Resonant enhancement of second harmonic generation in etchless thin film lithium niobate heteronanostructure

Lithium niobate has received interest in nonlinear frequency conversion due to its wide transparency window, from ultraviolet to mid-infrared spectral regions, and large second-order nonlinear susceptibility. However, its nanostructure is generally difficult to etch, resulting in low-Q resonance and lossy nanostructures for second harmonic generation. By applying the concept of bound states in the continuum, we performed theoretical and experimental investigations on high-Q resonant etchless thin-film lithium niobate with SiO2 nanostructures on top for highly efficient second harmonic generation. In the fabricated nanostructured devices, a resonance with a Q factor of 980 leads to the strong enhancement of second harmonic generation by over 1500 times compared with that in unpatterned lithium niobate thin film. Although the pump slightly deviates from central resonance, an absolute conversion efficiency of 6.87×10−7 can be achieved with the fundamental pump peak intensity of 44.65 MW/cm2, thus contributing to the normalized conversion efficiency of 1.54×10−5 cm2/GW. Our work establishes an etchless lithium niobate device for various applications, such as integrated nonlinear nanophotonics, terahertz frequency generation, and quantum information processing.