Vectorial beams have attracted great interests due to their broad applications in optical micromanipulation, optical imaging, optical micromachining, and optical communication. All methods to generate nonlinear vector beams are conducted in two steps: the first step is to generate the vectorial optical fields by using linear optical method, and the second step is to perform nonlinear transformation processes. However, these methods are not integrated and compact in experimental setups. We propose a method to generate nonlinear vector beams by using a nonlinear fork grating to simplify the experimental setups.

Fig. 1. Schematic of the experimental setup. Inset: shows the microscope image of fork grating etched on the LN surface.

Fig. 2. Stokes parameters of second harmonic (SH) vectorial light field. Columns 1, 3, and 5 represent theoretical results, and columns 2, 4, and 6 are the corresponding experimental results.

Fig. 3. SH vector beams of different initial phase with topological charge l=1, 2, and 3.

Fig. 4. The variation of generated SH vector beams power (P2) as the incident FF beams power (P1) increases with topological charge (a) l=1, (b) l=2, (c) l=3.

We have successfully realized the generation of SH vectorial optical fields of different topological charge l=1, 2, 3 by using nonlinear crystal etched with fork grating. The experimental setup is shown in Fig. 1. We analyze the vectorial properties of generated SH vector beams by Stokes parameters, as shown in Fig. 2. We can see that the experimental results are in good agreement with the theoretical predictions. As depicted in Fig. 3, we rotate the axis direction of the HWP2 to change the initial phase and manipulate the polarization states of the generated vector beams. We measure the polarization states of generated SH vector beams by a polarizer to verify the flexibility of the proposed method. In theory, we can obtain arbitrary linearly polarized vector beams by rotating the axis direction of the HWP2 and changing topological charge of nonlinear fork grating. Moreover, we also measure the nonlinear frequency conversion efficiency of our method of generating SH vector beams, which is shown in Fig. 4. We can see that the intensity of the generated SH vector beams increases with the FF beams as a quadratic function. And the intensity of generated SH vector beams will decrease slightly with the topological charge increasing. The conversion efficiency is about 5.09%〖 W〗^(-1) cm^(-2). The proposed method provides a new way to generate nonlinear vector beams by using microstructure of nonlinear crystal, which may also be applied in other nonlinear processes and promote all optical waveband applications of such vector beams.

This research was published by “Qian Yang, Yangfeifei Yang, Hao Li, Haigang Liu, and Xianfeng Chen, Nonlinear generation of vector beams by using compact nonlinear fork grating, Photonics Research, 12(5), 1036-1043 (2024)”.

Link: https://doi.org/10.1364/PRJ.515731