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. [ABSTRACT FROM AUTHOR]