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 interdot 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 interlevel relaxation around the point of maximum coupling. The radiative lifetimes of the coupled excitonic states can be tuned from $\sim 2\mathrm{ns}$ to $\sim 10\mathrm{ns}$ as the spatially direct and indirect character of the wave function is varied by detuning from resonance. Acoustic phonon mediated interlevel relaxation is shown to proceed over time scales comparable to the direct exciton radiative lifetime, indicative of a relaxation bottleneck for level spacings in the range $\mathrm{\Delta }E\sim 3-6\mathrm{meV}$.