Direct and indirect excitons in coupled GaAs/Al0.30Ga0.70As double quantum wells separated by AlAs barriers

Heavy-hole and light-hole excitons in symmetric, coupled GaAs/Al0.30Ga0.70As double quantum wells have been investigated by photoluminescence and photoluminescence excitation spectroscopy in magnetic fields B less than or equal to 13 T. The excitonic states in these structures can be controlled by varying the width of the AlAs barrier, which separates the GaAs quantum wells. The ground-state exciton binding energy shows a highly nonlinear dependence on the AlAs barrier width which originates from the redistribution of the electron and hole wave functions by the barrier. The barrier width also controls the spatial character of the ground-state excitons. In the case of relatively wide barriers (five monolayers of AlAs) the Coulomb interaction mixes symmetric and antisymmetric single-particle states and generates exciton states, which have predominantly direct or indirect character. With increasing magnetic field the enhancement of the exciton binding energy is stronger for the direct than for the indirect exciton, which leads to an enhanced splitting of indirect and direct excitons as well as to a strong redistribution of oscillator strength from the indirect to the direct exciton. For narrow barriers (one monolayer), on the other hand, spatially direct and indirect character are strongly mixed in all exciton states, leading to a very similar magnetic-field dependence of the energies and oscillator strengths of equivalent exciton states.