Customizing 3D Polarization Structures at Second Harmonic Frequency

3D polarization structures have received growing interest due to their unique optical properties and extra information-carrying capacity. Although the second harmonic generation (SHG) from nonlinear optical metasurfaces has enabled advanced polarization control, the realization of 3D polarization structures has not been reported. This study proposes and experimentally demonstrates a nonlinear metasurface approach to simultaneously generate SHG beams and create 3D polarization structures. Upon the illumination of a linearly polarized fundamental wave (FW), the efficacy of this method is exemplified through the simulation and experimental demonstration of various SHG polarization structures ranging from rings to 3D polarization knots. The SHG polarization distributions on these structures can be further modulated by continuously changing the linear polarization state of the FW. The 3D polarization control with nonlinear metasurfaces offers a new approach to generate SHG beams with customized 3D polarization distributions, which are promising for high-dimensional optical information processing and encryption.