Generation of sub-two-cycle mid-infrared pulses by four-wave mixing through filamentation in air

Generation of sub-two-cycle, microjoule pulses in the mid-infrared region is demonstrated. Fundamental and second-harmonic pulses of $25\mathrm{fs}$ Ti:sapphire amplifier output were focused into the air to produce extremely broadband mid-infrared pulses by four-wave difference-frequency generation through the filamentation. The full width at half-maximum of the spectral bandwidth reaches one octave $(2.5–5.5\mathrm{\mu }\mathrm{m})$, which is sufficiently broad for sub-single-cycle pulse generation. The pulse width was estimated to be $13\mathrm{fs}$, without any compressors, by cross-correlation frequency resolved optical gating. The output energy of more than a few microjoule is sufficient for spectroscopy.