Transformation process from quantum beats of miniband excitons to Bloch oscillations in aGaAs∕AlAssuperlattice under applied electric fields

Abstract

We have investigated the coherent dynamics of excitonic wave packets from the viewpoint of the electric-field-induced transformation from the minibands to the Wannier-Stark (WS) localization in a GaAs $(6.8\phantom{\rule{0.3em}{0ex}}\mathrm{nm})∕\mathrm{Al}\mathrm{As}$ $(0.9\phantom{\rule{0.3em}{0ex}}\mathrm{nm})$ superlattice embedded in a $p\text{\ensuremath{-}}i\text{\ensuremath{-}}n$ diode structure with the use of a reflection-type pump-probe technique. It is clearly confirmed that the coherent-dynamics profile changes from the quantum beat of the miniband excitons to the Bloch oscillation in the WS localization with an increase in an applied bias voltage, producing an internal electric field. The key finding is that the Bloch oscillation with the frequency of ${\ensuremath{\nu}}_{\mathrm{BO}}=2eFD∕h$ appears in a weak-localization regime in addition to the ordinary Bloch oscillation with ${\ensuremath{\nu}}_{\mathrm{BO}}=eFD∕h$ in a strong-localization regime, where $F$ is the electric field and $D$ is the superlattice period. The experimental results of the coherent dynamics are reasonably explained by the electric-field-strength dependence of the excitonic-transition energies observed by electroreflectance spectroscopy and that of the localization profiles of the envelope functions of the electron and hole states calculated by a transfer-matrix method.