1,572 research outputs found
The Role of Spatial Coherence and Orbital Angular Momentum of Light in Astronomy
The orbital angular momentum (OAM) of light is potentially interesting for
astronomical study of rotating objects such as black holes, but the effect of
reduced spatial coherence of astronomical light sources such as stars is
largely unknown. In a lab-scale experiment, we find that the detected OAM
spectrum depends strongly on the position of the light-twisting object along
the line of sight. We develop a simple intuitive model to predict the influence
of reduced spatial coherence in astronomical observations, and discuss
line-of-sight and intensity issues.Comment: updated versio
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
Shear-band arrest and stress overshoots during inhomogeneous flow in a metallic glass
At the transition from a static to a dynamic deformation regime of a shear band in bulk metallic glasses, stress transients in terms of overshoots are observed. We interpret this phenomenon with a repeated shear-melting transition and are able to access a characteristic time for a liquidlike to solidlike transition in the shear band as a function of temperature, enabling us to understand why shear bands arrest during inhomogenous serrated flow in bulk metallic glasses
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
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
Observation of OAM sidebands due to optical reflection
We investigate how the orbital angular momentum (OAM) of a paraxial light
beam is affected upon reflection at a planar interface. Theoretically, the
unavoidable angular spread of the (paraxial) beam leads to OAM sidebands which
are found to be already significant for modest beam spread (0.05). In analogy
to the polarization Fresnel coefficients we develop a theory based upon spatial
Fresnel coefficients; this allows straightforward prediction of the strength of
the sidebands. We confirm this by experiment.Comment: 5 page
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
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