Direct Generation of Radially Polarized X-Shape Terahertz Pulse via Focusing Cherenkov Radiation

Radially polarized X-shaped terahertz (THz) pulses (RPXTPs) have unique properties such as spatio-temporal localization, self-reconstruction, nondispersion, and superluminality, making them attractive for various applications. However, the direct generation of RPXTPs is a challenging task that has not yet been achieved. We propose a set of easily implementable schemes to directly generate RPXTPs using either a pencil beam or a ring beam of electrons to drive a modified axicon. Cherenkov radiation is excited in the modified axicon in the THz region and then focused in the forward region, allowing direct generation of RPXTPs. We demonstrate the generation of single-cycle broadband RPXTPs with tunable spectral range, focusing depth, and pulse intensity. The field strength reaches tens of MV/m, much higher than that obtained by most available indirect methods. This high-intensity RPXTP has potential applications in THz spectroscopy, THz imaging, light-matter interactions, and THz-driven particle accelerators.