Optically mapping the electronic structure of coupled quantum dots

Abstract

In a network of quantum dots embedded in a semiconductor structure, no two are the same, and so their individual and collective properties must be measured after fabrication. Here, we demonstrate a level anti-crossing spectroscopy (LACS) technique in which the ladder of orbital energy levels of one quantum dot is used to probe that of a nearby quantum dot. This optics-based technique can be applied in situ to a cluster of tunnel-coupled dots, in configurations similar to that predicted for new photonic or quantum information technologies. Although the lowest energy levels of a quantum dot are arranged approximately in a shell structure, asymmetries or intrinsic physicssuch as spin-orbit coupling for holesmay alter level splittings significantly. We use LACS on a diatomic molecule composed of vertically stacked InAs/GaAs quantum dots and obtain the excited-state level diagram of a hole with and without extra carriers. The observation of excited molecular orbitals, including and bonding states, provides fresh opportunities in solid-state molecular physics. Combined with atomic-resolution microscopy and electronic-structure theory for typical dots, the LACS technique could also enable reverse engineering of the level structure and the corresponding opticalresponse. © 2008 Nature Publishing Group.