Ben A. Schmid

Broadband terahertz study of excitonic resonances in the high-density regime in GaAs∕AlxGa1−xAs quantum wells

We report the first terahertz study of the intra-excitonic 1s-2p transition at high excitation densities in GaAs/AlGaAs quantum wells. A strong shift, broadening, and ultimately the disappearance of this resonance occurs with increasing density, after ultrafast photoexcitation at the near-infrared exciton line. Densities of excitons and unbound electron-hole pairs are followed quantitatively using a model of the composite terahertz dielectric response. Comparison with near-infrared absorption changes reveals a significantly enhanced energy shift and broadening of the intra-excitonic resonance.

Broadband THz study of excitonic resonances in the high-density regime

We report the first terahertz study of the intra-excitonic 1s-2p transition at high excitation densities in GaAs/AlGaAs quantum wells. A strong shift, broadening, and ultimately the disappearance of this resonance occurs with increasing density, after ultrafast photoexcitation at the near-infrared exciton line. Densities of excitons and unbound electron-hole pairs are followed quantitatively using a model of the composite terahertz dielectric response. Comparison with near-infrared absorption changes reveals a significantly enhanced energy shift and broadening of the intra-excitonic resonance.

Femtosecond terahertz studies of excitons

Excitons, Coulomb bound pairs of one electron with one hole, are among the most important elementary excitations in condensed matter physics. They are often seen as analogous to the hydrogen atom. However, a complex phase diagram due to intricate many-body interactions sets limits to this picture. Up to now, excitons have almost exclusively been investigated by optical techniques resonant to the band gap. Since these studies involve generation or annihilation of electron-hole pairs, they are only indirectly sensitive to existing populations. In contrast, femtosecond terahertz (THz) technology (1 THz ap 4.1 meV) has been advanced in recent years to directly trace the complex-valued dielectric response of low-energy resonances in extreme non-equilibrium systems (Huber et al., 2005). The internal quantum fine structure of excitons has thus become accessible (Kaindl et al., 2003). We exploit THz spectroscopy to test the quasi-particle concept of excitons in a direct, time-resolved way and explore new quantum optical processes.

Stimulated Terahertz Emission from Excitons in Cu 2 O

Using ultrashort THz pulses we observe stimulated emission between internal energy levels of excitons. Emission occurs in Cu2O due to the 3p to 2s transition at 6.6 meV, with a cross section of 10-14 cm2.

Resonantly excited high-density exciton gas studiedvia broadbandTHz spectroscopy

We report the first terahertz study of the intra-excitonic 1s-2p transition at high excitation densities in GaAs/AlGaAs quantum wells. A strong shift, broadening, and ultimately the disappearance of this resonance occurs with increasing density, after ultrafast photoexcitation at the near-infrared exciton line. Densities of excitons and unbound electron-hole pairs are followed quantitatively using a model of the composite terahertz dielectric response. Comparison with near-infrared absorption changes reveals a significantly enhanced energy shift and broadening of the intra-excitonic resonance.