Generation of tunable mid- and far-infrared pulses during gas ionization by a chirped two-color laser field

We propose and investigate a method for generating tunable and phase-controllable mid- and far-infrared pulses in gas ionized by an intense two-color laser field composed of the chirped fundamental and its second-harmonic pulses with group time delay. The generation frequency equals the difference between the second-harmonic and the doubled fundamental frequencies and is continuously tunable by varying chirp or time delay. The duration of the generated pulses is determined by the ionization duration, which is much shorter than the ionizing pulse and is controlled by laser-pulse stretching or changing its intensity. Our quantum-mechanical calculations and analytical description show that this method can provide a wide tuning range spanning from several to more than a hundred THz using femtosecond lasers.