Polarization-controlled efficient second harmonic generation in etchless lithium niobate photonic crystals with quasi-bound states in the continuum.

We numerically investigate a high-Q resonant and etchless lithium niobate on insulators (LNOI) nanostructure based on the quasi-bound states in the continuum (q-BICs) to achieve highly efficient second harmonic generation (SHG). The proposed structure consists of an etchless -cut thin film LN with an integrated silica photonic crystal on top. Due to the strong field confinement within the LN given by the q-BICs, the SHG efficiency is as high as 0.67% when the pump intensity is as low as 100 kW/cm. Meanwhile, owing to the very high Q-factor of the q-BICs, an ultra-narrow nonlinear process with the full width at half maximum less than 0.1 nm is also observed, indicating excellent monochromaticity. Besides, the results show that under the vertical incidence of linearly polarized light, the dual-band SHG supported by the structure is verified to be polarization-controlled. The proposed nanostructure exhibits good sensing performance, which could provide valuable insights for the development of sensors in various fields. Our scheme not only avoids the challenge associated with etching LN thin film but also achieves highly efficient nonlinear frequency conversion. It is suitable for various nonlinear effects, including sum-frequency generation, difference-frequency generation, and high-order harmonics.