Design method and application of rotationally symmetrical lithium niobate metasurface

Abstract

The invention belongs to the technical field of optical metasurfaces, and particularly relates to a rotationally symmetrical lithium niobate elliptical nano-column quaii-BIC metasurface and a design method and application of the rotationally symmetrical lithium niobate elliptical nano-column quaii-BIC metasurface. The method comprises the following steps: 1, providing a cylindrical tetramer symmetric BIC metasurface, wherein the metasurface comprises a plurality of square lattices and incident light; 2, when the incident light is x polarization, the radius length of the single side in the cylindrical tetramer is increased or decreased at the same time, asymmetric disturbance is introduced, and the cylindrical tetramer is changed into a windmill type elliptical nano-column tetramer; the elliptical nano-column tetramer is composed of four elliptical nano-columns, the radiuses of the fixed sides of the four elliptical nano-columns are 250nm, the heights of the four elliptical nano-columns are 450nm, and the windmill type elliptical nano-column tetramer is made of lithium niobate. The intensity and waveform of a transmission spectrum can be flexibly modulated by adjusting related structure parameters of the metasurface, and the formed metasurface is relatively high in sensitivity and can be applied to polarization-independent characteristic application and gas sensors.

Description

Technical Field

[0001] The present invention belongs to the technical field of optical metasurfaces, and particularly relates to a design method and application of a rotationally symmetric lithium niobate elliptical nanorod quasi-BIC metasurface. Background Art

[0002] The refractive index (RI) is an inherent property of materials and is often used in material identification, quality control, chemical analysis, optical design, and environmental monitoring. A metasurface is a discrete subwavelength structure composed of artificial meta-atoms. Due to its sensitivity to the refractive index change in the neighborhood, metasurface-based RI sensors have a wide range of applications in many fields such as sensors, optical switches, surface wave couplers, and filters. For example, as an RI sensor, the metasurface uses resonance characteristics to detect minute changes in the refractive index, thereby improving the accuracy of chemical analysis and environmental monitoring. In these applications, the quality factor (Q factor) and figure of merit (FOM) are key parameters reflecting the efficiency of the metasurface. Metal metasurfaces generate a large amount of radiation loss due to the oscillation of free electrons, resulting in a low Q value at the resonance point. In contrast, the resonance peaks of all-dielectric metasurfaces are sharper, and the FOM and Q factor are larger. Structures with bound states in the continuum (BIC) are natural high-Q resonators because, in the ideal case, the radiative Q value is equal to infinity. Recent reports have demonstrated the correlation between high-Q-factor metasurfaces and BIC.

[0003] In the prior art, a large number of metasurface transmission studies based on bound states in the continuum (BIC) have been proposed and analyzed, and their application ranges from efficient optical filters and sensor devices to laser cavities and enhanced light-matter interaction systems. Currently, metasurfaces made of high-refractive-index dielectric materials are widely used to construct enhanced optical nonlinear processes due to their large nonlinear coefficients. In addition, such metasurfaces also provide multipole electric and magnetic resonances called Mie resonances, and the moderate quality factor (Q factor) of Mie resonances is always accompanied by moderate electric field confinement, which may limit the light-matter coupling strength. A higher Q factor is expected to cause stronger field confinement in the resonator, which may promote light-matter interactions in applications such as surface-emitting lasers, biomedical sensors, and nonlinear frequency converters. However, bound states in the continuum (BIC) can provide a feasible solution to the major problem of achieving almost infinite Q factors. BIC is an electromagnetic eigenstate that coexists within the continuous spectrum but is completely confined within a resonant system without radiative energy. Ideal BIC exists in lossless or extreme structures with zero linewidth and infinite Q factors, and can be transformed into quasi-BIC by introducing finite coupling into the radiative continuum, thereby generating ultra-high Q factors and observable resonances in the far-field spectrum.

[0004] The generation of quasi-BIC metasurfaces usually requires the destruction of structural symmetry, and most symmetric BICs are polarization-sensitive. In the prior art, by controlling two phase change materials, graphene and vanadium dioxide, the conversion and regulation of BIC and absorber are achieved, but its biosensing ability is poor. Although the performance of the graphene-metal boat structure with a metal mirror structure has been significantly improved, the preparation process is complex and the light transmittance is poor. To improve the transmittance, sub-wavelength nanograting metasurfaces can be fabricated in a lithium niobate crystal thin film, and the spectral response can be adjusted by simply changing the geometric parameters. However, the full width at half maximum of the metasurface transmission spectrum is too large, resulting in the overlap of different transmissions and making it difficult to accurately distinguish. Summary of the Invention

[0005] The object of the present invention is to design a rotationally symmetric lithium niobate elliptical nanocolumn quasi-BIC metasurface. By adjusting the relevant structural parameters of the metasurface, the intensity and waveform of the transmission spectrum can be flexibly modulated. The formed metasurface has high sensitivity and can be applied in polarization-independent characteristic applications and gas sensors.

[0006] The technical solution adopted by the present invention is: a design method for a rotationally symmetric lithium niobate metasurface, and the design method includes the following steps: S1: Provide a symmetric BIC metasurface, the symmetric BIC metasurface includes a plurality of square lattices and incident light; each square lattice is composed of a nano-substrate and a cylindrical tetramer deposited on the nano-substrate. The nano-substrate is a nano-cube with a refractive index of 1.4-1.52, a thickness of 900-1000 nm, and the material is silica; the cylindrical tetramer is composed of four circular nanocolumns with the same radius, and the four circular nanocolumns are arranged symmetrically in pairs in the x-axis direction and the y-axis direction.

[0007] S2: When the incident light is x-polarized, simultaneously increase or decrease the unilateral radius length in the xy-plane of the cylindrical tetramer in the cylindrical tetramer to introduce an asymmetric perturbation, and the cylindrical tetramer is changed into a windmill-shaped elliptical nanocolumn tetramer to form an asymmetric square lattice; the windmill-shaped elliptical nanocolumn tetramer is composed of four first elliptical nanocolumns, second elliptical nanocolumns, third elliptical nanocolumns, and fourth elliptical nanocolumns of the same size. The four elliptical nanocolumns are arranged in a windmill shape, that is, the directions of the second elliptical nanocolumn and the fourth elliptical nanocolumn are obtained by rotating the first elliptical nanocolumn and the third elliptical nanocolumn by 90°; the fixed-side radius of the first elliptical nanocolumn, the second elliptical nanocolumn, the third elliptical nanocolumn, and the fourth elliptical nanocolumn is 250 nm, and its variable-side radius is 163-337 nm, and the height is 430-470 nm; the center distance between two adjacent elliptical nanocolumns in the first elliptical nanocolumn, the second elliptical nanocolumn, the third elliptical nanocolumn, and the fourth elliptical nanocolumn is 510 nm; the material of the windmill-shaped elliptical nanocolumn tetramer is lithium niobate.

[0008] When the incident light is x-polarized, in the xy-plane, only the length of the variable unilateral radius in the windmill-shaped elliptical nanocylinder tetramer is increased or decreased simultaneously, introducing an asymmetric perturbation while keeping the unilateral radius fixed.

[0009] A rotationally symmetric lithium niobate elliptical nanocylinder quasi-BIC metasurface is composed of several asymmetric square lattices.

[0010] Furthermore, the thickness of the nanosubstrate is 975 nm and its refractive index is 1.46; the variable-side radius of the first, second, third, and fourth elliptical nanocylinders is 163 nm and their height is 450 nm.

[0011] Furthermore, the incident light is a plane wave with its wave vector parallel to the z-axis direction, and the polarization angle is -90° to 90°. Different polarization angles result in the same transmission spectrum, and the four resonances always maintain a high transmission depth in different polarization directions, indicating that the rotationally symmetric lithium niobate elliptical nanocylinder quasi-BIC metasurface is polarization-independent.

[0012] Furthermore, when the rotationally symmetric lithium niobate elliptical nanocylinder quasi-BIC metasurface is excited by x-polarized incident light, the environmental refractive index is 1.001 - 1.01.

[0013] Furthermore, the period of the rotationally symmetric lithium niobate elliptical nanocylinder quasi-BIC metasurface is 1950 - 2050 nm.

[0014] Furthermore, the rotationally symmetric lithium niobate elliptical nanocylinder quasi-BIC metasurface is applied in devices related to polarization-independent characteristics.

[0015] Furthermore, the rotationally symmetric lithium niobate elliptical nanocylinder quasi-BIC metasurface is applied in gas sensors.

[0016] Furthermore, the polarization angle of the incident light is described by the angle between the incident electric field and the x-axis.

[0017] Furthermore, the asymmetric peaks of the quasi-BIC metasurface exhibit Fano resonance characteristics.

[0018] The beneficial effects of the present invention: To solve the technical problems of the present application, a rotationally symmetric lithium niobate elliptical nanocylinder quasi-BIC metasurface is designed. This metasurface changes the existing cylindrical tetramer to a windmill-shaped elliptical nanocylinder, forming an asymmetric square lattice. The metasurface exhibits obvious sharp Fano resonances near 1553 nm, 1608 nm, 1855 nm, and 1928 nm, and the Q factors are as high as 1.15×10 4 、0.302×10 4 、2.1×104 and 0.94×10 4 . By adjusting the relevant structural parameters of the metasurface (the polarization angle of the incident light, the background refractive index), the intensity and waveform of the transmission spectrum can be flexibly modulated. The maximum sensitivity S and figure of merit FOM at the optimal parameters are 753.33 nm / RIU and 5707 RIU, respectively -1 , and it can be widely applied in the fields of gas sensors and polarization-independent characteristic applications, etc. Description of the Drawings

[0019] Figure 1 is a three-dimensional structural schematic diagram of a square lattice in the elliptical nanocolumn quasi-BIC metasurface

[0020] Figure 2 is a top view of a square lattice in the elliptical nanocolumn quasi-BIC metasurface

[0021] Figure 3 is the transmission spectrum diagram of the quasi-BIC metasurface of Examples 1-6 and Comparative Example 1 in Test Example 1

[0022] Figure 4 is the TD1 multipole decomposition diagram of the quasi-BIC metasurface of Example 1 in Test Example 2

[0023] Figure 5 is the TD1 electric field distribution diagram of the quasi-BIC metasurface of Example 1 in Test Example 2

[0024] Figure 6 is the TD1 magnetic field distribution diagram of the quasi-BIC metasurface of Example 1 in Test Example 2

[0025] Figure 7 is the TD2 multipole decomposition diagram of the quasi-BIC metasurface of Example 1 in Test Example 2

[0026] Figure 8 is the TD2 electric field distribution diagram of the quasi-BIC metasurface of Example 1 in Test Example 2

[0027] Figure 9 is the TD2 magnetic field distribution diagram of the quasi-BIC metasurface of Example 1 in Test Example 2

[0028] Figure 10 is the TD3 multipole decomposition diagram of the quasi-BIC metasurface of Example 1 in Test Example 2

[0029] Figure 11 is the TD3 electric field distribution diagram of the quasi-BIC metasurface of Example 1 in Test Example 2

[0030] Figure 12 It is the TD3 magnetic field distribution diagram of the quasi-BIC metasurface in Example 1 of Test Example 2.

[0031] Figure 13 It is the TD4 multipole decomposition diagram of the quasi-BIC metasurface in Example 1 of Test Example 2.

[0032] Figure 14 It is the TD4 electric field distribution diagram of the quasi-BIC metasurface in Example 1 of Test Example 2.

[0033] Figure 15 It is the magnetic field distribution diagram of TD4 of the quasi-BIC metasurface in Example 1 of Test Example 2.

[0034] Figure 16 It is the schematic diagram of the polarization angle of the incident light in Test Example 3.

[0035] Figure 17 It is the transmission spectrum diagram of the quasi-BIC metasurface in Example 1 of Test Example 3 at different polarization angles.

[0036] Figure 18 It is the curve graph of the change of the TD1 and TD2 transmission spectra with the background refractive index of the quasi-BIC metasurface in Example 1 of Test Example 4 under x-polarized light.

[0037] Figure 19 It is the curve graph of the change of the TD3 and TD4 transmission spectra with the background refractive index of the quasi-BIC metasurface in Example 1 of Test Example 4 under x-polarized light.

[0038] Figure 20 It is the graph of the change of the resonance wavelengths of TD1, TD2, TD3 and TD4 with the thickness of the nano-substrate of the quasi-BIC metasurface in Example 1 of Test Example 5 under x-polarized light.

[0039] Figure 21 It is the curve graph of the change of the resonance wavelengths of TD1, TD2, TD3 and TD4 with the square lattice period of the quasi-BIC metasurface in Example 1 of Test Example 6 under x-polarized light.

[0040] Figure 22 It is the curve graph of the change of the resonance wavelengths of TD1, TD2, TD3 and TD4 with the height of the elliptical nano-pillars of the quasi-BIC metasurface in Example 1 of Test Example 7 under x-polarized light.

[0041] Reference numerals: 1, nano-substrate; 2, windmill-shaped elliptical nano-pillar tetramer; 3, first elliptical nano-pillar; 4, second elliptical nano-pillar; 5, third elliptical nano-pillar; 6, fourth elliptical nano-pillar. Detailed implementation mode

[0042] Example 1 The design method of the rotationally symmetric lithium niobate metasurface includes the following steps: S1: Provide a symmetric BIC metasurface. The symmetric BIC metasurface includes a number of square lattices and incident light. The incident light is a plane wave with its wave vector parallel to the z-axis direction and the polarization angle between -90° and 90°. Each square lattice is composed of a nano-substrate 1 and a cylindrical tetramer deposited on the nano-substrate. The nano-substrate is a nano-cube with a refractive index of 1.46, a thickness H of 900 - 1000 nm, and the material is silica; the cylindrical tetramer is composed of four circular nano-columns with the same radius. The four circular nano-columns are arranged symmetrically in pairs in the x-axis direction and the y-axis direction. The center distance between the two circular nano-columns arranged symmetrically in the x-axis and y-axis directions is 510 nm.

[0043] S2: When the incident light is x-polarized, simultaneously increase or decrease the length of the unilateral radius in the cylindrical tetramer in the xy-plane to introduce an asymmetric perturbation. The cylindrical tetramer is changed into a windmill-shaped elliptical nano-column tetramer 2, forming an asymmetric square lattice; the windmill-shaped elliptical nano-column tetramer is composed of four first elliptical nano-columns 3, second elliptical nano-columns 4, third elliptical nano-columns 5, and fourth elliptical nano-columns 6 of the same size. As Figure 1 shown, the four elliptical nano-columns are arranged in a windmill shape, that is, the directions of the second elliptical nano-column and the fourth elliptical nano-column are obtained by rotating the first elliptical nano-column and the second elliptical nano-column by 90°; the fixed-side radius R2 (fixed unilateral radius) of each elliptical nano-column is 250 nm, its variable-side radius R1 (variable unilateral radius) is 163 - 337 nm, and the height h1 is 430 - 470 nm; the center distance between two adjacent elliptical nano-columns among the four elliptical nano-columns is 510 nm; the material of the windmill-shaped elliptical nano-column tetramer is lithium niobate.

[0044] Specifically, when the rotationally symmetric lithium niobate elliptical nano-column quasi-BIC metasurface is excited by x-polarized incident light, the ambient refractive index is 1.001 - 1.01. The incident light is a plane wave with its wave vector parallel to the z-axis direction and the polarization parallel to the x-axis direction. As Figure 2 shown, in the xy-plane, simultaneously reduce the length of the unilateral radius in the cylindrical tetramer by 87 nm to introduce an asymmetric perturbation. The cylindrical tetramer is changed into a windmill-shaped elliptical nano-column tetramer, forming an asymmetric square lattice. A number of asymmetric square lattices form a rotationally symmetric lithium niobate elliptical nano-column quasi-BIC metasurface, and its period P is 2025 nm.

[0045] Numerical simulation of symmetric BIC metasurfaces is realized using COMSOL Multiphysics. By breaking the geometric symmetry of the symmetric BIC metasurface, the symmetric BIC metasurface is converted into an asymmetric quasi-BIC metasurface. In the xy-plane, when the unilateral radius length in the elliptical tetramer is increased or decreased simultaneously, an asymmetric perturbation is introduced, and a finite coupling is introduced into the radiative continuum, which can be converted into a quasi-BIC, thus generating a high Q factor and observable resonance in the transmission spectrum. At this time, the symmetric BIC metasurface is converted into an asymmetric quasi-BIC metasurface, and an increase in the change value of the radius will broaden the linewidth of the resonance peak.

[0046] Example 2 The unilateral radius length in the cylindrical tetramer of Example 1 is simultaneously reduced by 58 nm, introducing an asymmetric perturbation to form an asymmetric square lattice, and the rest is the same as in Example 1. Several asymmetric square lattices in this example form a rotationally symmetric lithium niobate elliptical nanorod quasi-BIC metasurface.

[0047] Example 3 The unilateral radius length in the cylindrical tetramer of Example 1 is simultaneously reduced by 29 nm, introducing an asymmetric perturbation to form an asymmetric square lattice, and the rest is the same as in Example 1. Several asymmetric square lattices in this example form a rotationally symmetric lithium niobate elliptical nanorod quasi-BIC metasurface.

[0048] Example 4 The unilateral radius length in the cylindrical tetramer of Example 1 is simultaneously increased by 29 nm, introducing an asymmetric perturbation to form an asymmetric square lattice, and the rest is the same as in Example 1. Several asymmetric square lattices in this example form a rotationally symmetric lithium niobate elliptical nanorod quasi-BIC metasurface.

[0049] Example 5 The unilateral radius length in the cylindrical tetramer of Example 1 is simultaneously increased by 58 nm, introducing an asymmetric perturbation to form an asymmetric square lattice, and the rest is the same as in Example 1. Several asymmetric square lattices in this example form a rotationally symmetric lithium niobate elliptical nanorod quasi-BIC metasurface.

[0050] Example 6 The unilateral radius length in the cylindrical tetramer of Example 1 is simultaneously increased by 87 nm, introducing an asymmetric perturbation to form an asymmetric square lattice, and the rest is the same as in Example 1. Several asymmetric square lattices in this example form a rotationally symmetric lithium niobate elliptical nanorod quasi-BIC metasurface.

[0051] Example 7 The rotationally symmetric lithium niobate elliptical nanocolumn quasi-BIC metasurface designed and formed in Example 1 is applied to technical fields such as gas sensors and polarization-independent property applications.

[0052] Comparative Example 1 This comparative example is a symmetric BIC metasurface, which includes a number of square lattices and incident light. Each square lattice consists of a nano-substrate and a cylindrical tetramer deposited on the nano-substrate. The material of the nano-substrate is silicon dioxide; the cylindrical tetramer is composed of four circular nanocolumns with the same radius, and the four circular nanocolumns are arranged symmetrically in pairs in the x-axis direction and the y-axis direction. Since the radius length in the cylindrical tetramer does not change and no asymmetric perturbation is introduced, it is a symmetric square lattice, and a number of symmetric square lattices constitute the symmetric BIC metasurface.

[0053] Test Example 1 (1) The transmission spectra of the quasi-BIC metasurfaces in Examples 1-6 and the symmetric BIC metasurface in Comparative Example 1 were tested, and the results are as Figure 3 shown. When the radii of the four circular nanocolumns do not change (Comparative Example 1), the transmission spectrum is almost horizontal in the wavelength ranges of 1500 nm to 1700 nm and 1800 to 2000 nm, that is, it shows a BIC with an infinitely high Q factor; this is a typical symmetry-protected BIC, characterized by extremely high transmittance and being invisible in the transmission spectrum.

[0054] When the unilateral diameter change R1 of the circular nanocolumns in Comparative Example 1 is reduced by 87 nm (Example 1), four sharp resonance modes (denoted as TD1, TD2, TD3, and TD4) are observed at wavelengths of 1553 nm, 1608 nm, 1855, and 1928 nm. The reason for this phenomenon is that the enhanced coupling between the ideal symmetry-protected BIC and the radiation mode is caused by breaking the structural symmetry, thus making it a quasi-BIC metasurface.

[0055] When the unilateral diameter change R1 of the circular nanocolumns in Comparative Example 1 varies from 163 nm to 337 nm (Examples 1-6), the line widths of these four resonance modes gradually increase.

[0056] Test Example 2 The multipole decomposition of TD1, TD2, TD3, and TD4 of the quasi-BIC metasurface in Example 1 was tested, and the results are as Figures 4 - 15 shown. Figure 4 、 Figure 7 、 Figure 10 and Figure 13Multi-pole decomposition diagrams of TD1, TD2, TD3, and TD4 of the quasi-BIC metasurface in Example 1 (with the variable-side radius R1 being 163 nm and the fixed-side radius R2 being 250 nm), respectively. In the figures, ED - electric dipole; EQ - electric quadrupole; MD - magnetic dipole; MQ - magnetic quadrupole; TD - toroidal dipole. Figure 5 , Figure 8 , Figure 11 and Figure 14 are the electric field distribution diagrams of TD1, TD2, TD3, and TD4, respectively. Figure 6 , Figure 9 , Figure 12 and Figure 15 are the magnetic field distribution diagrams of TD1, TD2, TD3, and TD4, respectively.

[0057] The total scattering efficiency of the resonant multi-poles of TD1 and TD3 is mainly determined by TD, followed by MQ, as shown in Figure 4 and Figure 10 respectively. The electric and magnetic field distributions of TD1 and TD3 are as shown in Figure 5 , Figure 6 and Figure 11 and Figure 12 respectively. The electric field of TD1 and TD3 comes from four longitudinal electric resonances, where the electric field is distributed between the outside and the gap of the tetramer and around the center of each column, forming a circular displacement current in the x-z plane. The x-y magnetic field directions are opposite, jointly forming a closed magnetic ring around the x-axis, indicating the presence of TD. At the same time, the reverse structure of the magnetic field indicates the presence of MQ.

[0058] As shown in Figure 7 and Figure 13 , the contribution of TD dominates the resonances of TD2 and TD4. Obviously, the TD contribution dominates the response, followed by MQ. The electric and magnetic field distributions of TD2 and TD4 are as shown in Figure 8 , Figure 9 and Figure 14 , Figure 15 respectively. The electric field in the nanodisk is small, and the electric field outside the nanodisk is strong. Two sets of counterclockwise and clockwise closed displacement current loops are obtained in the x-y plane respectively. In the x-z plane, the magnetic field vector directions on the cross-section of the nanocolumn are opposite, and this reverse distribution shows symmetry in multiple quadrants, indicating the presence of MQ. In addition, the direction arrows of the magnetic field vectors form a ring head-to-head and circulate clockwise between the z directions, which is a characteristic of TD.

[0059] Test Example 3 Test the variations of TD1, TD2, TD3, and TD4 of the quasi-BIC metasurface in Example 1 under different incident light polarization angles. The polarization angle of the incident light is described by the angle between the incident electric field and the x-axis, defined as θ, as shown in Figure 16As shown in the figure, the polarization angles θ of Example 1 were set to 0°, 15°, 30°, 45°, 60°, 75°, 90°, left-handed and right-handed, respectively, and the changes of TD1, TD2, TD3 and TD4 in the quasi-BIC metasurface of Example 1 under different polarization angles were observed. The results are as Figure 17 shown. The transmission spectra at different polarization angles did not change. TD1, TD2 and TD3 maintained large amplitudes in different polarization directions, indicating that this metasurface has polarization-independent characteristics. This unique characteristic can be used to eliminate or even significantly reduce the need for polarization calibration, thus providing greater flexibility for applications such as projectors, radar technology, microscope systems and solar cells.

[0060] Test Example 4 The effects of the background refractive index on the transmission spectrum, resonance wavelengths of TD1, TD2, TD3 and TD4 of the quasi-BIC metasurface of Example 1 under x-polarization were tested. The results are as Figure 18 and Figure 19 shown. As the background refractive index increased, TD1, TD2, TD3 and TD4 all underwent red shifts, and the amounts of red shift of the four resonances were different. TD1 shifted from 1534.1 nm to 1540.9 nm, TD2 shifted from 1608.1 nm to 1613.3 nm, TD3 shifted from 1855.5 nm to 1858.2 nm, and TD4 shifted from 1928.5 nm to 1930.2 nm. The red shift of TD1 was more significant. Through calculation, the sensitivities of TD1, TD2, TD3 and TD4 were 753.33, 579.39, 300 and 187.27 nm / RIU, respectively, and the corresponding FOMs were 5707, 1089, 3409 and 918 RIU -1 .

[0061] Test Example 5 The effects of the thickness of the nano-substrate on the transmission spectrum, resonance wavelengths of TD1, TD2, TD3 and TD4 of the quasi-BIC metasurface of Example 1 under x-polarization were tested. The results are as Figure 20 shown. As H increased from 900 nm to 1000 nm, the four resonances TD1, TD2, TD3 and TD4 underwent red shifts. However, when H changed in increments of 25 nm, the shift of TD4 was not as obvious as the shifts of the other three resonances.

[0062] Test Example 6 The effects of the square lattice period on the transmission spectrum, resonance wavelengths of TD1, TD2, TD3 and TD4 of the quasi-BIC metasurface of Example 1 under x-polarization were tested. The results are as Figure 21As shown. When P increases from 1950 nm to 2050 nm, the four resonances show significant redshifts.

[0063] Experimental Example 7 The influence of the nanocolumn height on the resonance wavelengths of the transmission spectrum, TD1, TD2, TD3, and TD4 of the quasi-BIC metasurface of Example 1 under x-polarization was tested, and the results are as Figure 22 shown. As h1 gradually increases from 430 nm to 470 nm in 10-nm steps, it is observed that the four resonances show slight redshifts.

[0064] Lithium niobate is used as the material of the windmill-shaped elliptical nanocolumn tetramer because it has high transparency in the near-infrared to near-ultraviolet spectral region. Designing and optimizing by designing lithium niobate nanostructures is crucial for improving the performance of nanophotonics and optoelectronics. As the most common low-refractive-index all-dielectric material, silica has been widely used in the fabrication of photonic crystals. Although the refractive index is low, the near-zero absorption rate makes silica an ideal material in the field of optical communication. On the basis of ensuring the low loss of the dielectric material, the output characteristics are efficiently expressed by promoting the coupling between different structures. Therefore, lithium niobate and silica are selected for the actual application effect.

[0065] The structural symmetry can be broken by changing the length of the unilateral radius of the elliptical nanocolumn to excite the quasi-BIC resonance. When all four elliptical nanocolumns change, the tetramer can coincide with itself after rotating a certain angle (0°-360°). Therefore, this structure is rotationally symmetric, which is a very simple and effective method to realize the transformation of the symmetry-protected BIC metasurface into a highly sensitive quasi-BIC metasurface, which is of great significance for improving the sensitivity of optical sensing.

Claims

1. A design method for a rotationally symmetric lithium niobate metasurface, characterized in that: The design method includes the following steps: S1: Provide a symmetric BIC metasurface, which includes a number of square lattices and incident light; each square lattice consists of a nano-substrate and a cylindrical tetramer deposited on the nano-substrate. The nano-substrate is a nano-cube with a refractive index of 1.4 - 1.52, a thickness of 900 - 1000 nm, and a material of silica; the cylindrical tetramer is composed of four circular nano-columns with the same radius, and the four circular nano-columns are arranged symmetrically in pairs in the x-axis direction and the y-axis direction. S2: When the incident light is x-polarized, simultaneously increase or decrease the unilateral radius length in the xy-plane of the cylindrical tetramer in the cylindrical tetramer to introduce an asymmetric perturbation, and the cylindrical tetramer is changed into a windmill-shaped elliptical nano-column tetramer, forming an asymmetric square lattice; the windmill-shaped elliptical nano-column tetramer is composed of four first elliptical nano-columns, second elliptical nano-columns, third elliptical nano-columns, and fourth elliptical nano-columns with the same size. The four elliptical nano-columns are arranged in a windmill shape. The fixed-side radius of the first elliptical nano-column, second elliptical nano-column, third elliptical nano-column, and fourth elliptical nano-column is 250 nm, and its variable-side radius is 163 - 337 nm, and the height is 430 - 470 nm; the center distance between two adjacent elliptical nano-columns in the first elliptical nano-column, second elliptical nano-column, third elliptical nano-column, and fourth elliptical nano-column is 510 nm; the material of the windmill-shaped elliptical nano-column tetramer is lithium niobate. A number of asymmetric square lattices form a rotationally symmetric lithium niobate elliptical nano-column quasi-BIC metasurface.

2. The design method of the rotationally symmetric lithium niobate metasurface according to claim 1, wherein: The thickness of the nano-substrate is 975 nm, and its refractive index is 1.46; the variable-side radius of the first elliptical nano-column, second elliptical nano-column, third elliptical nano-column, and fourth elliptical nano-column is 163 nm, and its height is 450 nm.

3. The design method of the rotationally symmetric lithium niobate metasurface according to claim 1 or 2, characterized in that: The incident light is a plane wave, and its wave vector is parallel to the z-axis direction, and the polarization angle is -90° to 90°.

4. The design method of the rotationally symmetric lithium niobate metasurface according to claim 1 or 2, characterized in that: When the rotationally symmetric lithium niobate elliptical nano-column quasi-BIC metasurface is excited by x-polarized incident light, the environmental refractive index is 1.001 - 1.

01.

5. The design method of the rotationally symmetric lithium niobate metasurface according to claim 1 or 2, characterized in that: The period of the rotationally symmetric lithium niobate elliptical nano-column quasi-BIC metasurface is 1950 - 2050 nm.

6. The application of the rotationally symmetric lithium niobate metasurface according to claim 1 in a device related to polarization-independent characteristics.

7. The application of the rotationally symmetric lithium niobate metasurface according to claim 1 in a gas sensor.

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