Polarization Shaping of Free‐Electron Radiation by Gradient Bianisotropic Metasurfaces

Abstract

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.

A Terahertz Vortex Beam Emitter With Tunable Topological Charge and Harmonic Excitation

“…Similar to the interest in OAM, Smith-Purcell radiation (SPR) also intrigues scientists and engineers in recent years and has been employed to achieve various functional sources [23][24][25][26]. In 2019, the helix tape is utilized to generate the vortex SPR for the first time with two possible values (±1) of topological charge (TC), while the possibility of generating the higherorder topological charge (i.e., ±2, ±3, …) is not discussed [27].…”

High-Performance Asymmetric Optical Transmission Based on a Dielectric–Metal Metasurface

“…Fortunately, the emergence of metamaterial provides an appealing alternative to control electromagnetic wave manipulations properties [12][13][14][15][16][17], and the discovery of the AT phenomenon based on metamaterial was first experimentally demonstrated in the microwave region by Fedotov et al in 2006 [18]. Since then, various AT devices based on artificial structures have been proposed which use photonic crystals [19,20], subwavelength asymmetric gratings [21][22][23][24], chiral metamaterials [25][26][27] and metasurfaces [28][29][30], and the operation wavelengths have been covered from microwave to visible light [31][32][33].…”

Broadband Janus Scattering from Tilted Dipolar Metagratings

“…The manipulation of the electromagnetic (EM) wave exploited by metamaterial has stimulated a huge interest in wave optics and microwave technology. [ 1–6 ] As the 2D counterparts of metamaterial, metasurfaces have realized many nontrivial and unprecedented functionalities, such as anomalous reflection or refraction, [ 7–12 ] asymmetric transmission, [ 13–17 ] focusing, [ 18–21 ] and absorption. [ 22–24 ] In contrast to traditional refractive optics, metasurfaces can introduce an effective refractive index and dispersion that are determined mainly by the shapes and arrangements of nanostructures, rather than the material's composition.…”