Generation of High Harmonics driven by Synchronized Ultra-Short UV and Mid-Infrared Pulses

We theoretically investigate high harmonic generation (HHG) in helium gas by employing a strong mid-infrared (MIR) field in conjunction with a delayed ultra-short UV pulse. A few-cycle 240-nm UV pulse is utilized to trigger low-energy electron emission via a multi-photon absorption process, while a 2 𝜇m MIR pulse drives the subsequent recollision dynamics. At pulse intensities where the ultra-short UV pulse governs the initial ionization step, we achieve selectivity of recollision trajectories in the time evolution of the long-wavelength IR pulse. We systematically analyze the main characteristics and potential control of the HHG emission spectrum by varying the time delay between the UV and MIR pulses. A simple semi-classical model is employed to interpret our numerical findings.