Time-resolved picosecond infrared absorption induced by high-density photogenerated carriers in Ge and CdSe

Picosecond time-resolved absorption at 2.2, 3.4, and 3.9 μm produced by photogenerated carriers in intrinsic Ge and semi-insulating CdSe has been measured at room temperature. For Ge, the peak absorbance changes as the square of the probe wavelength. The absorption profile as a function of delay time relative to the excitation pulse rises with a resolution-limited risetime (< 10 ps) and decays over the range from several hundred picoseconds to several nanoseconds. These observations can be accounted for by free-carrier absorption and Auger recombination. Intervalence transitions make a small contribution (∼ 17 percent) to the induced absorption only at 2.2 μm. For CdSe, the absorption change with time exhibits a more complex profile. Two temporal peaks occur for probing wavelengths at 3.4 and 3.9 μm and three peaks occur at 2.2 μm. The first peak occurring at about 5 ps after excitation for all three probe wavelengths with a resolution-limited risetime is due to free-carrier absorption. The other peaks have been attributed to transitions involving impurity (defect) levels by trapped photogenerated carriers.