Electron-phonon quantum kinetics in the strong coupling regime

The dynamic nonequilibrium properties of a crystal electron coupled to a bath of dispersionless phonons are investigated for the strong coupling regime. The four-wave mixing experiments on the semiconductor ZnSe at T = 77 K use blue optical pulses of 13 fs in duration and give evidence for an oscillation with the phonon period and also evidence for an additional oscillation with half the phonon period. The latter is observed here for the first time and is an immediate consequence of the fact that, even for simultaneous emission of two phonons, coherence of an electron is preserved for some time. (C) 1999 Elsevier Science B.V. All rights reserved.