Free-electron laser spectroscopy of exciton dynamics in GaAs quantum wells

Intraband excitation of optically injected carriers using strong mid-infrared or THz pulses provides interesting opportunities to study carrier and exciton dynamics in semiconductor quantum wells (QW). In particular, owing to the odd parity of the dipole operator, such intraband excitation transfers excitons located in radiative states into dark (or interband-forbidden) states as long as the symmetry of the QW ensures parity conservation. This property yields spectroscopic access to specific dark states, giving rise to resonant enhancement of intraband absorption as well as suppression of the photoluminescence (PL), and allows us to investigate associated dynamical processes.
We will report on time-resolved PL studies, where we investigate the dynamics of excitons and electron-hole plasma subsequent to pulsed irradiation using the free-electron laser (FEL) at the Helmholtz-Zentrum Dresden-Rossendorf. The FEL is tuned into resonance with, respectively, intraexcitonic and intersubband transitions. Specifically, Figs. 1(a)-(b) demonstrate the effect of FEL excitation of radiative excitons in resonance with the 1s-2p transition, located at 9 meV for 8.2 nm wide high-quality QWs (sample 1). Besides quenching and recovery of the 1s PL, the salient feature in Fig. 1(b) is the appearance of PL originating from the 2s exciton. As direct excitation from 1s into 2s is dipole forbidden, the latter has to be the consequence of rapid scattering from the 2p into the 2s state, which nicely confirms earlier theoretical predictions. This 2s PL is specific for resonant 1s-2p excitation and just seen in high-quality quantum wells. Figs. 1(c) and (d) show PL transients for FEL excitation at 173 meV, in resonance with the electron intersubband transition for 6 nm wide GaAs QWs (sample 2). Typically, the PL recovery after the FEL pulse obeys the same time constant as the initial PL formation after NIR excitation, and its value after recovery exceeds the reference value since the population transferred into nonradiative states eventually returns into the radiative ones, such that the time-integrated PL emission is conserved.