1,565 research outputs found
Local structural excitations in model glasses
Structural excitations of model Lennard-Jones glass systems are investigated
using the Activation-Relaxation-Technique (ART), which explores the potential
energy landscape of a local minimum energy configuration by converging to a
nearby saddle-point configuration. Performing ART results in a distribution of
barrier energies that is single-peaked for well relaxed samples. The present
work characterises such atomic scale excitations in terms of their local
structure and environment. It is found that, at zero applied stress, many of
the identified events consist of chain-like excitations that can either be
extended or ring-like in their geometry. The location and activation energy of
these saddle-point structures are found to correlate with the type of atom
involved, and with spatial regions that have low shear moduli and are close to
the excess free volume within the configuration. Such correlations are however
weak and more generally the identified local structural excitations are seen to
exist throughout the model glass sample. The work concludes with a discussion
within the framework of and relaxation processes that are
known to occur in the under-cooled liquid regime.Comment: 34 Pages, 13 Figure
Dephasing of Mollow Triplet Sideband Emission of a Resonantly Driven Quantum Dot in a Microcavity
Detailed properties of resonance fluorescence from a single quantum dot in a
micropillar cavity are investigated, with particular focus on emission
coherence in dependence on optical driving field power and detuning.
Power-dependent series over a wide range could trace characteristic Mollow
triplet spectra with large Rabi splittings of GHz. In
particular, the effect of dephasing in terms of systematic spectral broadening
of the Mollow sidebands is observed as a strong fingerprint
of excitation-induced dephasing. Our results are in excellent agreement with
predictions of a recently presented model on phonon-dressed QD Mollow triplet
emission in the cavity-QED regime
Indistinguishable photons from the resonance fluorescence of a single quantum dot in a microcavity
We demonstrate purely resonant continuous-wave optical laser excitation to
coherently prepare an excitonic state of a single semiconductor quantum dot
(QDs) inside a high quality pillar microcavity. As a direct proof of QD
resonance fluorescence, the evolution from a single emission line to the
characteristic Mollow triplet10 is observed under increasing pump power. By
controlled utilization of weak coupling between the emitter and the fundamental
cavity mode through Purcell-enhancement of the radiative decay, a strong
suppression of pure dephasing is achieved, which reflects in close to Fourier
transform-limited and highly indistinguishable photons with a visibility
contrast of 90%. Our experiments reveal the model-like character of the coupled
QD-microcavity system as a promising source for the generation of ideal photons
at the quantum limit. From a technological perspective, the vertical cavity
symmetry -- with optional dynamic tunability -- provides strongly directed
light emission which appears very desirable for future integrated emitter
devices.Comment: 24 pages, 6 figure
Sub-nanometer free electrons with topological charge
The holographic mask technique is used to create freely moving electrons with
quantized angular momentum. With electron optical elements they can be focused
to vortices with diameters below the nanometer range. The understanding of
these vortex beams is important for many applications. Here we present a theory
of focused free electron vortices. The agreement with experimental data is
excellent. As an immediate application, fundamental experimental parameters
like spherical aberration and partial coherence are determined.Comment: 4 pages, 5 figure
On the expected diameter, width, and complexity of a stochastic convex-hull
We investigate several computational problems related to the stochastic
convex hull (SCH). Given a stochastic dataset consisting of points in
each of which has an existence probability, a SCH refers to the
convex hull of a realization of the dataset, i.e., a random sample including
each point with its existence probability. We are interested in computing
certain expected statistics of a SCH, including diameter, width, and
combinatorial complexity. For diameter, we establish the first deterministic
1.633-approximation algorithm with a time complexity polynomial in both and
. For width, two approximation algorithms are provided: a deterministic
-approximation running in time, and a fully
polynomial-time randomized approximation scheme (FPRAS). For combinatorial
complexity, we propose an exact -time algorithm. Our solutions exploit
many geometric insights in Euclidean space, some of which might be of
independent interest
Detuning effects in the one-photon mazer
The quantum theory of the mazer in the non-resonant case (a detuning between
the cavity mode and the atomic transition frequencies is present) is written.
The generalization from the resonant case is far from being direct. Interesting
effects of the mazer physics are pointed out. In particular, it is shown that
the cavity may slow down or speed up the atoms according to the sign of the
detuning and that the induced emission process may be completely blocked by use
of a positive detuning. It is also shown that the detuning adds a potential
step effect not present at resonance and that the use of positive detunings
defines a well-controlled cooling mechanism. In the special case of a mesa
cavity mode function, generalized expressions for the reflection and
transmission coefficients have been obtained. The general properties of the
induced emission probability are finally discussed in the hot, intermediate and
cold atom regimes. Comparison with the resonant case is given.Comment: 9 pages, 8 figure
Multi-dimensional laser spectroscopy of exciton-polaritons with spatial light modulators
We describe an experimental system that allows one to easily access the
dispersion curve of exciton-polaritons in a microcavity. Our approach is based
on two spatial light modulators (SLM), one for changing the excitation angles
(momenta), and the other for tuning the excitation wavelength. We show that
with this setup, an arbitrary number of states can be excited accurately and
that re-configuration of the excitation scheme can be done at high speed.Comment: 4 pages, 5 figure
Approximate Minimum Diameter
We study the minimum diameter problem for a set of inexact points. By
inexact, we mean that the precise location of the points is not known. Instead,
the location of each point is restricted to a contineus region (\impre model)
or a finite set of points (\indec model). Given a set of inexact points in
one of \impre or \indec models, we wish to provide a lower-bound on the
diameter of the real points.
In the first part of the paper, we focus on \indec model. We present an
time
approximation algorithm of factor for finding minimum diameter
of a set of points in dimensions. This improves the previously proposed
algorithms for this problem substantially.
Next, we consider the problem in \impre model. In -dimensional space, we
propose a polynomial time -approximation algorithm. In addition, for
, we define the notion of -separability and use our algorithm for
\indec model to obtain -approximation algorithm for a set of
-separable regions in time
Spontaneously Localized Photonic Modes Due to Disorder in the Dielectric Constant
We present the first experimental evidence for the existence of strongly
localized photonic modes due to random two dimensional fluctuations in the
dielectric constant. In one direction, the modes are trapped by ordered Bragg
reflecting mirrors of a planar, one wavelength long, microcavity. In the cavity
plane, they are localized by disorder, which is due to randomness in the
position, composition and sizes of quantum dots located in the anti-node of the
cavity. We extend the theory of disorder induced strong localization of
electron states to optical modes and obtain quantitative agreement with the
main experimental observations.Comment: 6 page
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