Generation of Sub-Half-Cycle Mid-Infrared Pulses through Two-Color Filamentation

We have experimentally demonstrated the generation of sub-half-cycle phase-stable mid-infrared (MIR) pulses through two-color filamentation in nitrogen.Intense femtosecond pulses from a Ti:sapphire amplifier and the second harmonic pulses are focused into nitrogen at atmospheric pressure, and sub-halfcycle pulses with the carrier wavelength of 10.2 μm are generated through four-wave mixing processes.The carrier-envelope phase (CEP) of the MIR pulse is passively stabilized and controlled by the attosecond time delay between the two-color input pulses.The duration of the MIR pulse is 13.7 fs, which corresponds to 0.402 cycles.The standard deviation of the CEP instability is 0.0397π (124 mrad).The absolute value of the CEP of the generated sub-half-cycle pulse is consistent with an one dimensional fourwave difference frequency generation model.