Intersubband electro-absorption and retardation in coupled quantum wells: the role of interface scattering

In this work we present a systematic experimental study aimed at resolving the various contributions to electro-optical modulation in a multiple coupled quantum wells structure. Using a set of eight-cross/parallel polarizer–analyzer measurements we were able to resolve the spectral dependence of the DC electric-field-induced absorption and phase-retardation due to intersubband transitions. The results of our experiment were fitted to a model that allows all quantum properties of the structure to vary with the external DC electric field and estimate the contribution of each term to the overall modulation. The experimental results suggest that, apart from the Stark shift of the energy levels, a major contribution to electro-optical modulation comes from line width modulation. We propose a model that correlates this effect with alloy disorder and interface roughness scattering that gives rise to electron dephasing. The larger degree of electron localization near the interfaces in the presence of a DC electric field is responsible for this effect.