Efficient near UV-vacuum UV sources based on second-harmonic generation enhanced by high-Q quasi-BICs in all-dielectric metasurfaces of low-index materials

Ultraviolet (UV) nanophotonics is an increasingly important branch of modern photonics due to the fundamental importance and practical applications. This work presents the efficient generation of near UV-vacuum UV ra-diations based on the second-harmonic generation in all-dielectric metasurfaces exhibiting high-Q quasi-bound states in the continuum (quasi-BICs). For this purpose, lithium niobate (LN) is taken as the nonlinear material comprising the metasurface, which is transparent in the ultraviolet range. Since LN has much lower index compared with the dielectrics commonly used as nanophotonic materials, a specific measure should be taken for supporting high-Q quasi-BICs: the metasurface is constructed with periodic LN nanodisk array on the ultrathin film of the same material deposited on silica, such that the guiding wave couples to the far-field with high Q -factor, resulting in second-harmonic generation with significantly high efficiency up to 0.14 % in the range from near UV to vacuum UV for both transmitted and reflected radiations. The metasurface of similar structure, but with mirror symmetry-broken unit cells, exhibits nearly perfect second-harmonic generation circular dichroism with high efficiency. The presented results can find important applications for ultraviolet nanophotonics due to the significantly high efficiency and strong nonlinear circular dichroism.