Design method of multilayer annular dipole fabry-perot chiral metasurface

Abstract

This invention belongs to the field of optical metasurface technology, specifically relating to a design method for a multilayer toroidal dipole Fabry-Perot chiral metasurface. 1. A cylindrical tetramer symmetric metasurface is provided; 2. When the incident light is TM and TE polarized, the radii of the third and fourth nanocylinders are simultaneously reduced, followed by a reduction in the radius of the fourth nanocylinder, introducing an asymmetric perturbation to form a monolayer TD metasurface; an alternating all-dielectric layer is added under the monolayer TD metasurface nanosubstrate to form a TD-FP-BIC metasurface. Placing the nanocylinder tetramer in a chiral molecular environment, with multiple layers of all-dielectric nanocubes forming a mirror and an FP cavity, results in a multilayer TD-FP-BIC chiral metasurface. By adjusting the relevant structural parameters and analyzing the performance of the metasurface, the intensity and waveform of the transmission spectrum can be flexibly modulated, enabling applications in chiral sensing and ultrasensitive circular dichroic spectral amplifiers.

Description

Technical Field

[0001] This invention belongs to the field of optical metasurface technology, specifically relating to a design method for a multilayer toroidal dipole Bripperrow chiral metasurface. Background Technology

[0002] With the rapid development of nanophotonics and metamaterials technology, metasurfaces, as two-dimensional functional materials composed of subwavelength-scale artificial structural units, have become an important research direction for the miniaturization and integration of optical devices due to their ability to precisely control the amplitude, phase, polarization, and other dimensions of electromagnetic waves within extremely thin thicknesses. Compared with traditional bulk optical devices, metasurfaces have advantages such as compact structure, high design freedom, and strong functional integration capabilities, showing broad application prospects in polarization control, wavefront shaping, spectral selection, nonlinear enhancement, and sensing.

[0003] Among the many research directions in metasurfaces, chiral metasurfaces have become a research hotspot due to their ability to selectively respond to left-handed and right-handed circularly polarized light. Chiral response typically manifests as circular dichroism (CD) and optical rotation, holding significant application value in fields such as biomolecular detection, chiral recognition, polarization imaging, and information encryption. However, existing chiral metasurfaces mostly rely on geometrically asymmetric structures or three-dimensional configurations, resulting in complex structures and significant fabrication difficulties. Furthermore, their chiral response intensity is often limited by material loss and radiation loss, making it difficult to simultaneously achieve a high quality factor (Q factor) and strong chiral response.

[0004] To overcome the aforementioned problems, chiral metasurfaces based on Bound States in the Continuum (BIC) have been proposed and analyzed in existing technologies. Among BIC-related research, the toroidal dipole (TD) mode has gradually attracted attention. Compared to traditional electric or magnetic dipole modes, the TD mode exhibits stronger local field enhancement capabilities and lower radiation leakage characteristics, thus possessing unique advantages in high-Q resonance design. Combining the TD mode with the BIC mechanism can further improve resonance performance, achieving higher Q values ​​and stronger field confinement.

[0005] Although metasurfaces based on the TD-BIC mechanism have made significant progress in improving the resonance quality factor, existing research has mostly focused on single-layer structure design, which has limited degrees of freedom for manipulation. Furthermore, single-layer structures have limited capabilities in mode coupling and spectral manipulation, making it difficult to achieve synergistic enhancement of multiple modes. To further overcome performance bottlenecks, multilayer designs introduce additional degrees of freedom in the vertical direction, enabling near-field coupling and far-field interference between different layers, thus achieving more flexible electromagnetic response manipulation. This multilayer coupling mechanism not only helps to improve the resonance Q value but also enhances the local electromagnetic field distribution, providing favorable conditions for achieving a strong chiral response. By introducing appropriate symmetry breaking in the multilayer system, a significant chiral response can be induced while maintaining a high Q resonance. Although progress has been made in chiral metasurfaces, the BIC mechanism, and TD modes, there are still significant shortcomings in achieving multilayer TD-FP-BIC chiral metasurfaces that balance high Q value, strong chiral response, and structural controllability, requiring further in-depth research. Summary of the Invention

[0006] The purpose of this invention is to design a multilayer TD-FP-BIC chiral metasurface (TD: toroidal electric dipole, FP: Fabry-Perot, BIC: continuous domain bound state). By adjusting the relevant structural parameters of the metasurface, the intensity and waveform of the transmission spectrum can be flexibly modulated. By utilizing the mirror coupling between the multilayer structures to excite the TD-FP-BIC, the resulting chiral metasurface exhibits high chiral detection sensitivity. This provides a novel design strategy for manipulating near-field electromagnetic fields, which can be applied in ultrasensitive chiral sensing.

[0007] The technical solution adopted in this invention is as follows: the design method of multilayer toroidal dipole Bripperrow chiral metasurface includes the following steps:

[0008] S1: Provides a single-layer symmetric metasurface, which includes several square lattices and incident light. Each square lattice consists of a nano-substrate and cylindrical tetramers deposited on the nano-substrate. The nano-substrate is a nanocube with a refractive index of 1.4-1.6, a thickness of 730-808 nm, and is made of silicon dioxide. The cylindrical tetramer consists of four nanocylinders of the same radius: a first nanocylinder, a second nanocylinder, a third nanocylinder, and a fourth nanocylinder. The four nanocylinders are arranged symmetrically in the X-axis and Y-axis directions. The center-to-center distance between two adjacent nanocylinders in the X-axis and Y-axis directions is 200-236 nm. The refractive index of the four nanocylinders is 3-4, the radius is 80-96 nm, the height is 450-550 nm, and the material is silicon. The incident light is TM polarized and TE polarized, i.e., X-axis polarized and Y-axis polarized, with a polarization angle of -90° to 90°.

[0009] S2: Provide a single-layer Y-axis symmetric metasurface. Simultaneously reduce the radii of the third and fourth nano-cylinders of the single-layer symmetric metasurface in step S1 to destroy the structural symmetry in the X-axis direction and introduce asymmetric perturbation to form an X-axis asymmetric square lattice. Several X-axis asymmetric square lattices constitute a single-layer Y-axis symmetric metasurface. The radii of the third and fourth nano-cylinders are reduced to 55-65 nanometers, while other structural parameters remain unchanged. The incident light is TM polarized and TE polarized.

[0010] S3: Provide a single-layer asymmetric metasurface. Restore the radius of the fourth nano-cylinder of the single-layer Y-axis symmetric metasurface in step S2 to 80-96 nanometers, while keeping the radius of the third nano-cylinder unchanged. Simultaneously introduce X-axis and Y-axis asymmetric perturbations to form an asymmetric square lattice. Several asymmetric square lattices constitute a single-layer polarization-independent TD metasurface, which is the single-layer TD metasurface. Calculate the average chirality enhancement of the single-layer TD metasurface. Change its incident light to left-hand circularly polarized LCP with the polarization angle rotated counterclockwise and right-hand circularly polarized RCP with the polarization angle rotated clockwise.

[0011] S4: Provides a multilayered TD-FP-BIC chiral metasurface. Several square silicon nanolayers and magnesium fluoride nanolayers are alternately stacked beneath a single-layer TD metasurface nanosubstrate to form a multilayered TD-FP-BIC metasurface. This multilayered TD-FP-BIC chiral metasurface is then placed within chiral molecules, forming a multilayered TD-FP-BIC chiral metasurface. In other words, several asymmetric cylindrical lattices constitute the TD-FP-BIC chiral metasurface. The silicon nanolayers are made of silicon with a thickness of 60-80 nm, and the magnesium fluoride layers are made of magnesium fluoride with a thickness of 175-185 nm. The incident light is left-handed and right-handed circularly polarized. The refractive index of the silicon nanolayers is 3-4, and the refractive index of the magnesium fluoride nanolayers is 1.34-1.4. When the cylindrical tetramer is placed in the environment of chiral molecules, the ambient refractive index is 1.33 ± 10. -5 i, the Pasteur parameter of the chiral molecular layer is 10. -5 i. By placing nanocylindrical tetramers in a chiral molecular environment, multilayer all-dielectric nanocubes are formed as mirrors, creating FP cavities, thus making it a multilayer TD-FP-BIC chiral metasurface.

[0012] Further, in step S1, the thickness of the nanosubstrate is 730 nm, and its refractive index is 1.4; the radius of the first nanocylinder, the second nanocylinder, the third nanocylinder, and the fourth nanocylinder is 80 nm, their height is 450 nm, the refractive index of the nanocylinder is 3, and the center-to-center distance between two adjacent nanocylinders in the X-axis and Y-axis directions is 200 nm; in step S2, the radius of the third nanocylinder and the fourth nanocylinder is 55 nm; in step S3, the radius of the third nanocylinder is 55 nm, and the radius of the fourth nanocylinder is 80 nm; in step S4, the thickness of the silicon nanolayer is 60 nm, the thickness of the magnesium fluoride nanolayer is 175 nm, the refractive index of the silicon nanolayer is 3, and the refractive index of the magnesium fluoride nanolayer is 1.34.

[0013] Further, in step S1, the thickness of the nanosubstrate is 769 nm, and its refractive index is 1.5; the radius of the first, second, third, and fourth nanocylinders is 88 nm, their height is 500 nm, the refractive index of the nanocylinders is 3.5, and the center-to-center distance between two adjacent nanocylinders in the X-axis and Y-axis directions is 218 nm; in step S2, the radius of the third and fourth nanocylinders is 60 nm; in step S3, the radius of the third nanocylinder is 60 nm, and the radius of the fourth nanocylinder is 88 nm; in step S4, the thickness of the silicon nanolayer is 70 nm, the thickness of the magnesium fluoride nanolayer is 180 nm, the refractive index of the silicon nanolayer is 3.5, and the refractive index of the magnesium fluoride nanolayer is 1.37.

[0014] Further, in step S1, the thickness of the nanosubstrate is 808 nm, and its refractive index is 1.6; the radius of the first nanocylinder, the second nanocylinder, the third nanocylinder, and the fourth nanocylinder is 96 nm, their height is 550 nm, the refractive index of the nanocylinder is 4, and the center-to-center distance between two adjacent nanocylinders in the X-axis and Y-axis directions is 236 nm; in step S2, the radius of the third nanocylinder and the fourth nanocylinder is 65 nm; in step S3, the radius of the third nanocylinder is 65 nm, and the radius of the fourth nanocylinder is 96 nm; in step S4, the thickness of the silicon nanolayer is 80 nm, the thickness of the magnesium fluoride nanolayer is 185 nm, the refractive index of the silicon nanolayer is 4, and the refractive index of the magnesium fluoride nanolayer is 1.4.

[0015] Furthermore, in steps S1, S2, and S3, the wave vector of the incident light is parallel to the z-axis direction.

[0016] Furthermore, the period of both the single-layer TD metasurface and the multilayer TD-FP-BIC chiral metasurface is 550-750 nanometers, and the period of the metasurface is equal to the length and width of the nanosubstrate.

[0017] Furthermore, the asymmetric peaks of the single-layer TD metasurface and the multilayer TD-FP-BIC chiral metasurface exhibit Fano resonance characteristics.

[0018] Furthermore, the silicon nanolayers and magnesium fluoride nanolayers are arranged alternately, and when excited by left-hand circularly polarized (LCP) and right-hand circularly polarized (RCP) incident light, the ambient refractive index is 1.33.

[0019] Furthermore, both the silicon nanolayer and the magnesium fluoride nanolayer have 6 layers.

[0020] Furthermore, the multilayer TD-FP-BIC metasurface exhibits enhanced maximum and average chirality in a nonlocal manner.

[0021] Furthermore, the circular dichroism signal of the multilayer TD-FP-BIC chiral metasurface is enhanced, which can be applied in chiral sensing.

[0022] The beneficial effects of this invention are as follows: To solve the technical problems of this application, a TD-FP-BIC chiral metasurface is designed. This metasurface replaces the existing cylindrical tetramer TD metasurface with a TD-FP-BIC metasurface formed by multilayer mirror coupling, constituting an asymmetric square lattice. The metasurface exhibits a distinct sharp Fano resonance near 1046 nm, with a Q factor as high as 6.6 × 10⁻⁶. 4 The monolayer TD metasurface amplified the far-field circular dichroism signal of chiral molecules by 128 times, while the TD-FP-BIC chiral metasurface achieved an amplification of up to 1092 times. C measured in the unstructured region... max The value and average chiral enhancement value reached 5×10 4 This represents a 10-fold and 2011-fold increase. It provides a safe and efficient nanophotonics solution for ultrasensitive chiral drug screening and enantiomeric recognition, demonstrating broad application prospects. Attached Figure Description

[0023] Figure 1 It is a top view of a square lattice in a single-layer symmetric metasurface.

[0024] Figure 2 It is a top view of a square lattice in a single-layer asymmetric metasurface.

[0025] Figure 3 This is a schematic diagram of the three-dimensional structure of a square lattice in a multilayer TD-FP-BIC metasurface.

[0026] Figure 4 This is a top view of a square lattice in a multilayer TD-FP-BIC metasurface.

[0027] Figure 5 This is a front view of a square lattice in a multilayer TD-FP-BIC metasurface.

[0028] Figure 6 This is the transmission spectrum of the monolayer symmetric metasurface in Experiment Example 1.

[0029] Figure 7 This is a multipole decomposition diagram of TM polarization of a single-layer symmetric metasurface in Experiment Example 1.

[0030] Figure 8 This is a comparison diagram of the electric and magnetic field distributions of the TM polarization of a single-layer symmetric metasurface in Experiment Example 1.

[0031] Figure 9 This is a multipole decomposition diagram of TE polarization of a single-layer symmetric metasurface in Experiment Example 1.

[0032] Figure 10 This is a comparison diagram of the electric and magnetic field distributions of the TE polarization of a single-layer symmetric metasurface in Experiment Example 1.

[0033] Figure 11 This is the transmission spectrum of the monolayer Y-axis symmetric metasurface in Experiment Example 2.

[0034] Figure 12 This is a multipole decomposition diagram of TM polarization of a single-layer Y-axis symmetric metasurface in Experiment Example 2.

[0035] Figure 13 This is a comparison diagram of the electric and magnetic field distributions of the TM polarization of a single-layer Y-axis symmetric metasurface in Experiment Example 2.

[0036] Figure 14 This is a multipole decomposition diagram of TE polarization of a single-layer Y-axis symmetric metasurface in Experiment Example 2.

[0037] Figure 15 This is a comparison diagram of the electric and magnetic field distributions of the TE polarization of a single-layer Y-axis symmetric metasurface in Experiment Example 2.

[0038] Figure 16 This is the transmission spectrum of the monolayer asymmetric metasurface in Experiment Example 3.

[0039] Figure 17 This is a multipole decomposition diagram of TM polarization of a single-layer asymmetric metasurface in Experiment Example 3.

[0040] Figure 18 This is a comparison diagram of the electric and magnetic field distributions of the TM polarization of the monolayer asymmetric metasurface in Experiment Example 3.

[0041] Figure 19 This is a multipole decomposition diagram of TE polarization of a single-layer asymmetric metasurface in Experiment Example 3.

[0042] Figure 20 This is a comparison diagram of the electric and magnetic field distributions of TE polarization on a single-layer asymmetric metasurface in Experiment Example 3.

[0043] Figure 21 This is a multipole decomposition diagram of LCP polarization of a single-layer asymmetric metasurface in Experiment Example 3.

[0044] Figure 22 This is a comparison diagram of the electric and magnetic field distributions of LCP polarization in a single-layer asymmetric metasurface in Experiment Example 3.

[0045] Figure 23 This is a multipole decomposition diagram of RCP polarization of the monolayer asymmetric metasurface in Experiment Example 3.

[0046] Figure 24 This is a comparison diagram of the electric and magnetic field distributions of RCP polarization on a single-layer asymmetric metasurface in Experiment Example 3.

[0047] Figure 25 This is the average chirality enhancement diagram of LCP and RCP polarization of the monolayer asymmetric metasurface in Experiment Example 3.

[0048] Figure 26 This is a chiral enhancement diagram of LCP and RCP polarization of the monolayer asymmetric metasurface in Experiment Example 3.

[0049] Figure 27 This is the transmission spectrum of the 6-layer TD-FP-BIC metasurface in Experiment Example 4.

[0050] Figure 28 This is the transmission spectrum of the 3-layer TD-FP-BIC metasurface in Experiment Example 4.

[0051] Figure 29 This is the reflection spectrum of the 6-layer TD-FP-BIC metasurface in Experiment Example 4.

[0052] Figure 30 This is the reflection spectrum of the 3-layer TD-FP-BIC metasurface in Experiment Example 4.

[0053] Figure 31 This is the average chirality enhancement diagram of the TD-FP-BIC metasurface in Experiment Example 4.

[0054] Figure 32 This is the maximum chirality diagram of the TD-FP-BIC metasurface in Experiment Example 4.

[0055] Figure 33 This is the absorption spectrum of a single chiral molecule in Experimental Example 5.

[0056] Figure 34 This is the absorption spectrum of a single-layer TD metasurface in Experiment Example 5.

[0057] Figure 35 This is the absorption spectrum of the multilayer TD-FP-BIC chiral metasurface in Experiment Example 6.

[0058] Figure 36 These are the circular dichroism diagrams of a single chiral molecule, a single-layer TD metasurface, and a multilayer TD-FP-BIC chiral metasurface in Experimental Example 7.

[0059] Figure reference numerals: 1. Nano-substrate; 2. Cylindrical tetramer; 3. First nano-cylinder; 4. Second nano-cylinder; 5. Third nano-cylinder; 6. Fourth nano-cylinder; 7. Silicon nanolayer; 8. Magnesium fluoride nanolayer. Detailed Implementation

[0060] Example 1

[0061] The design method based on multilayer toroidal dipole Fabry-Perot chiral metasurfaces includes the following steps;

[0062] S1: Provides a single-layer symmetric metasurface, which includes several square lattices and incident light. Each square lattice consists of a nano-substrate 1 and cylindrical tetramers 2 deposited on the nano-substrate. The nano-substrate is a nanocube with a refractive index of 1.4, a thickness of 730 nm, and is made of silicon dioxide. The cylindrical tetramer consists of four nano-cylinders with the same radius: a first nano-cylinder 3, a second nano-cylinder 4, a third nano-cylinder 5, and a fourth nano-cylinder 6. The four nano-cylinders are arranged symmetrically in the X-axis and Y-axis directions. The center-to-center distance between two adjacent nano-cylinders in the X-axis and Y-axis directions is 200 nm. The refractive index of the four nano-cylinders is 3, the radius is 80 nm, the height is 450 nm, and the material is silicon. The incident light is TM polarized and TE polarized, i.e., X-axis polarized and Y-axis polarized, with a polarization angle of -90° to 90°.

[0063] S2: Provide a single-layer Y-axis symmetric metasurface. Simultaneously reduce the radii of the third and fourth nano-cylinders of the single-layer symmetric metasurface in step S1 to destroy the structural symmetry in the X-axis direction and introduce asymmetric perturbation to form an X-axis asymmetric square lattice. Several X-axis asymmetric square lattices constitute a single-layer Y-axis symmetric metasurface. The radii of the third and fourth nano-cylinders are reduced to 55 nanometers, while other structural parameters remain unchanged. The incident light is TM polarized and TE polarized.

[0064] S3: Provide a single-layer asymmetric metasurface. Restore the radius of the fourth nanometer cylinder of the single-layer Y-axis symmetric metasurface in step S2 to 80 nanometers, while keeping the radius of the third nanometer cylinder unchanged. Simultaneously introduce X-axis and Y-axis asymmetric perturbations to form an asymmetric square lattice. Several asymmetric square lattices constitute a single-layer polarization-independent TD metasurface, which is a single-layer TD metasurface. The incident light is left-handed circularly polarized (LCP) and right-handed circularly polarized (RCP).

[0065] S4: Provides a multilayer TD-FP-BIC chiral metasurface. Several square silicon nanolayers 7 and magnesium fluoride nanolayers 8 are alternately stacked under the nano-substrate of a single-layer TD metasurface to form a multilayer TD-FP-BIC metasurface. The multilayer TD-FP-BIC metasurface is placed in chiral molecules to form a multilayer TD-FP-BIC chiral metasurface. The silicon nanolayer is made of silicon and has a thickness of 60 nm. The magnesium fluoride layer is made of magnesium fluoride and has a thickness of 175 nm. The refractive index of the silicon nanolayer is 3, and the refractive index of the magnesium fluoride nanolayer is 1.34. The incident light is left-handed circularly polarized (LCP, polarization angle rotates counterclockwise) and right-handed circularly polarized (RCP, polarization angle rotates clockwise).

[0066] Example 2

[0067] The design method based on multilayer toroidal dipole Fabry-Perot chiral metasurfaces includes the following steps;

[0068] S1: Provides a single-layer symmetric metasurface, which includes several square lattices and incident light. Each square lattice consists of a nano-substrate and cylindrical tetramers deposited on the nano-substrate. The nano-substrate is a nanocube with a refractive index of 1.6, a thickness of 808 nm, and is made of silicon dioxide. The cylindrical tetramer consists of four nanocylinders of the same radius: a first nanocylinder, a second nanocylinder, a third nanocylinder, and a fourth nanocylinder. The four nanocylinders are arranged symmetrically in the X-axis and Y-axis directions. The center-to-center distance between two adjacent nanocylinders in the X-axis and Y-axis directions is 236 nm. The refractive index of the four nanocylinders is 4, the radius is 96 nm, the height is 550 nm, and the material is silicon. The incident light is TM polarized and TE polarized, i.e., X-axis polarized and Y-axis polarized, with a polarization angle of -90° to 90°.

[0069] S2: Provide a single-layer Y-axis symmetric metasurface. Simultaneously reduce the radii of the third and fourth nano-cylinders of the single-layer symmetric metasurface in step S1 to destroy the structural symmetry in the X-axis direction and introduce asymmetric perturbation to form an X-axis asymmetric square lattice. Several X-axis asymmetric square lattices constitute a single-layer Y-axis symmetric metasurface. The radii of the third and fourth nano-cylinders are reduced to 65 nanometers, while other structural parameters remain unchanged. The incident light is TM polarized and TE polarized.

[0070] S3: Provide a single-layer asymmetric metasurface. Restore the radius of the fourth nano-cylinder of the single-layer Y-axis symmetric metasurface in step S2 to 96 nanometers, while keeping the radius of the third nano-cylinder unchanged. Simultaneously introduce X-axis and Y-axis asymmetric perturbations to form an asymmetric square lattice. Several asymmetric square lattices constitute a single-layer polarization-independent TD metasurface, which is a single-layer TD metasurface. The incident light is left-handed circularly polarized (LCP) and right-handed circularly polarized (RCP).

[0071] S4: Provides a multilayer TD-FP-BIC chiral metasurface. Several square silicon nanolayers and magnesium fluoride nanolayers are alternately stacked under the nano-substrate of a single-layer TD metasurface to form a multilayer TD-FP-BIC metasurface. The multilayer TD-FP-BIC metasurface is placed in chiral molecules to form a multilayer TD-FP-BIC chiral metasurface. The silicon nanolayer is made of silicon and has a thickness of 80 nanometers. The magnesium fluoride layer is made of magnesium fluoride and has a thickness of 185 nanometers. The refractive index of the silicon nanolayer is 4, and the refractive index of the magnesium fluoride nanolayer is 1.4. The incident light is left-handed circularly polarized (LCP, polarization angle rotates counterclockwise) and right-handed circularly polarized (RCP, polarization angle rotates clockwise).

[0072] Example 3

[0073] The design method based on multilayer toroidal dipole Fabry-Perot chiral metasurfaces includes the following steps;

[0074] S1: Provides a single-layer symmetric metasurface, comprising several square lattices and incident light. Each square lattice consists of a nano-substrate and cylindrical tetramers deposited on the nano-substrate. The nano-substrate is a nanocube, such as... Figure 1 As shown, its refractive index is 1.5, its thickness is 769 nm, and its material is silicon dioxide. The cylindrical tetramer consists of four nanocylinders of the same radius: a first nanocylinder, a second nanocylinder, a third nanocylinder, and a fourth nanocylinder. The four nanocylinders are arranged symmetrically in the X-axis and Y-axis directions. The center-to-center distance between two adjacent nanocylinders in the X-axis and Y-axis directions is 218 nm. The refractive index of the four nanocylinders is 3.5, their radius is 88 nm, their height is 500 nm, and their material is silicon. The incident light is TM polarized and TE polarized, that is, X-axis polarized and Y-axis polarized, with a polarization angle of -90° to 90°.

[0075] S2: Provide a single-layer Y-axis symmetric metasurface. Simultaneously reduce the radii of the third and fourth nanocylinders of the single-layer symmetric metasurface in step S1, disrupting the structural symmetry in the X-axis direction and introducing asymmetric perturbations to form an X-axis asymmetric square lattice, such as... Figure 2 As shown, a single-layer Y-axis symmetric metasurface is formed by several X-axis asymmetric square lattices. The radii of the third and fourth nano-cylinders are reduced to 60 nanometers, while other structural parameters remain unchanged. The incident light is TM-polarized and TE-polarized.

[0076] S3: Provide a single-layer asymmetric metasurface, restoring the radius of the fourth nanocylinder of the single-layer Y-axis symmetric metasurface in step S2 to 88 nanometers, while keeping the radius of the third nanocylinder unchanged. Simultaneously, introduce X-axis and Y-axis asymmetric perturbations to form an asymmetric square lattice, such as... Figure 3-5As shown, several asymmetric square lattices constitute a single-layer polarization-independent TD metasurface, which is a single-layer TD metasurface. In this case, the incident light is TM and TE polarized. If the incident light is changed to left-hand circular polarization (LCP) and right-hand circular polarization (RCP), the average chirality enhancement of the single-layer TD metasurface is calculated.

[0077] S4: A multilayered TD-FP-BIC chiral metasurface is provided by alternately stacking several square silicon nanolayers and magnesium fluoride nanolayers beneath a single-layer TD metasurface nanosubstrate. This forms a multilayered TD-FP-BIC metasurface. The multilayered TD-FP-BIC chiral metasurface is then placed within chiral molecules to form a multilayered TD-FP-BIC chiral metasurface (based on a multilayered toroidal dipole Fabry-Perot chiral metasurface). The silicon nanolayers are made of silicon and have a thickness of 70 nm. The magnesium fluoride layers are made of magnesium fluoride and have a thickness of 180 nm. The refractive index of the silicon nanolayers is 3.5, and the refractive index of the magnesium fluoride nanolayers is 1.37. The incident light is left-handed circularly polarized (LCP) and right-handed circularly polarized (RCP). The cylindrical tetramer is placed in the environment of chiral molecules, where the environmental refractive index is 1.33 ± 10. -5 i, the Pasteur parameter of the chiral molecular layer is 10. -5 i.

[0078] A monolayer TD metasurface is excited by TM-polarized and TE-polarized incident light, with an ambient refractive index of 1.33. In the XY plane, the radii of the third and fourth nanocylinders in the cylindrical tetramer are simultaneously reduced, introducing an asymmetric perturbation along the X-axis to form an asymmetric square lattice. Then, after restoring the radius of the fourth nanocylinder, the radius of a single third nanocylinder is reduced again, resulting in a polarization-independent monolayer TD metasurface. Based on this, silicon nanocubes and magnesium fluoride nanocubes are alternately arranged on a nanomaterial substrate, with their cylindrical tetramers placed within chiral molecules, transforming the monolayer TD metasurface into a multilayer TD-FP-BIC chiral metasurface. Several asymmetric square lattices constitute the multilayer TD-FP-BIC chiral metasurface, with a period P of 600 nm.

[0079] Numerical simulations of symmetric BIC metasurfaces were performed using COMSOL MultiphYsics. By disrupting the geometric symmetry of the symmetric BIC metasurface, it was transformed into an asymmetric quasi-BIC metasurface. In the XY plane, by reducing the radius of the nanocylinders, an asymmetric perturbation was introduced, creating finite coupling to the radiation continuum, which transformed it into a quasi-BIC metasurface, resulting in a high Q-factor and observable resonances in the transmission spectrum. At this point, the symmetric BIC metasurface was transformed into an asymmetric quasi-BIC metasurface.

[0080] Example 4

[0081] The multilayer TD-FP-BIC chiral metasurfaces designed in Examples 1-3 are applied in the fields of chiral sensing and ultrasensitive circular dichroism spectral amplifiers.

[0082] Comparative Example 1

[0083] This comparative example provides a single-layer symmetrical metasurface (the parameters of this comparative example are the same as those in step S1 of Example 3). The metasurface comprises several square lattices and incident light. Each square lattice consists of a nano-substrate and cylindrical tetramers deposited on the nano-substrate. The nano-substrate is made of silicon dioxide. The cylindrical tetramers consist of four nanocylinders of the same radius, arranged symmetrically in the X-axis and Y-axis directions. Figure 1 As shown. Since the radius and length of the cylindrical tetramer do not change, no asymmetric perturbation is introduced, resulting in a symmetrical square lattice. Several symmetrical square lattices constitute a single-layer symmetrical metasurface.

[0084] Experimental Example 1

[0085] Transmission spectroscopy, scattering, and electromagnetic field analysis were performed on the monolayer TD metasurface of Example 3 and the monolayer symmetric metasurface of Comparative Example 1. When the radii of the four nanocylinders remained unchanged (Comparative Example 1), the transmission spectrum showed a transmission valley that coincided under TM-polarized and TE-polarized incident light in the wavelength range of 940 nm to 1100 nm, demonstrating a polarization-independent phenomenon. Figure 6 As shown, the multipole decomposition under TM-polarized and TE-polarized incident light was tested as follows: Figure 7 and Figure 9 As shown in the figure, ED is an electric dipole; EQ is an electric quadrupole; MD is a magnetic dipole; MQ is a magnetic quadrupole; ETD is a toroidal electric dipole; and MTD is a toroidal magnetic dipole. The total scattering efficiency is mainly dominated by TD.

[0086] Figure 8 and Figure 10 The figures show the electromagnetic field distributions under TM and TE polarization, respectively. The left side represents the electric field distribution, and the right side represents the magnetic field distribution. Both electromagnetic field distributions are derived from the XY section. Under TM and TE polarization, the electric field inside the nanocylinder is relatively small, while the electric field outside the nanocylinder is stronger. Two sets of counterclockwise and clockwise closed displacement current loops were obtained on the XY plane, respectively. The magnetic field is mainly distributed inside the nanocylinder and is weaker outside. Symmetrical metasurfaces with strong TD response are not suitable for chiral sensing due to the limitations of field enhancement in nanostructures.

[0087] Comparative Example 2

[0088] This comparative example provides a single-layer Y-axis symmetric metasurface (this comparative example has the same parameters as step S2 in Example 3). When the radii of the third and fourth nanocylinders in Comparative Example 1 change from 88 nanometers to 60 nanometers, the structural symmetry is broken, resulting in enhanced coupling between the ideal symmetric protected BIC and the radiation mode, which leaks the BIC into a quasi-BIC, thus making it a single-layer Y-axis symmetric metasurface.

[0089] Experimental Example 2

[0090] The transmission spectrum, scattering, and electromagnetic field analysis of the monolayer Y-axis symmetric metasurface of Comparative Example 2 under TM-polarized and TE-polarized incident light are shown in the figure below. Figure 11 As shown, the transmission spectra under TM and TE polarization do not coincide, indicating that the metasurface is polarization-dependent. The rightmost transmission valley under TM and TE polarization was tested, and its multipole decomposition is shown below. Figure 12 and Figure 14 As shown, the total scattering efficiency under TM polarization is mainly dominated by TD, while the total scattering efficiency under TE polarization is mainly dominated by MD.

[0091] Figure 13 and Figure 15 The figures show the electromagnetic field distributions at z=H / 2 under TM and TE polarization, respectively. The left side represents the electric field distribution, and the right side represents the magnetic field distribution. Both electromagnetic field distributions are from the XY section. Under TM polarization, the electric field is stronger in the gap between two adjacent nanocylinders and weaker outside the gap, resulting in a counterclockwise closed displacement current loop in the XY plane. The magnetic field is mainly distributed inside the nanocylinders and weaker outside. Under TE polarization, the electric field is mainly distributed outside the nanocylinders, with stronger electric fields in the gap between two adjacent nanocylinders and inside the nanocylinders, and weaker outside the gap, also resulting in a counterclockwise closed displacement current loop in the XY plane. However, polarization-dependent resonance is not conducive to enhancing the detection of chiral responses between light and matter.

[0092] Comparative Example 3

[0093] This comparative example provides a single-layer asymmetric metasurface (the parameters of this comparative example are the same as those in step S3 of Example 3). When the radius of the fourth nanocylinder in Comparative Example 2 is restored to 88 nanometers, and the radius of the third nanocylinder is still changed from 88 nanometers to 60 nanometers, asymmetric perturbations in the X-axis and Y-axis directions are introduced, thereby making it a single-layer asymmetric metasurface (single-layer TD metasurface).

[0094] Experimental Example 3

[0095] The transmission spectrum, scattering, and electromagnetic field analysis of the monolayer asymmetric metasurface of Comparative Example 3 under TM-polarized and TE-polarized incident light are shown in the figure below. Figure 16As shown, the transmission spectra under TM and TE polarizations coincide, indicating that the metasurface is polarization-independent. The rightmost transmission valley under TM and TE polarizations exhibits multipole decomposition as shown below. Figure 17 and Figure 19 As shown, the total scattering efficiency under both TM and TE polarization is mainly dominated by TD.

[0096] Figure 18 and Figure 20 The electromagnetic field distributions at z=H / 2 under TM and TE polarization are shown respectively. The left side represents the electric field distribution, and the right side represents the magnetic field distribution. Both electromagnetic field distribution diagrams are from the XY section. Under TM polarization, the electric field is stronger in the gap between two adjacent nanocylinders and weaker outside the gap, resulting in a counterclockwise closed displacement current loop in the XY plane. The magnetic field is mainly distributed within the nanocylinders and weaker inside and outside the gap, resulting in a counterclockwise closed displacement magnetic flux loop in the XY plane. Under TE polarization, the electric field is stronger in the gap between two adjacent nanocylinders and weaker outside the gap, resulting in a clockwise closed displacement current loop in the XY plane. The magnetic field is mainly distributed within the nanocylinders and weaker inside and outside the gap, resulting in a counterclockwise closed displacement magnetic flux loop in the XY plane, thus obtaining a single-layer TD metasurface.

[0097] The multipolar decomposition of a single-layer asymmetric metasurface (single-layer TD metasurface) under LCP and RCP polarization was tested as follows: Figure 21 and Figure 23 The total scattering efficiency is mainly dominated by TD, and the electromagnetic field distribution is as follows: Figure 22 and Figure 24 As shown, under LCP polarization, the electric field is stronger in the gap between two adjacent nanocylinders and weaker outside the gap, resulting in a counterclockwise closed displacement current loop in the XY plane. The magnetic field is mainly distributed within the nanocylinders and weaker inside and outside the gap, resulting in a counterclockwise closed displacement magnetic current loop in the XY plane. Under RCP polarization, the electric field is stronger in the gap between two adjacent nanocylinders and weaker outside the gap, resulting in a clockwise closed displacement current loop in the XY plane. The magnetic field is mainly distributed within the nanocylinders and weaker inside and outside the gap, resulting in a counterclockwise closed displacement magnetic current loop in the XY plane.

[0098] Figure 25 The average chirality enhancement under LCP and RCP polarization is shown, with the same enhancement factor but opposite directions. The chirality enhancement at z=H / 2 under LCP and RCP polarization is as follows: Figure 26 As shown, the chirality enhancement, on the contrary, proves that the monolayer TD metasurface is inherently achiral, ensuring that the subsequently observed CD signal originates entirely from chiral molecules, thus allowing the metasurface to act as an efficient CD signal amplifier.

[0099] Comparative Example 4

[0100] A multilayer TD-FP-BIC chiral metasurface is provided (the parameters of this comparative example are the same as those in step S4 of Example 3). When a fully dielectric alternating layer is added under a nanocube substrate to the single-layer TD metasurface of Comparative Example 3, the thickness of the silicon nanolayer is 70 nm and the thickness of the magnesium fluoride nanolayer is 180 nm, thus forming a TD-FP-BIC metasurface.

[0101] Test Example 4

[0102] Transmission, reflection, average chirality enhancement, and maximum chirality maps of the multilayer TD-FP-BIC chiral metasurface in Comparative Example 4 are shown. Transmission maps of the silicon nanolayer and magnesium fluoride nanolayer, which are 6-layer and 3-layer overlapping layers respectively, are shown below. Figure 27 and Figure 28 As shown, the reflection patterns of the silicon nanolayer and the magnesium fluoride nanolayer are 6-layer and 3-layer overlapping layers, respectively. Figure 29 and 30 As shown. Figure 31 The average chirality enhancement map of the TD-FP-BIC metasurface shows an average chirality enhancement of up to 2011 times. The maximum chirality map is shown below. Figure 32 As shown, increase by 5×10 4 The nanocylindrical tetramer is placed in a chiral molecular environment, and multilayer all-dielectric nanocubes form a mirror to create an FP cavity, thus making it a multilayer TD-FP-BIC chiral metasurface.

[0103] Experimental Example 5

[0104] The multilayer TD-FP-BIC chiral metasurface of Comparative Example 4 was tested under LCP and RCP polarization, and only the absorption spectra of the nano-substrate cube and the chiral molecular layer structure were observed. The results are as follows: Figure 33 As shown, the chiral molecular layer is simulated by constitutive equations, with a Pasteur parameter of 10. -5 The absorption values ​​under both LCP and RCP polarization are small, close to 0, and the maximum circular dichroism (CD) is 1.19E. -4 ,like Figure 36 As shown.

[0105] Experimental Example 6

[0106] The multilayer TD-FP-BIC chiral metasurface of Comparative Example 4 was tested under LCP and RCP polarization. Only the cubic nanosubstrate structure, the chiral molecular layer structure, and the tetramer nanocylinder structure were observed to exhibit absorption spectra. The results are as follows: Figure 34 As shown, the chiral molecular layer is consistent with that of Example 3 (its environmental refractive index is 1.33 ± 10). -5 i, the Pasteur parameter of the chiral molecular layer is 10. -5 i), compared to the absorbance value in Test Example 5 ( Figure 33The absorption values ​​under LCP and RCP polarizations increase slightly, with a maximum CD of 0.0153. Figure 36 As shown.

[0107] Experimental Example 7

[0108] The absorption spectra of the multilayer TD-FP-BIC chiral metasurface of Comparative Example 4 under LCP polarization and RCP polarization were tested, and the results are as follows: Figure 35 As shown, the chiral molecular layer is consistent with that of Example 3 (its environmental refractive index is 1.33 ± 10). -5 i, the Pasteur parameter of the chiral molecular layer is 10. -5 i), compared to the absorbance value in Test Example 5 ( Figure 33 The absorption values ​​increase under both LCP and RCP polarization, with a maximum CD of 0.130. Figure 36 As shown.

[0109] Silicon, as a nanocylindrical tetramer material, possesses an extremely high refractive index in the near-infrared band. This high refractive index effectively traps incident light within and near the nanostructure, a prerequisite for exciting strong ring dipole and other high-order multipole resonances. Silicon dioxide, as the most common low-refractive-index all-dielectric material, has been widely used in the fabrication of photonic crystals. Although its refractive index is low, its near-zero absorptivity makes silicon dioxide an ideal material for optical communication. Magnesium fluoride, with its extremely low refractive index, can be layered with silicon to form multilayer mirrors, creating a significant refractive index difference. While ensuring low loss in the dielectric material, it promotes efficient output characteristics by enhancing coupling between different structures. Therefore, silicon, silicon dioxide, and magnesium fluoride were chosen for practical applications due to their superior performance.

Claims

1. A design method based on a multilayer toroidal dipole Fabry-Perot chiral metasurface, characterized in that: The design methodology includes the following steps: S1: Provides a single-layer symmetric metasurface, comprising several square lattices and incident light. Each square lattice consists of a nano-substrate and cylindrical tetramers deposited on the nano-substrate. The nano-substrate is a nanocube with a refractive index of 1.4-1.6, a thickness of 730-808 nm, and is made of silicon dioxide. The cylindrical tetramer consists of four nanocylinders of the same radius: a first nanocylinder, a second nanocylinder, a third nanocylinder, and a fourth nanocylinder. The four nanocylinders are arranged symmetrically in the X-axis and Y-axis directions. The center-to-center distance between two adjacent nanocylinders in the X-axis and Y-axis directions is 200-236 nm. The refractive index of the four nanocylinders is 3-4, the radius is 80-96 nm, the height is 450-550 nm, the material is silicon, and the incident light is TM-polarized and TE-polarized. S2: Provide a single-layer Y-axis symmetric metasurface. Simultaneously reduce the radii of the third and fourth nano-cylinders of the single-layer symmetric metasurface in step S1, introduce asymmetric perturbation, and form an X-axis asymmetric square lattice. Several X-axis asymmetric square lattices constitute a single-layer Y-axis symmetric metasurface. The radii of the third and fourth nano-cylinders are reduced to 55-65 nanometers, and the incident light is TM polarized and TE polarized. S3: Provide a single-layer asymmetric metasurface. Restore the radius of the fourth nanometer cylinder of the single-layer Y-axis symmetric metasurface in step S2 to 80-96 nanometers, while keeping the radius of the third nanometer cylinder unchanged. Simultaneously introduce X-axis and Y-axis asymmetric perturbations to form an asymmetric square lattice. Several asymmetric square lattices constitute a single-layer polarization-independent TD metasurface, which is a single-layer TD metasurface. The incident light is left-handed circularly polarized and right-handed circularly polarized. S4: Provides a multilayer TD-FP-BIC chiral metasurface. Several square silicon nanolayers and magnesium fluoride nanolayers are alternately stacked under the nano-substrate of a single-layer TD metasurface to form a multilayer TD-FP-BIC metasurface. The multilayer TD-FP-BIC metasurface is placed in chiral molecules to form a multilayer TD-FP-BIC chiral metasurface. The silicon nanolayer is made of silicon and has a thickness of 60-80 nanometers. The magnesium fluoride layer is made of magnesium fluoride and has a thickness of 175-185 nanometers. The refractive index of the silicon nanolayer is 3-4, and the refractive index of the magnesium fluoride nanolayer is 1.34-1.

4. The incident light is left-handed and right-handed circularly polarized.

2. The design method based on a multilayer toroidal dipole Fabry-Perot chiral metasurface according to claim 1, characterized in that: In step S1, the thickness of the nanosubstrate is 730 nm, and its refractive index is 1.4; the first, second, third, and fourth nanocylinders have a radius of 80 nm and a height of 450 nm, and the refractive index of the nanocylinders is 3. The center-to-center distance between two adjacent nanocylinders in the X-axis and Y-axis directions is 200 nm; in step S2, the radius of the third and fourth nanocylinders is 55 nm; in step S3, the radius of the third nanocylinder is 55 nm, and the radius of the fourth nanocylinder is 80 nm; in step S4, the thickness of the silicon nanolayer is 60 nm, the thickness of the magnesium fluoride nanolayer is 175 nm, the refractive index of the silicon nanolayer is 3, and the refractive index of the magnesium fluoride nanolayer is 1.

34.

3. The design method based on a multilayer toroidal dipole Fabry-Perot chiral metasurface according to claim 1, characterized in that: In step S1, the thickness of the nanosubstrate is 769 nm, and its refractive index is 1.5; the first, second, third, and fourth nanocylinders have a radius of 88 nm, a height of 500 nm, and a refractive index of 3.

5. The center-to-center distance between two adjacent nanocylinders in the X-axis and Y-axis directions is 218 nm; in step S2, the radius of the third and fourth nanocylinders is 60 nm; in step S3, the radius of the third nanocylinder is 60 nm, and the radius of the fourth nanocylinder is 88 nm; in step S4, the thickness of the silicon nanolayer is 70 nm, the thickness of the magnesium fluoride nanolayer is 180 nm, the refractive index of the silicon nanolayer is 3.5, and the refractive index of the magnesium fluoride nanolayer is 1.

37.

4. The design method for a multilayer toroidal dipole Fabry-Perot chiral metasurface according to claim 1, characterized in that: In step S1, the thickness of the nanosubstrate is 808 nm, and its refractive index is 1.6; the radius of the first, second, third, and fourth nanocylinders is 96 nm, their height is 550 nm, the refractive index of the nanocylinders is 4, and the center-to-center distance between two adjacent nanocylinders in the X-axis and Y-axis directions is 236 nm; in step S2, the radius of the third and fourth nanocylinders is 65 nm; in step S3, the radius of the third nanocylinder is 65 nm, and the radius of the fourth nanocylinder is 96 nm; in step S4, the thickness of the silicon nanolayer is 80 nm, the thickness of the magnesium fluoride nanolayer is 185 nm, the refractive index of the silicon nanolayer is 4, and the refractive index of the magnesium fluoride nanolayer is 1.

4.

5. The design method based on a multilayer toroidal dipole Fabry-Perot chiral metasurface according to any one of claims 1-4, characterized in that: The wave vector of the incident light in steps S1, S2 and S3 is parallel to the z-axis direction.

6. The design method based on a multilayer toroidal dipole Fabry-Perot chiral metasurface according to any one of claims 1-4, characterized in that: Both the silicon nanolayer and the magnesium fluoride nanolayer have 6 layers.

7. The design method based on a multilayer toroidal dipole Fabry-Perot chiral metasurface according to any one of claims 1-4, characterized in that: The period of both the single-layer TD metasurface and the multilayer TD-FP-BIC chiral metasurface is 550-750 nanometers.

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