Watching bare charges get dressed in an ultrabroadband THz experiment

Abstract

The concept of quasiparticles is an essential tool to describe interacting many-body systems. A dense electron-hole plasma in a semiconductor constitutes a prime example: Detailing the ensemble in terms of isolated charge carriers which interact with all individual particles via the bare Coulomb potential would be far too complex. An intuitive description accounts for the fact that each charge attracts a polarization cloud of opposite sign forming a dressed quasiparticle. These new units interact via a dynamically screened potential. However, the model is justified only if the carrier distributions in momentum space are slowly changing. Over the last decade, there has been an intense discussion on how to treat Coulomb interactions during ultrafast relaxation of a many-body ensemble in the extreme nonequilibrium regime. Hot carriers excited in a semiconductor by a femtosecond light pulse are an important model system for these questions. It has been supposed that immediately after generation of an electron-hole plasma no screening of the Coulomb interaction should exist. Quantum kinetic calculations predict that many-body correlations dressing up bare charges develop on an ultrashort time scale of the order of the inverse plasma frequency [1,2].