Control of intra-excitonic scattering in semiconductor quantum wells by an external magnetic field

We report on the internal dynamics of excitons in high-quality GaAs quantum wells and on the control of intra-excitonic transitions by an external magnetic field. The free-electron laser FELBE in Dresden is ideally suited for selective excitation of intra-excitonic transitions, since it provides intense, spectrally narrow transform-limited terahertz pulses in a unique continuous pulse train, which also allows us to use a synchroscan streak camera system [1]. Subsequent to the production of excitons by pulsed interband excitation, we resonantly pump the 1s-2p intra-excitonic transition which is located at around 2 THz. Coulomb-mediated transfer from the optically "dark" 2p to the radiative 2s state and relaxation into the fundamental 1s state is investigated by time-resolved photoluminescence involving the 1s and 2s excitonic levels [2]. In particular, applying an external magnetic field strongly affects the observed behavior. Detailed analysis of the experimental behavior based on a newly developed microscopic theory allows us to demonstrate the remarkable impact of magnetic fields on the Coulomb and terahertz interactions in the excitonic system, which occurs as a consequence of magnetically induced changes of excitonic orbitals and energetic detuning of excitonic levels [3]. As an interesting application, we also discuss the possibility of observing terahertz gain induced by intra-excitonic transitions.
[1] J. Bhattacharyya et al., Rev. Sci. Instrum. 82, 103107 (2011)
[2] W. D. Rice et al., Phys. Rev. Lett. 110, 137404 (2013)
[3] J. Bhattacharyya et al., Phys. Rev. B 89, 125313 (2014)