Multipolar second-harmonic generation from high-Q quasi-BIC states in subwavelength resonators

Abstract

We put forward the multipolar model whichcaptures the physics behind linear and nonlinear responsedriven by high-quality (high-Q) supercavity modes in sub-wavelength particles. We show that the formation ofsuch trapped states associated with bound states in thecontinuum (quasi-BIC) can be understood through multi-polar transformations of coupled leaky modes. The quasi-BIC state appears with increasing the order of the domi-nating multipole, where dipolar losses are completely sup-pressed. The efficient optical coupling to this state in theAlGaAs nanodisk is implemented via azimuthally polarizedbeam illumination matching its multipolar origin. Weestablish a one-to-one correspondence between the stan-dard phenomenological non-Hermitian coupled-modetheory and multipolar models. The derived multipolarcomposition of the generated second-harmonic radiationfrom the AlGaAs nanodisk is then validated with full-wavenumerical simulations. Back-action of the second-harmonicradiation onto the fundamental frequency is taken into ac-count in the coupled nonlinear model with pump depletion.A hybrid metal-dielectric nanoantenna is proposed toaugment the conversion efficiency up to tens of per cent dueto increasing quality factors of the involved resonant states.Ourfindings delineate novel promising strategies in thedesign of functional elements for nonlinear nanophotonics applications. Show more