The Study Of Fano Resonance In All-Dielectric Nanostructures And Their Applications

Fano resonance is a special physical phenomenon first observed in atomic physics,which is formed by the coupling of continuum states and discrete states and exhibits an asymmetric spectrum.Recently,Fano resonance,widely developed in nanophotonics attributed to its sharp spectrum and strong near-field enhancement,provides prominent potentials in applications such as optical switching,sensors,lasing,nonlinear and slow-light devices.The early Fano-resonant nanostructures,always built from metal materials,are limited by the high inherent energy dissipation of metal and incompatibility with CMOS technologies.In order to reduce dissipative losses and enhance electromagnetic field intensity,recently,a variety of all-dielectric nanostructures associated with Ge,Si,GaAs and many other high refractive index materials have been developed,paving a way for nano-devices with good-performance,miniaturization and high-integration.In this work,the high-quality factor(Q)Fano resonance in all-dielectric nanostructures is further studied in theory and experiment,a deeper exploration of its tunability and application in nonlinear optics is carried out as well.The major results are summarized as follow:(1)The high Q-factor resonance is achieved in our designed all-dielectric metasurface and the THG conversion is boosted through the metasurface.The obtainment of a Q-factor as high as~1000 of the Fano resonance is attributed to the excitation of the bound states in the continuum(BIC),introduced by the broken symmetry imposed on the highly symmetric nanopillars.Such an asymmetric design with only a single building block arrayed periodically holds great fabrication tolerance compared to the complex structures comprised of several compositions,providing potentials for a higher Q-factor.Through analyzing the near-field distribution and valuing the far-field radiation of several primary multipoles,this Fano resonance is verified to be contributed predominantly by the magnetic dipole.Further employing this asymmetric metasurface in nonlinear applications,a THG enhancement of about~600 is observed in the transmitted light through the metasurface by contrast with the one through the unpatterned Si film,ascribed to the strong near-field confinement produced by the high Q-factor Fano resonance.(2)The tunability of the Fano resonance assisted by graphene is explored in theory and experiment.The influence of structure parameters on the positions and strengths of Fano peaks is analyzed in the photonic crystal slabs.Then transferring graphene layers on the photonic crystal slabs,we observed a modulated Fano resonance,with the modulation depth increasing when more layers are transferred and bring in more absorptions.With 3-layers of graphene on the photonic crystal slabs,the modulation depths of Fano resonance reach 60%and 44%in theory and experiment,respectively.In addition,we studied the modulation capacities of graphene with varied Fermi energy(E_F)and discovered that the transmission modulation depths of Fano resonance change crucially around E_F=0.472 eV.Further explanations ascribed this dramatical difference to the alterations of optical conductivity produced by the Pauli blocking.(3)The Fano resonance formed by toroidal dipole oscillations is investigated in silicon photonic crystal slabs.Breaking the mirror symmetry of the structures enables the excitation of non-degenerate modes,creating several sharp Fano peaks in the near-infrared region.The calculations of far-field radiation by multipole expansion method and near-field distributions at the resonant wavelengths demonstrate that there exist abundant toroidal dipoles in photonic crystal slabs devices.The transmission spectra extracted from the experiments match well with the theoretical expectations,with small deviations owing to the fabrication imperfections.