Focussing effects in laser-electron Thomson scattering

TL;DR

This paper investigates how laser pulse focussing influences the spectral characteristics of Thomson scattered radiation, highlighting the dominant role of pulse duration and phase effects, especially in ultra-short and tightly focused regimes.

Contribution

It introduces a comprehensive model combining paraxial and sub-cycle vector beam descriptions to analyze focusing effects on Thomson scattering spectra.

Findings

Temporal envelope dominates spectral effects over focussing in most cases

Ultra-short pulses cause blue shifts and broadening of emission harmonics

Focusing to sub-wavelength spots mimics effects of sub-cycle pulses

Abstract

We study the effects of laser pulse focussing on the spectral properties of Thomson scattered radiation. Modelling the laser as a paraxial beam we find that, in all but the most extreme cases of focussing, the temporal envelope has a much bigger effect on the spectrum than the focussing itself. For the case of ultra-short pulses, where the paraxial model is no longer valid, we adopt a sub-cycle vector beam description of the field. It is found that the emission harmonics are blue shifted and broaden out in frequency space as the pulse becomes shorter. Additionally the carrier envelope phase becomes important, resulting in an angular asymmetry in the spectrum. We then use the same model to study the effects of focussing beyond the limit where the paraxial expansion is valid. It is found that fields focussed to sub-wavelength spot sizes produce spectra that are qualitatively similar to those from sub-cycle pulses due to the shortening of the pulse with focussing. Finally, we study high-intensity fields and find that, in general, the focussing makes negligible difference to the spectra in the regime of radiation reaction.