Polarization is an important parameter to characterize the property of the light beams. Previous researches are mainly focused on the homogeneous polarization beams. Recently, vectorial optical fields with inhomogeneous polarization states have attracted great interest for their unique properties in both quantum and classical physics. Until now, a variety of methods have been proposed to generate the vectorial optical fields, such as metasurfaces, photoalignment liquid crystals, q-plates, and spatial light modulators and so on. However, most studies are all carried out in the linear optical realm and few attention are paid on the generation of nonlinear vectorial beams.

Simulated and experimental results of fundamental frequency and second harmonic Taiji (left) and clover (right) pattern

In this paper, we propose a method to realize nonlinear frequency conversion of the vectorial optical fields, which is by using a Mach-Zehnder interferometer. The two arms of the Mach-Zehnder interferometer enable nonlinear frequency conversion of the horizontal and vertical polarizations of the vectorial optical fields at the same time. In our experiment, nonlinear frequency conversion of fundamental frequency vectorial optical field is studied. To illustrate the flexibility of the method proposed here, nonlinear Taiji and clover patterns with different polarization states are also realized. This study proposes a more flexible method to realize nonlinear frequency conversion of the vectorial optical fields and may have potential applications in the generation of ultrafast vectorial optical field.

This research was published in “Hui Li, Haigang Liu, and Xianfeng Chen, Nonlinear frequency conversion of vectorial optical fields with a Mach-Zehnder interferometer, Applied Physics Letters, 114, 241901 (2019).”