Research Of High-Q Toroidal Dipole Resonators In Dual-SRR Silicon Dielectric Metasurface

In metamaterial systems,toroidal dipoles plays an important role in determining their optical properties.Like electric dipole and magnetic dipole,toroidal dipole metamaterials or metasurfaces have potential applications in the design of photonic devices such as polarization conversion,sensing,and resonant transparency.In particular,toroidal dipole is often associated with high quality factor(Q)resonance and plays an important role in ultrahigh sensitive sensing and nonlinear interactions,such as it can excite non-radiative anapole mode,thus achieving high Q resonance.Therefore,the excitation and manipulation of toroidal dipole by designing effective structure of metasurface have become a hot spot now.In addition,recent reports reveal a strong link between high Q toroidal dipole metasurface and the bound state in the continuum(BIC).Optical BIC offers a unique opportunity to manipulate the light-matter interaction within radiative continuum,due to its ultrahigh-Q properties and the associated giant enhancement of the electromagnetic near-fields.Based on the analysis and calculation of the multipole scattering characteristics of a single pair split-ring-resonator(SRR)nanoparticles,the thesis mainly studies the design of high-Q toroidal dipole all-dielectric metasurface and the realization of BIC based on toroidal dipole.The main research work of the thesis is as follows:(1)The multipole scattering characteristics of a pair of silicon split-ring structures facing each other and facing away from are studied.By adjusting the gap size of the SRRs,the coupling between the two SRRs and the effect on the toroidal dipole and other multipoles are analyzed.The calculation results show that the non-radiative anapole mode can be realized at relatively broad spectral range in the silicon dual-SRR facing away from.However,the anapole mode cannot be realized in the silicon dual-SRR facing each other.(2)Based on the dual-SRR structure,an all-dielectric silicon dual-SRR metasurface is designed.When the polarization direction of the electric field is parallel to the gap of the split-ring,the high-Q strong toroidal dipole resonance can be excited.The excitation of toroidal dipole resonance mainly attributed to contributions of the toroidal dipole moments both inside the unit cell(intra-toroidal dipole moment)and between the neighboring unit cells(inter-toroidal dipole moment).By changing the geometric parameters of the structure,the influence on the characteristics of the toroidal dipole resonance is studied,and the manipulations of the intra-toroidal dipole moment and the inter-toroidal dipole moment are further analyzed,thereby explaining the excitation of the high-Q toroidal dipole resonance.In addition,the research also shows that the anapole mode cannot be realized in the silicon dual-SRR metasurface.(3)An all-dielectric silicon closed-ring metasurface is designed in the range of near-infrared wavelength.The all-dielectric silicon closed-ring metasurface can realize ultra-high Q resonance,the Q value of the resonance can reach up to 10~8,with the strong electric field and magnetic field near-field enhancement,and the near-field enhancement factors reach up to 10~4.The analysis shows that the resonance is a symmetric unprotected toroidal dipole BIC.In addition,we emphatically analyze the effect of the distance between the closed rings in the unit cell on toroidal dipole BIC.