Abstract

Lithium niobate (LN) has been widely used for second-harmonic generation (SHG) from bulk crystals. Recent studies have reported improved SHG efficiency in LN micro-ring resonators and hybrid waveguiding structures, as well as in LN nanostructures supporting anapole modes and plasmon-assisted dipole resonances. Here we numerically demonstrate that high $ Q $-factor resonances associated with symmetry-protected bound states in the continuum can lead to highly efficient frequency doubling in LN metasurfaces. Simulations show that the radiative $ Q $-factor and on-resonance field enhancement factor observed in the metasurface are closely dependent on the asymmetric parameter $ \alpha $ of the system. Furthermore, high $ Q $-factor resonances boost the SH conversion process in the LN nanostructures. In particular, for a LN metasurface with a $ Q $-factor of $\sim {8} \times {{10}^4}$, a 0.49% peak SH conversion efficiency is achieved at a pump intensity of ${3.3}\;{{{\rm kW}/{\rm cm}}^2}$. This suggests that such high $ Q $-factor LN metasurfaces may be good candidates for practical blue–ultraviolet light sources. Our work provides insight into the possible implementation of metadevices based on nanoengineering of conventional nonlinear crystals.

© 2021 Optical Society of America

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