Extraordinary enhancement of nonlinear processes in a non-Hermitian PT-symmetric metasurface

Motivated by recent advancements in the extraordinary scattering characteristics of parity-time (PT)-symmetric metasurface, we investigate the optical scattering characteristics and nonlinear properties of a PT-symmetric metasurface by exploiting its unique non-Hermitian singular property, coherent perfect absorption and lasing point (CPA-LP). In particular, we study scattering properties of a PT-symmetric gain/loss AlGaAs nano-disk resonators using full-wave numerical simulations and scattering matrix theory. The proposed metasurface exhibits exceptional points, spectral singularities, and asymmetric reflection characteristics. Importantly, we theoretically demonstrate that in the PT-broken phase, the PTsymmetric metasurface can facilitate extraordinary enhancement of nonlinear processes, such as second- and third-harmonic generations, due to the strong scattering field at the CPA-LP, which allows the system to access a strongly nonlinear regime.