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.
全庞加莱光束（Full poincaré beams）作为一种特殊的矢量光场，其波前包含庞加莱球面上所描述的所有偏振状态。由于其特殊的性质，近年来在光通信、光学微操控、光学成像和偏量子信息处理等领域引起了广泛关注。但是该光束的产生及调控多集中在线性光学领域，而与非线性光学相结合的研究目前仍有待探索。
为此，厦门大学陈理想教授和佛山科学技术学院张莉博士等研究人员利用级联两块光轴正交的I型相匹配BBO晶体构建了全矢量特性的倍频效应，并实验实现了全庞加莱光束的倍频，即从红外全庞加莱光束倍频到可见光全庞加莱光束。他们通过分析倍频光场的空间矢量构造，揭示了倍频过程中低阶庞加莱光束可有效地转换为高阶庞加莱光束这一有趣的拓扑荷倍增效应，而光场的偏振奇异性，即C点和L线，却保持不变。相关研究结果发表于Chinese Optics Letters 2019年第17卷第9期（Li Zhang, et al., Full vectorial feature of second-harmonic generation with full Poincaré beams）。