Date of this Version
PHYSICAL REVIEW A 94, 013402 (2016)
Ionization by relativistically intense laser pulses of finite duration is considered in the framework of strongfield quantum electrodynamics. We show that the resulting ionization spectra change their behavior from the interference-dominated oscillatory pattern to the interference-free smooth supercontinuum, the latter being the main focus of this paper. More specifically, when studying the energy distributions of photoelectrons ionized by circularly polarized and short pulses, we observe the appearance of broad structures lacking the interference patterns. These supercontinua extend over hundreds of driving photon energies, thus corresponding to high-order nonlinear processes. Their positions on the electron energy scale can be controlled by changing the pulse duration. The corresponding polar-angle distributions show asymmetries which are attributed to the radiation pressure experienced by photoelectrons. Moreover, our time analysis shows that the electrons comprising the supercontinuum can form pulses of short duration. While we present the fully numerical results, their interpretation is based on the saddle-point approximation for the ionization probability amplitude.