Polarization Engineering of Second-Harmonic Generation in 3R-MoS2 Waveguides.

Chip-scale transition metal dichalcogenide (TMDC) waveguides offer giant material nonlinearity for integrated photonics. However, research has predominantly focused on maximizing conversion efficiency, leaving the polarization mechanisms of the generated nonlinear signal largely unexplored. Here, we establish a comprehensive framework for engineering the polarization of second-harmonic generation (SHG) within 3R-MoS2 planar waveguides. By synergizing polarization-resolved measurements with theoretical modeling, we reveal that SHG polarization is governed by guided-mode interactions constrained by waveguide geometry and crystal symmetry, and continuously reshaped during propagation. We demonstrate that thickness-dependent modal confinement and in-plane crystal symmetry provide robust static control, while propagation length serves as a dynamic tuning knob for tailoring the nonlinear output. Our findings establish a deterministic approach for on-chip polarization engineering, opening new avenues for reconfigurable nonlinear light sources and quantum photonic circuits.