Optically Mapping the Electronic Structure of Coupled Quantum Dots

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 physics such as spin orbit coupling for holes may 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 delta and pi bonding states, provides fresh opportunities in solid-state molecular physics. Combined with atomic-resolution microscopy and electronicstructure theory for typical dots, the LACS technique could also enable reverse engineering of the level structure and the corresponding optical response.
Published in Nature Physics, v4 p291-296, Apr 2008. Sponsored in part by ARO and NSA.