Generation of terahertz beam with longitudinally varied polarization state via coherent superposition based on...

Polarization is an important dimension in the research and applications of light waves. However, traditional polarization optics often only focus on the polarization characteristics in the transverse plane. Here, we demonstrate a new scheme for the generation of longitudinally varied polarization state in terahertz beam using all-silicon metasurface. We employ wavefront transformation designs with long-focal-depth for orthogonal circularly polarized terahertz waves, achieving varied amplitude and phase along the propagation direction in opposite spin states. Based on the principle of coherent superposition of polarized waves, different linear and elliptical polarization states are obtained in transverse planes along the propagation path, with variable ellipticity and azimuth angle. Simulation results show that a large-scale evolution of the elliptical polarization azimuth angle from 45° to -60° and ellipticity from 20° to -74° can be observed within a focal depth range of 0.45-0.8 mm. We also intuitively display the helical trajectory of the polarization state from left-hand elliptical ones to right-hand elliptical ones within the focal depth range, using the Poincaré sphere. This work expands the application of metasurface devices for multifunctional polarization devices and can be applied to polarization generation and transformation for optical imaging or terahertz communications.