Efficient second-harmonic generation in high Q-factor asymmetric lithium niobate metasurfaces

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 α 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 ∼8×10 4 , a 0.49% peak SH conversion efficiency is achieved at a pump intensity of 3.3 k W / c m 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.