Quasi-bound states in the continuum in all-dielectric metasurfaces for nonlinear optics (Invited Paper)

We develop a simple approach for designing high-quality quasi-bound states in the continuum (quasi-BICs) using all-dielectric asymmetry kite-shaped nanopillar arrays and investigated their third-harmonic generation (THG). By manipulating the asymmetric of the kite-shaped meta-atoms, the symmetry-protected quasi-BICs was excited showing a tunable resonant spectral profile between Fano and Lorentzian-lineshapes due to the interplay with the broadband magnetic dipole (MD) mode. The high quality-factor quasi-BICs renders a four order of magnitude THG enhancement in calculation and 46.4-fold THG in experiment. In addition, a hybrid system incorporating amorphous-silicon nanocubes and Indium-Tin-Oxide (ITO) film was developed to study the interplay among electric dipolar, quasi-BICs, and Epsilon-Near-Zero (ENZ) modes. The strong coupling between these resonances results in a Rabi splitting energy of 137 meV and also achieves a broadband THG enhancement in the fundamental wavelength range of 1100 to 1600 nm.