Mid-Infrared Diffuse Reflection on Ultrafast Time Scales
Diffuse reflection (DR) occurs when light penetrates a translucent scattering solid (e.g., a powder comprised of a finely ground crystalline material; e.g., KBr) and is re-emitted after undergoing multiple instances of transmission and reflection. The image on the cover is a schematic of an instrument designed to study DR in the mid-IR region of the spectrum on ultrafast time scales (from ~100 fs). A regeneratively amplified Ti:Sapphire laser pumps and optical parametric oscillator (OPA). Femtosecond mid-IR pulses are generated by difference frequency mixing the signal and idler outputs of the OPA. These are directed toward a sample (KBr powder mixed with varying amounts of carbon black) by an off-axis parabolic mirror (OAP). The diffusely reflected photons are collected by a second OAP and subsequently focused in a nonlinear crystal, AgGaS2 (AGS). A small portion of the Ti:Sapphire pump laser (~150 µJ) is passed through a variable optical delay and then overlapped spatially and temporally with the diffusely reflected mid-IR photons in the AGS crystal. This serves as an optical “gate” by producing the sum frequency only when both are present in the crystal simultaneously. By scanning the optical delay of the gate, the temporal profile of the diffusely reflected light is produced. Because of diffuse reflection, a pulse that is launched into the sample emerges with a “tail” of trailing photons that have traversed a longer distance due to repeated cycles of transmission/reflection. This is shown schematically in the lower part of the cover and is also clearly evident in the data. However, increasing the fraction of carbon black (a strongly absorbing analyte) reduces the observed effect.