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 σ and π 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. To begin, we recall the previously established spectroscopic features of quantum dot molecules (QDMs). We use the recombination of an electron–hole pair (that is, an exciton X) as an indicator for the state of the hole. In these structures, the electron is localized in the bottom dot (B dot) over the entire electric field range. Recombination with the hole within the same dot leads to an intense spectral line ( 1 0 1 0 ) in the photoluminescence spectrum (Fig. 1). As the electric field is scanned, this intradot transition goes through the first of a series of anticrossings at a field that we take as F=0. Here, there is a resonance with the lowest interdot transition, ( 1 0 0 1 ) , in which the hole is in the top-dot (T-dot) ground state. The interdot transition energy exhibits a strong field dependence (Stark shift), caused by a change in the relative level alignment between the two dots with electric field. The strength of the shift is determined by the dot separation. Its slope (1E/1F = 0.955meV kV cm) provides a built-in calibration for the conversion between electric field (1F) and energy (1E). This first resonance arises from the coherent tunnelling of the hole between the ‘s shells’ of the two dots (B0 and T0), and the corresponding formation of a bonding and an antibonding molecular state. Such resonances between the ground states of two quantum dots have been studied intensely in recent publications. En er gy (m eV )