129 research outputs found
GaN/AlN Quantum Dots for Single Qubit Emitters
We study theoretically the electronic properties of -plane GaN/AlN quantum
dots (QDs) with focus on their potential as sources of single polarized photons
for future quantum communication systems. Within the framework of eight-band
k.p theory we calculate the optical interband transitions of the QDs and their
polarization properties. We show that an anisotropy of the QD confinement
potential in the basal plane (e.g. QD elongation or strain anisotropy) leads to
a pronounced linear polarization of the ground state and excited state
transitions. An externally applied uniaxial stress can be used to either induce
a linear polarization of the ground-state transition for emission of single
polarized photons or even to compensate the polarization induced by the
structural elongation.Comment: 6 pages, 9 figures. Accepted at Journal of Physics: Condensed Matte
Impact of phonons on dephasing of individual excitons in deterministic quantum dot microlenses
Optimized light-matter coupling in semiconductor nanostructures is a key to
understand their optical properties and can be enabled by advanced fabrication
techniques. Using in-situ electron beam lithography combined with a
low-temperature cathodoluminescence imaging, we deterministically fabricate
microlenses above selected InAs quantum dots (QDs) achieving their efficient
coupling to the external light field. This enables to perform four-wave mixing
micro-spectroscopy of single QD excitons, revealing the exciton population and
coherence dynamics. We infer the temperature dependence of the dephasing in
order to address the impact of phonons on the decoherence of confined excitons.
The loss of the coherence over the first picoseconds is associated with the
emission of a phonon wave packet, also governing the phonon background in
photoluminescence (PL) spectra. Using theory based on the independent boson
model, we consistently explain the initial coherence decay, the zero-phonon
line fraction, and the lineshape of the phonon-assisted PL using realistic
quantum dot geometries
In-situ electron-beam lithography of deterministic single-quantum-dot mesa-structures using low-temperature cathodoluminescence spectroscopy
We report on the deterministic fabrication of sub-um mesa structures
containing single quantum dots by in-situ electron-beam lithography. The
fabrication method is based on a two-step lithography process using a
low-temperature cathodoluminescence (CL) spectroscopy setup. In the first step
the position and spectral features of single InGaAs quantum dots (QDs) are
detected by CL. Then circular sub-um mesa-structures are exactly defined by
high-resolution electron-beam lithography and subsequent etching in the second
step. CL spectroscopy and micro-photoluminscence spectroscopy demonstrate the
high optical quality of the single-QD mesa-structures with emission linewidths
below 15 ueV and g(2)(0) = 0.04. Our lithography method allows for an alignment
precision better than 100 nm which paves the way for a fully-deterministic
device technology using in-situ CL lithography.Comment: 4 pages, 4 figure
Immunomagnetic t-lymphocyte depletion (ITLD) of rat bone marrow using OX-19 monoclonal antibody
Graft versus host disease (GVHD) may be abrogated and host survival prolonged by in vitro depletion of T lymphocytes from bone marrow (BM) prior to allotransplantation. Using a mouse anti-rat pan T-lymphocyte monoclonal antibody (0×19) bound to monosized, magnetic, polymer beads, T lymphocytes were removed in vitro from normal bone marrow. The removal of the T lymphocytes was confirmed by flow cytometry. Injection of the T-lymphocyte-depleted bone marrow into fully allogeneic rats prevents the induction of GVHD and prolongs host survival. A highly efficient technique of T-lymphocyte depletion using rat bone marrow is described. It involves the binding of OX-19, a MoAb directed against all rat thy-mocytes and mature peripheral T lymphocytes, to monosized, magnetic polymer spheres. Magnetic separation of T lymphocytes after mixing the allogeneic bone marrow with the bead/OX-19 complex provides for a simple, rapid depletion of T lymphocytes from the bone marrow. In vitro studies using flow cytometry and the prevention of GVHD in a fully allogeneic rat bone marrow model have been used to demonstrate the effectiveness of the depletion procedure. © 1989 Informa UK Ltd All rights reserved: reproduction in whole or part not permitted
Energy-time entanglement from a resonantly driven quantum dot three-level system
Entanglement is a major resource in advanced quantum technology, where it can
enable secure exchange of information over large distances. Energy-time
entanglement is particularly attractive for its beneficial robustness in
fiber-based quantum communication and can be demonstrated in the Franson
interferometer. We report on Franson-type interference from a resonantly driven
biexciton cascade under continuous wave excitation. Our measurements yield a
maximum visibility of (73 2)% surpassing the limit of violation of Bell's
inequality (70.7%) by more than one standard deviation. Despite being unable to
satisfy a loophole free violation, our work demonstrates promising results
concerning future works on such a system. Furthermore, our systematical studies
on the impact of driving strength indicate that dephasing mechanisms and
deviations from the cascaded emission have major impact on the degree of the
measured energy-time entanglement
Gesture-controlled interactive three dimensional anatomy: a novel teaching tool in head and neck surgery
The politicisation of evaluation: constructing and contesting EU policy performance
Although systematic policy evaluation has been conducted for decades and has been growing strongly within the European Union (EU) institutions and in the member states, it remains largely underexplored in political science literatures. Extant work in political science and public policy typically focuses on elements such as agenda setting, policy shaping, decision making, or implementation rather than evaluation. Although individual pieces of research on evaluation in the EU have started to emerge, most often regarding policy “effectiveness” (one criterion among many in evaluation), a more structured approach is currently missing. This special issue aims to address this gap in political science by focusing on four key focal points: evaluation institutions (including rules and cultures), evaluation actors and interests (including competencies, power, roles and tasks), evaluation design (including research methods and theories, and their impact on policy design and legislation), and finally, evaluation purpose and use (including the relationships between discourse and scientific evidence, political attitudes and strategic use). The special issue considers how each of these elements contributes to an evolving governance system in the EU, where evaluation is playing an increasingly important role in decision making
Lateral positioning of InGaAs quantum dots using a buried stressor
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. 100, 093111 (2012) and may be found at https://doi.org/10.1063/1.3691251.We present a “bottom-up” approach for the lateral alignment of semiconductor quantum dots (QDs) based on strain-driven self-organization. A buried stressor formed by partial oxidation of (Al,Ga)As layers is employed in order to create a locally varying strain field at a GaAs(001) growth surface. During subsequent strained layer growth, local self-organization of (In,Ga)As QDs is controlled by the contour shape of the stressor. Large vertical separation of the QD growth plane from the buried stressor interface of 150 nm is achieved enabling high optical quality of QDs. Optical characterization confirms narrow QD emission lines without spectral diffusion.DFG, 43659573, SFB 787: Halbleiter - Nanophotonik: Materialien, Modelle, Bauelement
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