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

We have experimentally demonstrated the generation of sub-half-cycle pulses at 10 $\mu \mathrm{m}$ through two-color filamentation in nitrogen. The absolute value of the carrier-envelope phase (CEP) of the 10 $\mu \mathrm{m}$ pulse is passively stabilized and controlled by the time delay between the two-color input pulses. The duration of the generated pulse is 13.7 fs, which corresponds to 0.402 cycles. The absolute value of the CEP of the sub-half-cycle pulse is consistent with a simple four-wave difference frequency generation model.