High-energy ionization by relativistically intense laser pulses

High-energy ionization by relativistically intense, $30~\text{fs}$ laser pulses is investigated within the relativistic strong-field approximation. It is shown that the momentum distribution of photoelectrons is focused around a spiral, the analytical form of which is derived from the saddle-point analysis of probability amplitudes. The geometry of this spiral allows one to clearly interpret the exact numerical results of the ionization probability distributions and to identify the photoelectron momentum domains in which the interference- and the interference-free patterns are observed. It is also demonstrated that attosecond single-electron wave packets can be created in this process. Thus, it opens new research opportunities for the four-dimensional electron diffraction experiments, which can be conducted in large-scale laser facilities such as ELI or XCELS.