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HL: Fachverband Halbleiterphysik

HL 60: Poster II: Optical Properties, incl. Photonic Crystals and Ultrafast Phenomena

HL 60.35: Poster

Donnerstag, 25. März 2010, 18:00–20:00, Poster D1

Resonant generation of coherent LO phonons in optically excited biased quantum wells — •Thomas Papenkort¹, Tilmann Kuhn¹, and Vollrath Martin Axt² — ¹Institut für Festkörpertheorie, Universität Münster, Wilhelm-Klemm-Str. 10, 48149 Münster, Germany — ²Institut für Theoretische Physik III, Universität Bayreuth, 95440 Bayreuth, Germany

We study the generation of coherent and incoherent phonons in a quantum well. When two exciton lines are simultaneously excited with a short laser pulse, a quantum beat with a frequency determined by the splitting of the two lines is created. If spatial symmetry is broken by an external electric field, this leads to an oscillating polarization along the growth direction which in turn drives LO phonons. At the same time the electric field can be used to tune the splitting energy due to the quantum confined Stark effect. Experiments by Kojima et al. (Phys. Rev. B 70, 233306 (2004)) have shown that the generation of coherent phonons is resonantly enhanced when the splitting is tuned to the LO phonon energy. We have performed numerical simulations in the density matrix formalism which reproduce the resonant enhancement. As the splitting energy approaches the LO phonon energy, relaxation by emission of incoherent phonons becomes important. Our model includes this process on a quantum kinetic level, i.e. without using the Markov approximation. We find that although incoherent phonons are dominant in terms of the energy transferred to the lattice, this does not hinder the resonant generation of coherent phonons.