4,659 research outputs found
Anomalous Stark Shifts in Single Vertically Coupled Pairs of InGaAs Quantum Dots
Vertically coupled Stranski Krastanow QDs are predicted to exhibit strong
tunnelling interactions that lead to the formation of hybridised states. We
report the results of investigations into single pairs of coupled QDs in the
presence of an electric field that is able to bring individual carrier levels
into resonance and to investigate the Stark shift properties of the excitons
present. Pronounced changes in the Stark shift behaviour of exciton features
are identified and attributed to the significant redistribution of the carrier
wavefunctions as resonance between two QDs is achieved. At low electric fields
coherent tunnelling between the two QD ground states is identified from the
change in sign of the permanent dipole moment and dramatic increase of the
electron polarisability, and at higher electric fields a distortion of the
Stark shift is attributed to a coherent tunnelling effect between the ground
state of the upper QD and the excited state of the lower QD.Comment: Conference paper for QD2004 3 figure
Highly efficient single photon emission from single quantum dots within a two-dimensional photonic bandgap
We report highly efficient single photon generation from InGaAs
self-assembled quantum dots emitting within a two-dimensional photonic bandgap.
A strongly suppressed multiphoton probability is obtained for single quantum
dots in bulk GaAs and those emitting into the photonic bandgap. In the latter
case, photoluminescence saturation spectroscopy is employed to measure a ~17
times enhancement of the average photon extraction efficiency, when compared to
quantum dots in bulk GaAs. For quantum dots in the photonic crystal we measure
directly an external quantum efficiency up to 26%, much higher than for quantum
dots on the same sample without a tailored photonic environment. The results
show that highly efficient quantum dot single photon sources can be realized,
without the need for complex nanopositioning techniques
Efficient Spatial Redistribution of Quantum Dot Spontaneous Emission from 2D Photonic Crystals
We investigate the modification of the spontaneous emission dynamics and
external quantum efficiency for self-assembled InGaAs quantum dots coupled to
extended and localised photonic states in GaAs 2D-photonic crystals. The
2D-photonic bandgap is shown to give rise to a 5-10 times enhancement of the
external quantum efficiency whilst the spontaneous emission rate is
simultaneously reduced by a comparable factor. Our findings are quantitatively
explained by a modal redistribution of spontaneous emission due to the modified
local density of photonic states. The results suggest that quantum dots
embedded within 2D-photonic crystals are suitable for practical single photon
sources with high external efficiency
Human performance prediction in man-machine systems. Volume 1 - A technical review
Tests and test techniques for human performance prediction in man-machine systems task
Human performance prediction in man-machine systems. Part 2 - The test catalog
Human performance prediction in man machine systems - test catalog table
Evidence for the Galactic X-ray Bulge II
A mosaic of 5 \ros~PSPC pointed observations in the Galactic plane
() reveals X-ray shadows in the keV band cast by
distant molecular clouds. The observed on-cloud and off-cloud X-ray fluxes
indicate that % and % of the diffuse X-ray background in this
direction in the \tq~keV and 1.5 keV bands, respectively, originates behind the
molecular gas which is located at 3 kpc from the Sun. The implication of
the derived background X-ray flux beyond the absorbing molecular cloud is
consistent with, and lends further support to recent observations of a Galactic
X-ray bulge.Comment: 19 pages, 5 figures, 2 table
Solutions of the sDiff(2)Toda equation with SU(2) Symmetry
We present the general solution to the Plebanski equation for an H-space that
admits Killing vectors for an entire SU(2) of symmetries, which is therefore
also the general solution of the sDiff(2)Toda equation that allows these
symmetries. Desiring these solutions as a bridge toward the future for yet more
general solutions of the sDiff(2)Toda equation, we generalize the earlier work
of Olivier, on the Atiyah-Hitchin metric, and re-formulate work of Babich and
Korotkin, and Tod, on the Bianchi IX approach to a metric with an SU(2) of
symmetries. We also give careful delineations of the conformal transformations
required to ensure that a metric of Bianchi IX type has zero Ricci tensor, so
that it is a self-dual, vacuum solution of the complex-valued version of
Einstein's equations, as appropriate for the original Plebanski equation.Comment: 27 page
Cascaded exciton emission of an individual strain-induced quantum dot
Single strain-induced quantum dots are isolated for optical experiments by
selective removal of the inducing InP islands from the sample surface.
Unpolarized emission of single, bi- and triexciton transitions are identified
by power-dependent photoluminescence spectroscopy. Employing time-resolved
experiments performed at different excitation powers we find a pronounced shift
of the rise and decay times of these different transitions as expected from
cascaded emission. Good agreement is found for a rate equation model for a
three step cascade
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