The process of emission of electromagnetic radiation does not occur instantaneously, but is “formed” over a finite time known as the 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 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. Since the TFL is 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.

• Received 1 September 2020
• Revised 18 December 2020
• Accepted 22 December 2020

DOI:https://doi.org/10.1103/PhysRevA.103.012215

Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article's title, journal citation, and DOI. Open access publication funded by the Max Planck Society.

Published by the American Physical Society