**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 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. 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 can be ignored. However, we show analytically that if the focus in a flying focus beam with nL≫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 nL and can substantially alter the emission spectrum for feasible flying focus pulses.

**A. Di Piazza, “Unveiling the transverse formation length of nonlinear Compton scattering”, Phys. Rev. A 103, 012215 (2021).**

https://journals.aps.org/pra/abstract/10.1103/PhysRevA.103.012215

Related to Project B02