Twist-Dependent In-Plane Symmetry Breaking in the WSe2/Black Phosphorus Interface

The control of optical responses and electronic states via the twist angle, "twistronics", has revolutionized the research field of van der Waals heterostructures. Despite extensive advances in twisted interfaces composed of materials sharing identical or similar crystal symmetries, twist-dependent phenomena at symmetry-mismatched interfaces hosting unconventional moiré superlattices and symmetry breaking remain poorly understood. Here, we report twist-dependent in-plane symmetry breaking at a symmetry-mismatched heterointerface between WSe2 and black phosphorus. Employing polarization-resolved second-harmonic generation (SHG) microscopy, we characterize the systematic evolution of in-plane symmetry breaking in WSe2/black phosphorus interfaces by varying the twist angle. We propose the twist-dependent in-plane polarization as a possible interpretation, supported by theoretical model calculations and their good agreement with the observed evolution of the SHG response. Our findings extend twistronics to symmetry-mismatched interfaces, offering a strategy to engineer nonlinear optical responses and interfacial polarization in low-symmetry van der Waals systems.