Highly robust and efficient second harmonic generation device based on lithium niobate

Boosting the efficiency of second harmonic generation based on subwavelength lithium niobate films has long been a hot topic of concern. However, it is difficult to achieve steep sidewalls and high edge resolution during the fabrication of lithium niobate nanoresonators, leading to devices with poor performance. Here, we propose a second harmonic generation (SHG) device based on lithium niobate that is robust against fabrication errors. The SHG effect is significantly enhanced by constructing the Brillouin zone folding induced bound states in the continuum (BZF-BIC) in the lithium niobate nanoresonators. The device has a high conversion efficiency of 2.26% under only $2\text{\ensuremath{-}}\mathrm{kW}/{\mathrm{cm}}^{2}$ pump intensity, which, to the best of our knowledge, is the highest second harmonic generation efficiency at the lowest pump intensity. This efficiency is maintained despite fabrication errors with sidewall tilt angles of 82--86\ifmmode^\circ\else\textdegree\fi{} and pore size errors of \ifmmode\pm\else\textpm\fi{}15 nm. Furthermore, the device demonstrates high second harmonic generation efficiency in a large range of incident angles due to the high quality factor of BZF-BIC in the entire Brillouin zone. Our approach can be applied in various fields, including nonlinear optics, sensing, and optical communications.