Full Poincaré (FP) beams, possessing inhomogeneous polarization covering the entire surface of Poincaré sphere in the cross section of light, have attracted tremendous attentions in optical communication, optical micromanipulation, optical imaging, and quantum information processing. Until now, the generation and manipulation of FP beams were merely implemented within the frame of linear optics. Whereas, the manipulation of FP beams via nonlinear optical processes has not yet been fully explored, which, however, may greatly enrich the applications of FP beams.

The research group led by Professor Lixiang Chen from Xiamen University and Dr. Li Zhang from Foshan University realized the frequency conversion from an infrared FP beam to a visible FP beam, based on vectorial second-harmonic generation with two cascading type-I phase-matching beta barium borate crystals of orthogonal optical axes. They visualized the structured features of vectorial SHG fields and revealed the interesting doubling effect of polarization topological index, i.e., a low-order FP beam was converted to a high-order one, while keeping the polarization singularities of C-points and L-lines both invariant. These results are reported in Chinese Optics Letters Vol. 17, No. 9, 2019 (Li Zhang, et al., Full vectorial feature of second-harmonic generation with full Poincaré beams).

"This work could offer a deeper understanding on the interaction of vectorial light fields with media, and can be extended to other nonlinear optical effects." The lead author Xiaodong Qiu says. The research group will investigate the other nonlinear optical effects such as spontaneous parametric down-conversion (SPDC) further with the FP beams, and explore their full vectorial features for potential high-dimensional quantum information applications.

Schematic diagram of vector frequency multiplication with full Poincaré beams.