Resonantly enhanced second- and third-harmonic generation in dielectric nonlinear metasurfaces

Nonlinear dielectric metasurfaces provide a promising approach to control and manipulate frequency conversion optical processes at the nanoscale, thus facilitating both advances in fundamental research and the development of new practical applications in photonics, lasing, and sensing. Here, we employ symmetry-broken metasurfaces made of centrosymmetric amorphous silicon for resonantly enhanced second- and third-order nonlinear optical response. Exploiting the rich physics of optical quasi-bound states in the continuum and guided mode resonances, we comprehensively study through rigorous numerical calculations the relative contribution of surface and bulk effects to second-harmonic generation (SHG) and the bulk contribution to third-harmonic generation (THG) from the meta-atoms. Next, we experimentally achieve optical resonances with high quality factors, which greatly boosts light-matter interaction, resulting in about 550 times SHG enhancement and nearly 5000-fold increase of THG. A good agreement between theoretical predictions and experimental measurements is observed. To gain deeper insights into the physics of the investigated nonlinear optical processes, we further numerically study the relation between nonlinear emission and the structural asymmetry of the metasurface and reveal that the generated harmonic signals arising from linear sharp resonances are highly dependent on the asymmetry of the meta-atoms. Our work suggests a fruitful strategy to enhance the harmonic generation and effectively control different orders of harmonics in all-dielectric metasurfaces, enabling the development of efficient active photonic nanodevices.