Vector beams with the space-variant polarization distribution in the transverse plane provide powerful capabilities for applications in diverse areas of science and technology, such as high-resolution imaging, laser processing. From a historical perspective, research on vector beams or rather the special category of cylindrically symmetric vector beams could be traced back to the early 1960s, when E. Snitzer put forward the concept of the vectorial optical field. Thereafter, Dieter Pohl published the original experimental work that opened up the research on vector beams. In recent years, as an important research topic, numerous spatial-variant polarization vector beams, e.g., radial, azimuthal, and spiral polarization distributions, have been reported. To date, several methods have been proposed to produce the vector beams in the infrared-visible regime in linear optical field. By using the intracavity axicon, birefringence of the gain media, conical pump beam, or geometric phase control, laser cavities can be customized to generate cylindrical vector beams. The external cavity techniques to generate vector beams have been demonstrated by means of an interference approach or by using π-cylindrical lens mode converter, q-plates, spatial liquid modulators, photo-patterned liquid crystal, digital micro-mirrors device, and optical fibers.

In this paper, we propose a method to achieve ultraviolet waveband vector beams by using solid-state crystals through nonlinear optical processes. In our experiment, the ultraviolet vector beams can be achieved by using sum-frequency generation processes of infrared vector beams and a frequency doubled Gaussian beam. We experimentally verify the generation of vector beams in infrared and ultraviolet regime with topological charge varying from 1 to 3. And the polarization states of the generated ultraviolet vector beams are measured in the simulated and experimental environments as shown in Fig. 1. Besides, the relative mode weightage of the orthogonally components of the generated ultraviolet vector beams is measured. Finally, we measure the conversion efficiency during the nonlinear optical processes. The proposed method paves the way for the generation of extreme ultraviolet vector beams with solid-state nonlinear crystal, which is a more compact and stable way compared with gas nonlinear method. This work also opens a route for the photolithographic techniques and laser processing based on ultraviolet-waveband vector beams.

FIG. 1. The odd rows represent the simulated intensity profiles of the ultraviolet vector beams with different topological charges after passing through the Glen-Taylor prism. The corresponding experimental results are shown in even rows.

This research was published in "Hui Li, Haigang Liu, Yangfeifei Yang, Ruifeng Lu and Xianfeng Chen. Ultraviolet waveband vector beams generation assisted by the nonlinear frequency conversion. Applied Physics Letters 119, 011104 (2021)”.

Link：https://aip.scitation.org/doi/full/10.1063/5.0053781