Mid-infrared diffuse reflection on ultrafast time scales.

This paper describes an instrument capable of studying diffuse reflection of mid-infrared (mid-IR) photons on ultrafast time scales. Femtosecond mid-IR pulses are generated by difference frequency mixing the output of an optical parametric amplifier that is pumped using a regeneratively amplified Ti:Sapphire laser. Time resolution is achieved by up-converting the diffusely reflected photons with pulses from the Ti:Sapphire oscillator. Experiments were performed on a series of powdered KBr samples containing varying amounts of carbon black. The results suggest that diffusely reflected mid-IR photons fall into two distinct categories. A small fraction of the photons travel relatively long effective path lengths (1.3-2.3 mm), while the majority traverse a much shorter distance (0.2-0.05 mm).