Confinement of carriers in nanostructures entails strong modifications of their physical properties, offering a well-defined laboratory for investigating the complex many-body interactions between charge, lattice and spin degrees of freedom . In particular, electron-hole (e-h) gases photoexcited into low-dimensional semiconductors are characterized by new optical transitions and strongly enhanced Coulomb interactions. Terahertz radiation offers a unique tool to measure low-energy excitations and transport properties in nanostructures without the need for an electrical contact. Along this path, we have recently developed a new, sensitive scheme to probe time-varying Coulomb correlations in confined carrier plasmas.
Here, we discuss experiments that utilize this pulsed terahertz source to probe the dynamical interplay of bound and unbound e-h pairs on a picosecond timescale . A new low-energy oscillator is observed directly after resonant creation of heavy-hole excitons in GaAs quantum wells. This peak arises from transitions between the hydrogen-like exciton bound states, most notably the 1s-2p level transition. The terahertz field probes excitons in a large range of in-plane momenta K in contrast to the usual restriction of interband probes close to K ~ 0. Owing to the strongly correlated motion of electrons and holes, charge-neutral excitons are electrically insulating up to a frequency that matches the separation between their lowest internal states. Above-bandgap excitation at elevated temperatures however induces unbound e-h pairs which represent a conducting ionized gas with a Drude-like response.
The distinct responses of these extreme phases enable us to follow in time a metal-insulator transition that occurs upon formation of excitons out of a gas of unbound e-h pairs, as well as its reverse process of ionization. These are dynamical transitions which occur on different timescales. Ionization of excitons can occur within only a few picoseconds, and depends on the phonon occupation. Exciton formation is a more complex process with several timescales involved: the first is the surprising, quasi-instantaneous appearance of an excitonic peak in an otherwise conducting state after excitation of unbound pairs. On a much longer timescale of several 100 ps, this complex mixture evolves into an insulating exciton gas. Our results demonstrate the suitability of terahertz pulses as a contactless measure of correlations and dynamics, motivating their use for studies of fundamental processes in novel nanoscale materials.
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This work was supported by the US Department of Energy, Office of Science, the Deutsche Forschungsgemeinschaft and the Alexander von Humboldt Foundation.