Optical Resonances Supported By Silicon Metasurfaces And Their Applications

Optical resonances in metasurfaces can be used to tailor the optical spectrum and reshape spatial wavefront of lightwave.Silicon material with low optical loss in the telecommunication windows,featuring flexibility for structure design and compatibility for semiconductor fabrication technology,has been favored by researchers recently.As artificial structures with sub-wavelength scales,optical metasurfaces can control the resonances through precisely designing its structural geometric parameters,providing an important approach to exciting the resonance with specific dominant electromagnetic multipoles and solving the problem of weak radiation of the toroidal dipoles in natural structures.Meanwhile,the resonances and the sub-wavelength scales of optical metasurfaces create a vital condition for the high integration of optical vortex beam devices,giving a solution to the problem of large device footprints in the conventional methods.In this paper,the silicon metasurfaces have been designed and fabricated based on the optical resonances,and the applications of optical resonances have been explored for the manipulations of optical spectrum and spatial wavefront.The main contents and innovations of this paper include:(1)Aiming to overcome the difficulty of exciting strong toroidal dipoles in natural structures,the silicon metasurface based on lattice perturbation has been proposed to excite high Q-factor Fano resonance dominated by toroidal dipoles in the near-infrared region,and the transmission spectrum has been tailored by tuning lattice perturbation.Firstly,the generation mechanism of Fano resonance is analyzed theoretically,in which the lattice perturbation and Brillouin zone folding effects lead the bound state to couple with external radiation,giving rise to the Fano resonance eventually.The transmission spectrum of the structures with a series of lattice perturbations have been simulated by finite-difference time-domain method,and the evolutions of Q-factors of the Fano resonances for the different perturbations have been demonstrated.Then,it has been confirmed that the resonance is dominated by toroidal dipole through multipole decompositions and electromagnetic field distributions.The intensity distributions of the four anti-phase magnetic dipole resonances in the structure are altered while the nanohole positions in the metaurfaces move,which results in that the scattering power of the toroidal dipole and the Fano Q-factor of the spectrum can be tailored by the lattice perturbation.Finally,six metasurfaces with different perturbations have been fabricated and their transmission spectra have been measured.The experimental and simulated results are in good agreement,and the highest experimental Q-factor of 584 is obtained when the perturbation offset is 53 nm.(2)In order to solve the problem of large footprints and realize high integration of optical vortex beam devices,the silicon metasurfaces with pillar height of 340 nm have been proposed to generate four kinds of vortex beams,with which the linearly polarized incident lights with a wavelength of 2000 nm are converted into vortex beams with topological charges of ±1 and ±2,respectively.Firstly,eight unit cells of metasurfaces with ?/4 phase gradient have been designed and optimized,of which the optical resonances are evaluated by the near field distributions and multipole decompositions.The relationships between electromagnetic multipole resonances and phase delays of unit cells have been discussed.Then the metasurfaces with topological charges of ±1 and ±2 are designed,and simulation results have shown that the transmitted light of metasurfaces has corresponding spiral phase and dark-core shaped intensity distributions.All the metasurfaces have more than 56.5%transmittance ranging from 1950 nm to 2050 nm.Finally,the topological charge of vortex beam is analyzed through interference method,and the results of the interferences between the simulated transmitted light and the reference light are in good agreement with the theoretical results.