Generation of sub-two-cycle, microjoule pulses in the mid-infrared region is demonstrated. Fundamental and second-harmonic pulses of 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 , which is sufficiently broad for sub-single-cycle pulse generation. The pulse width was estimated to be , without any compressors, by cross-correlation frequency resolved optical gating. The output energy of more than a few microjoule is sufficient for spectroscopy.
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