Abstract

Excess LO phonons created by carrier cooling play an important role for the relaxation of hot electrons and holes in polar semiconductors. Recently, we have introduced a novel method for the direct study of hot phonons via transient changes of the intraband absorption of electrons /1/. Intraband absorption represents a three particle interaction of electron, photon and phonon, where the coupling to the lattice provides the conservation of the k-vector /2/. In our experiments, the electron gas in an n-doped sample is heated to a temperature of approximately 500 K by an intense infrared pump pulse. The resulting change of free carrier absorption was monitored by a weak probe pulse at the same wavelength. In InAs, a significant enhancement of intraband absorption was found, which relaxes on a time scale of several tens of picoseconds. The theoretical analysis of the data demonstrates, that this effect is mainly due to the occurrence of excess LO phonons, which couple in the absorption process and increase the absorption coefficient. In this paper, we discuss new frequency resolved measurements of the transient free carrier absorption in InAs and GaAs. Data taken at different wavelengths between 5 and 10 µm give detailed information on the spectral distribution of the excess phonons in k-space.

© 1990 Optical Society of America