Christoph Pauly

One- and two-dimensional photonic crystal microcavities in single crystal diamond

Diamond is an attractive material for photonic quantum technologies because its colour centres have a number of outstanding properties, including bright single photon emission and long spin coherence times. To take advantage of these properties it is favourable to directly fabricate optical microcavities in high-quality diamond samples. Such microcavities could be used to control the photons emitted by the colour centres or to couple widely separated spins. Here, we present a method for the fabrication of one- and two-dimensional photonic crystal microcavities with quality factors of up to 700 in single crystal diamond. Using a post-processing etching technique, we tune the cavity modes into resonance with the zero phonon line of an ensemble of silicon-vacancy colour centres, and we measure an intensity enhancement factor of 2.8. The controlled coupling of colour centres to photonic crystal microcavities could pave the way to larger-scale photonic quantum devices based on single crystal diamond.

Deterministic Coupling of a Single Silicon-Vacancy Color Center to a Photonic Crystal Cavity in Diamond

Deterministic coupling of single solid-state emitters to nanocavities is the key for integrated quantum information devices. We here fabricate a photonic crystal cavity around a preselected single silicon-vacancy color center in diamond and demonstrate modification of the emitters internal population dynamics and radiative quantum efficiency. The controlled, room-temperature cavity coupling gives rise to a resonant Purcell enhancement of the zero-phonon transition by a factor of 19, coming along with a 2.5-fold reduction of the emitters lifetime.

Narrow-band single photon emission at room temperature based on a single nitrogen-vacancy center coupled to an all-fiber-cavity

We report the realization of a device based on a single Nitrogen-vacancy (NV) center in diamond coupled to a fiber-cavity for use as single photon source (SPS). The device consists of two concave mirrors each directly fabricated on the facets of two optical fibers and a preselected nanodiamond containing a single NV center deposited onto one of these mirrors. Both, cavity in- and output are directly fiber-coupled and the emission wavelength is easily tunable by variation of the separation of the two mirrors with a piezo-electric crystal. By coupling to the cavity we achieve an increase of the spectral photon rate density by two orders of magnitude compared to free-space emission of the NV center. With this work we establish a simple all-fiber based SPS with promising prospects for the integration into photonic quantum networks.

Feature Adaptive Sampling for Scanning Electron Microscopy.

A new method for the image acquisition in scanning electron microscopy (SEM) was introduced. The method used adaptively increased pixel-dwell times to improve the signal-to-noise ratio (SNR) in areas of high detail. In areas of low detail, the electron dose was reduced on a per pixel basis, and a-posteriori image processing techniques were applied to remove the resulting noise. The technique was realized by scanning the sample twice. The first, quick scan used small pixel-dwell times to generate a first, noisy image using a low electron dose. This image was analyzed automatically, and a software algorithm generated a sparse pattern of regions of the image that require additional sampling. A second scan generated a sparse image of only these regions, but using a highly increased electron dose. By applying a selective low-pass filter and combining both datasets, a single image was generated. The resulting image exhibited a factor of ≈3 better SNR than an image acquired with uniform sampling on a Cartesian grid and the same total acquisition time. This result implies that the required electron dose (or acquisition time) for the adaptive scanning method is a factor of ten lower than for uniform scanning.

Ru/Al Multilayers Integrate Maximum Energy Density and Ductility for Reactive Materials

Established and already commercialized energetic materials, such as those based on Ni/Al for joining, lack the adequate combination of high energy density and ductile reaction products. To join components, this combination is required for mechanically reliable bonds. In addition to the improvement of existing technologies, expansion into new fields of application can also be anticipated which triggers the search for improved materials. Here, we present a comprehensive characterization of the key parameters that enables us to classify the Ru/Al system as new reactive material among other energetic systems. We finally found that Ru/Al exhibits the unusual integration of high energy density and ductility. For example, we measured reaction front velocities up to 10.9 (±0.33) ms−1 and peak reaction temperatures of about 2000 °C indicating the elevated energy density. To our knowledge, such high temperatures have never been reported in experiments for metallic multilayers. In situ experiments show the synthesis of a single-phase B2-RuAl microstructure ensuring improved ductility. Molecular dynamics simulations corroborate the transformation behavior to RuAl. This study fundamentally characterizes a Ru/Al system and demonstrates its enhanced properties fulfilling the identification requirements of a novel nanoscaled energetic material.

Numerical modeling of self-propagating reactions in Ru/Al nanoscale multilayer foils

The Ru/Al system integrates high energy density and high product ductility and serves as an alternative for utilization as nanoscale reactive multilayer. We present a modeling study that relates the Ru-Al phase transformations occurring during self-propagating reactions with macroscopic reaction parameters such as net front velocity and reaction temperature. We coupled equations for mass and thermal transport and used a numerical scheme to solve the differential equations. We calculated the temporal evolution of the temperature distribution in the reaction front as a function of the multilayer bilayer thickness. The calculated net velocities were between 4.2 m/s and 10.8 m/s, and maximal reaction temperatures were up to 2171 K, in good agreement with measured data. Interfacial premixing, estimated to be around 4 nm, had a large influence on reaction velocities and temperature at smaller bilayer thicknesses. Finally, the theoretical results of the present study help to explain the experimental findings and guide tailoring of reactive properties of Ru/Al multilayers for applications.

The role of transitional phase formation during ignition of reactive multilayers

The ignition processes of sputter-deposited reactive Ru/Al multilayers were studied, measuring temperatures and calculating activation energy of ignition for bilayer thicknesses between 22 nm and 222 nm. Microstructural investigations of a partially reacted sample show that the grain boundary-dominated formation of a transitional Al6Ru phase plays an important role during ignition by triggering a more exothermic formation of the final product phase. A model of hot-plate ignition is proposed, based and tested on a designed three-component Ru/Al/Cu multilayer showing a strongly reduced ignition temperature versus its binary counterpart. The results demonstrate the role of transitional phase formation during ignition and provide a further means to modify ignition temperatures of reactive systems.

Electron beam characterization techniques for the study of wear in sliding contacts

In this study different methods for studying phenomena of adhesive wear in sliding electrical contacts using electron beam characterization methods within a dual beam workstation are presented. The interface between the substrate and transferred material is complex exhibiting small grain size as well as mechanical deformation. For this purpose, a dual beam workstation is used in order to perform high resolution imaging, structural investigation by electron backscatter diffraction as well as FIB cross sectioning. Only a combination of different imaging techniques with different contrast mechanisms can provide a full understanding of the wear mechanism. During wear a prow is formed on the slider. The structure implies a multi-generation history. The deformation texture resembles a simple shear texture, which is in agreement with the presumed deformation mode. The study mainly gives a guideline for future work in the field of wear in sliding contacts.

“Smart Microscopy”: Feature Based Adaptive Sampling for Focused Ion Beam Scanning Electron Microscopy

A new method for the image acquisition in scanning electron microscopy (SEM) was introduced. The method used adaptively increased pixel-dwell times to improve the signal-to-noise ratio (SNR) in areas of high detail. In areas of low detail, the electron dose was reduced on a per pixel basis, and a-posteriori image processing techniques were applied to remove the resulting noise. The technique was realized by scanning the sample twice. The first, quick scan used small pixel-dwell times to generate a first, noisy image using a low electron dose. This image was analyzed automatically, and a software algorithm generated a sparse pattern of regions of the image that require additional sampling. A second scan generated a sparse image of only these regions, but using a highly increased electron dose. By applying a selective low-pass filter and combining both datasets, a single image was generated. The resulting image exhibited a factor of ≈3 better SNR than an image acquired with uniform sampling on a Cartesian grid and the same total acquisition time. This result implies that the required electron dose (or acquisition time) for the adaptive scanning method is a factor of ten lower than for uniform scanning.

SensMiLi: Absolutes Wegmess-System aus einer diffraktiven q-nären Pseudo-Zufalls-Sequenz

Zusammenfassung Es wird ein neuartiges Prinzip für ein optisch absolut codiertes Wegmess-System vorgestellt, das auf der Beugung von Licht beruht. Optimiert für die Integration in Miniatur-Linearmotoren soll es diese zu hochdynamischen, mikrometergenauen Bewegungen in Reinräumen befähigen. Zum ersten Mal wird gezeigt wie q-näre Pseudo-Zufalls-Sequenzen durch optische Beugung codiert werden können und so u. a. Verbesserungen bzgl. Code-Effizienz, Integrationsfähigkeit und Robustheit ermöglichen. Abstract A novel principle for an optical absolute position encoder is presented which is based on the diffraction of light. Optimized for the integration into miniature linear motors, the encoder is designed to enable highly dynamic, micrometer precise positioning in clean room environments. For the first time it is shown how q-ary pseudo-random-sequences can be coded by optical diffraction, enabling improvements in e. g. code-efficiency, ability of integration, and robustness.