138 / 2023-04-14 16:55:36

Coherent subcycle optical shock from superluminal plasma wake

subcycle pulse,coherent radiation,superluminal plasma wake,attosecond pulse

极端光下的基础物理学 > 辐射源

High-energy subcycle radiation pulses are useful for many applications. For many applications, it is desired to have high intensity, tunable frequency, stable carrier envelope phase (CEP), and foremost a simple setup. However, to our best knowledge, currently there is not such a radiation source that possesses all these desired properties simultaneously. We report a new regime of generating subcycle pulses by simply directing a relativistic electron beam (REB) into a plasma with slowly varying density up-ramp[1]. We show that the sheath electrons flowing along the boundary of the REB-induced plasma wake emit strong spontaneous radiations at the rear of

the bubble (ROB), and they are coherently overlapped at the Cherenkov angle when the wake phase velocity is superluminal. This new coherence mechanism[2] does not require bunching the radiating particles in a spatial region smaller than the radiation wavelength as that in existing coherent radiation sources[3]. Three-dimensional (3D) particle-in-cell (PIC) and far-field time-domain radiation simulations confirm that an isolated subcycle pulse in the form of an optical shock is generated at the rear of a superluminal wake bubble induced by the REB. In particular, such radiation has many interesting and unique features. It is CEP stable, as well as of subcycle attosecond-scale duration. It has also excellent directionality, namely at the Cherenkov angle with very small angular divergence, and high intensity that scales with the square of the propagation distance. In addition, the frequency can be readily tuned by adjusting the densities of the plasma and REB.