Successive Wannier-Stark localization and excitonic enhancement of intersubband absorption in a short-period GaAs/AlAs superlattice

The authors have investigated the electro-optical absorption properties of intersubband excitonic transitions as a function of an electric field F applied parallel to the growth-axis of a short-period GaAs/AlAs (0 0 1) superlattice by photocurrent spectroscopy. Direct evidence is presented for the evenly spaced Stark ladder spectra up to the 3rd order and for a successive excitonic enhancement of the oscillator-strengths preceding from the transition associated with higher order Stark-ladder states due to the gradual Wannier-Stark localization. The intensity ratio normalized by the 0th order transition indicates a clear F-2n dependence of the tunneling-assisted transitions between localized hole-states and nth order ladder electron-states in good agreement with the recent tight binding theory.