Relativistic calculation of nondipole effects in high-order harmonic generation

We present results of relativistic calculations of even-order harmonic generation from various atomic targets. The even-order harmonics appear due to the relativistic nondipole effects. We take these relativistic effects into account by using an approach based on the solution of the time-dependent Dirac equation. The spectra of the nondipole even harmonics look qualitatively similar to the spectra of the dipole harmonics obeying the same classical cutoff rule. The temporal dynamics of the formation of the nondipole harmonics is, however, distinctly different from the process of dipole harmonics formation. Even-order harmonics emission is strongly suppressed at the beginning of the laser pulse, and the emission times of the nondipole harmonics are shifted with respect to the bursts of the dipole emission. These features are partly explained by a simple modification of the classical three-step model which takes into account selection rules governing the emission of harmonic photons.