Fig. 1. Photonic grating slab with a pair of BICs for enhancing SHG. (a) Photonic grating slab that contains TE-type and TM-type BICs. (b) Spatial distribution of the electric field of TE and TM modes in a unit cell at K_x = 0, respectively. (c) Band structure of the target TE and TM modes of the photonic grating slab with thickness t = 0.37 ·a, respectively. (d) K_x dependent quality factor of the TE and TM modes shown in (c), respectively.

Fig. 2. Spatial overlap coefficient and SHG efficiency with a homogeneous WS₂ on top of the photonic grating slab. (a) The K_x dependent spatial overlap coefficient ${\text{Z}}_{2\omega \mathrm{,}\omega }^{\text{TMD}}$. (b) The K_x dependent SHG efficiency monitored at the reflected (top) and transmitted (bottom) side, respectively. The blue star is a reference point for showing the SHG efficiency with a freely-standing WS₂ monolayer. The FW is incident from the top side of the grating, and the intensity of FW is set to be 0.1 GW/cm².

Fig. 3. Spatial overlap coefficient and SHG efficiency with a patterned WS₂ on top of the photonic grating slab. (a) The K_x dependent spatial overlap coefficient ${\text{Z}}_{2\omega \mathrm{,}\omega }^{\text{TMD}}$. (b) The K_x dependent SHG efficiency monitored at the reflected (top) and transmitted (bottom) side, respectively. The blue star is a reference point for showing the maximum SHG efficiency achieved in the case with homogeneous WS₂ monolayer. The FW is incident from the top side of the grating, and the intensity of FW is set to be 0.1 GW/cm².