Optical spectroscopy of electronic states in a single pair of vertically coupled self-assembled quantum dots

Artificial molecules are studied using coupled quantum-dot (QD) ensembles and single QD spectroscopy. We demonstrate a significant improvement in the uniformity of the macroscopic ensembles of vertically self-aligned slacked QDs The coupling strength between the zero-dimensional states is varied by changing the distance between two layers of stacked self-assembled InAs/GaAs QDs. For strongly coupled QDs grown with a 4 nm spacer layer, slate filling spectroscopy reveals a shift of the QD symmetric state to lower energies by similar to 23 meV. The wetting layer states are also strongly coupled because of their shallower confinement. Spectra of single pairs of coupled QDs are compared to those of single QDs. Energy level splittings larger than 30 meV of the symmetric and antisymmetric states of the lowest confined shell are measured and compared with theory.