Control of Coulomb-mediated excitonic 2s emission by an external magnetic field

We report on time-resolved photoluminescence (PL) studies of exciton dynamics in undoped GaAs quantum wells in the presence of an external magnetic field. Subsequent to pulsed interband excitation, the photogenerated excitons are manipulated by applying time-delayed THz pulses from the free-electron laser (FEL) FELBE at the Helmholtz-Zentrum Dresden-Rossendorf. Simultaneous time and wavelength resolved spectroscopy using a synchroscan streak camera allows us to investigate the emitted PL with high detection efficiency [1]. If the FEL photon energy is tuned into resonance with the intra-excitonic 1s-2p transition, the PL originating from the 1s exciton is suppressed by the incident THz pulse and subsequently recovers to a value higher than the reference trace. Furthermore, the THz pulse leads to pronounced enhancement of the PL intensity at the energy of the excitonic 2s and 2p states. Since radiative recombination is dipole forbidden for the 2p state, this PL contribution is attributed to 2s emission [2], thus confirming earlier theoretical predictions of efficient Coulomb scattering between the nearly degenerate 2s and 2p states which allows for rapid 2p-2s population transfer [3].
In this presentation, we demonstrate the efficient manipulation of the 2p-2s scattering efficiency via an external magnetic field, which tunes the 2s and 2p exciton energies from resonant to non-resonant since these energies change differently with magnetic field. This leads to efficient suppression of the THz induced 2s PL signal upon increasing the magnetic field from 0 T to 2.5 T. These results demonstrate that the underlying Coulomb-mediated 2p-2s scattering process can be magnetically controlled. Our experiments confirm theoretical calculations which will also be discussed.