Control of harmonic generation using two-colour femtosecond-attosecond laser fields: quantum and classical perspectives

We investigate high-order harmonic generation in atoms and molecules using two-colour laser pulses: an infrared (IR) laser pulse of a few cycles superposed on an attosecond ultraviolet (UV) or vaccum ultraviolet laser Pulse. We adapt a standard semiclassical model to this process. We show that enhancement and additional control of harmonics via time delay between these pulses are possible because of a different ionization mechanism from the one-colour case. The electron is moved to the continuum via one-photon absorption of a UV photon (instead of tunnelling through the potential barrier) at a specific phase of the IR pulse (determined by the time delay between two pulses), with a non-zero initial velocity, leading to new possibilities for the control of harmonic generation.