Direct observation of acoustic phonon mediated relaxation between coupled exciton states in a single quantum dot molecule

TL;DR

This study demonstrates how acoustic phonons facilitate relaxation between coupled exciton states in a quantum dot molecule, with relaxation times influenced by electric field tuning and wavefunction character.

Contribution

It provides the first direct observation of phonon-mediated relaxation dynamics in a single quantum dot molecule with tunable inter-dot coupling.

Findings

Relaxation times range from ~2 ns to ~10 ns depending on wavefunction character.

Relaxation occurs over timescales comparable to radiative lifetimes, indicating a bottleneck.

Relaxation is prominent for energy spacings of 3-6 meV.

Abstract

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.