Ultrabroadband mid-infrared noncollinear difference frequency generation in a silver thiogallate crystal

We report the generation of ultrabroadband mid-infrared (mid-IR) pulses by noncollinear difference frequency mixing. The signal and the idler output beams of an optical parametric amplifier are combined in a silver thiogallate crystal (AgGaS 2 ) to generate mid-infrared radiation. We show that a noncollinear geometry facilitates broadband phase matching. Spectral bandwidths up to 1750  cm -1 were obtained at an external noncollinear angle of 4.2 deg, which is more than three times broader than in a collinear geometry. The broadband spectrum is tunable in the range of 1500-4500  cm -1 . Pulse energies up to 1 μJ were achieved. The broadband pulses were used in sum frequency generation in ZnSe and in vibrational absorption spectroscopy experiments of liquid samples.