Exploring the mode conversion of a vector vortex beam in second-harmonic generation using a periodically poled nonlinear photonic KTiOPO₄ crystal

The nonlinear interactions between structured beams with spatially varying polarization and matter extend the boundaries of fundamental nonlinear optics, opening new avenues for applications across various fields. In this work, the second-harmonic generation (SHG) of vector beams with two orthogonal polarization components of vortices in a periodically poled crystal (PPC) is demonstrated both theoretically and experimentally. Potassium titanyl phosphate KTiOPO 4 (KTP) is chosen as the PPC for its excellent nonlinear-optical properties. Notably, for a given vector light field, adjusting the initial phase of the vector beam allows for the varying outputs of SHG. In addition, SHG outputs with one, two, or three vortices are achieved by rotating a polarizer to modulate the fundamental light, and the proportion weights of the three vortex terms within the SHG are dependent on the rotating angles of the polarizer. Our findings provide a foundational understanding and offer new insights into the study of full-field selection rules in specific nonlinear processes within superlattices.