Enhanced Third‐Harmonic Generation in a Bound State in the Continuum Assisted Multiband All‐Dielectric Metasurface

Multiband Fano resonances are demonstrated in the near‐infrared (near‐IR) using an all‐dielectric metasurface whose unit cell consists of four silicon nanoblocks on a glass substrate. An in‐plane asymmetry triggers symmetry‐protected quasi‐bound states in the continuum (QBICs), producing multiple high‐ resonances. Their origin is identified through multipolar decomposition of the scattering cross section and field distributions at the resonances. The strong field localization at these resonances enables efficient multiband third‐harmonic (TH) generation in the ultraviolet (UV), with a maximum simulated conversion efficiency of at a peak pump intensity of . The metasurface is fabricated in symmetric and asymmetric configurations, and its linear and nonlinear responses are measured under normal incidence. A TH conversion efficiency of is obtained at a peak pump intensity of . These results establish a route to multiband photonic devices, including multiwavelength lasers, multiband harmonic generation, and single‐photon sources for quantum photonics.