Polarization Shaping of Free‐Electron Radiation by Gradient Bianisotropic Metasurfaces

Free‐electron radiation phenomena facilitate enticing potential to create light emission with highly tunable spectra, covering hard‐to‐reach frequencies ranging from microwave to X‐ray. Consequently, they take part in many applications such as on‐chip light sources, particle accelerators, and medical imaging. While their spectral tunability is extremely high, their polarizability is usually much harder to control. Such limitations are especially apparent in all free electron based spontaneous radiation sources, such as the Smith−Purcell (SP) radiation. Here, anomalous free‐electron radiation phenomenon is demonstrated at the microwave regime from gradient bianisotropic metasurfaces, by using a phased dipole array to mimics moving charged particles. The phase gradient and the bianisotropy in metasurfaces provide new degrees of freedom for the polarization shaping of free‐electron radiation, going beyond the common spectral and angular shaping. Remarkably, the observed anomalous free‐electron radiation obeys a generalized SP formula derived from Fermat's principle. Polarization shaping of Smith−Purcell (SP) radiation is experimentally demonstrated through the use of gradient bianisotropic metasurfaces. If the phase gradient is nonzero, the anomalous SP radiation obeys the generalized SP formula derived from Fermat's principle, instead of the regular SP formula. The phase gradient and bianisotropy in metasurfaces can provide extra degrees of freedom for the simultaneous polarization, spectral, and angular shaping of anomalous free‐electron radiation.