Unveiling the transverse formation length of nonlinear Compton scattering

TL;DR

This paper investigates the transverse formation length in nonlinear Compton scattering, revealing that a moving laser focus can significantly enhance quantum interference effects, impacting radiation emission predictions.

Contribution

It introduces the concept of transverse formation length in nonlinear Compton scattering and shows how a flying focus laser beam can amplify its effects.

Findings

Transverse formation length is typically of the order of the Compton wavelength.

Moving focus in a flying focus beam enhances TFL effects proportionally to the number of laser cycles.

Enhanced TFL effects can substantially alter differential emission probabilities.

Abstract

The process of emission of electromagnetic radiation does not occur instantaneously but it is "formed" over a finite time known as radiation formation time. In the ultrarelativistic regime, the corresponding (longitudinal) formation length is given by the formation time times the speed of light and controls several features of radiation. Here, we elucidate the importance of the transverse formation length (TFL) by investigating nonlinear Compton scattering by an electron initially counterpropagating with respect to a \emph{flying focus} laser beam. The TFL is related to the transverse size of the radiation formation "volume" and, unlike the longitudinal formation length, has a quantum origin. Being the TFL typically of the order of the Compton wavelength, where any laser field can be assumed to be approximately uniform, related quantum interference effects have been ignored. However, we show analytically that if the focus in a flying focus beam with $n_L\gg 1$ cycles moves at the speed of light and backwards with respect to the beam propagation direction, the effects of the TFL undergo a large enhancement proportional to $n_L$ and may substantially alter the differential emission probability for feasible flying focus pulses.