Cavity-enhanced polarization-independent frequency conversion for vector beams

Vector beams with spatially varying polarization, represented on the higher-order Poincaré sphere, are indispensable in optical communications, imaging, and quantum information science. However, nonlinear frequency conversion of vector beams remains challenging due to sensitivity and limited conversion efficiency. Here, we demonstrate a highly efficient, external-cavity-enhanced, polarization-independent frequency conversion based on difference-frequency generation for vector beams. Benefiting from cavity enhancement within the Sagnac configuration, we achieve external quantum efficiencies of 43.85% and 16.83% for vector beams with <i>l</i>=1 and <i>l</i>=2, respectively, demonstrating efficient continuous-wave frequency conversion. The preservation of the vectorial polarization structure in the converted beams is verified by spatial Stokes measurements. This work demonstrates an efficient approach for wavelength manipulation of vector beams and is potentially relevant to applications in structured-light control and nonlinear photonic interfaces.