Stark-ladder transitions and their oscillator strengths in GaAs/AlAs superlattices

We have studied Stark-ladder transitions and their oscillator strengths in GaAs/AlAs superlattices with different miniband widths 2Δ by low-temperature photocurrent spectroscopy. The measured intensity ratio as a function of an electric field F of the nth-order Stark-ladder transition normalized by the zeroth-order transition shows a clear ${\mathit{F}}^{\mathrm{-}2\mathit{n}}$ dependence in the limit of high field. A ${\mathrm{\Delta }}^2$ dependence of the measured intensity ratio on the miniband width is also obtained. These results support the tight-binding theory of Bleuse, Bastard, and Voisin [Phys. Rev. Lett. 60, 220 (1988)].