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 For the case of ultra-short pulses, where the paraxial model is no longer valid, we adopt a sub-cycle vector beam description of the field.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.
Highlights
There is currently a great deal of interest in the development of compact, tunable and well-collimated radiation sources
While there have been a number of works on individual aspects of focusing effects in classical Thomson scattering, and in quasiclassical Compton scattering (e.g., Refs. [18,19,20]), the time is ripe for a thorough study of how the structure of a focussed pulse alters the properties of the emitted radiation. (We note some promising techniques for tackling the fully quantum case, see Ref. [21].) In this work we aim to provide this by systematically analyzing the effects of the laser pulse focusing on the Thomson emission spectra
By comparing the spectra obtained using a paraxial field with a plane wave model we found that, in all but the most extreme focusing, the temporal envelope has a much bigger effect on the spectrum than the focusing itself
Summary
There is currently a great deal of interest in the development of compact, tunable and well-collimated radiation sources. Such sources have applications in a wide range of areas including X-ray radiography [1], medical and biological imaging [2], and in the study of ultrafast molecular processes. Laser-electron setups are much more compact than traditional alternatives such as undulator magnets and magnetic synchrotron rings, widening the range of potential applications. They show a great deal of promise due to the consistent, exponential increase in peak focal intensities over the past 30 years [3]. With the development of a number of new facilities such as the Vulcan 20 PW upgrade [4] and the Extreme Light Infrastructure (ELI) Facility [5] this trend is expected to continue into the foreseeable future
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