Ultrabroadband XFROG of few-cycle mid-infrared pulses by four-wave mixing in a gas

Cross-correlation frequency-resolved optical gating (XFROG) based on four-wave mixing (FWM) in a gas medium is shown to enable dispersion-free characterization of few-cycle mid-infrared (mid-IR) pulses tunable within a spectral range of more than two octaves. The FWM XFROG technique is used to measure spectra and pulse shapes, as well as to retrieve the phase of a few-cycle output of difference-frequency generation (DFG) tunable from 3 to 11 μm. With Ti:sapphire laser pulses used as a reference, this FWM process maps the entire tunability range of the DFG source, spanning over more than two octaves, onto a wavelength region of only 50 nm in the visible, allowing convenient XFROG measurements and revealing the reshaping of few-cycle mid-IR field waveforms by molecular rovibrational modes.