Many-body correlations and excitonic effects in semiconductor spectroscopy

The optically excited system of electronic excitations in semiconductor nanostructures is analyzed theoretically. A many-body theory based on an equation-of-motion approach for the interacting electron, hole, photon, and phonon system is reviewed. The infinite hierarchy of coupled equations for the relevant correlation functions is systematically truncated using a cluster-expansion scheme. The resulting system of equations describes the optical generation of semiconductor quasi-particle configurations with classical or quantum mechanical light sources, as well as their photon-assisted spontaneous recombination. The theory is evaluated numerically to study semiclassical and quantum excitation under different resonant and non-resonant conditions for a wide range of intensities. The generation of a correlated electron-hole plasma and exciton populations is investigated. It is shown how these states can be identified using direct quasi-particle spectroscopy with sources in the terahertz range of the electromagnetic spectrum. The concept of quantum-optical spectroscopy is introduced and it is predicted that semiconductor excitation with suitable incoherent light directly generates quantum-degenerate exciton states. The phase space for this exciton condensate is identified and its experimental signatures are discussed. (c) 2007 Elsevier Ltd. All rights reserved.