Investigation of Nonequilibrium Many-Body Effects in Semiconductors

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The optical properties of intrinsic semiconductors in the spectral vicinity of their fundamental absorption edge are determined by the interaction between photons and elementary excitations with the material. One of the most important processes is the absorption of a photon by means of simultaneous excitation of an electron from the originally full valence band into the originally empty conduction band. Conduction-band electrons and valence-band holes are quantum- mechanical quasi-particles. The aim of this proposal is an experimental and theoretical study of nonequilibrium properties of a high-density elecron-hole-pair system, which is generated resonantly by femtosecond laser excitation. The behavior of a high-density nonthermal carrier distribution in semiconductors is very complex and presently little is known. The studies will use state-of-the-art femtosecond laser and will focus on obtaining information on the following topics: (1) how effective is the screening attributable to a nonequilibrium electron-hole distribution, and how does it differ from the screening of a quasi-thermal equilibrium distribution; (2) how does one describe the dynamics of screening associated with the evolution from nonequilibrium to quasi-equilibrium carrier distributions; (3) what is the time scale for population thermalization, and when do the many-body Coulomb effects become dominant; (4) under what conditions is it possible to observe optical gain (non-equilibrium gain), and what are the relevant time scales to obtain a population inversion. Using laser pulses of approximately 10 fs, the dynamics of the semiconductors will be followed from the initial nonequilibrium state to quasi-thermal equilibrium. The semiconductors that will be investigated include CdS (or CdSe), GaAs, and GaAs-A1GaAs multiple quantum wells.