Dual bound states in the continuum enhanced second harmonic generation with transition metal dichalcogenides monolayer

The emergence of two dimensional (2D) materials has opened new possibilities for exhibiting second harmonic generation (SHG) at the nanoscale, due to their remarkable optical response related to stable excitons at room temperature. However, the ultimate atomic-scale interaction length with light makes the SHG of Transition Metal Dichalcogenides (TMDs) monolayers naturally weak. Here, we propose coupling a monolayer of TMDs with a photonic grating slab that works with doubly resonant bound states in the continuum (BIC). The BIC slabs are designed to exhibit a pair of BICs, resonant with both the fundamental wave (FW) and the second harmonic wave (SHW). Firstly, the spatial mode matching can be fulfilled by tilting FW's incident angle. We theoretically demonstrate that this strategy leads to more than four orders of magnitude enhancement of SHG efficiency than a sole monolayer of TMDs, under a pump light intensity of 0.1 GW/cm2. Moreover, we demonstrate that patterning the TMDs monolayer can further enhance the spatial overlap coefficient, which leads to an extra three orders of magnitude enhancement of SHG efficiency. These results demonstrate remarkable possibilities for enhancing SHG with nonlinear 2D materials, opening many opportunities for chip-based light sources, nanolasers, imaging, and biochemical sensing.