Multiple cutoffs in high harmonic generation via multi-XUV-photon absorption

High harmonic generation (HHG) is one of the most fundamental processes in the interaction of strong laser fields with atoms and molecules. Because of wide applications of HHG, for example, imaging atomic or molecular orbitals, visualizing chemical reactions, synthesizing a single attosecond pulse, the HHG attracts huge attentions in both theories and experiments. The HHG can be explained by the famous three-step model:first, the laser field bends the Coulomb potential and the electron tunnels out; second, the electron is accelerated in the laser field and gains kinetic energy; Third, the energetic electron recombines with the parent ion and release its energy as high energetic photons. The HHG can be tailored by controlling the each step. In this paper, we conceive a strategy to control the third step. We simulate the HHG when He+ is exposed to the combined few-cycle Ti-Sapphire (800 nm) IR femtosecond laser pulse and XUV laser pulse by numerically solving the time dependent Schrdinger equation. The simulation shows that after the electron tunnels out and gains energies from the infrared laser field, extra XUV photons may be absorbed during the electron and parent ion recombination, contributing multiple cutoffs separated by XUV photon energies in the high harmonic spectrum. This scenario is confirmed by time-delay-dependent HHG in the time-frequency representation, and by the power scaling of the cutoffs' intensities as a function of the XUV intensity.