Broadband circularly polarized terahertz source based on free-electron radiation

Recently, metasurfaces have been employed to manipulate free-electron radiation, rendering versatile light sources with unique functionalities. In this Letter, we propose a scheme to generate broadband circularly polarized (CP) radiation through the interaction of free electrons with stacked bianisotropic metasurfaces. The bianisotropy in metasurfaces provides new degrees of freedom for the polarizability shaping of free-electron radiation, extending beyond the common spectral and angular shaping. By simply layered stacking, the 3-dB axial ratio (AR) bandwidth spans the entire first-order diffraction of the Smith-Purcell radiation (SPR). The underlying mechanism is elucidated through detailed multipole analysis, encompassing the electric, magnetic, and toroidal dipoles and quadrupoles. Moreover, Stokes parameters are employed to characterize the polarization state of the emitted radiation. These findings provide an alternative pathway toward the development of compact, tunable, and high-power terahertz free-electron radiation sources.