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.