Direct observation of acoustic phonon mediated relaxation between   coupled exciton states in a single quantum dot molecule
  We probe acoustic phonon mediated relaxation between tunnel coupled exciton states in an individual quantum dot molecule in which the inter-dot quantum coupling and energy separation between exciton states is continuously tuned using static electric field. Time resolved and temperature dependent optical spectroscopy are used to probe inter-level relaxation around the point of maximum coupling. The radiative lifetimes of the coupled excitonic states can be tuned from ~2 ns to ~10 ns as the spatially direct and indirect character of the wavefunction is varied by detuning from resonance. Acoustic phonon mediated inter-level relaxation is shown to proceed over timescales comparable to the direct exciton radiative lifetime, indicative of a relaxation bottleneck for level spacings in the range $\Delta E\$ ~3-6 meV. 
T. Nakaoka, H. J. Krenner, E. C. Clark, M. Sabathil, M. Bichler,   Y.Arakawa, G. Abstreiter, and J. J. Finley