Tunable Second Harmonic Generation with Large Enhancement in A Nonlocal All-Dielectric Metasurface Over A Broad Spectral Range

Recently, all-dielectric metasurfaces are profoundly exploited to enhance light-matter interactions, resulting from the high quality-factor (Q-factor) optical resonances that based on novel concepts such as the bound states in the continuum (BIC). Unfortunately, BIC operates at a fixed resonance wavelength and a fixed wavevector for a specific structure. Here, the experimental demonstration of a dielectric nonlocal metasurface capable of robustly maintaining high-Q resonances is reported, over a broad spectral range, where the specific wavelength is selected by controlling the incident angle. This is enabled by steering infinite-Q guided modes (GMs) in subwavelength lattices into quasi-GMs (QGMs), which are accessible by external radiations while retaining the same dispersion as the GMs. Such invaluable characteristics are achieved by harnessing the period-doubling perturbation, implemented in a nonlocal metasurface structure on top of a lithium niobate (LN) film. Furthermore, spectrally tunable second-harmonic generations are demonstrated in this structure with an enhancement factor of ≈1200 compare to that of a bare LN film of the same thickness over a large bandwidth. These results suggest that the QGMs supported by all-dielectric nonlocal metasurfaces provide an excellent platform for enhancing light-matter interactions with additional desired functionalities of spectral tunability and random selection of the operation wavelength.