Twist- and Interface-Controlled Nonlinear Optical Interference in Janus MoSSe Bilayers

Featuring intrinsic out-of-plane dipoles and broken inversion symmetry, Janus transition metal dichalcogenides (TMDs) provide a unique platform for exploring nonlinear twistronics at the two-dimensional limit. However, how the macroscopic nonlinear polarization is governed by hierarchical modulation, where the atomic interfacial configuration acts as the primary structural switch and the twist angle provides secondary phase tuning, remains experimentally unresolved in polar Janus homostructures. Here, we construct Janus MoSSe artificial homostructures with precise angular control using a pristine transfer technique and systematically elucidate their coherent superposition mechanisms in nonlinear optical processes via polarization-resolved SHG spectroscopy. We show that in conventional MoSSe/SSeMo bilayers with an S–Se van der Waals interface the SHG response strictly follows a symmetry-governed vector superposition principle, enabling continuous modulation from constructive to destructive contributions through twist-angle tuning. More importantly, in anomalous MoSSe/SeSMo bilayers featuring a Se–Se interface with antiparallel out-of-plane dipole alignment, the SHG signal is profoundly suppressed. This pronounced quenching originates from the restoration of inversion symmetry in the Se–Se stacked configuration, which enforces the cancellation of second-order nonlinear polarizations within the electric-dipole approximation. These results demonstrate an effective hierarchical modulation of the nonlinear optical response in Janus materials enabled by interface engineering (Se–Se vs Se–S stacking) and angular control. More broadly, this work elucidates the physical origin of interlayer nonlinear optical interference in polar van der Waals systems and establishes Janus MoSSe homostructures as a versatile platform for tunable nonlinear photonics governed by dual degrees of freedom: twist angle and interfacial configuration.