Coherently controlled ionization of gases by three-color femtosecond laser pulses

Photoionization of atoms and molecules by intense femtosecond laser pulses is a fundamental process of strong-field physics. Using a three-color femtosecond laser scheme with attosecond phase control precision, we demonstrate coherently controlled ionization of nitrogen molecules with a modulation level up to 45% by varying the phase shifts between the fundamental laser frequency at 800 nm and its second and third harmonics. Furthermore, the phase dependence of the ionization degree qualitatively changes with the laser intensity ratios between the three colors. The observations are interpreted as a manifestation of the competition between different parametric channels contributing to the ionization process. Such coherent control of ionization opens different ways to finely tune and optimize various phenomena accompanying laser-material interactions: high-order harmonic and attosecond generation, nanofabrication, remote ablation of samples, and even guidance of discharge and control of lightning by lasers.