Coherence and localization in superlattices under electric fields

Using optical techniques, we have demonstrated that the quantum coherence of electron states in GaAsGaAlAs superlattices under electric fields increases drastically as the period, D, and the electric field, ∄, decrease, reaching at least ten periods at zero field when D = 60 A ̊. The electric field localizes the states, breaking the superlattice minibands into Stark ladders, and producing a transition from three- to quasi-two-dimensional excitons. These effects are manifested, respectively, by the appearance of oscillations in the absorption coefficient, which are periodic both in energy and in ∄^-1, and by a sharp increase in the intrawell exciton binding energy.