Twist-dependent second harmonic generation mediated by interfacial coupling in a vertical MoS ₂ /WS ₂ heterostructure

Second harmonic generation (SHG) in twisted van der Waals heterostructures is conventionally interpreted as the coherent superposition of nonlinear contributions from the constituent monolayers. Herein, the twist-angle-dependent SHG response is systematically investigated in vertically stacked MoS 2 /WS 2 heterostructures. Polarization-resolved SHG measurements reveal that the nonlinear response cannot be described by simple superposition of monolayer contributions. A physical model incorporating an interfacial nonlinear susceptibility term is proposed to account for interlayer coupling effects. The model, which includes coherent interference between monolayer and interface contributions, achieves excellent agreement with the experimental data ( R 2 =0.9908) across twist angles ranging from 20° to 100°. Compared to the classical model, the modified model exhibits an F-factor of 45.39 with a p-value below 10 −7 , confirming that interlayer coupling is physically authentic rather than an experimental artifact. This work provides a quantitative framework for engineering nonlinear optical properties in twisted van der Waals heterostructures.