Research On The Properties Of Toroidal Dipole Fano Resonance Based On All-dielectric Metasurface

Toroidal dipoles(TD)and higher toroidal multipoles are the subject of growing interest because of their unusual electromagnetic properties.In particular,it is expected to solve the radiation loss of plasma and nano-photonic crystal systems and realize many high-value applications such as low-loss nanoantennas,ultra-sensitive sensors,low-threshold lasers,low-loss waveguides,nonlinear optics and holographic imaging.In modern photonics,TD resonance interferes with electric multipole resonance or magnetic multipole resonance,which enables subwavelength ring molecular resonators to arbitrarily manipulate the polarization,phase,and amplitude of light,and generate near-field coupling effects to achieve Fano resonance.Fano resonance occurs when a discrete quantum state interferes with a continuum band of states.Electromagnetically induced transparency(EIT)can be viewed as a special case of Fano resonance when the frequencies of strongly and weakly damped oscillators match.Fano and EIT resonances have low loss and high locality in the near field,which have broad application prospects in the fields of holographic imaging,optical communication,lasers and optoelectronic modulators.As photonics research moves into high frequencies,especially in the near-infrared and visible wavelengths,the intrinsic heat loss of metallic structures severely limits the resonant quality and high-performance device design.Therefore,based on the design and simulation optimization of TD metasurfaces for dielectric materials,we realize Fano,EIT,and bound states in the continuum resonances,and study their intrinsic physical mechanisms.We reveal the coupling between electromagnetic modes by means of multipole interference theory and coupled mode theory.In this paper,lateral and directional enhanced scattering,TD Fano resonance,anapole mode,active optical switch based on EIT resonance,dual-band quasi-BIC resonance and its refractive index sensing are realized.The main contents and innovations of this paper are as follows:1.Based on the excitation of Mie resonance in high-refractive-index ceramic particles,the multipole moment interference inside the particles is carefully designed and studied to achieve nonpole modes,side scattering and unidirectional enhanced scattering that are less affected by the environment.In addition,a collective lattice coupling effect composed of rectangular ceramic dielectric blocks is designed to amplify the electromagnetic response of single particles,and a multi-band high-Q-factor TD Fano-resonant metasurface is realized,with a maximum Q-factor of 3×10~4.The reflection and transmission spectra numerically calculated are in excellent agreement with the corresponding spectra calculated with multipole decompositions.The spectral positions of Fano resonances can be tuned by controlling the topology of the particles.Finally,the effects of ambient temperature and dielectric loss on the Fano resonance strength and linewidth are investigated.We believe our results could be useful for analysis and understanding of the electromagnetic properties of nanoparticle arrays and pave the way for the design of novel metasurfaces for various optical applications.2.Based on the dielectric metasurface model with high refractive index,This study proposes an EIT resonance with a high quality factor,which can reach 7798,and low mode volume can reach 0.009μm~3,high contrast ratio can reach nearly 100%,in the near-infrared region,which is generated by the magnetic TD in a reverse-symmetric coupling spiral metasurface.A two-oscillator model can only explain the influence of near-field coupling at the EIT point for weak coupling.Moreover,a multipole decomposition method shows that the excitation mechanism of EIT resonances originates from the destructive interference between the subradiant modes(magnetic toroidal dipole-electric quadrupole)and magnetic dipole resonance.Consequently,a new general extinction spectrum interference model is applied to fit all coupling conditions for both weak and strong coupling results that perfectly correspond to the multipole decomposition method.The results of this study could be useful in the analysis and understanding of the electromagnetic coupling characteristics of nanoparticles and provide a design approach for novel metasurfaces for low-loss optical applications.We also studied the integration of graphene strips on glass substrates,and the modulation of the EIT window was achieved by tuning the Fermi level of graphene,with a modulation depth of up to 90%.Our proposed metasurface provides a feasible solution in the fields of optical filters,optoelectronic modulators,and switches.3.We investigate dual bound states in the continuum in a subwavelength all-dielectric metasurface consisting of an array of high-index dimer nanodisk that leak into Fano and EIT resonances.Eigenmode analysis reveals the existence of the polarization-dependent,symmetry-protected BICs.Numerical simulation verified the dual quasi-BICs by opening a leakage channel to probe the far-field spectral features.We also observe the diverging radiative quality factors and show the inverse square dependence on structural or permittivity asymmetry.In addition,we also investigate the effect of geometric parameters on Fano and EIT spectral quality and resonance peak positions.The low-loss TD Fano and EIT resonances with extremely narrow linewidth are very sensitive to the change in the refractive index of the surrounding media,achieving sensitivity level of 288 nm/RIU and ultrahigh the figure of merit is 25800.The proposed toroidal metasurface structure is simple and easy to realize experimentally,and can also be applied to laser,nonlinear and other fields.