Resonant generation of coherent LO phonons by charge oscillations in a biased quantum well

Papenkort T, Kuhn T, Axt VM

Abstract

A theoretical analysis of the generation of coherent and incoherent LO phonons after optical excitation of an electrically biased quantum well is presented. We show numerical calculations based on the density-matrix formalism where both coherent and incoherent phonons are included on a fully quantum kinetic level. Bulk phonon modes are used as is appropriate for the wide quantum well studied here. The generation of coherent phonons is resonantly enhanced when two exciton lines with the splitting tuned to the phonon energy are simultaneously excited; such a tuning is achievable by the quantum-confined Stark effect, where the exciton lines are shifted by varying the electric bias. However, there is also a strong emission of incoherent LO phonons due to intersubband transitions of the excited carriers; even in the resonant case this by far dominates the energy transfer to the lattice. The different contributions of the impulsive and the resonant coherent phonon generation mechanism are discussed. A good agreement with recent experiments is found.