Relativistic high-order harmonic generation
Abstract A relativistic generalization of the strong-field-approximation theory of high-order harmonic generation is presented. Numerical results for the harmonic spectra obtained by shining a strong laser pulse on multiply charged ions are analysed. Harmonic generation is explained using the tunnelling-propagation-recombination model. Relativistic effects are important in the intermediate phase of this process and lead to a suppression of both low-order and ultrahigh-order harmonic generation due to the magnetic-field-induced drift, which prevents the return of the electron to the nucleus. For higher laser frequencies and ionization potentials the non-tunnelling mechanism of generation of high harmonics with energies below the ionization potential becomes important. These harmonics are due to inner-atom dynamics, they are emitted with a much higher efficiency, and relativistic effects are less important. We analyse the difference between the plateau heights of the tunnelling and nontunnelling harmonic spectra as a function of the ionization potential and the laser intensity.