Fig. 3: Experimental demonstration of QPM enabled by van der Waals stacking of 3R-MoS₂.

a Optical microscope image of a 3R-MoS₂ homo-structure, formed by stacking two 3R-MoS₂ flakes, with a twist angle of 60°. The blue (purple) filled area highlights a uniform area with a measured thickness of 576 nm (564 nm) from bottom (top) flake A (B). The orange filled area highlights the homo-structure region AB with a measured thickness of 1140 nm. Inset: Measured height vs distance profile along the red dashed line. b Measured (dots) and simulated (solid line) co-polarized SHG as a function of the polarization angle of incident laser, from three areas A (blue), B (purple) and AB (orange) of the 3R-MoS₂ homo-structures with twist angles of 60°. The homo-structure with 60° twist angle is shown in (a). In experiment, the polarization of the incident laser was initially set to be along the armchair direction of the flake and controlled by rotating the \(\lambda /2\) waveplate. A linear polarizer was used to select the polarization component of the SH radiation parallel to the polarization of the pump beam. c Measured (black dots) enhancement factor as a function of the stack number (N) on the stacked van der Waals homostructure and the simulated curve \(f\left(N\right)={N}^{2}\)(Navy). The error bars represent the measurement uncertainty (standard deviations) obtained from more than ten data takes.