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 which is strongly influenced by the tuning electric field and inter-molecular Coulomb energies. A rate equation model is developed to gain more insight into the charge transfer and tunneling mechanisms. Non-resonant (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.Comment: 4 pages, 4 figure

A Cooper pair light emitting diode

We demonstrate Cooper-pair's drastic enhancement effect on band-to-band radiative recombination in a semiconductor. Electron Cooper pairs injected from a superconducting electrode into an active layer by the proximity effect recombine with holes injected from a p-type electrode and dramatically accelerate the photon generation rates of a light emitting diode in the optical-fiber communication band. Cooper pairs are the condensation of electrons at a spin-singlet quantum state and this condensation leads to the observed enhancement of the electric-dipole transitions. Our results indicate the possibility to open up new interdisciplinary fields between superconductivity and optoelectronics.Comment: 5 pages (4 figures

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

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.Comment: 14 pages, 4 figures; the following article has been submitted to Journal of Applied Physics (29th ICPS - invited paper); after it is published, it will be found at http://jap.aip.org

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 energy of the single QDM shows good agreement with the experimentally observed values. By decreasing the edge-to-edge distance between the two QDs within a QDM, a splitting of the electron (hole) wavefunction into symmetric and antisymmetric states is observed, indicating the presence of lateral coupling. Site control of such lateral QDMs obtained by growth on a pre-patterned substrate, combined with a technology to fabricate gate structures at well-defined positions with respect to the QDMs, could lead to deterministically controlled devices based on QDMs. © IOP Publishing Ltd and Deutsche Physikalische Gesellschaft

Skin, paper, tiles: a cross-cultural history of Kadiwéu art

This article focuses on the global traffic in images relating to Kadiwéu culture in South America, analyzing the extent to which they are entangled in the group’s continuing sense of presence. It begins with Kadiwéu designs as they appeared in the sketchbook of the artist-explorer Guido Boggiani in the late nineteenth century. It then explores the mapping of Kadiwéu territory and the practices and protocols informing a politics of land rights, cultural property and economic survival, looking in particular at the commissioning of Kadiwéu designs for a housing estate and an associated exhibition in Berlin early in the twentieth-first century. By developing a cross-cultural history of Kadiwéu art that considers the transnational networks across different times and spaces, including the case of a transcultural history of copyright, the article seeks to contribute to the ongoing re-thinking of the colonial archive and its afterlife

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

Inter-dot coupling and excitation transfer mechanisms of telecommunication band InAs quantum dots at elevated temperatures

We investigate the photoluminescence temperature dependence of individual InAs/InGaAlAs quantum dots emitting in the optical telecommunication bands. The high-density dots are grown on InP substrates and the selection of a smaller dot number is done by the processing of suitable nanometer-sized mesas. Using ensembles of only a few dots inside such mesas, their temperature stability, inter-dot charge transfer, as well as carrier capture and escape mechanisms out of the dots are investigated systematically. This includes the discussion of the dot ensemble and individual dots. Among the single-dot properties, we investigate the transition of emission lines from zero-phonon line to acoustic phonon sideband-dominated line shape with temperature. Moreover, the presence of single recombination lines up to temperatures of about 150K is demonstrated

Dynamics of cell adhesion and motility in living cells is altered by single amino acid change in E-cadherin fused to enhanced green fluorescent protein.

E-Cadherin regulates epithelial cell adhesion and is critical for the maintenance of tissue integrity. In sporadic diffuse-type gastric carcinoma, mutations of the E-cadherin gene are frequently observed that predominantly affect putative calcium binding motifs located in the linker region between the second and third extracellular domains. A single amino acid change (D370A) as found in a gastric carcinoma patient reduces cell adhesion and up-regulates cell motility. To study the effect of this mutation on the dynamics of cell adhesion and motility in living cells, enhanced green fluorescent protein (EGFP) was C-terminally fused to E-cadherin. The resulting mutant E-cadherin-EGFP fusion protein with a point mutation in exon 8 (p8-EcadEGFP) and a wild-type E-cadherin-EGFP fusion construct (wt-EcadEGFP) were expressed in human MDA-MB-435S cells. Fluorescent images were acquired by time-lapse laser scanning microscopy and E-cadherin was visualized during contact formation and in moving cells. Spatial and temporal localization of p8- and wt-EcadEGFP differed significantly. While wt-EcadEGFP was mainly localized at lateral membranes of contacting cells and formed E-cadherin puncta and plaques, p8-EcadEGFP-expressing cells frequently formed transient cell-cell contacts. During random cell migration, p8-EcadEGFP was found in lamellipodia. In contrast, wt-EcadEGFP localized at lateral cell-cell contact sites in low or non-motile cells. Inhibition of the epidermal growth factor (EGF) receptor, which plays a major role in lamellipodia formation and cell migration, reduced the motility of p8-EcadEGFP-expressing cells and caused lateral membrane staining of p8-EcadEGFP. Conversely, EGF induced cell motility and caused formation of lamellipodia that were E-cadherin positive. In conclusion, our data show that mutant E-cadherin significantly alters the dynamics of cell adhesion and motility in living cells and interferes with the formation of stable cell-cell contacts