Electron-phonon quantum kinetics in the strong-coupling regime

The quantum kinetics of optically excited crystal electrons coupled to a bath of dispersionless phonons is studied in the regime of strong electron-phonon coupling. Four wave-mixing experiments on a thin film of bulk ZnSe using 13 fs blue pulses give evidence for corresponding effects that are distinctly different from those found for GaAs which has proven to be a model system for the weak-coupling regime. The experimental results on ZnSe are compared with solutions of a simple model Hamiltonian of electron-phonon interaction suitable for both GaAs and ZnSe. Exact analytic solutions of this model for arbitrary electron-phonon coupling strengths are compared with numerical solutions of the same model following the usual approach in terms of an infinite hierarchy of correlation functions. Virtues and limitations of the simple model Hamiltonian with respect to the experiment are discussed.