Emission of a narrowband terahertz pulse from a radial plasma oscillator

Laser-driven plasma oscillation is a useful mechanism for generation of a very strong, narrowband, tunable terahertz (THz) wave from a plasma. The difficulty of using the plasma oscillation as a THz source is in that the plasma wave, which is a typical form the plasma oscillation exists, does not couple well with the radiation in vacuum. Several ideas have been proposed to achieve the coupling, e.g. linear mode conversion [1], Cherenkov wake [2], and superluminal plasma wave [3]. From our team, we have taken a totally different approach by generating an isolated, oscillating-in-phase bunch of electrons, that is, a plasma oscillator (in contrast to the plasma wave). The localized oscillator can readily emit a radiation just from multipole radiation mechanism. One of such methods is generating a plasma dipole oscillator (PDO) by colliding two detuned laser pulses in a plasma [4]. In this presentation, we introduce another novel idea, the radially oscillating plasma oscillator (RPO). RPO can be created on a density gradient of a plasma that is irradiated by a bi-frequency laser pulse. At the resonant position on the density gradient where the beat frequency matches the local plsma frequency, a local, strong plasma wave is excited. The accompanying radial oscillation acts as a planar antenna. We report kinetic simulation results and theoretical analysis of the idea. 

[1] Wang et al. PRL 132, 165002 (2004)

[2] Yoshii et al. PRL 79, 4194 (1997)

[3] Pukhov et al. PRL 127, 175001 (2021)

[4] Kwon et al. Sci. Rep. 8, 145 (2019).