C. Hermannstädter

Towards deterministically controlled InGaAs/GaAs lateral quantum dot molecules

We report on the fabrication, detailed characterization and modeling of lateral InGaAs quantum dot molecules (QDMs) embedded in a GaAs matrix and we discuss strategies to fully control their spatial configuration and electronic properties. The three-dimensional morphology of encapsulated QDMs was revealed by selective wet chemical etching of the GaAs top capping layer and subsequent imaging by atomic force microscopy (AFM). The AFM investigation showed that different overgrowth procedures have a profound consequence on the QDM height and shape. QDMs partially capped and annealed in situ for micro-photoluminescence spectroscopy consist of shallow but well-defined quantum dots (QDs) in contrast to misleading results usually provided by surface morphology measurements when they are buried by a thin GaAs layer. This uncapping approach is crucial for determining the QDM structural parameters, which are required for modeling the system. A single-band effective-mass approximation is employed to calculate the confined electron and heavy-hole energy levels, taking the geometry and structural information extracted from the uncapping experiments as inputs. The calculated transition

Polarization fine structure and enhanced single-photon emission of self-assembled lateral InGaAs quantum dot molecules embedded in a planar microcavity

Single lateral InGaAs quantum dot molecules have been embedded in a planar micro-cavity in order to increase the luminescence extraction efficiency. Using a combination of metal-organic vapor phase and molecular beam epitaxy samples could be produced that exhibit a 30 times enhanced single-photon emission rate. We also show that the single-photon emission is fully switchable between two different molecular excitonic recombination energies by applying a lateral electric field. Furthermore, the presence of a polarization fine-structure splitting of the molecular neutral excitonic states is reported which leads to two polarization-split classically correlated biexciton exciton cascades. The fine-structure splitting is found to be on the order of 10 micro-eV.

Influence of the charge carrier tunneling processes on the recombination dynamics in single lateral quantum dot molecules

We report on the charge carrier dynamics in single lateral quantum dot molecules and the effect of an applied electric field on the molecular states. Controllable electron tunneling manifests itself in a deviation from the typical excitonic decay behavior in dot molecules. It results in a faster population decay and can be strongly influenced by the tuning electric field and intermolecular Coulomb energies. A rate equation model is developed and compared to the experimental data to gain more insight into the charge transfer and tunneling mechanisms. Nonresonant (phonon-mediated) electron tunneling which changes the molecular exciton character from direct to indirect, and vice versa, is found to be the dominant tunable decay mechanism of excitons besides radiative recombination.

Polarization anisotropic luminescence of tunable single lateral quantum dot molecules

We investigate the photoluminescence polarization anisotropy of self-assembled individual lateral InGaAs/GaAs quantum dot molecules. In contrast to similarly grown single quantum dots, the dot molecules exhibit a remarkable degree of linear polarization, which remains almost unchanged when a lateral electric field is applied to tune the exciton wave function and, thus, the luminescence spectral properties. We discuss the nature of this polarization anisotropy and suggest possible causes based on the system’s symmetry and heterostructure alloy composition.

Time‐Resolved Optical Spectroscopy of Tunnel Coupled Lateral Quantum Dot Molecules

The two laterally coupled quantum dots, also referred to as lateral quantum dot molecules, exhibit a characteristic photoluminescence spectrum consisting of six dominant emission lines that are due to neutral and charged excitonic as well as biexcitonic recombination. All of these lines are found to originate from the same single quantum emitter following photon statistics measurements. Using a parallel electric field we are able to control the quantum coupling between the dots. This control manifests itself as an ability to reversibly switch the relative intensities of the two neutral excitonic transitions, which results in a possible application of the molecules as tunable single‐photon emitters. To further investigate the exact origin of the photoluminescence lines we have also investigated the decay times of the molecule emission.

Tunable lateral tunnel coupling between two self-assembled InGaAs quantum dots

We demonstrate direct control over the level of lateral quantum coupling between two self-assembled InGaAs/GaAs quantum dots. This coupled system, which we also refer to as a lateral quantum dot molecule, was produced using a unique technique which combines molecular beam epitaxy and in-situ atomic layer etching. Atomic force microscopy measurements show that each molecule consists of two structurally distinct dots, which are aligned along the [1-10] direction. Each molecule exhibits a characteristic photoluminescence spectrum primarily consisting of two neutral excitonic and two biexcitonic transitions. The various transitions have been investigated using micro-photoluminescence measurements as a function of excitation power density, time, and applied electric field. Photon statistics experiments between the excitonic emission lines display strong antibunching in the second-order cross-correlation function which confirms that the two dots are quantum coupled. Cascaded emission between corresponding biexcitonic and excitonic emission has also been observed. Using a parallel electric field we can control the quantum coupling between the dots. This control manifests itself as an ability to reversibly switch the relative intensities of the two neutral excitonic transitions. Furthermore, detailed studies of the emission energies of the two neutral excitonic transitions as a function of parallel lateral electric field show a clear anomalous Stark shift which further demonstrates the presence of quantum coupling between the dots. In addition, this shift allows for a reasonable estimate of the coupling energy. Finally, a simple one-dimensional model, which assumes that the coupling is due to electron tunneling, is used to qualitatively describe the observed effects.

Control of tunnel coupling using a lateral quantum dot molecule

We report lateral quantum coupling between two self-assembled InGaAs/GaAs quantum dots. Single-photon photoluminescence emission has been observed from this quantum dot molecule and the level of coupling can be controlled using a static electric field.