Design method and application of rotationally symmetric lithium niobate metasurface

Abstract

The present invention belongs to the technical field of optical metasurfaces, and specifically relates to a rotationally symmetric lithium niobate elliptical nanocolumn quasi-BIC metasurface and its design method and application. The method is: 1. Providing a cylindrical tetramer symmetric BIC metasurface, the metasurface includes a number of square lattices and incident light; 2. When the incident light is x-polarized, the length of the unilateral radius in the cylindrical tetramer increases or decreases at the same time, introducing an asymmetric perturbation, and the cylindrical tetramer changes to a windmill-type elliptical nanocolumn tetramer; the elliptical nanocolumn tetramer is composed of four elliptical nanocolumns, and the fixed side radius of the four elliptical nanocolumns is 250nm and the height is 450nm. The material of the windmill-type elliptical nanocolumn tetramer is lithium niobate. By adjusting the relevant structural parameters of the metasurface, the intensity and waveform of the transmission spectrum can be flexibly modulated. The sensitivity of the formed metasurface is high and can be used in polarization-independent characteristic applications and gas sensors.

Description

Technical Field

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

[0002] The refractive index (RI) is an intrinsic property of materials and is frequently used in material identification, quality control, chemical analysis, optical design, and environmental monitoring. Metasurfaces are discrete subwavelength structures composed of artificial meta-atoms. Due to their sensitivity to changes in the refractive index of their neighbors, metasurface-based RI sensors have broad applications in sensors, optical switches, surface wave couplers, and filters. For example, as RI sensors, metasurfaces utilize resonance 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 metasurfaces. Metallic metasurfaces generate significant radiative losses due to the oscillation of free electrons, resulting in low Q values ​​at the resonance point. In contrast, all-dielectric metasurfaces exhibit sharper resonance peaks, larger FOMs, and larger Q factors. Structures with bound states in the continuum (BICs) are natural high-Q resonators because, ideally, the radiative Q value is infinite. Recent reports have demonstrated a correlation between high-Q metasurfaces and BICs.

[0003] Numerous metasurface transmission studies based on bounded states in the continuum (BICs) have been proposed and analyzed, with applications ranging 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. Furthermore, such metasurfaces offer multipolar electric and magnetic resonances known as Mie resonances. The moderate quality factor (Q factor) of Mie resonances is always accompanied by moderate electric field confinement, which can limit the strength of light-matter coupling. Higher Q factors are expected to induce stronger field confinement within the resonator, potentially promoting light-matter interactions in applications such as surface-emitting lasers, biomedical sensors, and nonlinear frequency converters. However, bounded states in the continuum (BICs) could provide a viable solution to the significant challenge of achieving nearly infinite Q factors. BICs are electromagnetic eigenstates that coexist within the continuum but are completely confined within the resonant system, devoid of radiative energy. Ideal BICs exist in lossless or extreme structures with zero linewidth and infinite Q factor, which can be transformed into quasi-BICs by introducing finite coupling to the radiative continuum, resulting in ultrahigh Q factors and observable resonances in the far-field spectrum.

[0004] The creation of quasi-BIC metasurfaces usually requires the destruction of structural symmetry, and most symmetric BICs are polarization-sensitive. In the prior art, the conversion and regulation of BICs and absorbers are achieved by controlling two phase-change materials, graphene and vanadium dioxide, but their biosensing capabilities are poor. Although the performance of the graphene-metal boat structure with a metallic mirror structure has been significantly improved, the preparation process is complicated and the transmittance is poor. In order to improve the transmittance, a subwavelength nanograting metasurface can be prepared in a crystalline lithium niobate 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, which is difficult to accurately distinguish. Summary of the Invention

[0005] The purpose of this invention is to design a rotationally symmetric lithium niobate elliptical nanopillar quasi-BIC metasurface. By adjusting the metasurface's structural parameters, the intensity and waveform of the transmission spectrum can be flexibly modulated. The resulting metasurface has high sensitivity and is suitable for applications in polarization-independent properties and gas sensors.

[0006] The technical solution adopted by the present invention is: a design method for a rotationally symmetric lithium niobate metasurface, the design method comprising the following steps:

[0007] S1: Provide a symmetrical BIC metasurface, which includes several square lattices and incident light; each square lattice is composed of a nanosubstrate and a cylindrical tetramer deposited on the nanosubstrate, wherein the nanosubstrate is a nanocube with a refractive index of 1.4-1.52, a thickness of 900-1000nm, and a material of silicon dioxide; the cylindrical tetramer is composed of four circular nanocolumns with the same radius, and the four circular nanocolumns are arranged in a two-by-two symmetry in the x-axis direction and the y-axis direction.

[0008] S2: When the incident light is x-polarized, the length of the single-side radius of the cylindrical tetramer in the xy-plane is increased or decreased at the same time, introducing an asymmetric perturbation, and the cylindrical tetramer is changed into a windmill-type elliptical nanocolumn tetramer to form an asymmetric square lattice; the windmill-type elliptical nanocolumn tetramer is composed of four first elliptical nanocolumns, second elliptical nanocolumns, third elliptical nanocolumns and fourth elliptical nanocolumns of the same size, and the four elliptical nanocolumns are arranged in a windmill-type 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 second 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 250nm, the variable side radius thereof is 163-337nm, and the height is 430-470nm; 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 510nm; the material of the windmill-type elliptical nanocolumn tetramer is lithium niobate.

[0009] When the incident light is x-polarized, in the xy-plane, only the length of the radius on one side of the windmill-type elliptical nanorod tetramer is increased or decreased simultaneously, introducing an asymmetric perturbation, while the radius on one side remains fixed.

[0010] Several asymmetric square lattices constitute the rotationally symmetric lithium niobate elliptical nanocolumn quasi-BIC metasurface.

[0011] Furthermore, the thickness of the nano-substrate is 975 nm, and its refractive index is 1.46; the radius of the changing side of the first elliptical nano-pillar, the second elliptical nano-pillar, the third elliptical nano-pillar and the fourth elliptical nano-pillar is 163 nm, and its height is 450 nm.

[0012] Furthermore, the incident light is a plane wave with a wave vector parallel to the z-axis and a polarization angle of -90°~90°. The transmission spectrum remains unchanged despite different polarization angles, and the quadruple resonance always maintains a high transmission depth in different polarization directions, indicating that the rotationally symmetric lithium niobate elliptical nanocolumn quasi-BIC metasurface is polarization-independent.

[0013] Furthermore, when the rotationally symmetric lithium niobate elliptical nanorod quasi-BIC metasurface is excited by x-polarized incident light, the ambient refractive index is 1.001-1.01.

[0014] Furthermore, the period of the rotationally symmetric lithium niobate elliptical nanorod quasi-BIC metasurface is 1950-2050nm.

[0015] Furthermore, the rotationally symmetric lithium niobate elliptical nanorod quasi-BIC metasurface is applied in devices with polarization-independent characteristics.

[0016] Furthermore, the rotationally symmetric lithium niobate elliptical nanorod quasi-BIC metasurface is applied in gas sensors.

[0017] Furthermore, the polarization angle of the incident light is described by the angle between the incident electric field and the x-axis.

[0018] Furthermore, the asymmetric peak of the quasi-BIC metasurface exhibits Fano resonance characteristics.

[0019] Beneficial effects of the present invention: To address the technical problems of this application, a rotationally symmetric lithium niobate elliptical nanorod quasi-BIC metasurface was designed. This metasurface replaces the existing cylindrical tetramer with windmill-shaped elliptical nanorods, forming an asymmetric square lattice. The metasurface exhibits distinct sharp Fano resonances near 1553 nm, 1608 nm, 1855 nm, and 1928 nm, with Q factors as high as 1.15×10 4 , 0.302×104 , 2.1×10 4 and 0.94×10 4 By adjusting the metasurface-related structural parameters (incident light polarization angle, background refractive index), the intensity and waveform of the transmission spectrum can be flexibly modulated. The maximum sensitivity S and quality factor FOM at the optimal parameters are 753.33nm / RIU and 5707RIU, respectively. -1 , can be widely used in fields such as gas sensors and polarization-independent characteristics applications. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Figure 1 This is a schematic diagram of the three-dimensional structure of a square lattice in the elliptical nanocolumn quasi-BIC metasurface.

[0021] Figure 2 A top-down view of a square lattice in the quasi-BIC metasurface of elliptical nanopillars.

[0022] Figure 3 It is the transmission spectrum of the quasi-BIC metasurface of Examples 1-6 and Comparative Example 1 in Experimental Example 1.

[0023] Figure 4 This is the TD1 multipole decomposition diagram of the quasi-BIC metasurface of Example 1 in Experimental Example 2.

[0024] Figure 5 This is the TD1 electric field distribution diagram of the quasi-BIC metasurface of Example 1 in Experimental Example 2.

[0025] Figure 6 This is the TD1 magnetic field distribution diagram of the quasi-BIC metasurface of Example 1 in Experimental Example 2.

[0026] Figure 7 This is the TD2 multipole decomposition diagram of the quasi-BIC metasurface of Example 1 in Experimental Example 2.

[0027] Figure 8 This is the TD2 electric field distribution diagram of the quasi-BIC metasurface of Example 1 in Experimental Example 2.

[0028] Figure 9 This is the TD2 magnetic field distribution diagram of the quasi-BIC metasurface of Example 1 in Experimental Example 2.

[0029] Figure 10 This is the TD3 multipole decomposition diagram of the quasi-BIC metasurface of Example 1 in Experimental Example 2.

[0030] Figure 11 This is the TD3 electric field distribution diagram of the quasi-BIC metasurface of Example 1 in Experimental Example 2.

[0031] Figure 12 This is the TD3 magnetic field distribution diagram of the quasi-BIC metasurface of Example 1 in Experimental Example 2.

[0032] Figure 13 This is the TD4 multipole decomposition diagram of the quasi-BIC metasurface of Example 1 in Experimental Example 2.

[0033] Figure 14 This is the TD4 electric field distribution diagram of the quasi-BIC metasurface of Example 1 in Experimental Example 2.

[0034] Figure 15 This is the magnetic field distribution diagram of TD4 of the quasi-BIC metasurface of Example 1 in Experimental Example 2.

[0035] Figure 16 Schematic diagram of the polarization angle of the incident light in Experimental Example 3.

[0036] Figure 17 This is the transmission spectrum of the quasi-BIC metasurface of Example 1 in Experimental Example 3 at different polarization angles.

[0037] Figure 18 This is a graph showing the change in TD1 and TD2 transmission spectra of the quasi-BIC metasurface of Example 1 in Experimental Example 4 with the background refractive index under x-polarized light.

[0038] Figure 19 This is a graph showing the change in transmission spectra of TD3 and TD4 versus background refractive index of the quasi-BIC metasurface of Example 1 in Experimental Example 4 under x-polarized light.

[0039] Figure 20 This is a graph showing how the resonance wavelengths of TD 1, TD 2, TD 3, and TD 4 of the quasi-BIC metasurface of Example 1 in Experimental Example 5 change with the thickness of the nano-substrate under x-polarized light.

[0040] Figure 21 This is a graph showing how the resonance wavelengths of TD 1, TD 2, TD 3, and TD 4 of the quasi-BIC metasurface of Example 1 in Experimental Example 6 change with the square lattice period under x-polarized light.

[0041] Figure 22 This is a graph showing how the resonance wavelengths of TD 1, TD 2, TD 3, and TD 4 of the quasi-BIC metasurface of Example 1 in Experimental Example 7 change with the height of the elliptical nanocolumns under x-polarized light.

[0042] Figure numerals: 1. nano-substrate; 2. windmill-type 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 DESCRIPTION

[0043] Example 1

[0044] The design method of the rotationally symmetric lithium niobate metasurface includes the following steps:

[0045] S1: A symmetric BIC metasurface is provided. The symmetric BIC metasurface includes several square lattices and incident light. The incident light is a plane wave with a wave vector parallel to the z-axis and a polarization angle between -90° and 90°. Each square lattice is composed of a nanosubstrate 1 and a cylindrical tetramer deposited on the nanosubstrate. The nanosubstrate is a nanocube with a refractive index of 1.46 and a thickness H of 900-1000nm, made of silicon dioxide. The cylindrical tetramer is composed of four circular nanopillars of the same radius, arranged in a two-by-two symmetric pattern along the x- and y-axes. The center-to-center spacing between two circular nanopillars symmetrically arranged along the x- and y-axes is 510nm.

[0046] S2: When the incident light is x-polarized, the length of the radius on one side of the cylindrical tetramer in the xy-plane is increased or decreased simultaneously, introducing an asymmetric perturbation, and the cylindrical tetramer is changed into a windmill-type elliptical nanocolumn tetramer 2, forming an asymmetric square lattice; the windmill-type elliptical nanocolumn tetramer is composed of four first elliptical nanocolumns 3, second elliptical nanocolumns 4, third elliptical nanocolumns 5 and fourth elliptical nanocolumns 6 of the same size, as shown in FIG. Figure 1 As shown, the four elliptical nanocolumns are arranged in a windmill-like 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 second elliptical nanocolumn by 90°; the fixed side radius R2 (fixed single-side radius) of each elliptical nanocolumn is 250nm, its variable side radius R1 (variable single-side radius) is 163-337nm, and the height h1 is 430-470nm; the center distance between two adjacent elliptical nanocolumns in the four elliptical nanocolumns is 510nm; the material of the windmill-like elliptical nanocolumn tetramer is lithium niobate.

[0047] Specifically, when the rotationally symmetric lithium niobate elliptical nanorod 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 a wave vector parallel to the z-axis and a polarization parallel to the x-axis, as shown in Figure 2As shown in the figure, in the xy-plane, the radius of each side of the cylindrical tetramer is simultaneously reduced by 87 nm, introducing an asymmetric perturbation. The cylindrical tetramer transforms into a pinwheel-shaped elliptical nanopillar tetramer, forming an asymmetric square lattice. Several asymmetric square lattices form a rotationally symmetric lithium niobate elliptical nanopillar quasi-BIC metasurface with a period P of 2025 nm.

[0048] COMSOL Multiphysics was used to numerically simulate a symmetric BIC metasurface. By breaking its geometric symmetry, the symmetric BIC metasurface was transformed into an asymmetric quasi-BIC metasurface. By simultaneously increasing or decreasing the radius lengths on one side of the elliptical tetramer in the xy-plane, introducing asymmetric perturbations and finite coupling to the radiating continuum, the metasurface was transformed into a quasi-BIC, resulting in a high Q factor and observable resonances in the transmission spectrum. In this case, the symmetric BIC metasurface was transformed into an asymmetric quasi-BIC metasurface, and the increase in radius widened the linewidth of the resonance peak.

[0049] Example 2

[0050] The radius of one side of the cylindrical tetramer in Example 1 was simultaneously reduced by 58 nm, and an asymmetric perturbation was introduced to form an asymmetric square lattice. The rest was the same as Example 1. Several asymmetric square lattices in this example formed a rotationally symmetric lithium niobate elliptical nanocolumn quasi-BIC metasurface.

[0051] Example 3

[0052] The radius of one side of the cylindrical tetramer in Example 1 was simultaneously reduced by 29 nm, and an asymmetric perturbation was introduced to form an asymmetric square lattice. The rest was the same as Example 1. Several asymmetric square lattices in this example formed a rotationally symmetric lithium niobate elliptical nanocolumn quasi-BIC metasurface.

[0053] Example 4

[0054] The radius of one side of the cylindrical tetramer in Example 1 was increased by 29 nm to introduce asymmetric perturbations to form an asymmetric square lattice, and the rest was the same as Example 1. Several asymmetric square lattices in this example constituted a rotationally symmetric lithium niobate elliptical nanocolumn quasi-BIC metasurface.

[0055] Example 5

[0056] The radius of one side of the cylindrical tetramer in Example 1 was increased by 58 nm to introduce asymmetric perturbations to form an asymmetric square lattice, and the rest was the same as Example 1. Several asymmetric square lattices in this example constituted a rotationally symmetric lithium niobate elliptical nanocolumn quasi-BIC metasurface.

[0057] Example 6

[0058] The radius of one side of the cylindrical tetramer in Example 1 was increased by 87 nm to introduce asymmetric perturbations to form an asymmetric square lattice, and the rest was the same as Example 1. Several asymmetric square lattices in this example constituted a rotationally symmetric lithium niobate elliptical nanocolumn quasi-BIC metasurface.

[0059] Example 7

[0060] The rotationally symmetric lithium niobate elliptical nanocolumn quasi-BIC metasurface designed and formed in Example 1 is applied in technical fields such as gas sensors and polarization-independent characteristic applications.

[0061] Comparative Example 1

[0062] This comparative example demonstrates a symmetric BIC metasurface, consisting of several square lattices and incident light. Each square lattice consists of a nanostructured substrate made of silicon dioxide and a cylindrical tetramer deposited on the substrate. Each cylindrical tetramer is composed of four circular nanopillars of equal radius, arranged in a two-by-two symmetric pattern along the x- and y-axes. Because the radius of the cylindrical tetramer remains constant, no asymmetric perturbation is introduced, resulting in a symmetrical square lattice. Several symmetrical square lattices constitute the symmetric BIC metasurface.

[0063] Test Example 1

[0064] (1) The quasi-BIC metasurfaces of Examples 1-6 and the symmetrical BIC metasurface of Comparative Example 1 were tested for transmission spectra. The results are as follows: Figure 3 When the radius of the four circular nanopillars remains unchanged (Comparative Example 1), the transmission spectrum is nearly horizontal in the wavelength ranges of 1500 nm to 1700 nm and 1800 nm to 2000 nm, indicating a BIC with an infinitely high Q factor. This is a typical symmetry-protected BIC, characterized by extremely high transmittance and invisible in the transmission spectrum.

[0065] When the unilateral diameter reduction R1 of the circular nanopillars in Comparative Example 1 was reduced by 87 nm (Example 1), four sharp resonant modes (denoted as TD1, TD2, TD3, and TD4) were observed at wavelengths of 1553 nm, 1608 nm, 1855 nm, and 1928 nm. This phenomenon occurs because the broken structural symmetry enhances the coupling between the ideal symmetry-protected BIC and the radiative modes, thus forming a quasi-BIC metasurface.

[0066] When the single-side diameter R1 of the circular nanorod in Comparative Example 1 changes from 163 nm to 337 nm (Examples 1-6), the line widths of the four resonance modes gradually increase.

[0067] Test Example 2

[0068] The multipolar decomposition of TD1, TD2, TD3 and TD4 of the quasi-BIC metasurface of Example 1 was tested. Figure 4-15 shown. Figure 4 、 Figure 7 、 Figure 10 and Figure 13 These are the multipole decomposition diagrams of TD1, TD2, TD3 and TD4 of the quasi-BIC metasurface of Example 1 (the variable side radius R1 is 163 nm, the fixed side radius R2 is 250 nm), where ED is electric dipole, EQ is electric quadrupole, MD is magnetic dipole, MQ is magnetic quadrupole, and TD is toroidal dipole. Figure 5 、 Figure 8 、 Figure 11 and Figure 14 These are the electric field distribution diagrams of TD1, TD2, TD3 and TD4 respectively. Figure 6 、 Figure 9 、 Figure 12 and Figure 15 These are the magnetic field distribution diagrams of TD1, TD2, TD3 and TD4 respectively.

[0069] The total scattering efficiency of TD1 and TD3 resonant multipoles is mainly determined by TD, followed by MQ, as Figure 4 and Figure 10 The electric and magnetic field distributions of TD1 and TD3 are shown in Figure 5 、 Figure 6 and Figure 11 and Figure 12 As shown in Figure 3, the electric fields of TD1 and TD3 arise from four longitudinal electric resonances. The electric fields are distributed between the outer sides of the tetramers and the gaps, and around the center of each column, forming a circular displacement current in the xz plane. The xy magnetic fields have opposite directions, forming a closed magnetic loop around the x-axis, indicating the presence of TDs. Furthermore, the inverse structure of the magnetic fields indicates the presence of MQs.

[0070] like Figure 7 and Figure 13 As shown in Figure 2, the contribution of TD dominates the TD2 and TD4 resonances. Obviously, the TD contribution dominates the response, followed by MQ. The electric and magnetic field distributions of TD2 and TD4 are shown in Figure 2. Figure 8 、 Figure 9 and Figure 14 、 Figure 15As shown, the electric field is weak within the nanodisk and strong outside it. Two sets of counterclockwise and clockwise closed displacement current loops are obtained in the xy plane, respectively. In the xz plane, the magnetic field vectors on the nanopillar cross section have opposite directions. This inverse distribution exhibits symmetry across multiple quadrants, indicating the presence of a magnetic flux density (MD). Furthermore, the direction arrows of the magnetic field vectors, facing each other, form a loop, circulating clockwise in the z direction, a characteristic of TD.

[0071] Test Example 3

[0072] The changes of TD1, TD2, TD3 and TD4 of the quasi-BIC metasurface of Example 1 under different incident light polarization angles are tested. The polarization angle of the incident light is described by the angle between the incident light field and the x-axis, which is defined as θ, as follows: Figure 16 The polarization angle θ of Example 1 was set to 0°, 15°, 30°, 45°, 60°, 75°, 90°, left-handed and right-handed respectively, and the changes of TD1, TD2, TD3 and TD4 of the quasi-BIC metasurface in Example 1 at different polarization angles were observed. The results are shown in FIG. Figure 17 As shown, the transmission spectra at different polarization angles do not change, and TD1, TD2, and TD3 maintain large amplitudes in different polarization directions, indicating that the metasurface is polarization-independent. This unique property can be used to eliminate or even significantly reduce the need for polarization calibration, thereby providing greater flexibility for applications such as projectors, radar technology, microscopy systems, and solar cells.

[0073] Test Example 4

[0074] The effect of the background refractive index on the transmission spectrum, the resonance wavelengths of TD1, TD2, TD3 and TD4 of the quasi-BIC metasurface of Example 1 under x-polarization was tested. The results are shown in Figure 2. Figure 18 and Figure 19 As shown. With the increase of the background refractive index, TD1, TD2, TD3 and TD4 will all undergo red shifts, and the red shift amounts of the four resonances are different. TD1 shifts from 1534.1 nm to 1540.9 nm, TD2 shifts from 1608.1 nm to 1613.3 nm, TD3 shifts from 1855.5 nm to 1858.2 nm, and TD4 shifts from 1928.5 nm to 1930.2 nm. The red shift of TD1 is more significant. After calculation, it can be concluded that the sensitivities of TD1, TD2, TD3 and TD4 are 753.33, 579.39, 300 and 187.27 nm / RIU, respectively, and the corresponding FOMs are 5707, 1089, 3409 and 918 RIU, respectively. -1 .

[0075] Test Example 5

[0076] The effect of the thickness of the nano-substrate on the transmission spectrum, the resonance wavelengths of TD1, TD2, TD3 and TD4 of the quasi-BIC metasurface of Example 1 under x-polarization was tested. The results are as follows: Figure 20 As shown in Figure 3, the four resonances TD1, TD2, TD3, and TD4 red-shift as H increases from 900 to 1000 nm. However, the shift of TD4 is less pronounced than that of the other three resonances when H is varied in 25 nm increments.

[0077] Test Example 6

[0078] The effects of the square lattice period of the quasi-BIC metasurface of Example 1 on the transmission spectrum, the resonance wavelengths of TD1, TD2, TD3 and TD4 under x-polarization were tested. The results are shown in Figure 2. Figure 21 As shown in Figure 3, the four resonances show a significant red shift when P increases from 1950 nm to 2050 nm.

[0079] Test Example 7

[0080] The effect of the nanopillar height on the transmission spectrum, the resonance wavelengths of TD1, TD2, TD3 and TD4 of the quasi-BIC metasurface of Example 1 under x-polarization was tested. The results are shown in Figure 2. Figure 22 As h1 gradually increases from 430 to 470 nm in steps of 10 nm, four resonances are observed to show a slight red shift.

[0081] Lithium niobate (LN) was chosen as the material for windmill-type elliptical nanorod tetramers due to its high transparency in the near-infrared to near-ultraviolet spectral region. Designing and optimizing LN nanostructures is crucial for improving the performance of nanophotonics and optoelectronics. Silicon dioxide, the most common low-refractive-index all-dielectric material, has been widely used in the fabrication of photonic crystals. Despite its low refractive index, its near-zero absorption makes it an ideal material for optical communications. While ensuring low loss in dielectric materials, LN and SiO were chosen for their effectiveness in practical applications, as they facilitate coupling between different structures to achieve efficient output characteristics.

[0082] By varying the radius of one side of the elliptical nanopillars, the structural symmetry can be disrupted, stimulating quasi-BIC resonance. When all four elliptical nanopillars are altered, the tetramer can rejoin itself after rotating through a certain angle (0°-360°). Therefore, the structure is rotationally symmetric. This is an extremely simple and effective method for transforming a 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 method for designing a rotationally symmetric lithium niobate metasurface, characterized by: The design methodology includes the following steps: S1: providing a symmetrical BIC metasurface, which 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, wherein the nano-substrate is a nanocube having a refractive index of 1.4-1.52, a thickness of 900-1000 nm, and a material of silicon dioxide; the cylindrical tetramer is composed of four circular nano-pillars with the same radius, and the four circular nano-pillars are arranged in a two-by-two symmetry in the x-axis direction and the y-axis direction; S2: When the incident light is x-polarized, the length of the variable unilateral radius in the cylindrical tetramer in the xy-plane is increased or decreased at the same time, and the fixed unilateral radius remains unchanged, and an asymmetric perturbation is introduced, so that the cylindrical tetramer is changed into a windmill-type elliptical nanocolumn tetramer to form an asymmetric square lattice; the windmill-type elliptical nanocolumn tetramer is composed of four first elliptical nanocolumns, second elliptical nanocolumns, third elliptical nanocolumns and fourth elliptical nanocolumns of the same size, and the four elliptical nanocolumns are arranged in a windmill-type shape, and the fixed side radius of the first elliptical nanocolumn, the second elliptical nanocolumn, the third elliptical nanocolumn and the fourth elliptical nanocolumn is 250nm, the variable side radius thereof is 163-337nm, and the height is 430-470nm; 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 510nm; the material of the windmill-type elliptical nanocolumn tetramer is lithium niobate; Several asymmetric square lattices constitute the rotationally symmetric lithium niobate elliptical nanocolumn quasi-BIC metasurface.

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

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

4. The method for designing a rotationally symmetric lithium niobate metasurface according to claim 1 or 2, characterized in that: When the rotationally symmetric lithium niobate elliptical nanocolumn quasi-BIC metasurface is excited by x-polarized incident light, the ambient refractive index is 1.001-1.

01.

5. The method for designing a rotationally symmetric lithium niobate metasurface according to claim 1 or 2, characterized in that: The period of the rotationally symmetric lithium niobate elliptical nanocolumn quasi-BIC supersurface is 1950-2050 nm.

6. Application of the rotationally symmetric lithium niobate metasurface according to claim 1 in a polarization-independent device.

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

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