Flexible chiral metasurface with tunable circular dichroism and polarization conversion function

Abstract

The invention discloses a circular dichroism and polarization conversion function tunable flexible chiral metamaterial surface, and belongs to the technical field of electromagnetic metamaterials. The invention aims to solve the problems that the conventional polarization conversion metasurface uses a rigid medium, the function is single, the working frequency band is relatively fixed, and the function and the working efficiency cannot be changed once the polarization conversion metasurface is designed and manufactured. Each chiral super-structure surface unit structure is composed of a vanadium dioxide pattern top layer, an upper polyimide dielectric layer, a middle vanadium dioxide adjustable layer, a middle open metal square ring, a lower polyimide dielectric layer and a metal bottom plate. A phase change material is utilized, namely, the conductivity of vanadium dioxide is changed to realize dynamic regulation and control of circular dichroism and a polarization conversion function, and potential application values are provided in the fields of wearable sensing, imaging, detection and the like.

Description

Technical Field

[0001] The present invention belongs to the technical field of electromagnetic metamaterials, and particularly relates to a tunable flexible chiral metasurface with circular dichroism and polarization conversion functions. Background Art

[0002] Terahertz waves are located between microwaves and infrared spectra in the electromagnetic spectrum and have special electromagnetic properties. Due to the strong penetration of terahertz waves in materials and their strong sensitivity to molecules, they have a wide range of applications in medical imaging, security detection, communication, non-destructive testing and other fields. In the terahertz band, the regulation of electromagnetic waves is crucial for achieving efficient transmission, high-precision imaging and secure communication. However, the regulation and utilization of terahertz waves face technical challenges. In the regulation of terahertz waves, traditional technologies are often limited by aspects such as frequency, material properties and structural complexity. Traditional optical elements have a weak response to terahertz waves, a short transmission distance, and poor performance in the high-frequency band. This situation has given rise to the demand for new and efficient electromagnetic regulation technologies and new materials.

[0003] As a two-dimensional ultra-thin planar structure of metamaterials, metasurfaces have electromagnetic effects not available in nature. By carefully designing the arrangement of structural units, efficient regulation of electromagnetic waves can be achieved. Polarization is an important characteristic of electromagnetic regulation. Polarization conversion metasurfaces are emerging electromagnetic wave regulation devices, and the polarization conversion ability of metasurfaces is measured by the polarization conversion efficiency.

[0004] Traditional metasurfaces mainly focus on phase regulation. However, in practical applications, the demand for the regulation of electromagnetic waves is more complex. Electromagnetic waves need to be regulated with multiple characteristics in different application scenarios. In recent years, metasurfaces with multiple electromagnetic regulation functions have attracted more extensive attention. These metasurfaces can not only achieve traditional phase and amplitude regulation, but also realize multifunctional regulation of electromagnetic wave absorption, transmission, reflection, phase regulation and amplitude adjustment in the same structure.

[0005] Chiral metasurfaces, as an important branch of metasurfaces, also face the same problems in the devices prepared by using them. The research on chiral metasurfaces with multiple electromagnetic regulation functions also has profound significance in technology, application and science. By continuously expanding the regulation functions of chiral metasurfaces, improving their performance and scope of application, it is expected to achieve technological innovation in multiple fields such as communication, medicine and radar. In addition, the research on multifunctional chiral metasurfaces will promote cross-cooperation between different disciplines and promote the continuous in-depth of scientific research. Among them, most of the currently proposed chiral metasurfaces are based on rigid dielectric layers, and the dielectric materials used are limited by the non-flexible nature. Using flexible materials in the same unit structure and achieving dual functions, high polarization conversion efficiency and wide relative bandwidth are still the hotspots and difficulties in the current research of chiral metasurfaces. Summary of the invention

[0006] In view of the fact that rigid dielectric materials are difficult to bend conformally, the narrow working band of linear polarization wave absorption and linear polarization conversion in chiral metasurfaces, and the need to achieve tunable circular polarization absorption and linear polarization conversion functions in the terahertz band, the present invention proposes a flexible chiral metasurface with tunable circular dichroism and polarization conversion functions. To achieve the above purpose, the technical solution adopted by the present invention is:

[0007] A flexible chiral metasurface with tunable circular dichroism and polarization conversion functions, the structure of which includes six layers, from the top to the bottom, a 4-Z vanadium dioxide pattern layer, an upper polyimide dielectric layer, a middle vanadium dioxide adjustable layer, a middle open metal square ring, a lower polyimide dielectric layer and a metal bottom plate. The unit structure is periodically arranged along the x and y directions in the o-xyz coordinate system to form a metasurface microstructure, with o being the origin of the coordinate.

[0008] According to the functionally tunable flexible chiral metasurface design method of the present invention, the number of the vanadium dioxide patterns is 4, two of which are identical with respect to the diagonal and the oblique diagonal, and are attached together on the polyimide flexible dielectric substrate.

[0009] According to the functionally tunable flexible chiral metasurface design method of the present invention, the upper and lower polyimide flexible dielectric layers, the middle vanadium dioxide adjustable layer and the metal base plate have the same size but different thicknesses.

[0010] According to the functionally tunable flexible chiral metasurface design method described in the present invention, the resonant state of the 4-Z vanadium dioxide pattern and the intermediate vanadium dioxide adjustable layer can achieve a reversible transition from an insulating state to a metallic state by changing the electrical conductivity, and its electrical conductivity can change with the external temperature. When the external temperature increases to 68°C, vanadium dioxide will change from a high-resistance insulating state to a low-resistance metallic state.

[0011] The dielectric constant of the vanadium dioxide in the terahertz band It can be expressed as follows:

[0012]

[0013] Among them, ε VO2 is the phase change characteristic function of vanadium dioxide, ε i and ε m Represent the dielectric constants of vanadium dioxide in the insulating state and metallic state respectively. V is the volume fraction of vanadium dioxide, V≈0 in the insulating state, and V in the metallic state is a maximum of 0.95. In this phase transition process, V is expressed as:

[0014]

[0015] Where T is the outside temperature, T 0 =68°C is the phase transition temperature of vanadium dioxide, and ΔT is the phase transition temperature. In the present invention, the dielectric constant of vanadium dioxide is is a constant value,

[0016] According to the functionally tunable flexible chiral metasurface design method of the present invention, the circular dichroism Φ of the chiral metasurface unit satisfies:

[0017]

[0018] The α is the circular dichroism value of the high-order polynomial curve fitting, the subscript n represents the nth sampling point, and α n-1 and α n They represent the curve fitting circular dichroism values ​​obtained by simulation at the n-1th and nth sampling points, δ 1 is the relative error between the circular dichroism value and the circular dichroism value fitted by the high-order polynomial curve, ζ 1 is the variance of the circular dichroism value and the circular dichroism value fitted by the high-order polynomial curve.

[0019] According to the functionally tunable flexible chiral metasurface design method of the present invention, the circular dichroism Φ of the chiral metasurface unit is obtained by:

[0020] Φ=A rcp -A lcp

[0021] Among them, A rcp and A lcp They represent the absorption rate of right-hand polarized waves and left-hand circularly polarized waves respectively when circularly polarized electromagnetic waves are incident. The methods for obtaining the two can be expressed as follows:

[0022]

[0023] Among them, R rr (R ll ) represents the co-polarization reflection coefficient formed by the right (left) circular polarization wave incident on the chiral metasurface and converted into the right (left) circular polarization reflection; R lr (R rl ) represents the cross-polarization reflection coefficient formed by the right (left) circular polarization wave incident on the chiral metasurface and converted into left (right) circular polarization reflection. Therefore, the circular dichroism Φ can be expressed as:

[0024] Φ=R ll +R rl -R rr -R lr

[0025] According to the functionally tunable flexible chiral metasurface design method of the present invention, the circular dichroism Φ of the chiral metasurface unit can be equivalent to the circular dichroism fitting error control function M(Φ) expressed as:

[0026]

[0027] According to the functionally tunable flexible chiral metasurface design method of the present invention, the polarization conversion efficiency Ψ of the chiral metasurface unit satisfies:

[0028]

[0029] The β is the high-order polynomial curve fitting polarization conversion efficiency. In the following table, n represents the nth sampling point, β n-1 and β n They represent the curve fitting polarization conversion efficiency obtained by simulation at the n-1th and nth sampling points, δ 2 is the relative error between the polarization conversion efficiency value and the polarization conversion efficiency fitted by the high-order polynomial curve, ζ 2 is the variance of the polarization conversion efficiency fitted with a high-order polynomial curve.

[0030] According to the functionally tunable flexible chiral metasurface design method of the present invention, the polarization conversion efficiency Ψ of the chiral metasurface unit is obtained by:

[0031]

[0032] Among them, R xy It represents the cross-polarization reflection coefficient formed by the electromagnetic wave incident along the y-axis direction and then converted into the x-axis polarization reflection, R yy It represents the co-polarization reflection coefficient formed by the electromagnetic wave incident along the y-axis direction and converted into the polarization reflection along the y-axis direction. The methods of obtaining the two can be expressed as follows:

[0033]

[0034] Among them, i represents the incident wave, r represents the reflected wave, represents the electric field component of the reflected electric field along the coordinate axis x, represents the electric field component of the incident electric field along the y-axis, represents the electric field component of the reflected electric field along the coordinate axis y; the coordinate axes x and y are rotated counterclockwise by 45° to obtain the u and v axes respectively; is the phase, m represents the cross-polarization reflection coefficient at any frequency point within the working bandwidth, and n represents the co-polarization reflection coefficient at any frequency point within the working bandwidth; They represent the reflection phases along the u and v axes at any frequency point m respectively; They represent the reflection phase along the u and v axes at any frequency point n respectively.

[0035] Then the polarization conversion efficiency Ψ of the chiral metasurface unit is expressed as:

[0036]

[0037] According to the functionally tunable flexible chiral metasurface design method of the present invention, the chiral metasurface unit polarization conversion rate Ψ can be equivalent to the polarization conversion rate fitting error control function N(Ψ) expressed as:

[0038]

[0039] According to the functionally tunable flexible chiral metasurface design method of the present invention, the chiral metasurface can simultaneously manipulate circular dichroism and polarization conversion rate using a control model F(Φ,Ψ), which is expressed as follows: F(Φ,Ψ)=λ 1 M(Φ)+λ 2 N(Ψ)

[0040] Among them, λ 1 and λ 2 They represent the circular dichroism influence factor and the polarization conversion rate influence factor respectively. Further, the control model F(Φ,Ψ) of the chiral metasurface can be rewritten as:

[0041]

[0042] Beneficial effects of the present invention

[0043] (1) The functionally tunable flexible chiral metasurface proposed in the present invention can realize the transition from an insulating state to a metallic state by simply controlling the conductivity of vanadium dioxide while ensuring that the structural units are fixed, and can also realize dynamic controllable functions of circular dichroism and polarization conversion efficiency during the phase transition process.

[0044] (2) The functionally tunable flexible chiral metasurface proposed in the present invention can independently control the circular dichroism and polarization conversion rate through control functions. In the circular dichroism function mode, under the incident circularly polarized wave, based on the reflection coefficients of right-handed circularly polarized wave and left-handed circularly polarized wave, the circularly polarized wave is selectively absorbed, and a large difference is generated to obtain circular dichroism; in the polarization conversion function mode, based on the relationship between phase difference and reflection coefficient, the cross-polarization and co-polarization reflection coefficients are independently controlled, so that the ability of the metasurface to control the polarization conversion of the incident electromagnetic wave is further improved.

[0045] (3) The functionally tunable flexible chiral metasurface proposed in the present invention can achieve quantitative controllable circularly polarized wave absorption and polarization conversion through the polarization conversion rate influencing factor and circular dichroism influencing factor.

[0046] (4) The functionally tunable flexible chiral metasurface proposed in the present invention can achieve the advantages of bending conformality that is difficult to achieve with ordinary rigid media by using a flexible intermediate dielectric layer. The chiral metasurface has a simple structure and is easy to implement, which provides a basis for the design of metasurface devices that need to consider deformation.

[0047] (5) The present invention adopts two error control methods: relative error and variance, which improves the control accuracy and thus better realizes quantitative conversion. BRIEF DESCRIPTION OF THE DRAWINGS

[0048] Figure 1 Schematic diagram of the top layer vanadium dioxide 4-Z pattern of the designed functionally tunable flexible chiral metasurface unit structure;

[0049] Figure 2 Schematic diagram of the middle metal open square ring pattern of the designed functionally tunable flexible chiral metasurface unit structure;

[0050] Figure 3 Side view of the designed functionally tunable flexible chiral metasurface unit structure;

[0051] Figure 4 A three-dimensional image of the designed functionally tunable flexible chiral metasurface unit structure periodically arranged (5×5) on a flat plate;

[0052] Figure 5 A three-dimensional image of the (5×5) bending conformal of the designed functionally tunable flexible chiral metasurface unit structure arranged periodically;

[0053] Figure 6 When the vanadium dioxide is in the metallic state, the designed functionally tunable flexible chiral metasurface adopts the method of the present invention under the condition of circularly polarized wave incidence, and the co-polarization and cross-polarization reflection coefficient diagrams are obtained;

[0054] Figure 7 When the vanadium dioxide is in the metallic state, the designed functionally tunable flexible chiral metasurface adopts the method of the present invention under the condition of circularly polarized wave incidence, and the circularly polarized wave absorptivity and CD curves are obtained on the flat plate;

[0055] Figure 8 When the vanadium dioxide is in the metallic state, the CD curve obtained by bending conformally using the method of the present invention under the condition of circularly polarized wave incidence on the designed functionally tunable flexible chiral metasurface;

[0056] Fig. 9 When the vanadium dioxide is in the insulating state, the co-polarization and cross-polarization reflection coefficient diagrams obtained by the method of the present invention are obtained under the condition of vertical incidence of linear polarization waves on the designed functionally tunable flexible chiral metasurface;

[0057] Fig.10 When the vanadium dioxide is in the insulating state, the polarization conversion efficiency curve of the designed functionally tunable flexible chiral metasurface obtained on a flat plate by using the method of the present invention under the condition of vertical incidence of linear polarized waves;

[0058] Fig.11 When the vanadium dioxide is in the insulating state, the polarization conversion efficiency curve obtained by bending conformally using the method of the present invention under the condition of vertical incidence of linear polarization wave on the designed functionally tunable flexible chiral metasurface;

[0059] Fig.12 The figure is a transformation efficiency curve magnified 100 times. DETAILED DESCRIPTION

[0060] 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.

[0061] Combination Figures 1 to 4 As shown, the present invention provides a functionally tunable flexible chiral metasurface, the unit structure includes six layers from top to bottom: the first top layer is 4 Z-shaped vanadium dioxide pattern layers, the second upper medium is polyimide, the third middle medium is a vanadium dioxide adjustable layer, the fourth middle layer is a metal open square ring pattern layer, the fifth lower medium is polyimide, and the bottom layer is a square metal plate. The chiral metasurface microstructure is periodically arranged in the x and y axis directions.

[0062] The top pattern of the unit structure is the same about the diagonal line. The frequency domain solver in CST electromagnetic simulation software is used to simulate and analyze the model. The unit structure is set to periodic boundary conditions along the x-axis and y-axis directions, and to open boundary conditions in the z-axis direction. The incident electromagnetic wave is along the negative direction of the z-axis.

[0063] Taking into account the circular dichroism and polarization conversion rate, the vertical width and length of the top layer 4-Z in this embodiment are l 1 =1μm, l 2 =4.7μm, the horizontal dimensions of Z are d 1 =1μm, d 2 =1.8μm, d 3 =2μm, d 4 =0.8 μm. The two pairs of Z are the same with respect to the diagonal line.

[0064] The second upper dielectric layer has a length of p=10 μm, a relative dielectric constant of 3.5, a loss tangent of 0.0027, and a thickness of h 1Polyimide, considering the optimal circular dichroism and polarization conversion rate, thickness h 1 The optimal value is 3.5μm.

[0065] The fourth layer intermediate pattern adopts a conductivity of 4.5×10 7 The outer ring length and inner ring length of the outer open square ring are p = 10μm and m respectively. 1 =8.15μm, opening length is m 2 =4.5μm.

[0066] The fifth lower dielectric layer has a length of p=10 μm, a relative dielectric constant of 3.5, a loss tangent of 0.0027, and a thickness of h 2 Polyimide, considering the optimal circular dichroism and polarization conversion rate, thickness h 2 The optimal value is 2.85μm.

[0067] The bottom plate is a square metal plate made of gold with a length of p=10 μm and a thickness of 0.2 μm, and its conductivity is 4.5×10 7 S / m.

[0068] Combination Figure 1 As shown, the four vanadium dioxide Z-shaped films are attached together on the second upper dielectric layer.

[0069] Furthermore, when the temperature is lower than the phase transition temperature of vanadium dioxide, 68°C, vanadium dioxide is in an insulating state, and when the temperature is higher than 68°C, vanadium dioxide is in a metallic state; when vanadium dioxide is in an insulating state, the conductivity is set to σ 1 =10S / m, the conductivity is set to σ in the metallic state 2 =2×10 5 S / m.

[0070] According to the functionally tunable flexible chiral metasurface design method of the present invention, the circular dichroism Φ of the chiral metasurface unit satisfies:

[0071]

[0072] The α is the circular dichroism value of the high-order polynomial curve fitting, the subscript n represents the nth sampling point, and α n-1 and α n They represent the curve fitting circular dichroism values ​​obtained by simulation at the n-1th and nth sampling points, respectively, 1 is the relative error between the circular dichroism value and the circular dichroism value fitted by the high-order polynomial curve, ζ 1 is the variance of the circular dichroism value and the circular dichroism value fitted by the high-order polynomial curve.

[0073] According to the functionally tunable flexible chiral metasurface design method of the present invention, the circular dichroism Φ of the chiral metasurface unit is obtained by:

[0074] Φ=A rcp -A lcp

[0075] Among them, A rcp and A lcp They represent the absorption rate of right-hand polarized waves and left-hand circularly polarized waves respectively when circularly polarized electromagnetic waves are incident. The methods for obtaining the two can be expressed as follows:

[0076]

[0077] Among them, R rr (R ll ) represents the co-polarization reflection coefficient formed by the right (left) circular polarization wave incident on the chiral metasurface and converted into the right (left) circular polarization reflection; R lr (R rl ) represents the cross-polarization reflection coefficient formed by the right (left) circular polarization wave incident on the chiral metasurface and converted into left (right) circular polarization reflection. Therefore, the circular dichroism Φ can be expressed as:

[0078] Φ=R ll +R rl -R rr -R lr

[0079] According to the functionally tunable flexible chiral metasurface design method of the present invention, the circular dichroism Φ of the chiral metasurface unit can be equivalent to the circular dichroism fitting error control function M(Φ) expressed as:

[0080]

[0081] According to the functionally tunable flexible chiral metasurface design method of the present invention, the polarization conversion efficiency Ψ of the chiral metasurface unit satisfies:

[0082]

[0083] The β is the high-order polynomial curve fitting polarization conversion efficiency. In the following table, n represents the nth sampling point, β n-1 and β n They represent the curve fitting polarization conversion efficiency obtained by simulation at the n-1th and nth sampling points, δ 2 is the relative error between the polarization conversion efficiency value and the polarization conversion efficiency fitted by the high-order polynomial curve, ζ 2 is the variance of the polarization conversion efficiency fitted with a high-order polynomial curve.

[0084] According to the functionally tunable flexible chiral metasurface design method of the present invention, the polarization conversion efficiency Ψ of the chiral metasurface unit is obtained by:

[0085]

[0086] Among them, R xy It represents the cross-polarization reflection coefficient formed by the electromagnetic wave incident along the y-axis direction and then converted into the polarization reflection along the x-axis direction, R yy It represents the co-polarization reflection coefficient formed by the electromagnetic wave incident along the y-axis direction and converted into the polarization reflection along the y-axis direction. The methods of obtaining the two can be expressed as follows:

[0087]

[0088] Among them, i represents the incident wave, r represents the reflected wave, represents the electric field component of the reflected electric field along the coordinate axis x, represents the electric field component of the incident electric field along the y-axis, represents the electric field component of the reflected electric field along the coordinate axis y; the coordinate axes x and y are rotated counterclockwise by 45° to obtain the u and v axes respectively; is the phase, m represents the cross-polarization reflection coefficient at any frequency point within the working bandwidth, and n represents the co-polarization reflection coefficient at any frequency point within the working bandwidth; They represent the reflection phases along the u and v axes at any frequency point m respectively; They represent the reflection phase along the u and v axes at any frequency point n respectively.

[0089] Then the polarization conversion efficiency Ψ of the chiral metasurface unit is expressed as:

[0090]

[0091] According to the functionally tunable flexible chiral metasurface design method of the present invention, the chiral metasurface unit polarization conversion rate Ψ can be equivalent to the polarization conversion rate fitting error control function N(Ψ) expressed as:

[0092]

[0093] According to the functionally tunable flexible chiral metasurface design method of the present invention, the chiral metasurface can simultaneously manipulate circular dichroism and polarization conversion rate using a control model F(Φ,Ψ), which is expressed as follows: F(Φ,Ψ)=λ 1 M(Φ)+λ 2 N(Ψ)

[0094] Among them, λ 1 and λ 2They represent the circular dichroism influence factor and the polarization conversion rate influence factor respectively. Further, the control model F(Φ,Ψ) of the chiral metasurface can be rewritten as:

[0095] Specific embodiment:

[0097] 1) Design of chiral metasurface units:

[0098] 2) Design the top layer to be a 4-Z-shaped vanadium dioxide film pattern structure: the width and length of the Z-shaped vertical are l 1 =1μm, l 2 =4.7μm, the horizontal dimensions of the Z are d 1 =1μm, d 2 =1.8μm, d 3 =2μm, d 4 =0.8μm. The two pairs of Z are symmetrical about the diagonal line. In the insulating state, the conductivity is set to σ 1 =10S / m, in the metallic state, the conductivity is set to σ 2 =2×10 5 S / m, the thickness of the vanadium dioxide film is 0.2μm, such as Figure 1 As shown;

[0099] 3) Design the fourth metal open square ring pattern layer: the material has a conductivity of 4.5×10 7 The outer ring length and inner ring length of the outer open square ring are p = 10μm and m respectively. 1 =8.15μm, opening length is m 2 =4.5μm, thickness is 0.2μm, such as Figure 2 As shown;

[0100] 4) Design the second flexible upper dielectric layer and the fifth flexible lower dielectric layer: Create a polyimide dielectric material with a relative dielectric constant of 3.5, a loss tangent of 0.0027, a side length of p = 10 μm, and a thickness of h 1 =3.5μm and h 2 =2.85μm, such as Figure 3 As shown;

[0101] 5) Design the sixth square metal base plate: the material has a conductivity of 4.5×10 7 The side length of the gold plate with a thickness of S / m is the same as that of the second, third and fifth layers of dielectrics, which is p = 10 μm and the thickness is 0.2 μm. Figure 3 As shown;

[0102] 6) Design a 5×5 array unit structure on a flat panel, such as Figure 4 As shown;

[0103] 7) Design a 5×5 array unit structure on a curved surface, such as Figure 5 As shown;

[0104] 8) The simulation frequency of the embodiment of the present invention is set to 5-11THz;

[0105] 9) The periodic boundary conditions of the unit structure of the example of the present invention are set as periodic boundary conditions along the x-axis and y-axis directions, and are set as open boundary conditions in the z-axis direction, and the incident electromagnetic wave is along the negative direction of the z-axis;

[0106] 10) By setting the conductivity of vanadium dioxide to 2×10 5 S / m, so that the chiral metasurface has circular dichroism function in the metallic state. The cross-polarization reflection coefficient R obtained by the method of the present invention is lr and R rl , co-polarization reflection coefficient R rr and R ll Simulation results, such as Figure 6 As shown. At 8.73THz, the cross-polarization coefficient R lr and R rl Basically coincident, and the co-polarization coefficient R rr <0.1, R ll >0.9, there are obvious differences in the co-polarization coefficients, showing huge polarization-selective reflection, creating conditions for achieving huge circular dichroism.

[0107] 11) The circular dichroism curve calculated by the method of the present invention is as follows: Figure 7 As shown:

[0108] Set the relative circular dichroism error δ 1 is 5%, and the variance ζ 1 The circular dichroism obtained by the method of the present invention is 0.83 at a frequency of 8.73 THz, and the circular dichroism relative error and variance results are always less than 4% after calculation, indicating that the method of the present invention can show large circular dichroism.

[0109] 12) The circular dichroism curve obtained by using the method of the present invention in the chiral metasurface bending conformal state is as follows: Figure 8 As shown:

[0110] When a circularly polarized wave is incident, the circular dichroism peak shifts to the left, and a maximum value of 0.64 is obtained near 7.73 THz, indicating that the method of the present invention can exhibit relatively good circular dichroism in a conformal state.

[0111] 13) By setting the conductivity of vanadium dioxide to 10S / m, the chiral metasurface has a polarization conversion function in an insulating state. The co-polarization reflection coefficient R obtained by the method of the present invention isyy and the cross-polarization reflection coefficient R xy Simulation results, such as Fig. 9 In the frequency range of 6.11-9.46THz, R yy Less than 0.2, and R xy is greater than 0.8, indicating that the y-polarized waves in this frequency band are converted into x-polarized waves through the metasurface.

[0112] 14) The polarization conversion rate curve is calculated by the method of the present invention as follows: Fig.10 As shown;

[0113] Set the relative error of polarization conversion rate δ 2 is 5%, and the variance ζ 2 The polarization conversion rate obtained by the method of the present invention is higher than 0.9 in the 6.11-9.46THz frequency band, the relative bandwidth is 43.03%, and the average polarization conversion rate is as high as 0.95. The relative error and variance of the polarization conversion rate are always less than 4% after calculation, indicating that the method of the present invention can achieve efficient polarization conversion.

[0114] 15) The polarization conversion rate curve obtained by using the method of the present invention in the curved conformal state of the chiral metasurface is as follows: Fig.11 As shown:

[0115] When y-polarized wave is incident, a polarization conversion of more than 0.9 is achieved in the 7.89-8.26 THz band, indicating that the method of the present invention can still exhibit good polarization conversion in a conformal state.

[0116] 16) To verify the superiority of the present invention, the polarization conversion efficiency was set to 98% in a certain implementation as the ideal conversion efficiency. The conversion efficiency curve magnified 100 times by using the present invention and the traditional single error control method was as follows: Fig.12 As shown;

[0117] Depend on Fig.12 It can be seen that the conversion efficiency of the present invention is higher than that of the traditional single error control method and is closer to the ideal polarization conversion efficiency.

[0118] 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 flexible chiral metasurface with tunable circular dichroism and polarization conversion functions, characterized in that: A chiral metasurface unit structure is designed, which includes six layers: from the top to the bottom, there are 4-Z vanadium dioxide pattern layer, upper polyimide dielectric layer, middle vanadium dioxide adjustable layer, middle open metal square ring, lower polyimide dielectric layer and metal base plate; the unit structure is periodically arranged along the x and y directions in the o-xyz coordinate axis to form a metasurface microstructure, and o is the coordinate origin; the number of Z-shaped vanadium dioxide patterns is 4, and they are identical in pairs with respect to diagonals and write diagonals, and are attached to an upper polyimide flexible dielectric substrate together; the upper and lower polyimide flexible dielectric layers, the middle vanadium dioxide adjustable layer and the metal base plate have the same size but different thicknesses.

2. The circular dichroism and polarization conversion tunable flexible chiral metasurface according to claim 1, characterized in that: The resonant state of the 4-Z vanadium dioxide pattern and the intermediate vanadium dioxide adjustable layer can achieve a reversible transition from an insulating state to a metallic state by changing the electrical conductivity, and its electrical conductivity can change with the external temperature. When the external temperature increases to 68°C, vanadium dioxide will change from a high-resistance insulating state to a low-resistance metallic state.

3. The circular dichroism and polarization conversion tunable flexible chiral metasurface according to claim 2, characterized in that: The dielectric constant of the vanadium dioxide in the terahertz band It can be expressed as follows: in, is the phase change characteristic function of vanadium dioxide, ε i and ε m Respectively represent the dielectric constants of vanadium dioxide in the insulating state and the metallic state; V is the volume fraction of vanadium dioxide, V≈0 in the insulating state, and V in the metallic state is a maximum of 0.95; in this phase transition process, V is expressed as: Wherein, T is the ambient temperature, T0=68°C is the phase transition temperature of vanadium dioxide, ΔT is the phase transition temperature, and the dielectric constant of vanadium dioxide in the present invention is is a constant value, 4. The circular dichroism and polarization conversion tunable flexible chiral metasurface according to claim 3, characterized in that: The circular dichroism Φ of the chiral metasurface unit satisfies: Where α is the circular dichroism value of the high-order polynomial curve fitting, the subscript n represents the nth sampling point, and α n-1 and α n They represent the curve-fitted circular dichroism values ​​obtained by simulation at the n-1th and nth sampling points respectively, δ1 is the relative error between the circular dichroism value and the circular dichroism value fitted by the high-order polynomial curve, and ζ1 is the variance between the circular dichroism value and the circular dichroism value fitted by the high-order polynomial curve.

5. The circular dichroism and polarization conversion tunable flexible chiral metasurface according to claim 4, characterized in that: The circular dichroism Φ of the chiral metasurface unit is obtained by: Φ=A rcp -A lcp Among them, A rcp and A lcp They represent the absorption rates of right-hand polarized waves and left-hand circularly polarized waves respectively when circularly polarized electromagnetic waves are incident. The methods for obtaining the two can be expressed as follows: Among them, R rr (R ll ) represents the co-polarization reflection coefficient formed by the right (left) circular polarization wave incident on the chiral metasurface and converted into the right (left) circular polarization reflection; R lr (R rl ) represents the cross-polarization reflection coefficient formed by the conversion of right (left) circularly polarized wave incident on the chiral metasurface into left (right) circularly polarized reflection; the circular dichroism Φ can be expressed as: Φ=R ll +R rl -R rr -R lr 。 6. The circular dichroism and polarization conversion tunable flexible chiral metasurface according to claim 5, characterized in that: The circular dichroism Φ of the chiral metasurface unit can be equivalent to the circular dichroism fitting error control function M(Φ):

7. The circular dichroism and polarization conversion tunable flexible chiral metasurface according to claim 6, characterized in that: The polarization conversion efficiency Ψ of the chiral metasurface unit satisfies: Where β is the high-order polynomial curve fitting polarization conversion efficiency, the subscript n represents the nth sampling point, and β n-1 and β n They represent the curve fitting polarization conversion efficiency obtained by simulation at the n-1th and nth sampling points respectively, δ2 is the relative error between the polarization conversion efficiency value and the polarization conversion efficiency fitted by the high-order polynomial curve, and ζ2 is the variance of the polarization conversion efficiency and the polarization conversion efficiency fitted by the high-order polynomial curve.

8. The circular dichroism and polarization conversion tunable flexible chiral metasurface according to claim 7, characterized in that: The method for obtaining the polarization conversion efficiency Ψ of the chiral metasurface unit is: In the formula, R xy It represents the cross-polarization reflection coefficient formed by the electromagnetic wave incident along the y-axis direction and then converted into the polarization reflection along the x-axis direction, R yy It represents the co-polarization reflection coefficient formed by the electromagnetic wave polarized along the y-axis direction and then converted into the polarized reflection along the y-axis direction. The methods of obtaining the two can be expressed as follows: The superscripts r and i represent the reflected wave and the incident wave, respectively. represents the electric field component of the reflected electric field along the coordinate axis x, represents the electric field component of the incident electric field along the y-axis, represents the electric field component of the reflected electric field along the coordinate axis y; the coordinate axes x and y are rotated counterclockwise by 45° to obtain the u and v axes respectively; is the phase, m represents the cross-polarization reflection coefficient at any frequency point within the working bandwidth, and n represents the co-polarization reflection coefficient at any frequency point within the working bandwidth; and They represent the reflection phases along the u and v axes at any frequency point m respectively; and They represent the reflection phases along the u and v axes at any frequency point n, respectively; Then the polarization conversion efficiency Ψ of the chiral metasurface unit is expressed as:

9. The circular dichroism and polarization conversion tunable flexible chiral metasurface according to claim 8, characterized in that: The chiral metasurface unit polarization conversion rate Ψ can be equivalent to the polarization conversion rate fitting error control function N(Ψ) expressed as:

10. The circular dichroism and polarization conversion tunable flexible chiral metasurface according to claim 9, characterized in that: The chiral metasurface can simultaneously manipulate circular dichroism and polarization conversion rate using the control model F(Φ,Ψ), which is expressed as follows: F(Φ,Ψ) = λ1M(Φ) + λ2N(Ψ) Wherein, λ1 and λ2 represent the circular dichroism influence factor and the polarization conversion rate influence factor, respectively; further, the control model F(Φ,Ψ) of the chiral metasurface can be rewritten as:

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