Contactless terahertz probes of correlations and dynamics in low-dimensional electron-hole gases
1Dept. Physics, UC Berkeley and Lawrence Berkeley National Lab,
Berkeley, CA 94720 USA
2Dept Physics, UC Berkeley, Lawrence Berkeley National Lab, Universitaet Hannover
This is an abstract
for a presentation given at the
Foresight Conference on Molecular Nanotechnology
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.
 D. S. Chemla and J. Shah, Nature 411, 549 (2001) and references therein.
 R. A. Kaindl, et al Nature 423, 734 (2003).
This work was supported by the US Department of Energy, Office of Science, the Deutsche Forschungsgemeinschaft and the Alexander von Humboldt Foundation.
Robert A. Kaindl
Dept. Physics, University of California at Berkeley and Lawrence Berkeley National Laboratory
1 Cyclotron Road
Berkeley, CA 94720 USA
Phone: 510 486 5264 Fax: 510 486 5530