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 $\alpha$ and $\beta$ 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 $|\Omega| \leq 15$ GHz. In particular, the effect of dephasing in terms of systematic spectral broadening $\propto \Omega^2$ 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

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