Temporal Structure of Attosecond Pulses from Intense Laser-Atom Interactions

We find that the high harmonics have a power-law spectrum $I_{\omega }\sim {\omega }^{-3.3\pm 0.25}$ in a wide frequency domain starting at the ionization potential $I_p$ and down to the plateau beginning. Our spectrotemporal analysis of the emitted radiation displays clear bowlike structures in the $(t,\omega )$ plane. These “bows” correspond to Corkum’s reencounters of the freed electron with the atom. We find that the bows are not filled and thus cannot be due to any bremsstrahlung. Rather, it is a resonant process that we call stimulated recombination (SR). It occurs when an electron with momentum $p$ reencounters the incompletely ionized atom, and interferes with itself still remaining in the ground state. The SR leads to a highly efficient resonant emission at $\hslash \omega =p^2/2m+I_p$ in the form of attosecond pulses. The SR relies on a low level of ionization and strongly benefits from the use of few-cycle laser pulses.