XUV generation with several-cycle laser pulse in barrier-suppression regime

We study XUV generation with several-cycle laser pulses of intensity up to 10¹⁵ W cm^-2 using numerical solution of the 3D Schrödinger equation for a hydrogen atom. Ionization of the atom mainly takes place in a barrier-suppression regime for such driving intensities. We find that in this regime XUV yield stops growing with the laser intensity and then even essentially decreases. The calculated yield dependence on the laser intensity agrees well with predictions of our theory. The latter shows several factors that lead to the decrease of the XUV yield in the barrier-suppression regime. The calculated cut-off in the XUV spectrum is displaced to slightly lower energies than those predicted by the I_p + 3.17U_p law. The change in the cut-off can be due to the non-zero initial velocity of the electron detached under the barrier-suppression ionization. We find that essential population of both the free wave packet returning to the origin and the ground state at the instant of the return is required for effective XUV generation. Rapid ground state depopulation leads to shortening of the attopulse train generated by the two-cycle laser pulse when laser intensity increases. In particular, we find that the cut-off XUV generated under essential barrier-suppression with sine-like laser pulse provides an isolated attopulse, while two attopulses are generated for lower laser intensity.