Excited-state indirect excitons in GaAs quantum dot molecules

We demonstrate the fabrication of strain-free and widely adjustable GaAs quantum-dot molecules (QDMs) by filling of droplet etched nanoholes in AlGaAs. Gate-voltage dependent optical spectra of highly asymmetric QDMs exhibit anticrossings that clearly indicate strong coupling with delocalized molecule states. Furthermore, indirect excitons are observed that are related to recombinations of excited-state electrons and ground-state holes both located in different dots. Simple numerical simulations reproduce the electric-field dependent energy shifts of direct and indirect transitions and predict their radiative lifetimes. The visibility of excited-state indirect excitons even for strong off-resonant energy detuning indicates the presence of a phonon bottleneck which suppresses the relaxation of excited electrons into lower levels.