Controlling light-matter interaction with mesoscopic quantum dots

Semiconductor quantum dots (QDs) enable efficient coupling between light and matter, which is useful in applications such as light-harvesting and all-solid-state quantum information processing. This coupling can be increased by placing QDs in nanostructured optical environments such as photonic crystals or plasmonic nanostructures, which enable strong confinement of light. An alternative approach to efficient light-matter coupling exploits the emitter and here large QDs have been proposed to have a giant oscillator strength but so far conclusive experimental evidence is lacking since non-radiative recombination can mask such effects [1]. The ability to separate the light-matter coupling efficiency into parts relating to either the emitter or the electromagnetic field relies on treating the QDs in the same way as atomic photon emitters, i.e., as point sources with wavefunctions whose spatial extent can be disregarded, which is known as the dipole approximation.
Here we present recent experimental results on the breakdown of the dipole approximation for QDs near plasmonic nanostructures [2]. We observe an eightfold enhancement of the plasmon excitation rate, depending on QD orientation as a result of their mesoscopic character as shown in Fig. 1. Moreover, we show that the interaction can be enhanced or suppressed, determined by the geometry of the nanostructure. This behaviour has no equivalence in atomic systems and offers new opportunities to exploit the unique mesoscopic characteristics of QDs [3]. Finally we present recent theoretical results, which show that a surprising relation holds between the symmetry of the exciton wavefunction and the validity of the dipole approximation [4].