Speaker
Description
Recently, electron accelerator in terahertz band has been suggested as a next-generation, compact accelerator [1]. Compared to conventional RF, higher frequency THz wave considerably increases the threshold of field strength for the vacuum breakdown, making much stronger accelerating field gradient available. However, there has been no good enough idea for generation of powerful, narrowband terahertz wave suitable for particle acceleration. Laser-driven plasma can serve as a powerful source of terahertz (THz) radiation. Unlike solid-based materials, which have inherent limitations of material damage, plasma—being in a broken-down state—can sustain extremely strong driving laser pulses. However, efficiently converting laser energy into lower-frequency THz radiation using plasma remains a challenging problem. Most plasma-based THz generation schemes utilize plasma as a reservoir of charge carriers (i.e., electrons). Typically, laser pulses induce a single-pass drift motion of electrons, leading to the emission of low-frequency radiation in the form of a half-cycle or a few-cycle pulse. Approaches such as two-color schemes and coherent transition radiation (CTR) fall into this category. Alternatively, plasma oscillations—specifically, plasma waves—can act as radiation sources. However, since plasma waves typically propagate more slowly than electromagnetic waves in a vacuum, their coupling efficiency is limited. To enhance coupling, methods such as mode conversion, magnetization, and density gradients have been proposed, yielding promising results. In this presentation, I introduce a different approach to utilizing plasma oscillations as a THz source. Instead of generating plasma waves, this method localizes plasma oscillations into harmonic-like oscillators. Electrons oscillating in-phase within a finite-sized plasma oscillator can emit radiation via a multipole antenna mechanism [2]. These localized plasma oscillators enable the generation of narrowband THz radiation with remarkably high efficiency. In an optimized case, a conversion efficiency approaches 1%, with a spectral narrowness of less than 5%, and a high output field strength reaching several GV/m. [1] D. Zhang et al., “Segmented terahertz electron accelerator and manipulator (STEAM)”, Nat. Phys. 12, 336 (2018). [2] M. Kumar et al., “Narrowband Terahertz Emission from a Plasma Oscillator Imbedded in a Plasma Density Gradient”, Phys. Rev. Lett. 134, 015001 (2025).
