Research On Structural Designs And Applications Based On All-dielectric Metasurface

Subwavelength-scale metasurfaces have become ideal platforms for hot research topics in optical localization,light-matter interaction enhancement,and high-Q optical response by virtue of their unique optical field manipulation capabilities,which have shown great potential for application in nextgeneration multifunctional planar integrated optical devices.In recent years,metasurfaces based on bound state in the continuum(BIC)have attracted wide attention in lasing,high-sensitivity sensing and other applications due to their superb light localization capability.By designing the structural units in the metasurface,the ideal BIC can be converted into quasi--BIC,achieving high Qfactor optical response while localizing light and enhancing the light-matter interaction.However,current metasurfaces suffer from limitations such as single operating band,incomplete multi-resonance excitation mechanism,unstable resonant wavelength when the Q-factor of resonance is adjusted,and high optical losses in metallic metasurfaces at optical band.To address these issues,in this thesis,all-dielectric metasurfaces will be studied to explore the formation mechanism of BIC and new methods for generating high Q-factor multiple resonance modes,and explore their applications in high-sensitivity sensing,optical switching and so on.The main research contents are as follows:1.We investigate the toroidal dipole modes in metasurfaces based on BIC to explore their physical mechanisms and controlling methods.A high-Q resonance mode with strong near-field enhancement is excited in a spherical periodic nanoarray,which originates from the interaction between toroidal dipole and magnetic quadrupole modes by multipole decomposition method and near-field analysis.By introducing an air gap in the nanodisk,the high Qfactor resonance mode is successfully extended in the planar nanodisk metasurface while other resonance modes are suppressed and strong electric field enhancement in the free space and good magnetoelectric separation are achieved.The physical mechanism of the resonance mode is analyzed from the symmetry point of view,then three different resonance modes are excited in a single nanodisk structure by further reducing the symmetry,and the strong coupling between toroidal dipole and magnetic dipole modes is found to cause anomalous changes for Q-factors.The proposed metasurface is evaluated as a refractive index sensor,achieving a high figure of merit of 3155.5 and a sensitivity of 142 nm/RTU,which is also verified by perturbation theory.2.We propose a BIC-based polarization-insensitive metasurface supporting multiple resonances.By introducing in-plane perturbations in adjacent periodic cells,the period of the metasurface is changed without affecting C4v symmetry of the structure,which causes the folding of the First Brillouin Zone in momentum space,quadrupling the eigenmode at the Γ point.The four degenerate modes folded to the Γ point can be excited by linearly x or y polarized light simultaneously,and the transformation of the four modes from nonradiative BIC to high-Q quasi-BIC can be observed by changing the magnitude of the perturbation.The physical mechanism of the four resonant modes,which originate from four different multipole sources,is also investigated by near-field analysis and multipole decomposition methods.The full-angle polarization insensitive characteristic of the proposed metasurface is also confirmed by varying the incident polarization angle,and the robustness of the metasurface to the fabrication is confirmed by considering the etching offset.The metasurface is applied to a refractive index sensor with a maximum sensitivity of 510 nm/RIU and a maximum figure of merit of 21287.3.We propose a new method for designing metasurfaces with stabilized resonant wavelength,which support multiple resonances based on BICs.By introducing perturbations in adjacent nanocells to produce dimerization,the First Brillouin Zone in the momentum space is folded and two non degenerate eigenmodes are successfully excited.Since such perturbations do not perturb the refractive index of the system,the resonant wavelengths of the resonant modes are very stabilized in the process of adjusting the Q-factor by changing the magnitude of the perturbation.The cross periodic unit provides sufficient degrees of freedom for the introduction of the perturbation,and after further reducing the symmetry of the structure,multiple Fano resonances are successfully excited in the metasurface.The physical mechanism of the formation of different resonance modes is investigated by multipole decomposition and symmetry analysis.The robustness and flexible tunability of the structure is also investigated by varying the geometrical parameters.The proposed metasurface is used for optical switches to achieve 0%to 95%amplitude control,and for a refractive index sensor to achieve a maximum sensitivity of 367.7 nm/RIU and a maximum figure of merit of 17922.2,which has the potential to be applied to a variety of devices dependent to high Qfactors.