Enhancement of second harmonic generation in two-dimensional monolayer materials empowered by merging BICs of photonic crystals

We investigate second harmonic generation (SHG) in two-dimensional (2D) monolayer materials on silicon photonic crystals by van der Waals contacts. By finely tuning the thickness of the photonic crystal, merging bound states in the continuum (BICs) are realized, which have more confined local fields and higher Q factors compared with the sole BICs at the same in-plane wavevector. Efficient SHG in 2D monolayer materials is achieved at relatively low pump intensities, and the conversion efficiency of SHG is several orders of magnitude larger than that on photonic crystals of isolated BICs. For typical transition metal dichalcogenide monolayers, such as MoS2, the maximum SHG conversion efficiency achieves 1% at a pump intensity of 104 kW/cm2, which is approximately five orders of magnitude greater than that on a photonic crystal of isolated BICs. The results indicate that merging BICs in resonators possess important applications in the enhancement of nonlinear responses in 2D monolayers.