High efficiency and tunable second harmonic generation in electro-optic Q-modulated metasurface with etchless lithium niobate

Second harmonic generation (SHG) at nanoscales have broad applications in integrated optoelectronic devices. In this paper, we numerically demonstrate highly efficient and electrically tunable SHG in metasurface with etchless lithium niobate (LN), based on symmetry-protected bound states in the continuum (BIC). Since the excellent electro-optic (EO) properties of LN, the symmetry of structural refractive index can be very slightly broken under an external voltage, resulting in the excitation of quasi-BIC with an ultrahigh quality factor ($Q$ factor). When asymmetry parameter is set to be $\Delta n=0.001$, a quasi-BIC with $Q$ factor as high as $5.4\times 10^7$ magnitude is excited, achieving 1.2% conversion efficiency of SHG under 0.1 kW/cm${}^2$ input intensity. We also show that the $Q$ factor and conversion efficiency can be actively and finely modulated by tuning asymmetry factor $\Delta n$ based on EO effects of LN. Our work provides a feasible idea to excite quasi-BIC with ultrahigh and accurately tunable $Q$ factor, and paves a way in designing electrically tunable ultracompact light sources.