℃ for 30 min-40 min. Step 1.2, Metal Thin Film Deposition: Gold thin films are deposited using electron beam evaporation. Step 1.3, Annealing: Anneal at 300℃-350℃ for 30min-40min to improve the crystal quality and conductivity of the gold film, and obtain the bottom metal reflective layer 1; Step 2: Deposition of the lower dielectric layer 2; Step 2 is as follows: Step 2.1, Spin coating of PTFE dielectric layer: Spin coat the PTFE precursor solution onto the bottom metal reflective layer 1. By precisely controlling the spin coating speed and time, the required dielectric layer thickness is obtained. Step 2.2, Curing treatment: Curing at 150℃-160℃ for 1h-2h to ensure the formation of a stable PTFE film, resulting in the lower dielectric layer 2 on the bottom metal reflective layer 1; Step 3: Preparation of intermediate gold polarization conversion layer 3; Step 3 specifically involves: Step 3.1, Photolithography Patterning: Using electron beam lithography, a structural pattern is defined by two parallel "I"-shaped gold patches arranged diagonally at 45°. Step 3.2, Gold Thin Film Deposition: Gold thin films are deposited using electron beam evaporation; Step 3.3, Lifting and Shaping: The photoresist and excess gold film on it are removed by the lift-off method to obtain the intermediate gold polarization conversion layer 3; Step 4: Preparation of vanadium dioxide thin film layer 4; Step 4 is as follows: Step 4.1, Mask preparation: Based on the designed periodic array pattern, prepare a mask for vanadium dioxide thin film deposition to ensure that the deposition area covers the entire unit cell and forms a continuous thin film layer; Step 4.2, Vanadium dioxide thin film deposition: A continuous vanadium dioxide thin film is deposited on the surface of the intermediate gold polarization conversion layer 3 using pulsed laser deposition (PLD) or magnetron sputtering. Step 4.3, Annealing and crystallization treatment: Annealing is performed at 400℃-450℃ for 20-30 minutes in an oxygen atmosphere to obtain a vanadium dioxide thin film layer with uniform thickness 4; Step 5: Deposition of the upper dielectric layer 5; Step 5 specifically involves: Step 5.1, Spin coating of PTFE dielectric layer: Spin coating of PTFE precursor solution onto vanadium dioxide thin film layer 4, and obtaining the required dielectric layer thickness by precisely controlling the spin coating speed and time; Step 5.2, Curing treatment: Curing at 150℃-160℃ for 1h-2h to ensure the formation of a stable PTFE film, and obtaining the upper dielectric layer 5 on the vanadium dioxide film layer 4; Step 6: Preparation of the top vanadium dioxide resonant layer 6.

[0028] Step 6 specifically involves: Step 6.1, Photolithography defines the vanadium dioxide region: The square vanadium dioxide region and its internal "I"-shaped groove pattern region are defined at the specified location by photolithography; Step 6.2, Vanadium dioxide thin film deposition: Deposit a vanadium dioxide thin film in the designated area using pulsed laser deposition; Step 6.3, Lifting and Shaping: Use the lift-off method to remove the photoresist and the excess vanadium dioxide film on it to obtain the top vanadium dioxide resonant layer 6.

[0029] Example 1 A terahertz absorber based on vanadium dioxide ultrawideband / polarization-switching, such as Figure 1-4 As shown, it is composed of a periodic array of 3×3 switchable ultra-wideband absorption-polarization conversion units with multi-layer structure, and the switchable ultra-wideband absorption-polarization conversion units are distributed in a 3×3 two-dimensional pattern. The ultra-wideband absorption-polarization conversion function switchable unit includes, from bottom to top, a bottom metal reflective layer 1, a lower dielectric layer 2, an intermediate gold polarization conversion layer 3, a vanadium dioxide thin film layer 4, an upper dielectric layer 5, and a top vanadium dioxide resonant layer 6.

[0030] The bottom metal reflective layer 1 is a continuous metal thin film; the material of the bottom metal reflective layer 1 is gold, with an electrical conductivity of 4.56 × 10⁻⁶. 7 The absorber has a thickness t1 of 0.2 µm and a bottom metal reflective layer 1 that is a continuous metal thin film with a length and width equal to the cell period p of the ultra-wideband absorption-polarization conversion switchable unit cell, where p is 20 µm. The thickness t1 is much greater than the skin depth of gold in the terahertz band to ensure that the absorber's transmittance T=0.

[0031] Both the lower dielectric layer 2 and the upper dielectric layer 5 are made of PTFE with a relative permittivity of 2.1. The thickness h1 of the lower dielectric layer 2 is 4µm, and the thickness h2 of the upper dielectric layer 5 is 5.5µm. The length and width of both the lower dielectric layer 2 and the upper dielectric layer 5 are equal to the cell period p.

[0032] The intermediate gold polarization conversion layer 3 is made of gold with an electrical conductivity of 4.56 × 10⁻⁶. 7S / m, thickness t2 is 1.5µm, the intermediate gold polarization conversion layer 3 is composed of two parallel "I" shaped gold patches 3-1 arranged along a 45° diagonal line, wherein the distance l1 between the two "I" shaped gold patches 3-1 is 9.0µm, and the "I" shaped gold patch 3-1 is composed of a first straight arm patch 3-1-1 and second straight arm patches 3-1-2 respectively set at both ends of the first straight arm patch 3-1-1; The length l2 of the first straight arm patch 3-1-1 is 10.0µm, the length l3 of the second straight arm patch 3-1-2 is 5.0µm, the width w1 of the first straight arm patch 3-1-1 is 1.2µm, and the width w2 of the second straight arm patch 3-1-2 is 1.2µm.

[0033] The vanadium dioxide thin film layer 4 is made of vanadium dioxide with a thickness d1 of 0.2µm. The vanadium dioxide thin film layer 4 is a continuous, unpatterned thin film that covers the entire unit cell region. Its length and width are both equal to the unit cell period p.

[0034] The thickness d2 of the top vanadium dioxide resonant layer 6 is 1.5µm. The top vanadium dioxide resonant layer 6 is a square vanadium dioxide thin film with a side length a of 10.0µm. Two mutually perpendicular "I"-shaped grooves 6-1 are opened in the middle of the square vanadium dioxide thin film. The "I"-shaped groove 6-1 is composed of a first straight arm groove 6-1-1 and a second straight arm groove 6-1-2 respectively set at both ends of the first straight arm groove 6-1-1. The first straight arm groove 6-1-1 has an arm length b of 6.0µm and an arm width s1 of 1.0µm. The second straight arm groove 6-1-2 has an arm length c of 4.0µm and an arm width s2 of 1.0µm.

[0035] The fabrication method of a vanadium dioxide-based ultrawideband / polarization-switchable terahertz absorber specifically includes the following steps: Step 1: Preparation of the bottom metal reflective layer 1; Step 1 is as follows: Step 1.1, Substrate cleaning: High-purity silicon was selected as the substrate and ultrasonically cleaned with acetone, ethanol and deionized water for 10 minutes in sequence to remove surface contaminants; after drying with nitrogen, it was dried in an oven at 110℃ for 30 minutes. Step 1.2, Metal Thin Film Deposition: Gold thin films are deposited using electron beam evaporation. Step 1.3, Annealing treatment: Anneal at 300℃ for 30 min to improve the crystal quality and conductivity of the gold film, and obtain the bottom metal reflective layer 1; Step 2: Deposition of the lower dielectric layer 2; Step 2 is as follows: Step 2.1, Spin coating of PTFE dielectric layer: Spin coat the PTFE precursor solution onto the bottom metal reflective layer 1. By precisely controlling the spin coating speed and time, the required dielectric layer thickness is obtained. Step 2.2, Curing treatment: Curing at 150℃ for 1 hour to ensure the formation of a stable PTFE film, resulting in the lower dielectric layer 2 on the bottom metal reflective layer 1; Step 3: Preparation of intermediate gold polarization conversion layer 3; Step 3 specifically involves: Step 3.1, Photolithography Patterning: Using electron beam lithography, a structural pattern is defined by two parallel "I"-shaped gold patches arranged diagonally at 45°. Step 3.2, Gold Thin Film Deposition: Gold thin films are deposited using electron beam evaporation; Step 3.3, Lifting and Shaping: The photoresist and excess gold film on it are removed by the lift-off method to obtain the intermediate gold polarization conversion layer 3; Step 4: Preparation of vanadium dioxide thin film layer 4; Step 4 is as follows: Step 4.1, Mask preparation: Based on the designed periodic array pattern, prepare a mask for vanadium dioxide thin film deposition to ensure that the deposition area covers the entire unit cell and forms a continuous thin film layer; Step 4.2, Vanadium dioxide thin film deposition: A continuous vanadium dioxide thin film is deposited on the surface of the intermediate gold polarization conversion layer 3 using pulsed laser deposition (PLD) or magnetron sputtering. Step 4.3, Annealing and crystallization treatment: Annealing is performed at 400℃ for 20 minutes in an oxygen atmosphere to obtain a vanadium dioxide thin film layer with uniform thickness 4; Step 5: Deposition of the upper dielectric layer 5; Step 5 specifically involves: Step 5.1, Spin coating of PTFE dielectric layer: Spin coating of PTFE precursor solution onto vanadium dioxide thin film layer 4, and obtaining the required dielectric layer thickness by precisely controlling the spin coating speed and time; Step 5.2, Curing treatment: Curing at 150℃ for 1-2 hours to ensure the formation of a stable PTFE film, resulting in the upper dielectric layer 5 on the vanadium dioxide film layer 4; Step 6: Preparation of the top vanadium dioxide resonant layer 6.

[0036] Step 6 specifically involves: Step 6.1, Photolithography defines the vanadium dioxide region: The square vanadium dioxide region and its internal "I"-shaped groove pattern region are defined at the specified location by photolithography; Step 6.2, Vanadium dioxide thin film deposition: Deposit a vanadium dioxide thin film in the designated area using pulsed laser deposition; Step 6.3, Lifting and Shaping: Use the lift-off method to remove the photoresist and the excess vanadium dioxide film on it to obtain the top vanadium dioxide resonant layer 6.

[0037] Figures 5-6 This document describes the cross-polarization reflection coefficient and co-polarization reflection coefficient, as well as the polarization conversion efficiency (PCR) curve of the terahertz device designed in this embodiment, under the vanadium dioxide insulating state. Based on... Figure 5 It can be seen that in the range of 3.96–9.58 THz, the cross-polarization reflection coefficient r_xy remains above 0.8, while the co-polarization reflection coefficient r_yy is always below 0.2, indicating that the incident y-polarized wave is efficiently converted into an x-polarized wave. Therefore, the device proposed in this embodiment has excellent cross-polarization conversion capability in this frequency band. According to Figure 6 It can be seen that the PCR value exceeds 90% within the above frequency band, with a bandwidth of 5.62 THz and a relative bandwidth of about 83%. In particular, the PCR reaches more than 99% at 4.88 THz and 8.20 THz, indicating that efficient broadband linear cross-polarization conversion has been achieved.

[0038] Figures 7-8 This embodiment shows the elliptic angle and polarization rotation azimuth angle of the reflected wave in polarization conversion mode, as well as the electric field decomposition diagrams of the incident and reflected waves. According to... Figure 7 It can be seen that in the range of 3.96–9.58 THz, the polarization rotation azimuth angle θ is approximately -90°, and the elliptic angle η is close to 0°. This further proves that the incident y-polarized electromagnetic wave is almost completely converted into an x-polarized reflected wave, while still maintaining a linear polarization state. According to Figure 8 The diagram showing the electric field decomposition illustrates that after the incident wave is decomposed along the u-axis and v-axis, the reflected wave has equal amplitudes (r_u) and r_v, with a phase difference of -180°. This is the physical basis for achieving cross-polarization conversion.

[0039] Figures 9-11 This embodiment shows the reflection coefficient and phase difference under u-polarized and v-polarized incident light in polarization conversion mode, as well as the surface current distribution of the gold polarization unit layer and the gold substrate. According to... Figure 9 It can be seen that in the range of 3.96–9.58 THz, the amplitudes of r_u and r_v are approximately equal (about 0.8), opposite in sign, and have a phase difference of -180°, satisfying the condition for cross-polarization conversion. According to Figure 10 As shown in the surface current distribution, at 4.88 THz, the currents in the gold layer and on the gold substrate are antiparallel, exciting magnetic resonance; according to Figure 11 As shown, at 8.20 THz, the current is mainly distributed on the "I"-shaped gold unit, which excites electric resonance. The superposition of the two resonances is the fundamental reason for realizing broadband cross-polarization conversion.

[0040] Figures 12-13This embodiment illustrates the effect of polarization angle and incident angle on polarization conversion rate PCR in polarization conversion mode. According to... Figure 12 It can be seen that, since the gold polarization conversion unit makes a 45° angle with the x / y axis, the PCR is symmetrically distributed about the 45° polarization angle. When the polarization angle is less than 10°, the PCR is >90%, and when it is less than 30°, the PCR is >50%, indicating that the device has good tolerance to polarization angle. According to Figure 13 It can be seen that within the incident angle range of 0°–40°, PCR maintains a high level, with no significant changes in operating frequency and bandwidth. In particular, PCR is not sensitive to the incident angle near 4.88 THz and 8.20 THz, showing good stability over a wide incident angle.

[0041] To explain how the device achieves the functional switching from "polarization conversion" to "perfect absorption" through the phase transition of vanadium dioxide, we compared and analyzed the equivalent dielectric parameters and resonance characteristics of the structure in the two states. Figure 14 The absorptivity A, reflectivity R, and transmittance T of the device in the vanadium dioxide metallic state in this embodiment are given. According to... Figure 14 It can be seen that in the range of 5.52–12.60 THz, the absorption rate A exceeds 90%, the bandwidth is about 7.08 THz, and the relative bandwidth is 78%. The absorption rates reach more than 99% at 6.26 THz, 9.77 THz and 12.04 THz, respectively, which is close to perfect absorption. Figure 15 The real and imaginary parts of the normalized equivalent impedance Z_r are given. Within the absorption band, the real part of the normalized impedance is close to 1 and the imaginary part is close to 0, indicating that the equivalent input impedance of the device is perfectly matched with the free space impedance, thus achieving reflection-free absorption. Figure 16 and Figure 17 These are the real and imaginary parts of the equivalent permittivity ε and equivalent permeability μ, respectively. According to Figure 16 and Figure 17 It can be seen that within the absorption band, the real part of the equivalent dielectric constant is almost equal to the real part of the equivalent permeability, the imaginary part of the dielectric constant is close to 0, and the real and imaginary parts of the permeability are similar in value, which fully satisfies the electromagnetic parameter conditions for perfect absorption, especially at the three absorption peak frequencies.

[0042] Figures 18-19 These are top and side views of the electric field distribution in the metallic state of vanadium dioxide in this embodiment. According to... Figure 18 The top view shows that at 6.26 THz, the electric field is concentrated at the top and bottom edges of the square vanadium dioxide; at 9.77 THz, the electric field is confined inside the "I"-shaped groove, resulting in strong terahertz trapping; and at 12.04 THz, the electric field is distributed within the groove and on the dielectric layer. Combined with... Figure 19The side view provides a clearer view of the localized distribution of electromagnetic waves, demonstrating that localized surface plasmon resonance (SPR) is excited. The incident terahertz wave is confined within the vanadium dioxide resonant layer and gradually consumed by the dielectric layer and the underlying vanadium dioxide film, thus achieving perfect absorption.

[0043] Figures 20-21 This embodiment illustrates the effect of polarization angle and incident angle on the absorptivity in absorption mode. According to... Figure 20 It can be seen that, due to the rotational symmetry of the structure, changing the polarization angle has almost no effect on the absorption rate, and the absorber exhibits polarization insensitivity. According to Figure 21 It can be seen that when the incident angle is below 40°, the device can still maintain its wide bandwidth and high absorption (A>90%) characteristics. The good wide-angle absorption characteristics are attributed to the subwavelength structure and simple top-level design, which effectively suppresses parasitic resonance and diffraction effects.

[0044] Example 2 A terahertz absorber based on vanadium dioxide ultrawideband / polarization-switching, such as Figure 1-4 As shown, it is composed of a periodic array of 4×4 switchable ultra-wideband absorption-polarization conversion units with multi-layer structure, and the switchable ultra-wideband absorption-polarization conversion units are distributed in a 4×4 two-dimensional pattern. The ultra-wideband absorption-polarization conversion function switchable unit includes, from bottom to top, a bottom metal reflective layer 1, a lower dielectric layer 2, an intermediate gold polarization conversion layer 3, a vanadium dioxide thin film layer 4, an upper dielectric layer 5, and a top vanadium dioxide resonant layer 6.

[0045] The bottom metal reflective layer 1 is a continuous metal thin film; the material of the bottom metal reflective layer 1 is gold, with an electrical conductivity of 4.56 × 10⁻⁶. 7 The absorber has a thickness t1 of 0.18 µm and a bottom metal reflective layer 1 that is a continuous metal thin film. The length and width of this layer are equal to the cell period p of the ultra-wideband absorption-polarization conversion switchable unit, where p is 19 µm. The thickness t1 is much greater than the skin depth of gold in the terahertz band to ensure that the absorber's transmittance T=0.

[0046] Both the lower dielectric layer 2 and the upper dielectric layer 5 are made of PTFE with a relative permittivity of 2.1. The thickness h1 of the lower dielectric layer 2 is 3.8µm, and the thickness h2 of the upper dielectric layer 5 is 5.3µm. The length and width of both the lower dielectric layer 2 and the upper dielectric layer 5 are equal to the cell period p.

[0047] The intermediate gold polarization conversion layer 3 is made of gold with an electrical conductivity of 4.56 × 10⁻⁶. 7S / m, thickness t2 is 1.4µm, the intermediate gold polarization conversion layer 3 is composed of two parallel "I" shaped gold patches 3-1 arranged along a 45° diagonal line, wherein the distance l1 between the two "I" shaped gold patches 3-1 is 8.8µm, and the "I" shaped gold patch 3-1 is composed of a first straight arm patch 3-1-1 and second straight arm patches 3-1-2 respectively set at both ends of the first straight arm patch 3-1-1; The length l2 of the first straight arm patch 3-1-1 is 9.8µm, the length l3 of the second straight arm patch 3-1-2 is 4.8µm, the width w1 of the first straight arm patch 3-1-1 is 1.1µm, and the width w2 of the second straight arm patch 3-1-2 is 1.1µm.

[0048] The vanadium dioxide thin film layer 4 is made of vanadium dioxide with a thickness d1 of 0.18µm. The vanadium dioxide thin film layer 4 is a continuous, unpatterned thin film that covers the entire unit cell region. Its length and width are both equal to the unit cell period p.

[0049] The thickness d2 of the top vanadium dioxide resonant layer 6 is 1.4µm. The top vanadium dioxide resonant layer 6 is a square vanadium dioxide thin film with a side length a of 9.8µm. Two mutually perpendicular "I"-shaped grooves 6-1 are opened in the middle of the square vanadium dioxide thin film. The "I"-shaped groove 6-1 is composed of a first straight arm groove 6-1-1 and a second straight arm groove 6-1-2 respectively set at both ends of the first straight arm groove 6-1-1. The first straight arm groove 6-1-1 has an arm length b of 5.8µm and an arm width s1 of 0.9µm. The second straight arm groove 6-1-2 has an arm length c of 3.8µm and an arm width s2 of 0.9µm.

[0050] When the ambient temperature is below 340K, vanadium dioxide is in the insulating phase with a conductivity σ of 200S / m, and the terahertz absorber operates in broadband cross-polarization conversion mode; when the ambient temperature is above 340K, vanadium dioxide is in the metallic phase with a conductivity σ of 200000S / m, and the terahertz absorber operates in ultra-wideband absorption mode.

[0051] The fabrication method of a vanadium dioxide-based ultrawideband / polarization-switchable terahertz absorber specifically includes the following steps: Step 1: Preparation of the bottom metal reflective layer 1; Step 1 is as follows: Step 1.1, Substrate cleaning: High-purity silicon was selected as the substrate and ultrasonically cleaned with acetone, ethanol and deionized water for 10 minutes in sequence to remove surface contaminants; after drying with nitrogen, it was dried in an oven at 110℃ for 30 minutes. Step 1.2, Metal Thin Film Deposition: Gold thin films are deposited using electron beam evaporation. Step 1.3, Annealing treatment: Anneal at 300℃ for 30 min to improve the crystal quality and conductivity of the gold film, and obtain the bottom metal reflective layer 1; Step 2: Deposition of the lower dielectric layer 2; Step 2 is as follows: Step 2.1, Spin coating of PTFE dielectric layer: Spin coat the PTFE precursor solution onto the bottom metal reflective layer 1. By precisely controlling the spin coating speed and time, the required dielectric layer thickness is obtained. Step 2.2, Curing treatment: Curing at 150℃ for 1-2 hours to ensure the formation of a stable PTFE film, resulting in the lower dielectric layer 2 on the bottom metal reflective layer 1; Step 3: Preparation of intermediate gold polarization conversion layer 3; Step 3 specifically involves: Step 3.1, Photolithography Patterning: Using electron beam lithography, a structural pattern is defined by two parallel "I"-shaped gold patches arranged diagonally at 45°. Step 3.2, Gold Thin Film Deposition: Gold thin films are deposited using electron beam evaporation; Step 3.3, Lifting and Shaping: The photoresist and excess gold film on it are removed by the lift-off method to obtain the intermediate gold polarization conversion layer 3; Step 4: Preparation of vanadium dioxide thin film layer 4; Step 4 is as follows: Step 4.1, Mask preparation: Based on the designed periodic array pattern, prepare a mask for vanadium dioxide thin film deposition to ensure that the deposition area covers the entire unit cell and forms a continuous thin film layer; Step 4.2, Vanadium dioxide thin film deposition: A continuous vanadium dioxide thin film is deposited on the surface of the intermediate gold polarization conversion layer 3 using pulsed laser deposition (PLD) or magnetron sputtering. Step 4.3, Annealing and crystallization treatment: Annealing is performed at 400℃ for 20 minutes in an oxygen atmosphere to obtain a vanadium dioxide thin film layer with uniform thickness 4; Step 5: Deposition of the upper dielectric layer 5; Step 5 specifically involves: Step 5.1, Spin coating of PTFE dielectric layer: Spin coating of PTFE precursor solution onto vanadium dioxide thin film layer 4, and obtaining the required dielectric layer thickness by precisely controlling the spin coating speed and time; Step 5.2, Curing treatment: Curing at 150℃ for 1-2 hours to ensure the formation of a stable PTFE film, resulting in the upper dielectric layer 5 on the vanadium dioxide film layer 4; Step 6: Preparation of the top vanadium dioxide resonant layer 6.

[0052] Step 6 specifically involves: Step 6.1, Photolithography defines the vanadium dioxide region: The square vanadium dioxide region and its internal "I"-shaped groove pattern region are defined at the specified location by photolithography; Step 6.2, Vanadium dioxide thin film deposition: Deposit a vanadium dioxide thin film in the designated area using pulsed laser deposition; Step 6.3, Lifting and Shaping: Use the lift-off method to remove the photoresist and the excess vanadium dioxide film on it to obtain the top vanadium dioxide resonant layer 6.

[0053] Example 3 A terahertz absorber based on vanadium dioxide ultrawideband / polarization-switching, such as Figure 1-4 As shown, it is composed of 3×4 switchable ultra-wideband absorption-polarization conversion units with multi-layer structure arranged in a periodic array, and the switchable ultra-wideband absorption-polarization conversion units are distributed in a 3×4 two-dimensional pattern. The ultra-wideband absorption-polarization conversion function switchable unit includes, from bottom to top, a bottom metal reflective layer 1, a lower dielectric layer 2, an intermediate gold polarization conversion layer 3, a vanadium dioxide thin film layer 4, an upper dielectric layer 5, and a top vanadium dioxide resonant layer 6.

[0054] The bottom metal reflective layer 1 is a continuous metal thin film; the material of the bottom metal reflective layer 1 is gold, with an electrical conductivity of 4.56 × 10⁻⁶. 7 The absorber has a thickness t1 of 0.22 µm and a bottom metal reflective layer 1 that is a continuous metal thin film. The length and width of this layer are equal to the cell period p of the ultra-wideband absorption-polarization conversion switchable unit, where p is 21 µm. The thickness t1 is much greater than the skin depth of gold in the terahertz band to ensure that the absorber's transmittance T=0.

[0055] Both the lower dielectric layer 2 and the upper dielectric layer 5 are made of PTFE with a relative permittivity of 2.1. The thickness h1 of the lower dielectric layer 2 is 4.2µm, and the thickness h2 of the upper dielectric layer 5 is 5.7µm. The length and width of both the lower dielectric layer 2 and the upper dielectric layer 5 are equal to the cell period p.

[0056] The intermediate gold polarization conversion layer 3 is made of gold with an electrical conductivity of 4.56 × 10⁻⁶. 7 S / m, thickness t2 is 1.6µm, the intermediate gold polarization conversion layer 3 is composed of two parallel "I" shaped gold patches 3-1 arranged along a 45° diagonal line, wherein the distance l1 between the two "I" shaped gold patches 3-1 is 9.2µm, and the "I" shaped gold patch 3-1 is composed of a first straight arm patch 3-1-1 and second straight arm patches 3-1-2 respectively set at both ends of the first straight arm patch 3-1-1; The length l2 of the first straight arm patch 3-1-1 is 10.2µm, the length l3 of the second straight arm patch 3-1-2 is 5.2µm, the width w1 of the first straight arm patch 3-1-1 is 1.3µm, and the width w2 of the second straight arm patch 3-1-2 is 1.3µm.

[0057] The vanadium dioxide thin film layer 4 is made of vanadium dioxide with a thickness d1 of 0.22µm. The vanadium dioxide thin film layer 4 is a continuous, unpatterned thin film that covers the entire unit cell region. Its length and width are both equal to the unit cell period p.

[0058] The thickness d2 of the top vanadium dioxide resonant layer 6 is 1.6µm. The top vanadium dioxide resonant layer 6 is a square vanadium dioxide thin film with a side length a of 10.2µm. Two mutually perpendicular "I"-shaped grooves 6-1 are opened in the middle of the square vanadium dioxide thin film. The "I"-shaped groove 6-1 is composed of a first straight arm groove 6-1-1 and a second straight arm groove 6-1-2 respectively set at both ends of the first straight arm groove 6-1-1. The first straight arm groove 6-1-1 has an arm length b of 6.2 and an arm width s1 of 1.1µm. The second straight arm groove 6-1-2 has an arm length c of 4.2µm and an arm width s2 of 1.1µm.

[0059] When the ambient temperature is below 340K, vanadium dioxide is in the insulating phase with a conductivity σ of 200S / m, and the terahertz absorber operates in broadband cross-polarization conversion mode; when the ambient temperature is above 340K, vanadium dioxide is in the metallic phase with a conductivity σ of 200000S / m, and the terahertz absorber operates in ultra-wideband absorption mode.

[0060] The fabrication method of a vanadium dioxide-based ultrawideband / polarization-switchable terahertz absorber specifically includes the following steps: Step 1: Preparation of the bottom metal reflective layer 1; Step 1 is as follows: Step 1.1, Substrate cleaning: High-purity silicon was selected as the substrate and ultrasonically cleaned with acetone, ethanol and deionized water for 10 minutes in sequence to remove surface contaminants; after drying with nitrogen, it was dried in an oven at 110℃ for 30 minutes. Step 1.2, Metal Thin Film Deposition: Gold thin films are deposited using electron beam evaporation. Step 1.3, Annealing treatment: Anneal at 300℃ for 30 min to improve the crystal quality and conductivity of the gold film, and obtain the bottom metal reflective layer 1; Step 2: Deposition of the lower dielectric layer 2; Step 2 is as follows: Step 2.1, Spin coating of PTFE dielectric layer: Spin coat the PTFE precursor solution onto the bottom metal reflective layer 1. By precisely controlling the spin coating speed and time, the required dielectric layer thickness is obtained. Step 2.2, Curing treatment: Curing at 150℃ for 1-2 hours to ensure the formation of a stable PTFE film, resulting in the lower dielectric layer 2 on the bottom metal reflective layer 1; Step 3: Preparation of intermediate gold polarization conversion layer 3; Step 3 specifically involves: Step 3.1, Photolithography Patterning: Using electron beam lithography, a structural pattern is defined by two parallel "I"-shaped gold patches arranged diagonally at 45°. Step 3.2, Gold Thin Film Deposition: Gold thin films are deposited using electron beam evaporation; Step 3.3, Lifting and Shaping: The photoresist and excess gold film on it are removed by the lift-off method to obtain the intermediate gold polarization conversion layer 3; Step 4: Preparation of vanadium dioxide thin film layer 4; Step 4 is as follows: Step 4.1, Mask preparation: Based on the designed periodic array pattern, prepare a mask for vanadium dioxide thin film deposition to ensure that the deposition area covers the entire unit cell and forms a continuous thin film layer; Step 4.2, Vanadium dioxide thin film deposition: A continuous vanadium dioxide thin film is deposited on the surface of the intermediate gold polarization conversion layer 3 using pulsed laser deposition (PLD) or magnetron sputtering. Step 4.3, Annealing and crystallization treatment: Annealing is performed at 400℃ for 20 minutes in an oxygen atmosphere to obtain a vanadium dioxide thin film layer with uniform thickness 4; Step 5: Deposition of the upper dielectric layer 5; Step 5 specifically involves: Step 5.1, Spin coating of PTFE dielectric layer: Spin coating of PTFE precursor solution onto vanadium dioxide thin film layer 4, and obtaining the required dielectric layer thickness by precisely controlling the spin coating speed and time; Step 5.2, Curing treatment: Curing at 150℃ for 1-2 hours to ensure the formation of a stable PTFE film, resulting in the upper dielectric layer 5 on the vanadium dioxide film layer 4; Step 6: Preparation of the top vanadium dioxide resonant layer 6.

[0061] Step 6 specifically involves: Step 6.1, Photolithography defines the vanadium dioxide region: The square vanadium dioxide region and its internal "I"-shaped groove pattern region are defined at the specified location by photolithography; Step 6.2, Vanadium dioxide thin film deposition: Deposit a vanadium dioxide thin film in the designated area using pulsed laser deposition; Step 6.3, Lifting and Shaping: Use the lift-off method to remove the photoresist and the excess vanadium dioxide film on it to obtain the top vanadium dioxide resonant layer 6.

[0062] Example 4 A terahertz absorber based on vanadium dioxide ultrawideband / polarization-switching, such as Figure 1-4 As shown, it is composed of 3×4 switchable ultra-wideband absorption-polarization conversion units with multi-layer structure arranged in a periodic array, and the switchable ultra-wideband absorption-polarization conversion units are distributed in a 3×4 two-dimensional pattern. The ultra-wideband absorption-polarization conversion function switchable unit includes, from bottom to top, a bottom metal reflective layer 1, a lower dielectric layer 2, an intermediate gold polarization conversion layer 3, a vanadium dioxide thin film layer 4, an upper dielectric layer 5, and a top vanadium dioxide resonant layer 6.

[0063] The bottom metal reflective layer 1 is a continuous metal thin film; the material of the bottom metal reflective layer 1 is gold, with an electrical conductivity of 4.56 × 10⁻⁶. 7 The absorber has a thickness t1 of 0.19 µm and a bottom metal reflective layer 1 that is a continuous metal thin film. The length and width of this layer are equal to the cell period p of the ultra-wideband absorption-polarization conversion switchable unit, which is 19.5 µm. The thickness t1 is much greater than the skin depth of gold in the terahertz band to ensure that the absorber's transmittance T=0.

[0064] Both the lower dielectric layer 2 and the upper dielectric layer 5 are made of PTFE with a relative permittivity of 2.1. The thickness h1 of the lower dielectric layer 2 is 3.9µm, and the thickness h2 of the upper dielectric layer 5 is 5.4µm. The length and width of both the lower dielectric layer 2 and the upper dielectric layer 5 are equal to the cell period p.

[0065] The intermediate gold polarization conversion layer 3 is made of gold with an electrical conductivity of 4.56 × 10⁻⁶. 7 S / m, thickness t2 is 1.45µm, the intermediate gold polarization conversion layer 3 is composed of two parallel "I" shaped gold patches 3-1 arranged along a 45° diagonal line, wherein the distance l1 between the two "I" shaped gold patches 3-1 is 8.9µm, and the "I" shaped gold patch 3-1 is composed of a first straight arm patch 3-1-1 and second straight arm patches 3-1-2 respectively set at both ends of the first straight arm patch 3-1-1; The length l2 of the first straight arm patch 3-1-1 is 9.9µm, the length l3 of the second straight arm patch 3-1-2 is 4.9µm, the width w1 of the first straight arm patch 3-1-1 is 1.15µm, and the width w2 of the second straight arm patch 3-1-2 is 1.15µm.

[0066] The vanadium dioxide thin film layer 4 is made of vanadium dioxide with a thickness d1 of 0.19µm. The vanadium dioxide thin film layer 4 is a continuous, unpatterned thin film that covers the entire unit cell region. Its length and width are both equal to the unit cell period p.

[0067] The thickness d2 of the top vanadium dioxide resonant layer 6 is 1.45µm. The top vanadium dioxide resonant layer 6 is a square vanadium dioxide thin film with a side length a of 9.9µm. Two mutually perpendicular "I"-shaped grooves 6-1 are opened in the middle of the square vanadium dioxide thin film. The "I"-shaped groove 6-1 is composed of a first straight arm groove 6-1-1 and a second straight arm groove 6-1-2 respectively set at both ends of the first straight arm groove 6-1-1. The first straight arm groove 6-1-1 has an arm length b of 5.9µm and an arm width s1 of 0.95µm. The second straight arm groove 6-1-2 has an arm length c of 3.9µm and an arm width s2 of 0.95µm.

[0068] When the ambient temperature is below 340K, vanadium dioxide is in the insulating phase with a conductivity σ of 200S / m, and the terahertz absorber operates in broadband cross-polarization conversion mode; when the ambient temperature is above 340K, vanadium dioxide is in the metallic phase with a conductivity σ of 200000S / m, and the terahertz absorber operates in ultra-wideband absorption mode.

[0069] Example 5 A terahertz absorber based on vanadium dioxide ultrawideband / polarization-switching, such as Figure 1-4 As shown, it is composed of 3×4 switchable ultra-wideband absorption-polarization conversion units with multi-layer structure arranged in a periodic array, and the switchable ultra-wideband absorption-polarization conversion units are distributed in a 3×4 two-dimensional pattern. The ultra-wideband absorption-polarization conversion function switchable unit includes, from bottom to top, a bottom metal reflective layer 1, a lower dielectric layer 2, an intermediate gold polarization conversion layer 3, a vanadium dioxide thin film layer 4, an upper dielectric layer 5, and a top vanadium dioxide resonant layer 6.

[0070] The bottom metal reflective layer 1 is a continuous metal thin film; the material of the bottom metal reflective layer 1 is gold, with an electrical conductivity of 4.56 × 10⁻⁶. 7 The absorber has a thickness t1 of 0.21 µm and a bottom metal reflective layer 1 that is a continuous metal thin film. The length and width of this layer are equal to the cell period p of the ultra-wideband absorption-polarization conversion switchable unit, which is 20.5 µm. The thickness t1 is much greater than the skin depth of gold in the terahertz band to ensure that the absorber's transmittance T = 0.

[0071] Both the lower dielectric layer 2 and the upper dielectric layer 5 are made of PTFE with a relative permittivity of 2.1. The thickness h1 of the lower dielectric layer 2 is 4.1µm, and the thickness h2 of the upper dielectric layer 5 is 5.6µm. The length and width of both the lower dielectric layer 2 and the upper dielectric layer 5 are equal to the cell period p.

[0072] The intermediate gold polarization conversion layer 3 is made of gold with an electrical conductivity of 4.56 × 10⁻⁶. 7S / m, thickness t2 is 1.55µm, the intermediate gold polarization conversion layer 3 is composed of two parallel "I" shaped gold patches 3-1 arranged along a 45° diagonal line, wherein the distance l1 between the two "I" shaped gold patches 3-1 is 9.1µm, and the "I" shaped gold patch 3-1 is composed of a first straight arm patch 3-1-1 and second straight arm patches 3-1-2 respectively set at both ends of the first straight arm patch 3-1-1; The length l2 of the first straight arm patch 3-1-1 is 10.1µm, the length l3 of the second straight arm patch 3-1-2 is 5.1µm, the width w1 of the first straight arm patch 3-1-1 is 1.25µm, and the width w2 of the second straight arm patch 3-1-2 is 1.25µm.

[0073] The vanadium dioxide thin film layer 4 is made of vanadium dioxide with a thickness d1 of 0.21µm. The vanadium dioxide thin film layer 4 is a continuous, unpatterned thin film that covers the entire unit cell region. Its length and width are both equal to the unit cell period p.

[0074] The thickness d2 of the top vanadium dioxide resonant layer 6 is 1.55µm. The top vanadium dioxide resonant layer 6 is a square vanadium dioxide thin film with a side length a of 10.1µm. Two mutually perpendicular "I"-shaped grooves 6-1 are opened in the middle of the square vanadium dioxide thin film. The "I"-shaped groove 6-1 is composed of a first straight arm groove 6-1-1 and a second straight arm groove 6-1-2 respectively set at both ends of the first straight arm groove 6-1-1. The first straight arm groove 6-1-1 has an arm length b of 6.1µm and an arm width s1 of 1.05µm. The second straight arm groove 6-1-2 has an arm length c of 4.1µm and an arm width s2 of 1.05µm.

[0075] When the ambient temperature is below 340K, vanadium dioxide is in the insulating phase with a conductivity σ of 200S / m, and the terahertz absorber operates in broadband cross-polarization conversion mode; when the ambient temperature is above 340K, vanadium dioxide is in the metallic phase with a conductivity σ of 200000S / m, and the terahertz absorber operates in ultra-wideband absorption mode.

[0076] Example 6 A terahertz absorber based on vanadium dioxide ultrawideband / polarization-switching, such as Figure 1-4 As shown, it is composed of 3×4 switchable ultra-wideband absorption-polarization conversion units with multi-layer structure arranged in a periodic array, and the switchable ultra-wideband absorption-polarization conversion units are distributed in a 3×4 two-dimensional pattern. The ultra-wideband absorption-polarization conversion function switchable unit includes, from bottom to top, a bottom metal reflective layer 1, a lower dielectric layer 2, an intermediate gold polarization conversion layer 3, a vanadium dioxide thin film layer 4, an upper dielectric layer 5, and a top vanadium dioxide resonant layer 6.

[0077] The bottom metal reflective layer 1 is a continuous metal thin film; the material of the bottom metal reflective layer 1 is gold, with an electrical conductivity of 4.56 × 10⁻⁶. 7 The absorber has a thickness t1 of 0.21 µm and a bottom metal reflective layer 1 that is a continuous metal thin film. The length and width of this layer are equal to the cell period p of the ultra-wideband absorption-polarization conversion switchable unit, which is 19.5 µm. The thickness t1 is much greater than the skin depth of gold in the terahertz band to ensure that the absorber's transmittance T = 0.

[0078] Both the lower dielectric layer 2 and the upper dielectric layer 5 are made of PTFE with a relative permittivity of 2.1. The thickness h1 of the lower dielectric layer 2 is 3.9µm, and the thickness h2 of the upper dielectric layer 5 is 5.6µm. The length and width of both the lower dielectric layer 2 and the upper dielectric layer 5 are equal to the cell period p.

[0079] The intermediate gold polarization conversion layer 3 is made of gold with an electrical conductivity of 4.56 × 10⁻⁶. 7 S / m, thickness t2 is 1.45µm, the intermediate gold polarization conversion layer 3 is composed of two parallel "I" shaped gold patches 3-1 arranged along a 45° diagonal, wherein the distance l1 between the two "I" shaped gold patches 3-1 is 9.1µm, and the "I" shaped gold patch 3-1 is composed of a first straight arm patch 3-1-1 and second straight arm patches 3-1-2 respectively set at both ends of the first straight arm patch 3-1-1; The length l2 of the first straight arm patch 3-1-1 is 9.9µm, the length l3 of the second straight arm patch 3-1-2 is 4.9µm, the width w1 of the first straight arm patch 3-1-1 is 1.25µm, and the width w2 of the second straight arm patch 3-1-2 is 1.15µm.

[0080] The vanadium dioxide thin film layer 4 is made of vanadium dioxide with a thickness d1 of 0.19µm. The vanadium dioxide thin film layer 4 is a continuous, unpatterned thin film that covers the entire unit cell region. Its length and width are both equal to the unit cell period p.

[0081] The thickness d2 of the top vanadium dioxide resonant layer 6 is 1.55µm. The top vanadium dioxide resonant layer 6 is a square vanadium dioxide thin film with a side length a of 9.9µm. Two mutually perpendicular "I"-shaped grooves 6-1 are opened in the middle of the square vanadium dioxide thin film. The "I"-shaped grooves 6-1 are composed of the first straight arm groove 6-1-1 and the second straight arm grooves 6-1-2 respectively set at both ends of the first straight arm groove 6-1-1. The first straight arm groove 6-1-1 has an arm length b of 6.1µm and an arm width s1 of 0.95µm. The second straight arm groove 6-1-2 has an arm length c of 3.9µm and an arm width s2 of 1.05µm.

[0082] When the ambient temperature is below 340K, vanadium dioxide is in the insulating phase with a conductivity σ of 200S / m, and the terahertz absorber operates in broadband cross-polarization conversion mode; when the ambient temperature is above 340K, vanadium dioxide is in the metallic phase with a conductivity σ of 200000S / m, and the terahertz absorber operates in ultra-wideband absorption mode.

Claims

1. A terahertz absorber based on vanadium dioxide ultrawideband / polarization-switching, characterized in that, It is composed of M×N switchable ultra-wideband absorption-polarization conversion function units with multi-layer structure arranged in a periodic array. The switchable ultra-wideband absorption-polarization conversion function units are distributed in an M×N two-dimensional distribution, where M and N are both positive integers. The ultra-wideband absorption-polarization conversion function switchable unit includes, from bottom to top, a bottom metal reflective layer (1), a lower dielectric layer (2), an intermediate gold polarization conversion layer (3), a vanadium dioxide thin film layer (4), an upper dielectric layer (5), and a top vanadium dioxide resonant layer (6).

2. The vanadium dioxide-based ultrawideband / polarization-switchable terahertz absorber according to claim 1, characterized in that, The bottom metal reflective layer (1) is a continuous metal thin film; the material of the bottom metal reflective layer (1) is gold, and its electrical conductivity is 4.56 × 10⁻⁶. 7 S / m, thickness t1 is 0.18µm-0.22µm, the bottom metal reflective layer (1) is a continuous metal thin film, the length and width of which are equal to the cell period p of the ultra-wideband absorption-polarization conversion function switchable unit, p is 19µm-21µm.

3. The vanadium dioxide-based ultrawideband / polarization-switchable terahertz absorber according to claim 1, characterized in that, The lower dielectric layer (2) and the upper dielectric layer (5) are both made of PTFE with a relative permittivity of 2.

1. The thickness h1 of the lower dielectric layer (2) is 3.8µm-4.2µm, and the thickness h2 of the upper dielectric layer (5) is 5.3µm-5.7µm. The length and width of the lower dielectric layer (2) and the upper dielectric layer (5) are both equal to the cell period p.

4. The vanadium dioxide-based ultrawideband / polarization-switchable terahertz absorber according to claim 1, characterized in that, The intermediate gold polarization conversion layer (3) is made of gold with an electrical conductivity of 4.56 × 10⁻⁶. 7 S / m, thickness t2 is 1.4µm-1.6µm, the intermediate gold polarization conversion layer (3) is composed of two parallel "I" shaped gold patches (3-1) arranged along a 45° diagonal line, wherein the distance l1 between the two "I" shaped gold patches (3-1) is 8.8µm-9.2µm, and the "I" shaped gold patch (3-1) is composed of a first straight arm patch (3-1-1) and a second straight arm patch (3-1-2) respectively set at both ends of the first straight arm patch (3-1-1); The length l2 of the first straight arm patch (3-1-1) is 9.8µm-10.2µm, the length l3 of the second straight arm patch (3-1-2) is 4.8µm-5.2µm, the width w1 of the first straight arm patch (3-1-1) is 1.1µm-1.3µm, and the width w2 of the second straight arm patch (3-1-2) is 1.1µm-1.3µm.

5. The vanadium dioxide-based ultrawideband / polarization-switchable terahertz absorber according to claim 1, characterized in that, The vanadium dioxide thin film layer (4) is made of vanadium dioxide, with a thickness d1 of 0.18µm-0.22µm. The vanadium dioxide thin film layer (4) is a continuous unpatterned film that covers the entire unit cell region, and its length and width are both equal to the unit cell period p.

6. The vanadium dioxide-based ultrawideband / polarization-switchable terahertz absorber according to claim 1, characterized in that, The thickness d2 of the top vanadium dioxide resonant layer (6) is 1.4µm-1.6µm. The top vanadium dioxide resonant layer (6) is a square vanadium dioxide film with a side length a of 9.8µm-10.2µm. Two mutually perpendicular "I" shaped grooves (6-1) are opened in the middle of the square vanadium dioxide film. The "I" shaped groove (6-1) is composed of a first straight arm groove (6-1-1) and a second straight arm groove (6-1-2) respectively set at both ends of the first straight arm groove (6-1-1). The first straight arm groove (6-1-1) has an arm length b of 5.8µm-6.2µm and an arm width s1 of 0.9µm-1.1µm. The second straight arm groove (6-1-2) has an arm length c of 3.8µm-4.2µm and an arm width s2 of 0.9µm-1.1µm.

7. The method for preparing a vanadium dioxide-based ultrawideband / polarization-switchable terahertz absorber according to any one of claims 1-6, characterized in that, Specifically, the following steps are included: Step 1: Preparation of the bottom metal reflective layer (1); Step 2, deposition of the lower dielectric layer (2); Step 3: Preparation of the intermediate gold polarization conversion layer (3); Step 4: Preparation of vanadium dioxide thin film layer (4); Step 5: Deposition of the upper dielectric layer (5); Step 6: Preparation of the top vanadium dioxide resonant layer (6).

8. The method for preparing a vanadium dioxide-based ultrawideband / polarization-switchable terahertz absorber according to claim 7, characterized in that, Step 1 is as follows: Step 1.1, Substrate cleaning: High-purity silicon is selected as the substrate, and ultrasonically cleaned with acetone, ethanol and deionized water for 10 min-15 min in sequence to remove surface contaminants; after drying with nitrogen, it is dried in an oven at 110℃-120℃ for 30 min-40 min. Step 1.2, Metal Thin Film Deposition: Gold thin films are deposited using electron beam evaporation. Step 1.3, Annealing: Anneal at 300℃-350℃ for 30min-40min to improve the crystal quality and conductivity of the gold film and obtain the bottom metal reflective layer (1). Step 2 is as follows: Step 2.1, Spin coating of PTFE dielectric layer: Spin coating of PTFE precursor solution onto bottom metal reflective layer (1), and obtaining the required dielectric layer thickness by precisely controlling the spin coating speed and time; Step 2.2, Curing treatment: Curing at 150℃-160℃ for 1h-2h to ensure the formation of a stable PTFE film, and obtaining the lower dielectric layer (2) on the bottom metal reflective layer (1). Step 3 specifically involves: Step 3.1, Photolithography Patterning: Using electron beam lithography, a structural pattern is defined by two parallel "I"-shaped gold patches arranged diagonally at 45°. Step 3.2, Gold Thin Film Deposition: Gold thin films are deposited using electron beam evaporation; Step 3.3, Lifting and Shaping: Use the lift-off method to remove the photoresist and the excess gold film on it to obtain the intermediate gold polarization conversion layer (3); Step 4 is as follows: Step 4.1, Mask preparation: Based on the designed periodic array pattern, prepare a mask for vanadium dioxide thin film deposition to ensure that the deposition area covers the entire unit cell and forms a continuous thin film layer; Step 4.2, Vanadium dioxide thin film deposition: A continuous vanadium dioxide thin film is deposited on the surface of the intermediate gold polarization conversion layer (3) by pulsed laser deposition or magnetron sputtering. Step 4.3, Annealing and crystallization treatment: Annealing is performed at 400℃-450℃ for 20-30 minutes in an oxygen atmosphere to obtain a vanadium dioxide thin film layer with uniform thickness (4). Step 5 specifically involves: Step 5.1, Spin coating of PTFE dielectric layer: Spin coating of PTFE precursor solution onto vanadium dioxide thin film layer (4), and obtaining the required dielectric layer thickness by precisely controlling the spin coating speed and time; Step 5.2, Curing treatment: Curing at 150℃-160℃ for 1h-2h to ensure the formation of a stable PTFE film, and obtaining the upper dielectric layer (5) on the vanadium dioxide film layer (4). Step 6 specifically involves: Step 6.1, Photolithography to define the vanadium dioxide region: The square vanadium dioxide region and its internal "I"-shaped groove pattern region are defined at the specified location by photolithography; Step 6.2, Vanadium dioxide thin film deposition: Deposit a vanadium dioxide thin film in the designated area using pulsed laser deposition; Step 6.3, stripping and shaping: Use the stripping method to remove the photoresist and the excess vanadium dioxide film on it to obtain the top vanadium dioxide resonant layer (6).

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