Terahertz multifunctional metasurface capable of tuning and switching functions

Abstract

The invention provides a terahertz multifunctional metasurface capable of tuning and switching functions, which is used for solving the problem that an existing metasurface cannot integrate and realize multifunctional switching of absorption and polarization conversion and dynamic regulation and control in a single device. The design has a six-layer structure from top to bottom: the first layer is a photosensitive silicon pattern layer, the second layer is a silicon dioxide dielectric layer, the third layer is a vanadium dioxide regulation and control layer, the fourth layer is a gold pattern layer, the fifth layer is a silicon dioxide dielectric layer, and the sixth layer is a metal bottom plate. By changing the state of vanadium dioxide, switching from the wave absorber to the polarization converter function can be achieved, by adjusting the conductivity of photosensitive silicon, dynamic regulation and control over the broadband absorption rate, the linear polarization conversion rate and the ellipticity can be achieved, and the wide application value is achieved in the aspect of multifunctional terahertz devices such as communication, sensing and electromagnetic stealth.

Description

Technical Field

[0001] The present invention belongs to the technical field of terahertz electromagnetic metasurfaces, and in particular relates to a terahertz multifunctional metasurface capable of tuning and function switching. Background Art

[0002] Terahertz technology has attracted widespread attention due to its unique applications in imaging, communication, and sensing. However, due to the non-ionizing nature of terahertz radiation, the electromagnetic response of traditional materials in nature is very weak. Fortunately, the emergence of artificial composite metasurfaces has effectively solved the above problems. Metasurfaces are two-dimensional metamaterials composed of a planar inhomogeneous particle array containing subwavelength dielectrics and metal elements. Due to their unique electromagnetic properties, such as negative magnetic permeability and inverse Doppler effect, metasurfaces have a wide range of applications and broad development prospects in infrared imaging, electromagnetic cloaks, and modulators. At present, many scholars have studied various terahertz devices based on metasurfaces, such as absorbers and polarization converters. The absorber can absorb and convert most of the electromagnetic waves incident on the metasurface into other forms with almost no reflection. The polarization converter can control the polarization mode of the electromagnetic wave according to actual needs to achieve line-to-line or line-to-circular polarization conversion.

[0003] However, most terahertz devices are not easy to adjust after manufacturing and have relatively simple functions. They are mainly used to solve the shortcomings of traditional materials and devices in some aspects. For example, metasurface absorbers mainly solve the problems of absorptivity, bandwidth and thickness, and polarization converters mainly study the problems of conversion efficiency, bandwidth and compact structure, which seriously limit their practical applications. However, with the continuous deepening of metasurface research, people use metasurfaces to control the amplitude and wavefront of electromagnetic waves to achieve different functions. In particular, researchers have innovatively arranged some subwavelength structural units on metasurfaces to achieve flexible control of electromagnetic wave amplitude and polarization. So far, although many researchers have explored dual-function devices that can switch between absorption and polarization conversion or between line-to-line and line-to-circular polarization conversion, the research on multifunctional, high-performance and dynamically adjustable metasurface devices is still insufficient. Therefore, for practical applications, it is crucial to study multifunctional metasurfaces with wider working bandwidth, simple structure and dynamic adjustability to develop highly integrated devices. Summary of the invention

[0004] In view of the problems of the single function, non-dynamic regulation and polarization sensitivity of the currently proposed terahertz metasurface, the present invention proposes a tunable and function-switching terahertz multifunctional metasurface. By adjusting the conductivity value of vanadium dioxide, the absorber, linear-to-linear polarization converter and linear-to-circular polarization converter can be freely switched in the same structure. At the same time, by adjusting the conductivity of photosensitive silicon, the absorption rate, linear polarization conversion rate and ellipticity can be dynamically adjusted.

[0005] To achieve the above object, the technical solution adopted by the present invention is:

[0006] A tunable and function-switchable terahertz multifunctional metasurface, the structure includes six layers: from top to bottom, the first layer is a photosensitive silicon pattern layer, the second layer is a silicon dioxide dielectric layer, the third layer is a vanadium dioxide control layer, the fourth layer is a gold pattern layer, the fifth layer is a silicon dioxide dielectric layer, and the sixth layer is a metal bottom plate. The unit structure is periodically arranged along the x and y coordinate axes in the o-xyz space coordinate system, and a multifunctional metasurface is formed by extension, with o being the origin of the coordinate axis.

[0007] According to the multifunctional metasurface design method of the present invention, the first photosensitive silicon pattern layer is composed of a ring and a "flower shape", and the ring is located in the middle of the outer "flower shape", so that the photosensitive silicon pattern layer presents a centrally symmetrical shape.

[0008] According to the multifunctional metasurface design method of the present invention, the fourth gold pattern layer is composed of two "bracket"-type patterns, making them diagonally symmetrical.

[0009] According to the multifunctional metasurface design method of the present invention, the second and fifth silicon dioxide dielectric layers and the third vanadium dioxide control layer are consistent with the length and width of the underlying metal plate, but have different thicknesses.

[0010] According to the multifunctional metasurface design method of the present invention, the second and fifth dielectric layers are made of silicon dioxide material with a dielectric constant of 3.75.

[0011] According to the multifunctional metasurface design method of the present invention, the relative dielectric constant of the vanadium dioxide is expressed as follows:

[0012]

[0013] Where ε represents the relative dielectric constant, VO 2 represents vanadium dioxide, ε VO2 Represents the relative dielectric constant of vanadium dioxide, τ represents the volume fraction of vanadium dioxide in the metal phase that changes with the external temperature, which is approximately 0 in the insulating stage and its maximum value is 0.95. ε 1represents the relative dielectric constant of vanadium dioxide in the insulating state, ε 2 represents the relative dielectric constant of vanadium dioxide in the metallic state.

[0014]

[0015] Where σ represents the conductivity, represents the conductivity of vanadium dioxide, ω represents the incident angle frequency, i represents the imaginary unit, and when vanadium dioxide is in an insulating state, When vanadium dioxide is in the metallic state, 10 5 S / m.

[0016] According to the multifunctional metasurface design method of the present invention, the conductivity of the photosensitive silicon is expressed as follows: σ PSi =4.863×10 -4 ×q 2 +0.1856×q+1.569

[0017] Where σ represents conductivity, PSi represents photosensitive silicon, σ PSi represents the conductivity of photosensitive silicon, q represents the energy flux of the pump beam, and when the photosensitive silicon is in an insulating state, σ PSi =1S / m, when the photosensitive silicon is in the metallic state, σ PSi =1×10 5 S / m.

[0018] According to the multifunctional metasurface design method of the present invention, the absorptivity A(ω) of the multifunctional metasurface satisfies:

[0019]

[0020] Where R(ω) and T(ω) represent reflectivity and transmittance respectively, i represents the imaginary unit, and Z 1 represents the impedance of the metasurface, n represents the equivalent medium complex refractive index, k represents the wave vector in the medium, and l represents the thickness of the medium layer. Since the bottom layer of the metasurface is a metal base plate, electromagnetic waves will not be transmitted, that is, T(ω) = 0.

[0021] According to the multifunctional metasurface design method of the present invention, the reflectivity R(ω) of the multifunctional metasurface is obtained as follows:

[0022]

[0023] Where Z 0 is the impedance in free space, μ r and ε r represent the equivalent magnetic permeability and equivalent dielectric constant of the metasurface, respectively.

[0024] According to the multifunctional metasurface design method of the present invention, the error control function C of the multifunctional metasurface reflectivity R(ω) R (μ r ,ε r ,S 11 ) can be expressed as:

[0025]

[0026] Where S 11 represents the reflection coefficient, and further the control function F of the multifunctional metasurface absorptivity can be obtained C (A(ω)):

[0027]

[0028] According to the multifunctional metasurface design method of the present invention, the linear polarization conversion rate ξ of the multifunctional metasurface satisfies:

[0029]

[0030] Where r xy represents the cross-polarization reflection coefficient, r yy Represents the co-polarization reflection coefficient. Linear polarization conversion rate ξ error control function C ξ It can be expressed as follows:

[0031]

[0032] In the formula represents the reflection phase difference, β represents the expected value of the linear polarization conversion rate, and the control function Γ of the linear polarization conversion rate of the multifunctional metasurface can be further obtained. C (ξ):

[0033]

[0034] According to the multifunctional metasurface design method of the present invention, the linear-to-circular polarization conversion degree of the multifunctional metasurface is analyzed by the ellipticity χ, and the ellipticity χ satisfies:

[0035]

[0036] Where, the subscripts m and m+1 represent any two adjacent frequency points within the working bandwidth, and χ m and χ m+1 They respectively represent the ellipticity fitting values ​​obtained by simulation at the mth and m+1th frequency points, and α represents the relative error value of the ellipticity.

[0037] According to the multifunctional metasurface design method of the present invention, the method for obtaining the ellipticity χ of the multifunctional metasurface is:

[0038]

[0039] In the formula, r xy represents the cross-polarization reflection coefficient, r yy represents the co-polarization reflection coefficient, represents the reflection phase difference,

[0040] We can further obtain the control function E of the ellipticity C (x):

[0041]

[0042] According to the multifunctional metasurface design method of the present invention, the multifunctional metasurface can use a control model Φ(A(ω),ξ,χ) to simultaneously manipulate the absorptivity, linear polarization conversion rate, and ellipticity. The control model Φ(A(ω),ξ,χ) of the multifunctional metasurface is obtained as follows:

[0043] Φ(A(w),x,c)=l 1 F C (A(w))+l 2 Γ C (x)+l 3 E C (c)

[0044] In the formula, λ 1 represents the absorption rate influencing factor, λ 2 represents the linear polarization conversion rate influencing factor, λ 3 represents the ellipticity influence factor. Further, the multifunctional metasurface control model Φ(A(ω),ξ,χ) can be rewritten as:

[0045]

[0046] Beneficial effects of the present invention

[0047] (1) The present invention proposes a tunable and function-switchable terahertz multifunctional metasurface, which introduces the tunable metamaterial vanadium dioxide into the metasurface, i.e., does not change the structure of the metasurface itself. By adjusting the conductivity of vanadium dioxide, the designed multifunctional metasurface can realize switching between the multiple functions of absorber, linear-to-linear polarization converter and linear-to-circular polarization converter.

[0048] (2) The present invention proposes a tunable and function-switchable terahertz multifunctional metasurface, which introduces the adjustable metamaterial photosensitive silicon into the metasurface. By adjusting the conductivity of the photosensitive silicon, the designed multifunctional metasurface can achieve dynamic regulation of the absorptivity, linear polarization conversion rate and ellipticity.

[0049] (3) The tunable and switchable terahertz multifunctional metasurface proposed in the present invention can independently respond to the absorptivity, linear polarization conversion rate and ellipticity through control functions. In the absorption working mode, the relationship between the electromagnetic wave reflectivity, reflection coefficient and absorptivity is used to achieve high-value absorptivity; in the linear to linear polarization conversion working mode, the orthogonal polarization rotation is maximized based on the relationship between the phase difference and the reflection coefficient; in the linear to circular polarization conversion working mode, the fitting value of the ellipticity is obtained by using two adjacent frequency points, making the result more accurate, so that the multifunctional metasurface structure can absorb and polarize electromagnetic waves to the maximum extent in the three working modes.

[0050] (4) The tunable and function-switchable terahertz multifunctional metasurface proposed in the present invention realizes numerical simulation calculation of the absorptivity, linear polarization conversion rate and ellipticity of the multifunctional metasurface according to the given reflection coefficient, cross-polarization reflection coefficient, co-polarization reflection coefficient and reflection phase in the working frequency band. Based on the multifunctional metasurface control model, the absorptivity, linear polarization conversion rate and ellipticity can be dynamically controlled by the influencing factors.

[0051] (5) The tunable and function-switchable terahertz multifunctional metasurface proposed in the present invention has the ability to switch between absorber to linear polarization converter and linear to circular polarization converter in a single device, and can dynamically tune the absorption rate, linear polarization conversion rate and ellipticity. In addition, the present invention has a wide relative bandwidth in both the absorption mode and the two polarization conversion modes. The relative bandwidths in the absorption, linear polarization conversion, and linear to circular polarization conversion modes reach 72.42%, 97.92% and 22.07%, respectively, and has potential application value in modulation, sensing, photoelectric detection and other multifunctional devices. BRIEF DESCRIPTION OF THE DRAWINGS

[0052] Figure 1 Schematic diagram of the three-dimensional structure of the designed tunable and function-switchable terahertz multifunctional metasurface unit;

[0053] Figure 2 A top-down view of the top pattern layer of the designed tunable and function-switchable terahertz multifunctional metasurface;

[0054] Figure 3 A side view schematic diagram of the designed tunable and function-switchable terahertz multifunctional metasurface unit structure;

[0055] Figure 4 A top-down view of the fourth gold pattern layer of the designed tunable and function-switchable terahertz multifunctional metasurface;

[0056] Figure 5The change curve of the absorption rate of the designed tunable and switchable terahertz multifunctional metasurface working in the absorption mode when the conductivity of the photosensitive silicon is different;

[0057] Figure 6 The variation curve of the wave absorption rate of the designed tunable and switchable terahertz multifunctional metasurface when it works in the wave absorption mode when the thickness of the second silicon dioxide dielectric layer is different;

[0058] Figure 7 The variation curve of the wave absorption rate of the designed tunable and switchable terahertz multifunctional metasurface when it works in the wave absorption mode, when the thickness of the fifth silicon dioxide dielectric layer is different;

[0059] Figure 8 The co-polarization reflection coefficient and cross-polarization reflection coefficient obtained when the designed tunable and function-switchable terahertz multifunctional metasurface works in polarization conversion mode;

[0060] Fig. 9 The linear polarization conversion rate curve obtained for the designed tunable and function-switchable terahertz multifunctional metasurface working in the linear-to-linear polarization conversion mode;

[0061] Fig.10 Phase difference curve obtained for the designed tunable and function-switchable terahertz multifunctional metasurface working in linear to circular polarization conversion mode;

[0062] Fig.11 The ellipticity curve obtained for the designed tunable and switchable terahertz multifunctional metasurface working in linear to circular polarization conversion mode;

[0063] Fig.12 The curve of linear polarization conversion rate variation in the linear-to-linear polarization conversion mode when vanadium dioxide is in an insulating state and the conductivity of photosensitive silicon is different;

[0064] Fig.13 The curve of ellipticity change in linear to circular polarization conversion mode when vanadium dioxide is in insulating state and the conductivity of photosensitive silicon is different. DETAILED DESCRIPTION

[0065] In order to make the main purpose, technical solutions and advantages of the present invention clearer, the following will be more detailed and comprehensive description of the implementation method of the present invention according to the accompanying drawings in the embodiments of the present invention. It should be understood that the specific embodiments introduced here are only used to explain the present invention and are not used to limit the present invention.

[0066] Combination Figures 1 to 4The present invention designs a tunable and function-switchable terahertz multifunctional metasurface, the structure of which includes six layers: the first layer is a photosensitive silicon pattern layer, the second layer is a silicon dioxide dielectric layer, the third layer is a vanadium dioxide regulation layer, the fourth layer is a gold pattern layer, the fifth layer is a silicon dioxide dielectric layer, and the sixth layer is a metal base plate.

[0067] The present invention uses the frequency domain solver in the CST electromagnetic simulation software to simulate and analyze the model. The unit structure is set to periodic boundary conditions along the x-axis and y-axis directions, and is set to open boundary conditions in the z-axis direction. The incident electromagnetic wave is along the negative direction of the z-axis.

[0068] Taking into account the absorption rate, linear polarization conversion rate, ellipticity and working bandwidth in the absorption and polarization conversion modes, the first photosensitive silicon pattern layer described in this embodiment is photosensitive silicon with a relative dielectric constant of 11.7, and the external "flower-shaped" pattern range is r 1 10μm, r 2 The outer diameter of the inner ring is 15 μm. 3 14μm, inner diameter r 4 The thickness of the entire top pattern is h 1 It is 0.2μm and is a centrally symmetrical pattern.

[0069] The length of the second silicon dioxide layer is p=50 μm, and the relative dielectric constant is ε SiO2 =3.75, considering the optimal absorption rate and working bandwidth, the thickness d 1 The optimal value is 15μm.

[0070] The length of the third vanadium dioxide control layer is p=50 μm, and considering the optimal absorption rate, linear polarization conversion rate, ellipticity and working bandwidth, the thickness h 2 The optimal value is 0.2μm.

[0071] The fourth gold pattern layer, the outer diameter of the "bracket" pattern is r 5 25μm, inner diameter r 6 The distance w between the "brackets" is 11 μm, and the thickness h is 20 μm. 3 0.1μm.

[0072] The fifth silicon dioxide layer has a length of p=50 μm and a relative dielectric constant of ε SiO2 =3.75, taking into account the optimal absorption rate, linear polarization conversion rate, ellipticity and working bandwidth, the thickness d 2 The optimal value is 15μm.

[0073] The sixth metal plate has a length of p=50 μm and a conductivity of 4.56×10 7 S / m of gold, considering the best results, thickness h4 The optimal value is 0.3μm.

[0074] Furthermore, when the photosensitive silicon is in an insulating state, σ PSi =1S / m, when the photosensitive silicon is in the metallic state, σ PSi =1×10 5 S / m; when vanadium dioxide is in an insulating state, When vanadium dioxide is in the metallic state, ×10 5 S / m.

[0075] According to the multifunctional metasurface design method of the present invention, the absorptivity A(ω) of the multifunctional metasurface satisfies:

[0076]

[0077] Where R(ω) and T(ω) represent reflectivity and transmittance respectively, i represents the imaginary unit, and Z 1 represents the impedance of the metasurface, n represents the equivalent medium complex refractive index, k represents the wave vector in the medium, and l represents the thickness of the medium layer. Since the bottom layer of the metasurface is a metal base plate, electromagnetic waves will not be transmitted, that is, T(ω) = 0.

[0078] According to the multifunctional metasurface design method of the present invention, the reflectivity R(ω) of the multifunctional metasurface is obtained as follows:

[0079]

[0080] Where Z 0 is the impedance in free space, μ r and ε r represent the equivalent magnetic permeability and equivalent dielectric constant of the metasurface, respectively.

[0081] According to the multifunctional metasurface design method of the present invention, the error control function C of the multifunctional metasurface reflectivity R(ω) R (μ r ,ε r ,S 11 ) can be expressed as:

[0082]

[0083] Where S 11 represents the reflection coefficient, and further the control function F of the multifunctional metasurface absorptivity can be obtained C (A(ω)):

[0084]

[0085] According to the multifunctional metasurface design method of the present invention, the linear polarization conversion rate ξ of the multifunctional metasurface satisfies:

[0086]

[0087] Where r xy represents the cross-polarization reflection coefficient, r yy Represents the co-polarization reflection coefficient. Linear polarization conversion rate ξ error control function C ξ It can be expressed as follows:

[0088]

[0089] In the formula represents the reflection phase difference, β represents the expected value of the linear polarization conversion rate, and the control function Γ of the linear polarization conversion rate of the multifunctional metasurface can be further obtained. C (ξ):

[0090]

[0091] According to the multifunctional metasurface design method of the present invention, the linear-to-circular polarization conversion degree of the multifunctional metasurface is analyzed by the ellipticity χ, and the ellipticity χ satisfies:

[0092]

[0093] Where, the subscripts m and m+1 represent any two adjacent frequency points within the working bandwidth, and χ m and χ m+1 They respectively represent the ellipticity fitting values ​​obtained by simulation at the mth and m+1th frequency points, and α represents the relative error value of the ellipticity.

[0094] According to the multifunctional metasurface design method of the present invention, the method for obtaining the ellipticity χ of the multifunctional metasurface is:

[0095]

[0096] Where r xy represents the cross-polarization reflection coefficient, r yy represents the co-polarization reflection coefficient, represents the reflection phase difference,

[0097] We can further obtain the control function E of the ellipticity C (x):

[0098]

[0099] According to the multifunctional metasurface design method of the present invention, the multifunctional metasurface can use a control model Φ(A(ω),ξ,χ) to simultaneously manipulate the absorptivity, linear polarization conversion rate, and ellipticity. The control model Φ(A(ω),ξ,χ) of the multifunctional metasurface is obtained as follows:

[0100] Φ(A(w),x,c)=l 1 F C (A(w))+l 2 Γ C (x)+l 3 E C (c)

[0101] In the formula, λ 1 represents the absorption rate influencing factor, λ 2 represents the linear polarization conversion rate influencing factor, λ 3 represents the ellipticity influence factor. Further, the multifunctional metasurface control model Φ(A(ω),ξ,χ) can be rewritten as:

[0102] Specific embodiment:

[0104] 1) Design of metasurface unit:

[0105] 2) Design the first layer as a photosensitive silicon pattern structure: The "flower pattern" and the embedded ring are made of photosensitive silicon with a dielectric constant of 11.7. In the wave absorption mode, the photosensitive silicon is in a metallic state and its conductivity is 1×10 5 S / m, in the two polarization conversion modes, the photosensitive silicon is in an insulating state, its conductivity is 1S / m, and the external "flower-shaped" pattern range is r 1 10μm, r 2 The outer diameter of the inner ring is 15 μm. 3 14μm, inner diameter r 4 The thickness of the entire top pattern is h 1 is 0.2μm and is a centrally symmetrical pattern, such as Figure 2 As shown;

[0106] 3) Design the second dielectric layer: Create a second layer of silicon dioxide dielectric material with a relative dielectric constant of 3.75 and a thickness of d 1 is 15 μm, length p = 50 μm, such as Figure 3 As shown;

[0107] 4) Design the third regulation layer: Create the third vanadium dioxide regulation layer material. In the wave absorption mode, vanadium dioxide is in a metallic state, and its conductivity is 2×10 5 S / m, in the two polarization conversion modes, vanadium dioxide is in an insulating state, its conductivity is 10S / m, and its thickness is h 2is 0.2 μm, length p = 50 μm, such as Figure 3 As shown;

[0108] 5) Design the fourth layer gold pattern structure: Create a conductivity of 4.56×10 7 S / m gold material, "bracket" pattern outer diameter r 5 25μm, inner diameter r 6 The distance w between the "brackets" is 11 μm, and the thickness h is 20 μm. 3 is 0.1μm, such as Figure 4 As shown;

[0109] 6) Design the fifth dielectric layer: Create the fifth layer of silicon dioxide dielectric material with a relative dielectric constant of 3.75 and a thickness of d 2 is 15 μm, length p = 50 μm, such as Figure 3 As shown;

[0110] 7) Design the bottom metal plate: Create a conductivity of 4.56×10 7 S / m gold material, thickness h 2 is 0.2 μm, length p = 50 μm, such as Figure 3 As shown;

[0111] 8) The simulation working frequency of the embodiment of the present invention is set to 0-5THz;

[0112] 9) The unit structure of the embodiment of the present invention is set to periodic boundary conditions along the x-axis and y-axis directions, and to open boundary conditions in the z-axis direction, and the incident electromagnetic wave is along the negative direction of the z-axis;

[0113] 10) By making vanadium dioxide and photosensitive silicon in a metallic state, the multifunctional metasurface exhibits broadband wave absorption function, and the absorption rate can be tuned by adjusting the conductivity of photosensitive silicon, such as Figure 5 As shown:

[0114] When σ PSi =1S / m, the absorption rate of 0-5THz is less than 52.35%. The absorption rate and absorption bandwidth vary with σ PSi When σ PSi =1×10 5 S / m, in the wide spectrum range of 1.53-3.25THz, the multifunctional metasurface can absorb more than 90% of the normally incident terahertz waves, with a relative bandwidth of 72.42%. There is an absorption peak with an absorption rate of 99.14% at 1.99THz, indicating that the embodiment of the present invention achieves a perfect effect in the wave absorption mode;

[0115] 11) When vanadium dioxide and photosensitive silicon are in a metallic state, the multifunctional metasurface exhibits broadband wave absorption function, and changing the thickness of different dielectric layers affects the absorption rate. Figure 6 As shown, with d 1 Increase, the embodiment of the present invention has the characteristic of wave absorption, but the absorption peak shows red shift phenomenon, such as Figure 7 As shown, with d 2 Increase, the absorption rate remains unchanged;

[0116] 12) By making vanadium dioxide and photosensitive silicon in an insulating state, the multifunctional metasurface exhibits linear-to-linear polarization conversion and linear-to-circular polarization conversion functions, and the cross-polarization reflection coefficient r xy and the co-polarization reflection coefficient r yy The results are as follows Figure 8 As shown. The cross-polarization reflection coefficient r xy In the range of 0.86-2.51THz, the co-polarization reflection coefficient r is greater than 0.9. yy Less than 0.3. Moreover, within the range of 0.86-2.51THz, there are three resonant frequencies of 0.92THz, 1.31THz and 2.51THz, which creates conditions for the embodiment of the present invention to achieve the linear-to-linear polarization conversion function within the range of 0.86-2.51THz. In addition, within the range of 0.80THz and 2.58-3.22THz, r yy ≈r xy , creating conditions for the embodiment of the present invention to realize the linear to circular polarization conversion function;

[0117] 13) When vanadium dioxide and photosensitive silicon are in an insulating state, the linear polarization conversion rate curve obtained by the method of the present invention is as follows: Fig. 9 As shown:

[0118] The linear polarization conversion rate of the embodiment of the present invention is greater than 90% in the range of 0.86-2.51THz, and the relative bandwidth is 97.92%. The results show that in the 0.86-2.51THz frequency band, all incident waves are converted into cross-polarized waves. The linear polarization conversion rates at 0.92THz, 1.31THz and 2.51THz are 99.92%, 99.99% and 99.97% respectively, indicating that the method of the present invention can achieve efficient linear-to-linear polarization conversion;

[0119] 14) When vanadium dioxide and photosensitive silicon are in an insulating state, the phase difference of the embodiment of the present invention in the range of 0.80TH and 2.58-3.22THz is 90° and -90° respectively. Fig.10 As shown, conditions are created for realizing efficient linear to circular polarization conversion;

[0120] 15) When vanadium dioxide and photosensitive silicon are in an insulating state, the ellipticity curve of linear to circular polarization conversion obtained by the method of the present invention is as follows: Fig.11 As shown:

[0121] In the embodiment of the present invention, at 0.80 THz, the ellipticity is close to 1, indicating that a left-hand circularly polarized wave is realized. In addition, in the range of 2.58-3.22 THz, the relative bandwidth is 22.07%, and the ellipticity is close to -1, indicating that the reflected wave is a right-hand circularly polarized wave;

[0122] 16) When vanadium dioxide is in an insulating state, the linear polarization conversion rate and working bandwidth can be dynamically controlled by changing the conductivity of photosensitive silicon, such as Fig.12 As shown:

[0123] In the embodiment of the present invention, the conductivity of vanadium dioxide is kept at 10 S / m, and the conductivity of photosensitive silicon is set to be 1 S / m, 1×10 2 S / m, 1×10 3 S / m, 1×10 4 S / m, 1×10 5 S / m, when the conductivity of photosensitive silicon increases from 1S / m to 1×10 4 S / m, the multifunctional metasurface maintains a high linear polarization conversion rate and a wide relative bandwidth. When the conductivity of the photosensitive silicon increases to 1×10 5 When S / m, the maximum value of linear polarization conversion efficiency is 89.10%, and the working bandwidth narrows rapidly. This shows that the method of the present invention can achieve dynamic regulation of linear polarization conversion efficiency and working bandwidth by changing the conductivity of different photosensitive silicon;

[0124] 17) When vanadium dioxide is in an insulating state, the ellipticity and working bandwidth of linear to circular polarization conversion can be dynamically controlled by changing the conductivity of photosensitive silicon, such as Fig.13 As shown:

[0125] In the embodiment of the present invention, the conductivity of vanadium dioxide is kept at 10 S / m, and the conductivity of photosensitive silicon is set to be 1 S / m, 1×10 2 S / m, 1×10 3 S / m, 1×10 4 S / m, 1×10 5 S / m, when the conductivity of photosensitive silicon increases to 1×10 4 S / m, the ellipticity remains almost unchanged. When the conductivity of photosensitive silicon increases to 1×10 5 When S / m, the left-hand circular polarization frequency point red-shifts, and the relative bandwidth with ellipticity less than -0.9 decreases from 22.07% to 5.40%. This shows that the method of the present invention can achieve dynamic regulation of ellipticity and relative bandwidth by changing the conductivity of different photosensitive silicon;

[0126] The above content is only used to introduce the specific examples of the present invention. It should be understood by those skilled in the art that without departing from the spirit and principle of the embodiments of the present invention, the exemplary embodiments may be improved, or some features of the embodiments may be arranged and replaced in other ways, which does not affect the essential content of the present invention.

Claims

1. A tunable and function-switchable terahertz multifunctional metasurface, characterized by: A metasurface unit structure, the structure comprising six layers: from top to bottom, the first layer is a photosensitive silicon pattern layer, the second layer is a silicon dioxide dielectric layer, the third layer is a vanadium dioxide regulation layer, the fourth layer is a gold pattern layer, the fifth layer is a silicon dioxide dielectric layer, and the sixth layer is a metal bottom plate; the unit structure is periodically arranged along the x and y coordinate axes in an o-xyz spatial coordinate system, and a multifunctional metasurface is formed by extension, with o being the origin of the coordinate axis; The first photosensitive silicon pattern layer is composed of a ring and a "flower shape", and the ring is located in the middle of the outer "flower shape", so that the photosensitive silicon pattern layer presents a centrally symmetrical pattern; The fourth gold pattern layer is composed of two "bracket"-shaped patterns, which are diagonally symmetrical; The second and fifth silicon dioxide dielectric layers and the third vanadium dioxide control layer are consistent with the length and width of the bottom metal plate, but have different thicknesses.

2. The tunable and function-switchable terahertz multifunctional metasurface according to claim 1, characterized in that: The second and fifth dielectric layers are made of silicon dioxide material with a dielectric constant of 3.

75.

3. The tunable and function-switchable terahertz multifunctional metasurface according to claim 1, characterized in that: Where ε represents the relative dielectric constant, VO2 represents vanadium dioxide, ε VO2 represents the relative dielectric constant of vanadium dioxide, τ represents the volume fraction of vanadium dioxide in the metal phase that changes with the external temperature, which is approximately 0 in the insulating stage and has a maximum value of 0.95, ε1 represents the relative dielectric constant of vanadium dioxide in the insulating state, and ε2 represents the relative dielectric constant of vanadium dioxide in the metallic state; Where σ represents the conductivity, represents the conductivity of vanadium dioxide, ω represents the incident angle frequency, i represents the imaginary unit, and when vanadium dioxide is in an insulating state, When vanadium dioxide is in the metallic state, 4. The tunable and function-switchable terahertz multifunctional metasurface according to any one of claims 1 to 3, characterized in that: s PSi =4.863×10 -4 ×q 2 +0.1856×q+1.569 Where σ represents conductivity, PSi represents photosensitive silicon, σ PSi represents the conductivity of photosensitive silicon, q represents the energy flux of the pump beam, and when the photosensitive silicon is in an insulating state, σ PSi =1S / m, when the photosensitive silicon is in the metallic state, σ PSi =1×105S / m.

5. The tunable and function-switchable terahertz multifunctional metasurface according to any one of claims 1 to 3, characterized in that: The absorptivity A(ω) of the multifunctional metasurface satisfies: Where R(ω) and T(ω) represent reflectivity and transmittance respectively, i represents the imaginary unit, Z1 represents the metasurface impedance, n represents the equivalent medium complex refractive index, k represents the wave vector in the medium, and l represents the thickness of the medium layer. Since the bottom layer of the metasurface is a metal bottom plate, electromagnetic waves will not be transmitted, that is, T(ω) = 0; The reflectivity R(ω) of the multifunctional metasurface is obtained by: Where Z0 represents the impedance in free space, μ r and ε r represent the equivalent magnetic permeability and equivalent dielectric constant of the metasurface respectively; The error control function C of the reflectivity R(ω) of the multifunctional metasurface R (μ r ,ε r ,S 11 ) can be expressed as: Where S 11 represents the reflection coefficient, and further the control function F of the multifunctional metasurface absorptivity can be obtained C (A(ω)):

6. The tunable and function-switchable terahertz multifunctional metasurface according to any one of claims 1 to 3, characterized in that: The linear polarization conversion rate ξ of the multifunctional metasurface satisfies: In the formula, r xy represents the cross-polarization reflection coefficient, r yy represents the co-polarization reflection coefficient, linear polarization conversion rate ξ error control function C ξ It can be expressed as follows: In the formula represents the reflection phase difference, β represents the expected value of the linear polarization conversion rate, and the control function Γ of the linear polarization conversion rate of the multifunctional metasurface can be further obtained. C (ξ):

7. The tunable and function-switchable terahertz multifunctional metasurface according to any one of claims 1 to 3, characterized in that: The degree of linear to circular polarization conversion of the multifunctional metasurface is analyzed by the ellipticity χ, which satisfies: Where, the subscripts m and m+1 represent any two adjacent frequency points within the working bandwidth, and χ m and χ m+1 They represent the ellipticity fitting values ​​obtained by simulation at the mth and m+1th frequency points respectively, and α represents the relative error value of the ellipticity; The method for obtaining the ellipticity χ of the multifunctional metasurface is: Where r xy represents the cross-polarization reflection coefficient, r yy represents the co-polarization reflection coefficient, represents the reflection phase difference, and further the control function E of the ellipticity can be obtained C (x):

8. The tunable and function-switchable terahertz multifunctional metasurface according to any one of claims 1 to 3, characterized in that: The multifunctional metasurface can use a control model Φ(A(ω),ξ,χ) to simultaneously manipulate the absorptivity, linear polarization conversion rate, and ellipticity. The control model Φ(A(ω),ξ,χ) of the multifunctional metasurface is obtained as follows: Φ(A(w),x,c)=l1F C (A(w))+l2Γ C (x)+l3E C (c) In the formula, λ1 represents the influence factor of the absorption rate, λ2 represents the influence factor of the linear polarization conversion rate, and λ3 represents the influence factor of the ellipticity. Furthermore, the multifunctional metasurface control model Φ(A(ω),ξ,χ) can be rewritten as:

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