C2-Symmetric Chiral Metasurface and Its Applications in Phase Control and Verification Methods

Abstract

The present invention discloses a C2 symmetric chiral metasurface and its application and verification method for phase control, belonging to the field of micro-nano optics. It includes a surface layer, an intermediate layer, and a backplane layer arranged in sequence from top to bottom. The surface layer is composed of a left arc ring and a right arc ring arranged centrosymmetrically. On the opposite sides of the left arc ring and the right arc ring, there are protrusions, and the protrusions on the left arc ring and the protrusions on the right arc ring are arranged symmetrically about the center of the circle. The notch central angle β of the left arc ring and the right arc ring is an obtuse angle. By adopting the above C2 symmetric chiral metasurface and its application and verification method for phase control, the spin decoupling phase control of co-polarized circularly polarized terahertz waves can be realized only through chiral phases by using superatoms without mirror symmetry and having second-order rotation (C2) symmetry and their mirror atoms.

Description

Technical Field

[0001] The present invention relates to the field of micro-nano optical technologies, and particularly to C2-symmetric chiral metasurfaces and applications and verification methods for phase control. Background Art

[0002] Chirality is a phenomenon commonly existing in nature. The chirality of a substance refers to a geometric feature of the substance structure, that is, it cannot be superimposed with its mirror image through translation and in-plane rotation operations.

[0003] At the same time, the polarization state characterizes the electric field oscillation of light in a plane perpendicular to the propagation direction. In circularly polarized light, the electric field vector can be decomposed into two linearly polarized components. The two perpendicular electric field vectors oscillate with a 90° phase shift and propagate along a helical trajectory in the clockwise or counterclockwise direction. Due to the different helical trajectories of the electric field vectors, circularly polarized light can be divided into left-handed circularly polarized light (LCP) and right-handed circularly polarized light (RCP).

[0004] Chiral media exhibit unique optical responses to LCP and RCP. However, the interaction between natural chiral substances and chiral light fields is very weak. In order to perform efficient chiral light field regulation and develop it into optical devices, the academic community has proposed and studied artificially designed chiral metamaterials. In recent years, a two-dimensional version has been developed, which is called a chiral metasurface. It can achieve magneto-electric resonance coupling and be excited with strong chiral optical responses such as optical activity and circular dichroism, and is then used in technical fields such as biochemical substance detection and circularly polarized optical display.

[0005] From the perspective of light field regulation, chiral metasurfaces are one of the important ways to independently control orthogonal circularly polarized electromagnetic waves. Exploring the electromagnetic theory for spin-decoupled manipulation of LCP and RCP wave components and developing corresponding metasurface devices have become one of the research hotspots in the academic community.

[0006] In the early stage, the almost only method for independent phase control of circularly polarized electromagnetic waves was the Pancharatnam-Berry (PB) phase, that is, when the unit structure rotates by an angle θ, an additional phase of ±2θ conjugate to LCP and RCP is introduced in the cross-polarized components of its reflection or transmission. Since the phase response of the geometric phase to LCP and RCP shows a fixed conjugate change, this greatly limits the spin-decoupling function of the metasurface.

[0007] In 2015, Professor Arbabi disclosed in "Dielectric metasurfaces for complete control of phase and polarization with subwavelength spatial resolution and high transmission" that by combining the polarization space-dependent PB phase with the parameter space-dependent resonance phase, the independent phase control of LCP and RCP was realized for the first time through the joint design of the two types of phases, and various wavefront designs were demonstrated. Subsequently, by introducing the local interference of adjacent meta-atoms, this scheme was extended to the phase and amplitude manipulation of any orthogonal polarization components. However, the above reports all considered achiral structures with mirror symmetry and needed to take into account the design requirements of geometric phase and resonance phase.

[0008] In recent years, the chiral phase induced by azimuthal rotation and structural parameter adjustment of chiral meta-atoms has become a new method for the design of spin-decoupled metasurfaces. In 2020, Yuan et al. disclosed in "Independent phase modulation for quadruplex polarization channels enabled by chirality-assisted geometric-phase metasurfaces" that the chiral-induced phase was introduced by using the relative azimuth difference of multilayer metal resonators, and the four-channel circular polarization wavefront design was completed by combining the resonance phase; in 2021, Chen et al. proposed in "Metasurfaces with Planar Chiral Meta-Atoms for Spin Light Manipulation" to use all-dielectric meta-atoms with C2 symmetry and their geometric phase to control spin waves; in the same year, Song et al. reported in "Spin-Selective Full-Dimensional Manipulation of Optical Waves with Chiral Mirror" the topological phase triggered by singular points in non-Hermitian chiral meta-atoms, and realized spin-decoupled wavefront control by combining the P-B phase.

[0009] However, the above methods for controlling circularly polarized electromagnetic waves based on achiral or chiral meta-atoms still have room for improvement in terms of theoretical complexity, design difficulty, computational amount, etc. Moreover, the combination of resonance phase or chiral phase with P-B phase requires more theoretical descriptions and needs to consider the analysis of polarization space while exploring the parameter space. In addition, it is also very difficult to control the amplitude difference of wave components. Summary of the Invention

[0010] The object of the present invention is to provide a C2-symmetric chiral metasurface and its application and verification method for phase control, so as to solve the above technical problems.

[0011] To achieve the above object, the present invention provides a C2-symmetric chiral metasurface, which includes a surface layer, an intermediate layer and a backplane layer arranged in sequence from top to bottom. The surface layer is composed of a left arc ring and a right arc ring arranged centrosymmetrically. Raised portions are provided on one side of the left arc ring and the right arc ring that face each other, and the raised portions on the left arc ring and the raised portions on the right arc ring are arranged symmetrically about the center of the circle. The notch central angle β of the left arc ring and the right arc ring is an obtuse angle.

[0012] Preferably, the central angle β of the left arc ring and the right arc ring is 40°;

[0013] The distance d between the left arc ring and the right arc ring is 10 μm;

[0014] The widths w of the left arc ring, the right arc ring and the raised portions are 25 μm;

[0015] The length h of the raised portions is 60 μm.

[0016] Preferably, the material of the intermediate layer is polyimide, the dielectric constant ε is 3.4, the loss tangent tanδ is 0.002, and the thickness t1 is 70 μm.

[0017] Preferably, the material of the backplane layer is gold, and the thickness t is 200 nm.

[0018] Preferably, the reflection phase responses of the chiral metasurface to left-handed polarized light and right-handed polarized light are mirror images, and the reflection efficiencies of the chiral metasurface to left-handed polarized light and right-handed polarized light are both greater than 71.7%.

[0019] An application of a C2-symmetric chiral metasurface in phase control, which uses chiral phase to perform spin decoupling phase control on co-polarized circularly polarized terahertz waves.

[0020] A verification method for the application of a C2-symmetric chiral metasurface in phase control includes the following steps:

[0021] S1. Establish a simulation model: Determine the geometric parameters of the chiral metasurface, and then perform frequency-domain simulation on the chiral metasurface using electromagnetic simulation software;

[0022] S2. Use electromagnetic simulation software to scan the raised portions, the left arc ring and the right arc ring of the chiral metasurface to obtain chiral metasurface simulation units under multiple parameter configurations;

[0023] S3. Simulation analysis: Calculate the reflection amplitude and phase of the chiral metasurface simulation unit under each parameter configuration. Based on the calculated reflection amplitude and phase, select the chiral metasurface simulation unit that meets the expectations as the final structure of the chiral metasurface;

[0024] S4. Conduct functional verification based on the final structure of the chiral metasurface in the dual-focus scenario.

[0025] Preferably, in step S1, the incident wave is defined as left-handed circularly polarized light or right-handed circularly polarized light, and the boundary conditions in the x and y directions are set to unit cell, the boundary condition in the z direction is set to open, and the number of polarization modes of the maximum value Zmax and the minimum value Zmin of the electromagnetic simulation space are both set to 2;

[0026] In step S2, scan the protrusions in steps of 2 μm from 0 μm to 160 μm, and simultaneously scan the left and right arc rings in steps of 2° from 0° to 116°.

[0027] Preferably, in step S3, the calculation formula for the reflection amplitude is as follows:

[0028]

[0029] In the formula, R represents the reflection amplitude; R circ represents the reflection coefficient matrix of the circularly polarized wave; r LL and r RR respectively represent the reflection coefficients of left-handed circularly polarized light and right-handed circularly polarized light; r LR and r RL respectively represent the cross reflection coefficients from left-handed circular polarization to right-handed circular polarization and from right-handed circular polarization to left-handed circular polarization; r xx 、r yy 、r xy and r yx all represent the elements of the reflection coefficient matrix of the linearly polarized wave; i represents the imaginary unit;

[0030] The phase calculation formula is as follows:

[0031]

[0032] In the formula, and respectively represent the phases of left-handed circularly polarized light and right-handed circularly polarized light; λ represents the working wavelength; X and Y represent the spatial rectangular coordinates; D represents the lateral offset of the focus; f represents the designed focal length.

[0033] Preferably, step S4 specifically includes the following steps:

[0034] S41. In the parameter space, select 10 chiral metasurfaces with a 90° phase gradient, including 4 pairs and chiral metasurfaces that produce the same response;

[0035] At the same time, select 6 chiral metasurfaces with opposite phases from the mirror-symmetric chiral metasurfaces to establish a library of spin decoupling structural units with a 90° phase gradient using chiral phases;

[0036] S42. Call 16 superatoms in the spin decoupling structural unit library to generate a chiral metasurface composed of 60*60 supercells. Define the parameters of the chiral metasurface using matlab and import the generated chiral metasurface into an electromagnetic simulation software for electromagnetic characteristic simulation;

[0037] S43. Regulate the left-handed polarized light and right-handed polarized light components respectively to regulate the focusing directions of the left-handed polarized light and right-handed polarized light in the 7000um plane;

[0038] S44. Under the conditions of respectively incident left-handed circularly polarized light and right-handed circularly polarized light, the boundary conditions in the x, y, and z directions are all set to open, the calculation frequency range is set to 0.5 THz - 0.7 THz, and the electric field monitoring frequency is 0.58 THz. Monitor the electric field intensity distribution in the focal plane and the yoz plane to generate a focusing effect diagram;

[0039] S45. Read the best focal plane position from the focusing effect diagram and compare the best focal plane position with the expected position to determine whether the expected position is reached.

[0040] Therefore, the present invention adopts the above C2 symmetric chiral metasurface and the application and verification methods of phase control, and the beneficial effects are as follows:

[0041] 1. Precise control of structural design: Multilayer structure: The chiral metasurface is composed of a surface layer, an intermediate layer, and a backplane layer. Each layer has clear material and geometric parameter requirements. This multi-layer design makes the structure more stable and can effectively control the propagation characteristics of electromagnetic waves;

[0042] Combination of symmetric and asymmetric elements: The left and right arc rings on the surface layer are arranged centrosymmetrically and have protrusions. These designs enhance the chiral characteristics of the structure and ensure that the reflection phase responses of left and right circularly polarized lights show a mirror relationship;

[0043] 2. High-performance optical characteristics: High reflection efficiency: The reflection efficiencies of the chiral metasurface for left-handed polarized light and right-handed polarized light are both greater than 71.7%, indicating its efficient reflection performance in the terahertz frequency band;

[0044] Phase control ability: The chiral metasurface can achieve spin-decoupled phase control of co-polarized circularly polarized terahertz waves, thus being applied to the fine adjustment of electromagnetic wave phases;

[0045] 3. Detailed simulation verification process: Establishment of simulation model: Perform frequency-domain simulation through electromagnetic simulation software to ensure that all geometric parameters are accurately calculated and optimized;

[0046] Parameter scanning: Scan the protrusions from 0um to 160um with a step size of 2um, and simultaneously scan the left and right arc-shaped rings from 0° to 116° with a step size of 2°, ensuring that a wide range of parameter configurations are covered, improving the comprehensiveness and reliability of the simulation;

[0047] Calculation of reflection amplitude and phase: Provide specific formulas for calculating reflection amplitude and phase, ensuring the accuracy and repeatability of the simulation;

[0048] 4. Practical application potential: Spin-decoupled structure unit library: By selecting 10 chiral metasurfaces with a 90° phase gradient and using these unit libraries to generate a chiral metasurface composed of 60*60 supercells, efficient phase gradient regulation is achieved;

[0049] Flexible boundary condition setting: During the simulation, the boundary conditions in the x, y, and z directions are all set to open, the calculation frequency range is set to 0.5THz - 0.7THz, and the electric field monitoring frequency is 0.58THz, ensuring the flexibility and accuracy of the simulation;

[0050] 5. Innovation and practicality: Mirror-symmetric chiral metasurface: By selecting 6 chiral metasurfaces with opposite phases, more complex phase gradient regulation is achieved, which is of great significance for the development of new optical devices and communication systems;

[0051] Determination of the optimal focal plane position: By comparing the simulation results with the expected position, it is convenient to precisely adjust the structural parameters to ensure the best focusing effect.

[0052] In summary, the C2-symmetric chiral metasurface described in the present invention not only has unique innovations in design, but also performs excellently in terms of performance and applications, and has broad practical application prospects.

[0053] Next, through the accompanying drawings and embodiments, the technical solutions of the present invention will be further described in detail. Description of the Drawings

[0054] Figure 1 It is a schematic diagram of the surface structure of a chiral metasurface of the present invention;

[0055] Figure 2Schematic diagram of the spin decoupling phase response for the simulation experiment; among which, (a) is the reflection amplitude manipulation diagram; (b) is the phase manipulation diagram; (c) is the circular polarization phase distribution diagram when the LCP wave (left-handed circularly polarized light) is incident; (d) is the circular polarization phase distribution diagram when the RCP wave (right-handed circularly polarized light) is incident; (e) is the circular polarization amplitude distribution diagram when the LCP wave (left-handed circularly polarized light) is incident; (f) is the circular polarization amplitude distribution diagram when the RCP wave (right-handed circularly polarized light) is incident;

[0056] Figure 3 Diagram of the functional verification result for the simulation experiment, among which, (a) is the electric field intensity distribution diagram on the xoz plane when the LCP wave (left-handed circularly polarized light) is incident; (b) is the electric field intensity distribution diagram on the xoz plane when the RCP wave (right-handed circularly polarized light) is incident; (c) is the electric field intensity distribution diagram on the plane where the best focal plane position Z = 7161 microns at the working frequency f = 0.58 THz when the LCP wave (left-handed circularly polarized light) is incident; (d) is the electric field intensity distribution diagram on the plane where the best focal plane position Z = 7161 microns at the working frequency f = 0.58 THz when the RCP wave (right-handed circularly polarized light) is incident.

[0057] Reference numerals

[0058] 1. Left arc ring; 2. Right arc ring; 3. Protrusion. Detailed implementation manners

[0059] In order to make the purpose, technical solutions and advantages of the embodiments of the present invention more clearly understood, the following further details the embodiments of the present invention in conjunction with the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are only used to explain the embodiments of the present invention, and are not used to limit the embodiments of the present invention. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative efforts belong to the scope of protection of this application. The examples of the embodiments are shown in the accompanying drawings, where the same or similar reference numerals represent the same or similar elements or elements with the same or similar functions from beginning to end.

[0060] It should be noted that the terms "include" and "have" and any variations thereof are intended to cover non-exclusive inclusion. For example, a process, method, system, product or server that includes a series of steps or units does not necessarily have to be limited to those clearly listed steps or units, but may include other steps or units that are not clearly listed or are inherent to these processes, methods, products or devices.

[0061] The following details the implementation manners of the present invention in conjunction with the accompanying drawings.

[0062] Conventional double split-ring resonators have mirror symmetry and can accumulate geometric phases or resonant phases of linear polarization in the paths of the polarization space or parameter space, but do not have chiral responses. When the structure is sheared to break the mirror symmetry while maintaining second-order rotational symmetry, chirality may exist; at this time, chiral phases can be accumulated through the path in the parameter space to achieve spin-decoupled phase control while keeping the amplitudes almost the same.

[0063] Based on the above analysis, the present invention is designed as follows: As Figure 1 shown, a C2-symmetric chiral metasurface includes a surface layer, an intermediate layer, and a backplane layer arranged in sequence from top to bottom. The surface layer is composed of a left arc ring 1 and a right arc ring 2 arranged centrosymmetrically. Protrusions 3 are provided on one side of the left arc ring 1 and the right arc ring 2 that face each other, and the protrusions 3 on the left arc ring 1 and the protrusions 3 on the right arc ring 2 are arranged centrosymmetrically about the center of the circle. The notch central angle β of the left arc ring 1 and the right arc ring 2 is an obtuse angle.

[0064] Specifically, the central angle β of the left arc ring 1 and the right arc ring 2 is 40°; the distance d between the left arc ring 1 and the right arc ring 2 is 10 μm; the width w of the left arc ring 1, the right arc ring 2, and the protrusion 3 is 25 μm; the length h of the protrusion 3 is 60 μm. The material of the intermediate layer is polyimide, with a dielectric constant ε = 3.4, a loss tangent tanδ = 0.002, and a thickness t1 = 70 μm. The material of the backplane layer is gold, with a thickness t = 200 nm.

[0065] The reflection phase responses of the chiral metasurface to left-handed polarized light and right-handed polarized light are mirror images, and the reflection efficiencies of the chiral metasurface to left-handed polarized light and right-handed polarized light are both greater than 71.7%.

[0066] An application of a C2-symmetric chiral metasurface in phase control, using chiral phases for spin-decoupled phase control of co-polarized circularly polarized terahertz waves.

[0067] A verification method for an application of a C2-symmetric chiral metasurface in phase control includes the following steps:

[0068] S1. Establish a simulation model: Determine the geometric parameters of the chiral metasurface, and then perform frequency-domain simulation on the chiral metasurface using electromagnetic simulation software;

[0069] In step S1, define the incident wave as left-handed circularly polarized light or right-handed circularly polarized light, set the boundary conditions in the x and y directions to unit cell (periodic), set the boundary condition in the z direction to open, and set the polarization mode numbers of the maximum value Zmax and the minimum value Zmin of the electromagnetic simulation space to 2;

[0070] S2. Use electromagnetic simulation software to scan the protrusions, left arc rings, and right arc rings of the chiral metasurface to obtain the chiral metasurface simulation units under multiple parameter configurations;

[0071] In step S2, the protrusions are scanned from 0 um to 160 um in steps of 2 um, and at the same time, the left arc ring and the right arc ring are scanned from 0° to 116° in steps of 2°.

[0072] S3. Simulation analysis: Calculate the reflection amplitude and phase of the chiral metasurface simulation unit under each parameter configuration, and based on the calculated reflection amplitude and phase, select the chiral metasurface simulation unit that meets the expectations as the final structure of the chiral metasurface;

[0073] In step S3, the calculation formula for the reflection amplitude is as follows:

[0074]

[0075] In the formula, R represents the reflection amplitude; R circ represents the reflection coefficient matrix of the circularly polarized wave; r LL and r RR respectively represent the reflection coefficients of the left-handed circularly polarized light and the right-handed circularly polarized light; r LR and r RL respectively represent the cross-reflection coefficients from left-handed circular polarization to right-handed circular polarization and from right-handed circular polarization to left-handed circular polarization; r xx , r yy , r xy and r yx all represent the elements of the reflection coefficient matrix of the linearly polarized wave; i represents the imaginary unit;

[0076] The phase calculation formula is as follows:

[0077]

[0078] In the formula, and respectively represent the phases of the left-handed circularly polarized light and the right-handed circularly polarized light; λ represents the working wavelength; X and Y represent the spatial rectangular coordinates; D represents the transverse offset of the focus; f represents the designed focal length. In this embodiment, λ = 517 um, f = 7000 um, d = 800 um.

[0079] S4. Perform functional verification based on the final structure of the chiral metasurface in a double-focus scenario.

[0080] Step S4 specifically includes the following steps:

[0081] S41. In the parameter space, select 10 chiral metasurfaces with a 90° phase gradient, including 4 pairs and Chiral metasurface generating the same response

[0082] Simultaneously select 6 chiral metasurfaces with opposite phases from the mirror-symmetric chiral metasurface to establish a library of spin decoupling structural units with a 90° phase gradient using chiral phases;

[0083] S42. Call 16 superatoms in the library of spin decoupling structural units to generate a chiral metasurface composed of 60*60 supercells, define the parameters of the chiral metasurface using matlab, and import the generated chiral metasurface into an electromagnetic simulation software for electromagnetic characteristic simulation;

[0084] S43. Regulate the left-handed polarized light and right-handed polarized light components respectively to control the focusing directions of the left-handed polarized light and right-handed polarized light in the 7000um plane;

[0085] S44. When left-handed circularly polarized light and right-handed circularly polarized light are incident respectively, the boundary conditions in the x, y, and z directions are all set to open, the calculation frequency range is set to 0.5 THz - 0.7 THz, and the electric field monitoring frequency is 0.58 THz. Monitor the electric field intensity distribution in the focal plane and the yoz plane to generate a focusing effect diagram;

[0086] S45. Read the best focal plane position from the focusing effect diagram and compare the best focal plane position with the expected position to determine whether the expected position is reached.

[0087] Simulation experiment

[0088] Based on the present invention, a simulation experiment is carried out, and the results are as Figure 2 and Figure 3 shown. From Figure 2 (a), it can be seen that when h = 60um and β = 40°, by performing frequency-domain solution simulation on the chiral metasurface using electromagnetic simulation software, it can be clearly observed that in the frequency range of 0.2 THz to 0.8 THz, the reflectivity (the reflectivity is the square of the reflection coefficient) curves of the reflected co-polarized circularly polarized waves R LL and R RR are basically coincident.

[0089] From Figure 2 (b), it can be seen that in the frequency range of 0.2 THz to 0.8 THz, by observing the reflected co-polarized circularly polarized phases and of the unit structure, it can be seen that and There are very obvious phase differences within the range of interest. The phase difference at the operating frequency of 0.58 THz is close to π. The inset shows the local electric field distributions when LCP (left-handed circularly polarized light) and RCP (right-handed circularly polarized light) are incident at this frequency point, which provides a more intuitive demonstration of the reason for the phase difference.

[0090] It can be seen from Figure 2 (c) and (d) that at the operating frequency of 0.58 THz, it can be clearly observed that the left arc ring achieves a 360° full coverage of the phase of the reflected co-polarized LCP, while for the phase of the reflected RCP wave, only about 180° of phase coverage is achieved. However, its mirror chiral structure (right arc ring) can achieve 360° coverage of and 180° phase coverage of . In addition, when the LCP wave is incident, changing the value of h has a great influence on but basically has no influence on . When the RCP wave is incident, there is basically no resonance at the raised h. Changing the notch central angle β has a great influence on but basically has no effect on .

[0091] It can be seen from Figure 2 (e) and (f) that at the operating frequency of 0.58 THz, due to the rotational symmetry of the chiral metasurface, the amplitudes of the R LL and R RR components are basically equal, and the R LL and R RR of each chiral metasurface basically remain above 0.7 in intensity, and the amplitude of the reflected co-polarized beam can be maintained at a relatively high level.

[0092] It can be seen from Figure 3 (c) and (d) that it can be seen that the different circularly polarized components form an eccentrically focused beam after being reflected by the metasurface, which is in line with the designed function, thus proving the effectiveness of the present invention.

[0093] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit them. Although the present invention has been described in detail with reference to the preferred embodiments, those of ordinary skill in the art should understand that they can still modify the technical solutions of the present invention or make equivalent replacements, and these modifications or equivalent replacements cannot make the modified technical solutions deviate from the spirit and scope of the technical solutions of the present invention.

Claims

1. A C2-symmetric chiral metasurface, characterized in that: It includes a surface layer, an intermediate layer, and a backplane layer arranged in sequence from top to bottom. The surface layer is composed of a left arc ring and a right arc ring arranged symmetrically about the center. On the opposite sides of the left arc ring and the right arc ring, there are protrusions, and the protrusions on the left arc ring and the protrusions on the right arc ring are arranged symmetrically about the center of the circle. The notch central angles of the left arc ring and the right arc ring are obtuse angles; Central angles of the left arc ring and the right arc ring ; The distance between the left arc ring and the right arc ring ; Width of the left arc ring, right arc ring, and protrusion ; Length of the protrusion .

2. The C2-symmetric chiral metasurface according to claim 1, wherein: The material of the intermediate layer is polyimide, with a dielectric constant , and a tangent of the loss angle , and a thickness .

3. The C2-symmetric chiral metasurface according to claim 1, wherein: The material of the backplane layer is gold and the thickness .

4. The C2-symmetric chiral metasurface according to claim 1, characterized in that: The reflection phase responses of the chiral metasurface to left-handed polarized light and right-handed polarized light present a mirror image, and the reflection efficiencies of the chiral metasurface to left-handed polarized light and right-handed polarized light are both greater than 71.7%.

5. Application of the C2-symmetric chiral metasurface according to any one of the above claims 1-4 in phase control, characterized in that: Utilize chiral phase for spin decoupling phase control of co-polarized circularly polarized terahertz waves.

6. A verification method for the application of the C2-symmetric chiral metasurface in phase control according to claim 5 above, characterized in that: It includes the following steps: S1. Establish a simulation model: Determine the geometric parameters of the chiral metasurface, and then perform frequency-domain simulation on the chiral metasurface using electromagnetic simulation software; S2. Use electromagnetic simulation software to scan the protrusions, left arc rings, and right arc rings of the chiral metasurface to obtain chiral metasurface simulation units under multiple parameter configurations; S3. Simulation analysis: Calculate the reflection amplitude and phase of the chiral metasurface simulation units under each parameter configuration, and based on the calculated reflection amplitude and phase, select the chiral metasurface simulation units that meet the expectations as the final structure of the chiral metasurface; S4. Conduct functional verification based on the final structure of the chiral metasurface in a double-focusing scenario.

7. The verification method for the application of the C2-symmetric chiral metasurface in phase control according to claim 6, characterized in that: In step S1, the incident wave is defined as left-handed circularly polarized light or right-handed circularly polarized light, the boundary conditions in the x and y directions are set to unit cell, the boundary condition in the z direction is set to open, and the maximum value of the electromagnetic simulation space and the minimum value of the electromagnetic simulation space are both set to 2 for the number of polarization modes; In step S2, the protrusions are scanned from 0 um to 160 um with a step size of 2 um, and at the same time, the left arc rings and right arc rings are scanned from 0° to 116° with a step size of 2°.

8. The verification method for the application of the C2-symmetric chiral metasurface in phase control according to claim 6, characterized in that: In step S3, the calculation formula for the reflection amplitude is as follows: （1）； In the formula, represents the reflection amplitude; represents the reflection coefficient matrix of the circularly polarized wave; and represent the reflection coefficients of the left-handed circularly polarized light and the right-handed circularly polarized light respectively; and represent the cross reflection coefficients from the left-handed circular polarization to the right-handed circular polarization and from the right-handed circular polarization to the left-handed circular polarization respectively; , , and all represent the elements of the reflection coefficient matrix of the linearly polarized wave; represents the imaginary unit; The calculation formula for the phase is as follows: （2）； In the formula, and respectively represent the phases of left-handed circularly polarized light and right-handed circularly polarized light; represents the working wavelength; and represent the spatial rectangular coordinates; represents the lateral offset of the focus; represents the designed focal length.

9. The verification method for the application of the C2-symmetric chiral metasurface in phase control according to claim 6, wherein: Step S4 Specifically, it includes the following steps: S41. In the parameter space, 10 chiral metasurfaces with a 90° phase gradient are selected, including 4 pairs and of chiral metasurfaces that produce the same response. Simultaneously select 6 chiral metasurfaces with opposite phases from the mirror-symmetric chiral metasurface to establish a spin decoupling structure unit library with a 90° phase gradient using chiral phase; S42. Call 16 superatoms in the spin decoupling structure unit library to generate a chiral metasurface composed of supercells, define the parameters of the chiral metasurface using matlab, and import the generated chiral metasurface into an electromagnetic simulation software for electromagnetic characteristic simulation; S43. Regulate the left-handed polarized light and right-handed polarized light components respectively to control the focusing directions of the left-handed polarized light and right-handed polarized light in the 7000 um plane; S44. Under the conditions that left-handed circularly polarized light and right-handed circularly polarized light are incident respectively, the boundary conditions in the x, y, and z directions are all set to open, the calculation frequency range is set to 0.5 THz - 0.7 THz, and the electric field monitoring frequency is 0.58 THz. Monitor the electric field intensity distribution in the focal plane and the yoz plane to generate a focusing effect diagram; S45. Read the optimal focal plane position from the focusing effect diagram and compare the optimal focal plane position with the expected position to determine whether the expected position is reached.

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