Micropillars with a controlled number of site-controlled quantum dots

This article may be downloaded for personal use only. Any other use requires prior permission of the author and AIP Publishing. This article appeared in Appl. Phys. Lett. 112, 071101 (2018) and may be found at https://doi.org/10.1063/1.5017692.We report on the realization of micropillars with site-controlled quantum dots (SCQDs) in the active layer. The SCQDs are grown via the buried stressor approach which allows for the positioned growth and device integration of a controllable number of QDs with high optical quality. This concept is very powerful as the number and the position of SCQDs in the cavity can be simultaneously controlled by the design of the buried-stressor. The fabricated micropillars exhibit a high degree of position control for the QDs above the buried stressor and Q-factors of up to 12 000 at an emission wavelength of around 930 nm. We experimentally analyze and numerically model the cavity Q-factor, the mode volume, the Purcell factor, and the photon-extraction efficiency as a function of the aperture diameter of the buried stressor. Exploiting these SCQD micropillars, we experimentally observe a Purcell enhancement in the single-QD regime with FP = 4.3 ± 0.3.EC/FP7/615613/EU/External Quantum Control of Photonic Semiconductor Nanostructures/EXQUISITEDFG, SFB 787, Halbleiter - Nanophotonik: Materialien, Modelle, Bauelement

Abusive supervision, public service motivation, and employee deviance: The moderating role of employment sector

Purpose – The purpose of this paper is to examine abusive supervision and public service motivation (PSM) as antecedents of deviant workplace behaviours. Design/methodology/approach – The study was conducted in a cross-sectional research design with survey data from 150 employees in the public, private and non-profit sector in Germany and the USA. Findings – Abusive supervision is positively associated with employee deviance, whereas PSM is negatively related to deviant behaviours. The employment sector moderates the negative relationship between PSM and employee deviance such that this relationship is stronger in the public and non-profit sector. Research limitations/implications – Limitations arise from the convenience sampling approach and the cross-sectional nature of the dataset. Practical implications – Human resource managers should consider behavioural integrity in the attraction, selection and training of both supervisors and subordinates. Private organisations can address the needs of strongly public service motivated employees by integrating associated goals and values into organisational missions and policies. Originality/value – This is the first study to introduce PSM into research on employee deviance. It shows that a pro-social motivation can drive anti-social behaviours when employees with high levels of PSM are members of profit-seeking organisations. Keywords Counterproductive work behaviour, Dark side of leadership, Destructive leadership, Person-organisation fit Paper type Research pape

Efficient single-photon source based on a deterministically fabricated single quantum dot - microstructure with backside gold mirror

This article may be downloaded for personal use only. Any other use requires prior permission of the author and AIP Publishing. This article appeared in Appl. Phys. Lett. 111, 011106 (2017) and may be found at https://doi.org/10.1063/1.4991389.We present an efficient broadband single-photon source which is fabricated by a flip-chip gold-bonding technique and in-situ electron beam lithography. The device comprises a single InGaAs quantum dot that is centered at the bottom of a monolithic mesa structure and located above a gold mirror for enhanced photon-extraction efficiency. We show a photon-extraction efficiency of ηext=(18±2) % into a numerical aperture of 0.4 and a high suppression of multi-photon events from this source with g(2)(0)=0.015±0.009. Our deterministic device with a backside gold mirror can be combined with electrical contacts and piezo-tuning capabilities in future refinements, which represents an important step towards a spectrally tunable plug-and-play quantum-light source with broadband enhancement for photonic quantum networks.DFG, SFB 787, Halbleiter - Nanophotonik: Materialien, Modelle, BauelementeBMBF, 03V0630TIB, Entwicklung einer Halbleiterbasierten Einzelphotonenquelle für die Quanteninformationstechnologi

Elevated intraocular pressure induces neuron-specific β-III-tubulin expression in non-neuronal vascular cells

PURPOSE Pathological alterations within optic nerve axons and progressive loss of the parental retinal ganglion cell (RGC) bodies are characteristics of glaucomatous neuropathy. Abnormally elevated intraocular pressure (IOP) is thought to be the major risk factor for most forms of glaucomatous changes, while lowering of the IOP is the mainstream of treatment. However, the pathophysiological mechanisms involved in neurodegenerative changes are poorly understood. It remains still a matter of debate whether elevated IOP harms the neurons directly or indirectly through alterations in the retinal vascularization. METHODS We analysed morphological and molecular changes within the retina exposed to elevated IOP in an animal model of glaucoma in vivo, in retinal explants and in cultured dissociated retinal cells each incubated under elevated air pressure in vitro, imitating elevated IOP. RESULTS Although ß-III-tubulin expressing RGCs decreased within the course of the disease, total amount of ß-III-tubulin protein within the retina increased, leading to the assumption that other cells than RGCs abnormally express ß-III-tubulin due to elevated IOP. Surprisingly, we found that β-III-tubulin, a marker developmentally regulated and specifically expressed in neurons under normal conditions, was strongly up-regulated in desmin-, PDGFR-β- and α-SMA-positive pericytes as well as in endothelin-1-positive endothelial cells both in vivo under elevated IOP and in vitro under elevated culture atmosphere pressure that simulated IOP elevation. Beta-III-tubulin-driven signalling pathways (ERK 1/2, pERK1/2 and cdc42/Rac) were also regulated. CONCLUSION The unprecedented regulation of neuron-specific β-III-tubulin in pericytes and endothelial cells is likely associated with a role of the retinal vasculature in the IOP-induced development and manifestation of glaucomatous degenerative optic nerve response

Lasing effects in micropillar resonators with few quantum emitters

Im Gegensatz zu klassischen Lasern, bei denen viele Emitter mit der Resonatormode wechselwirken, tragen bei Mikrolasern zum Teil nur wenige Quantenemitter zur Emission bei. Bei Mikrolasern mit geringem Modenvolumen ist die elektromagnetische Wechselwirkung zwischen den Quantenemitter und der Resonatormode sehr hoch und kann sich sogar im Regime der starken Kopplung befinden. Die Beschreibung der starken Kopplung erfolgt üblicherweise im Bereich schwacher Anregung und im Rahmen der Quantenelektrodynamik in Resonatoren. Klassische Laser hingegen arbeiten in der Regel bei hoher Anregung im Regime der schwachen Kopplung. Der Übergangsbereich zwischen dem Regime der schwachen Anregung und dem der hohen Anregung ist für Mikrolaser mit wenigen Quantenpunkten kaum erforscht. Dieser Bereich ist von großer Relevanz für die Charakterisierung eines Lasers, da sich die Laserschwelle in diesem Bereich befindet. Bei klassischen Lasern kann die Laserschwelle unter anderem anhand einer ausgeprägten Nichtlinearität in der Eingangs/Ausgangs-Kennlinie ermittelt werden. Für Mikrolaser hingegen zeigen diese und andere Größen kontinuierliche Übergänge von thermischer zu kohärenter Emission und können somit nicht mehr zur eindeutigen Bestimmung der Laserschwelle verwendet werden. Im ersten Teil dieser Arbeit werden Mikrolaser auf Basis von Quantenpunkt-Mikrosäulenresonatoren, bei denen sich im Niederanregungsregime ein einzelner Quantenpunkt in starker Kopplung mit der Resonatormode befindet, hinsichtlich ihrer Lasereigenschaften untersucht. Dazu wird ein Modell vorgestellt, welches die Emissionsspektren im Bereich der Laserschwelle eines stark gekoppelten Systems korrekt beschreiben kann. Dieses Modell kann genutzt werden, um den Übergang vom Regime der starken Kopplung zum Regime der schwachen Kopplung in Abhängigkeit der Laserschwelle zu quantifizieren. Darüber hinaus kann die Kopplungsstärke zwischen dem einzelnen Quantenpunkt und der Resonatormode extrahiert werden. Dieses Modell wurde in Zusammenarbeit mit PD Dr. Christopher Gies und seinen Kollegen an der Universität Bremen erstellt. Die Kopplungsstärke und weiteren Systemparameter werden genutzt, um im Rahmen von numerischen Simulationen, die Messungen zu beschreiben und der Fragestellung nachzugehen: Kann ein einzelner Quantenpunkt die benötigte Verstärkung liefern, um einen Einzel-Quantenpunkt-Laser zu realisieren? Dabei wird das aktive Medium auf zwei Arten simuliert. Die erste Simulation beschreibt einen einzelnen resonant gekoppelten Quantenpunkt als aktives Medium. Dieses simulierte System ist nicht in der Lage, die Laserschwelle zu erreichen oder die in Rahmen dieser Arbeit gewonnenen experimentellen Daten zu beschreiben. Das zweite simulierte System entspricht dem Ersten, wobei zusätzlich zum resonanten Quantenpunkt noch wenige Hintergrundemitter zur Emission beitragen. Dieses simulierte System kann die Gegebenheiten des Experiments quantitativ beschreiben und die Verstärkung des aktiven Mediums genügt, um über die Laserschwelle zu gelangen. In diesem Fall befindet sich das gekoppelte System aus Quantenpunkt und Resonatormode oberhalb der Laserschwelle weiterhin im Regime der starken Kopplung. Damit konnte demonstriert werden, dass starke Kopplung und Lasing in einem Quantenpunkt-Mikroresonatorsystem koexistieren können. Der zweite Teil dieser Arbeit untersucht den Einfluss der Hintergrundemitter auf das Emissionsverhalten eines Quantenpunkt-Mikrolasers. Durch eine umfassende experimentelle und theoretische Studie werden die Beiträge eines einzelnen, resonanten Quantenpunktes und von einem Ensemble von 160 Hintergrundemittern separat betrachtet. Experimentell wird diese Studie durch ein zwei-Farben Anregungsschema ermöglicht. Dabei kann zwischen der Verstärkung eines Quantenpunkt-Ensemble und der Verstärkung von wenige Emitter mit einer begrenzten Verstärkung graduell gewechselt werden. Dieses spiegelt sich wider in einem graduellen Wechsel der Charakteristiken eines makroskopischen Lasers zu denen eines Mikrolasers. Die theoretische Beschreibung des resonanten Quantenpunktes und der Hintergrundemitter erfolgt auf einer mikroskopischen Ebene. Aus dieser mikroskopischen Beschreibung kann ein effektiver Beta-Faktor abgeleitet werden, der den Anteil der spontanen Emission in die Lasermode beschreibt und die verschiedenen Beiträge zur Verstärkung berücksichtigt. Diese theoretische Beschreibung wurde in Zusammenarbeit mit Mawussey Segnon, PD Dr. Christopher Gies und ihren Kollegen an der Universität Bremen erstellt. Der dritte Teil dieser Arbeit untersucht deterministisch hergestellte Quantenpunkt-Laser hinsichtlich ihrer Lasereigenschaften. Bei diesen Strukturen werden die Quantenpunkte mithilfe einer Oxidapertur gezielt in das Maximum des elektrischen Felds innerhalb des Resonators integriert. Mithilfe von Messungen der Photonenzahlstatistik und der Autokorrelationsfunktion zweiter und höherer Ordnung kann gezeigt werden, dass ca. zehn dieser selbstorganisierten Quantenpunkte mit hoher optischer Qualität genügend Verstärkung bereitstellen, um über die Laserschwelle zu gelangen. Ermöglicht wird die Messung der Photonenzahlstatistik und der Autokorrelationsfunktion zweiter und höherer Ordnung durch einen Photonenzahl-auflösenden Detektor. Zusammenfassend eignen sich Laser auf Basis von Quantenpunkt-Mikrosäulenresonatoren vorzüglich, um Effekte der Quantenelektrodynamik in Resonatoren im Halbleitermaterialsystem zu untersuchen. Darüber hinaus besitzen sie großes Potential für neuartige Bauelemente auf Basis von Effekten der Quantenelektrodynamik und sind vergleichsweise einfach in bewährte elektronische und optische Schaltungen auf Halbleiterbasis integrierbar. Bezüglich einer hohen Ausbeute bei der Herstellung dieser Bauelemente, ist ein deterministisches Herstellungsverfahren von außerordentlicher Wichtigkeit.In classical semiconductor lasers, lasing is sustained by stacked quantum wells or by a large number of quantum dots. In contrast, microlasers base their lasing action on only a few quantum emitters which strongly interact with the cavity mode. This interaction of few quantum emitters with the cavity mode is described in the framework of cavity quantum electrodynamics and can take place in the regime of strong coupling, which is usually studied under weak excitation. Classical lasers operate in the regime of weak emitter-cavity coupling but under strong excitation conditions. This region of excitation is crucial for the characterization of a lasers, as it contains the threshold for all kinds of lasers. The region of intermediate excitation is widely unexplored for microlasers. For classical lasers the threshold can be determined e.g. from the nonlinearity in the input-output curve. In the case of microlasers this and other quantities can exhibit a smooth transition from thermal to coherent emission without a pronounced nonlinearity and cannot be used to non-ambiguously determine the laser threshold. The first part of this work examines the laser properties of a quantum dot microlaser were a single quantum dot is strongly coupled to the cavity mode in the weak excitation regime. A theoretical model is introduced to describe the spectra of this strongly coupled system at the lasing threshold. This sophisticated description of the spectra can be used to quantify the regions were the single quantum dot is in the strong and weak coupling regime and the laser threshold. Additionally the system parameters can be extracted and can be used to numerically simulate a single quantum dot inside a cavity. This simulation shows, that the quantum dot under study is not able to provide sufficient gain to cross the laser threshold. To perfectly match experimental observations with simulations a few background quantum dots have to be added to the gain material. With this gain material the structure is able to reach the lasing threshold. The strongly coupled system can cross the laser threshold while staying in the regime of strong coupling. Therefore, this work demonstrates that strong coupling and lasing can coexist in quantum dot microlasers. The theoretical model was developed in cooperation with PD Dr. Christopher Gies and his colleagues from the University of Bremen. The second part of this work analyzes in detail the contribution of the background emitters to lasing. A comprehensive experimental and theoretical study is used to examine separately the contribution of the resonant quantum dot and the contributions of an ensamble of 160 background emitters to laser emission. Experimentally, the study is based on a two-color excitation scheme that allows to switch continuously between the characteristics of a classical laser and of a single-quantum-dot microlaser. The former is composed by a quantum dot ensemble as gain medium while the latter exhibits only few quantum dots contributing to lasing. The theoretical description is based on a microscopic theory with an effective description of the background emitter. With that description an effective Beta-factor can be introduced, which describes the fraction of spontaneous emission, funneled into laser mode by the different gain contributions. This theoretical description was developed in cooperation with Mawussey Segnon, D Dr. Christopher Gies and their colleagues from the University of Bremen. The third part of this work examines micropillar lasers with site-controlled quantum dots as active medium. The fabrication of these lasers is based on an oxide aperture as a burried stressor to deterministically grow quantum dots in the maximum of the electrical field distribution inside the cavity. The examined structure possesses roughly ten quantum dots with a high optical quality. Measurements of the autocorrelation function of second and higher orders and the photon number distribution prove the crossing of the laser threhold. The former measurement was executed with the help of a photon number resolving transition-edge sensor. Quantum dot micropillars are well suited to study the effects of cavity quantum electrodynamics in solid state systems. They are perfectly suited for future solid-state nanophotonic integrated circuits based on cavity quantum electrodynamics effects. Especially the deterministically production of this high quality devices with a high yield is from extraordinary importance

Connecting element with spanner-less release comprises bolt and geometrically defined balls which by housing and tube are brought into defined position and held there

The connecting element comprises a bolt (1) and geometrically defined balls (2) which by a housing (4) and a tube (3) are brought into a defined position and held there. The bolt is introduced into the mechanism consisting of the balls, housing and tube by its contour which conforms to the geometrically defined balls, with a form fit being created between the bolt and the aforesaid mechanism. As the diameter of the mechanism is greater than that of the bore in the connecting halves (6,7) a form fit is created between the connecting element and connecting partners like a screw head

A method for determining the torque loading of a bolt has an expanding ring around radially moveable segments forming a countersunk contact with the bolt head

The bolt (4) has a countersunk profile within a ring of separate segments (1) surrounded by a collar the strain in which is measured by a sensor element (2) to determine the clamping torque of the bolt in a workpiece