Dresden 2003 – wissenschaftliches Programm
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HL: Halbleiterphysik
HL 39: Ultrakurzzeitph
änomene I
HL 39.2: Vortrag
Donnerstag, 27. März 2003, 11:15–11:30, BEY/81
Time resolved THz response of excitons and free carriers. An new look on exciton dynamics. — •Daniel Hägele1,2, Robert A. Kaindl1, Marc A. Carnahan1, and Daniel S. Chemla1 — 1Department of Physics, University of California at Berkeley and Materials Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, CA 94720 — 2for correspondence: Institute for Solid State Physics, University of Hannover, Appelstraße 2, D-30167 Hannover, Germany
The exciton response in optically excited semiconductors was extensively explored in recent years by photoluminescence and pump probe techniques with photon energies around the band-gap energy. In contrast, studies of transitions between internal exciton states - which typically occur at a few THz - remain scarce [1]. We reveal the dynamic interplay of excitons and free carriers in a 14nm GaAs/AlGaAs multiple quantum well by measuring the THz absorption and refractive index changes with ps resolution after creation of carriers with an optical pump at or above the exciton energy. The internal 1s-2p transition of excitons is observed in the THz absorption spectrum as a peak at 1.7 THz (7 meV). Temperature ionization of excitons in a hot lattice manifests itself in a transient reduction of the 1s-2p peak, an increasing (Drude like) background of free carrier absorption, and a negative change of the refractive index. The opposite process of exciton formation is observed for initially hot carriers in a cold lattice. The dynamic transformation of conducting carriers into charge neutral low energy excitons will be discussed.