Quantum dynamics in semiconductors studied via ultrafast optical techniques

The quasiparticle model is one of the fundamental concepts in many-body physics. It is usually too complicated to regard each isolated ("bare") particle and to take into account its interactions with all the other individual components of the system. More physical insight is gained by introducing new units which are composed of the bare particle plus some average surrounding. Usually, these quasiparticles are assumed to form instantly. However, this picture turns out to be valid only on timescales which are long compared to the oscillation cycle of the collective mode of the system. Very recently, the quantum dynamical phenomena which occur during the formation of interparticle correlations in systems far from thermal equilibrium have become accessible. As a first example, the dynamics of an isolated charge coupled to a highly polar crystal lattice via the Frohlich interaction is discussed. The experimental access employed is femtosecond transmission spectroscopy of interband transitions The second part of the contribution deals with carrier-carrier interactions. The ultrafast buildup of Coulomb screening and collective behavior in an electron-hole plasma of a density of 2 × 10¹⁸ cm^-3 photogenerated in GaAs within 10 fs are demonstrated.