Fully relativistic laser-induced ionization and recollision processes

Relativistic above-threshold ionization (ATI) and high-harmonic generation in the long pulse regime are investigated via the Klein-Gordon equation employing the strong-field approximation. The energy and angular spectra of the rescattering electrons for ATI in the relativistic low frequency tunneling regime are examined. While the relativistic drift significantly reduces the rescattering efficiency in conventional sinusoidal laser waves, the employment of laser pulses specially tailored as a strong attosecond pulse train (APT) allows to counteract the drift. The optimal conditions for high-energy rescattering are found in relation to the duration of the attosecond pulse, to the time delay between pulses in the APT, as well as to the spectral content of the APT. The stability of the harmonic yield vs deformations of the tailored pulse shape is shown. The proposed method renders energies of more than 1 MeV possible for the rescattering atomic electron.