Ultrashort Excitations in Semiconductors

Abstract

In keeping with the goals of the Advanced Study Institute, the following is a tutorial review of recent research accomplishments in the field of ultrashort excitations, and the subsequent return of the excited material to thermal equilibrium. The designation of “ultrashort” in the present context refers, for the most part, to events which last of the order of femtoseconds to picoseconds. Electrical circuits are inherently several orders of magnitude slower. Thus, observation of ultrashort events, directly in this time domain, had to await the development of ultrashort optical pulses. Even though readily useful laser sources are now only a decade or so old, the literature on the application of these laser systems is very large. No attempt is made here to cover a major part of the pertinent literature. I choose instead a subset of experiments, which illustrate the richness of the scientific results which have been obtained so far. Specifically, I trace the ultrashort dynamics in semiconductors of nonequilibrium distributions of optical phonons and polaritons in contact with thermal baths, with impurities, with, electronic carriers, or with each other. The discussion here includes the experimental apparatus whenever it is unique in some way, and I include discussions of the theoretical basis behind the experimentation, whenever available.