Broadband (8.5-13.5 µm) intra-pulse difference frequency generation in a LiGaS2 crystal of a 90 fs 744 nm laser pulse after its continuous redshift in air.

Broadband frequency downconversion of a 90 fs 744 nm Ti:sapphire laser pulse into the mid-infrared (IR) was demonstrated via its filamentation-induced self-frequency shift in air and subsequent intra-pulse difference frequency generation in a LiGaS2 crystal. The filamentation of the laser pulse in air provided its continuous spectral broadening to the Stokes wing with spectral humps separated by ∼1000cm-1 that was appropriate for the laser pulse difference frequency conversion into the mid-IR. The difference frequency emission spectrum spanned from 8.5 to 13.5 µm at the e-2 level. The transform limited pulse duration of the mid-IR pulse was 47 fs, which corresponded to a 1.3-cycle laser pulse. Energy conversion efficiency was up to 10-4 and 5⋅10-4 without and with chirp compensation, respectively.