A novel and interesting approach to detect microfluidic dynamics at a very small scale is given by optically trapped particles that are used as optofluidic sensors for microfluidic flows. These flows are generated by artificial as well as living microobjects, which possess their own dynamics at the nanoscale. Optical forces acting on a small particle in a laser beam can evoke a three dimensional trapping of the particle. This phenomenon is called optical tweezing and is a consequence of the momentum transfer from incident photons to the confined object. An optically confined particle shows Brownian motion in an optical tweezer, but is prevented from long term diffusion. A careful analysis of the motion of the confined particle allows a precise detection of microfluidic flows generated by an artificial or living source in the close vicinity of the particle. Thus, the particle can be used as a sensitive optofluidic detector. For this aim, several optical tweezers at different wavelengths are integrated into a dark-field microscope, combined with a high speed camera, to achieve a precise detection of the motion of the center-of-mass of the trapped particle. With this unique experimental system, a gold sphere is used as an optofluidic nanosensor to analyze for the first time the microfluidic oscillations generated by a biological sample. Here, a freely swimming larva of Copepods serves as the living source of flow. However, even if the trapping laser wavelength is off-resonant to the plasmon resonance of the flow detector, a finite heating of the gold nanoparticle occurs which reduces the sensitivity of detection. To increase the sensitivity of the optofluidic detection, a non-absorbing, dielectric microparticle is introduced as the optofluidic sensor for the microflows. It enables a quantitative, two dimensional mapping of the vectorial velocity field around a microscale oscillator in an aqueous environment. This paves the way for an alternative and sensitive detection approach for the microfluidic dynamics of artificial and living objects at a very small scale. To this aim and as a first step, an optically trapped microhelix serves as a model system for the mechanical and dynamical properties of a living microorganism. An optical tweezer is implemented for initiating a light-driven rotation of the chiral microobject in an aqueous environment and the optofluidic detection of its flow field is established. The method is then adopted for the measurement of the microfluidic flow generated by a biological system with similar dynamics, in this case a bacterium. The experimental approach is used to quantify the time-dependent changes of the flow generated by the flagella bundle rotation at a single cell level. This is achieved by observing the hydrodynamic interaction between a dielectric particle and a bacterium that are both trapped next to each other in a dual beam optical tweezer. This novel experimental technique allows the extraction of quantitative information on bacterial motility without the necessity of observing the bacterium directly. These findings can be of great relevance for an understanding of the response of different strains of bacteria to environmental changes and to discriminate between different states of bacterial activity.

No description available.

Large X-ray surveys are a powerful tool to study the large-scale structure (LSS) of the Universe. The scientific impact of LSS studies using active galactic nuclei (AGN) in X-ray surveys can be significantly increased by conducting wider and deeper X-ray surveys and studying the surface brightness fluctuations of the unresolved cosmic X-ray background (CXB). In the first part of this Thesis, we have investigated the prospects of using the AGN sample to be detected by the upcoming eROSITA all-sky survey (eRASS) for LSS studies. We show that eRASS will detect about 3 million AGN in the 0.5-2.0 keV band. This will result in a ~30 times larger number of sources and a ~30 times better sensitivity than its 25 year old predecessor, the ROSAT all-sky survey (RASS). We show that this unprecedented AGN sample will have a median luminosity of ~10^44 erg/s, which is typical for the entire AGN population in this energy band. It will have a median redshift of z ~ 1 and approximately 40% of the objects will be in the redshift range of z = 1-2, where the bulk of the X-ray emission of AGN is produced. About 10^4 - 10^5 AGN are predicted to be beyond redshift z = 3 and about 2 000 - 30 000 beyond z = 4, which will potentially include some of the earliest AGN in the Universe. We demonstrate that, given these unique properties, the eRASS-AGN sample will be able to significantly improve our current knowledge of the AGN spatial density as a function of redshift and luminosity over a wide range of cosmic time. Further, we show that it will enable us, for the first time, to perform detailed redshift- and luminosity-resolved studies of the clustering strength of X-ray selected AGN. All these measurements will dramatically improve our understanding of the growth of supermassive black holes over cosmic time and its implications for galaxy evolution. We demonstrate for the first time that, given the breadth and depth of eRASS, it will be possible to use AGN as a cosmological probe via baryon acoustic oscillation (BAO) measurements. We will be able to convincingly detect BAOs in the currently uncharted redshift range of z ~ 1-2, which will improve the constraints on the current cosmological model. In the second part of this Thesis, we have conducted the most accurate measurement to date of the brightness fluctuations of the unresolved CXB in the 0.5-2.0 keV band for angular scales of < ~17'. For this we used the XBOOTES survey, the currently largest continuous survey of the X-ray telescope Chandra. We find that on small angular scales (< ~2') the observed power spectrum of the brightness fluctuations is broadly consistent with the conventional AGN clustering model, although with a 30% deviation. This deviation nevertheless presents a good opportunity to improve our understanding of clustering properties of unresolved AGN by testing more sophisticated clustering models with our measurement. For angular scales of > ~2' we measure a significant excess with up to an order of magnitude difference in comparison to the standard AGN clustering model. We demonstrate that an instrumental origin can be excluded. However, we also show that the excess can neither be explained with any known X-ray source population by looking at strength of its clustering signal and the shape of its energy spectrum. It might be caused by more than one type of source but the dominant source appears to have extragalactic origin. Finally, we make predictions on how eRASS will be able to advance the studies of the unresolved CXB.

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Why do we exist? CP violation is an integral part of this question as its understanding is crucial to explain the matter-antimatter asymmetry observed in our universe. Several experiments were designed and carried out to precisely measure CP violation, especially in the B meson system where large asymmetries where predicted and found. With Belle II and LHCb, two new experiments are going to improve the existing measurements. Belle II will be based on the very successful Belle experiment at the KEKB collider, currently holding the world record on luminosity with 2.11×10³⁴ cm⁻²2s⁻¹. The B meson system has a very rich decay topology and many of theses decay modes and their CP asymmetry parameters have already been measured at Belle. The most famous decay channel, B0⟶J/ψK0s, poses very tight constraints on sin 2φ₁ but leaves a twofold ambiguity on the actual value of the angle φ₁ in the CKM triangle. The decay mode B0⟶D*-D*+K0s, while experimentally much more challenging, offers the unique possibility to also extract cos2φ₁ and thus resolve this ambiguity. In the first chapters of this thesis we present the principle of this measurement and the results for the branching fraction and the time-dependent CP violation parameters of B0⟶D*-D*+K0s decays. These results are obtained from the final data sample of the Belle experiment containing 772 million BBbar pairs collected at the Υ(4S) resonance with the Belle detector at the KEKB asymmetric-energy e+e- collider. We obtain the branching fraction BR(B0⟶D*-D*+K0s) = (5.35+0.35−0.34(stat) ± 0.57(syst))×10⁻³, which is in agreement with the current world average. In a 3 parameter fit sensitive to cos2φ₁, we extract the currently most precise values for the CP parameters Jc/J0 = 0.37 ± 0.10(stat) ± 0.02(syst), (2Js1/J0) sin(2φ₁) = 0.30 ± 0.16(stat) ± 0.03(syst), (2Js2/J0) cos(2φ₁) = 0.16 ± 0.16(stat) ± 0.03(syst). This allows us to exclude a negative value for cos2φ₁ at a 85% confidence under the assumption that that (2Js2/J0) is positive. Finally, we describe the implementation of the vertex detector geometry for the upcoming Belle II experiment. The upgrade to Belle aims to increase the integrated luminosity by a factor of 50 and will receive, among other upgrades, a completely new vertex detector. To produce simulated events, a precise description of the sensor geometry and material budget is needed.

When the optical pulses emitted by a laser become so short in time that they encompass only a few cycles of the carrier wave, the phase between carrier and envelope becomes a crucial parameter. The ability to control this carrier-envelope phase (CEP) is elemental to experiments probing the fastest processes in the microcosm, occurring on the time-scale of attoseconds. More than a decade into the attosecond era, the limitations of the established CEP stabilisation technique have begun to curtail experimental progress. First, increasingly complex experiments require many hours of uninterrupted operation at the same waveform. Second, the pulses used in experiments are approaching the single-cycle boundary, calling for ever-decreasing CEP noise. With the conventional stabilisation technique, already these two requirements cannot be fulfilled simultaneously. Ultimately, the low efficiency of the underlying nonlinear processes can only be compensated by driver lasers at a higher repetition rate than available at present. In order to advance attosecond pulse generation, novel approaches to CEP control thus face a threefold challenge that outlines this thesis: To simultaneously provide low CEP noise and long-term operation to present-day few-cycle lasers and amplifiers, and to investigate CEP control capability in high average power sources that are currently under development. This thesis describes the adaptation of cavity-external CEP stabilisation for use with few-cycle pulses. The intrinsic limitations of the conventional feed-back technique are lifted. A laser oscillator is demonstrated to maintain record-low CEP noise for tens of hours of operation free from phase discontinuities. In addition, a modification of the technique is presented that further enhances the applicability to amplified systems. Two routes are investigated to achieve CEP control in system architectures that represent potential megahertz repetition rate driver sources. In combination with temporal pulse compression, a thin-disk laser is shown to yield few-cycle pulses. Experiments are presented that provide the groundwork towards the first CEP-stabilised thin-disk oscillator. The second approach targets the seed oscillator of a fibre chirped-pulse amplifier. The CEP noise properties of different amplification regimes are examined. Intensity enhancement of the output pulses in a passive resonator is shown to benefit greatly even from a coarse lock of the CEP slip rate. For few-cycle pulse energy to reach the millijoule level and above, amplification and temporal compression will remain indispensable in the foreseeable future. Maintaining CEP stability across such stages is crucial, irrespective of the technology employed. Cavity-external CEP control is demonstrated to enable more than 24 hours of constant-CEP operation in chirped-pulse amplifiers. Furthermore, a novel actuator is introduced that, in conjunction with a fast means of measuring the CEP, is able to provide phase correction of the amplified waveform up to several kilohertz bandwidth. The result is a train of millijoule-level pulses with residual CEP noise comparable to that of state-of-the-art nanojoule oscillators. Eventually, an experiment is presented to examine the influence of different types of hollow-core fibre-based temporal compression on the CEP. The findings shed new light on the origin of adverse effects introduced by this technique, and point out ways towards effective compensation. / Wenn die von einem Laser emittierten Lichtpulse so kurz werden, dass ihre Dauer nur noch wenige Schwingungszyklen des elektrischen Feldes umfasst, kommt der Phase zwischen Trägerwelle und Einhüllender (CEP) eine entscheidende Rolle zu. Ihre Regelung ist essentiell für jene Experimente, die die schnellsten Prozesse in der Natur auf der Zeitskala von Attosekunden ausloten. Mehr als zehn Jahre nach Beginn der Attosekunden-Ära ist die etablierte Methode der CEP-Regelung zum Hindernis für experimentelle Fortschritte geworden. Einerseits erfordern immer komplexere Experimente, dass das elektrische Feld der Pulse über viele Stunden konstant bleibt. Andererseits zeichnet sich eine Entwicklung der Pulsdauer zu immer kürzerer Dauer in Richtung eines einzigen Zyklus ab, was eine steigende Präzision der Regelung erfordert. Die gleichzeitige Erfüllung schon dieser beiden Anforderungen ist mit der konventionellen Methode nicht zu erreichen. Schlussendlich kann die niedrige Effizienz der zugrunde liegenden nichtlinearen Prozesse nur die Verwendung von Lasersystemen mit deutlich erhöhter Wiederholrate ausgeglichen werden. Um die Erzeugung von Attosekunden-Pulsen voranzutreiben, müssen neue Ansätze zur CEP-Regelung einer dreifachen Herausforderung gerecht werden, die dieser Dissertation ihren Rahmen gibt: Einerseits hohe Präzision und andererseits hohe Langzeittauglichkeit zur Verfügung zu stellen, und überdies neue Wege zur CEP-Regelung von derzeit in Entwicklung befindlichen Laserquellen mit hoher Durchschnittsleistung aufzuzeigen. Diese Dissertation beschreibt die Anpassung einer alternativen Methode der CEP-Regelung auf Pulse mit einer Dauer von wenigen Zyklen. Die intrinsischen Beschränkungen der konventionellen Technik werden damit behoben. Der solchermaßen stabilisierte Oszillator bietet geringstes CEP-Rauschen über mehrere zehn Stunden Laufzeit ohne Phasensprünge. Zusätzlich wird eine Abwandlung der Methode beschreiben, die deren Anwendbarkeit für Verstärkersysteme erweitert. Die CEP-Regelung in Systemarchitekturen für hohe Durchschnittsleistungen wird an zwei Lasersystemen untersucht, die exemplarisch für potentielle Attosekunden-Quellen mit Megahertz-Wiederholrate stehen. Es wird gezeigt, dass ein Scheibenlaser in Kombination mit zeitlicher Pulskompression genutzt werden kann, um Pulse in der Größenordnung von 10 fs zu erzeugen. Erste Experimente zu deren CEP-Stabilisierung ebnen den Weg für den ersten CEP-stabilen Scheibenlaser. Der zweite Ansatz betrifft die CEP-Regelung eines Oszillator-Verstärker-Systems. Das CEP-Rauschverhalten verschiedener Faserverstärker wird untersucht. Es wird gezeigt, dass die Überhöhung des Pulszugs in einem passiven Resonator auch von einer groben Stabilisierung der CEP-Änderungsrate deutlich profitiert. Um Pulse von wenigen Zyklen Dauer auf eine Energie von Millijoule und darüber hinaus zu bringen, wird Verstärkung und zeitliche Kompression auf absehbare Zeit unverzichtbar bleiben. Unabhängig von der hierzu gewählten Technologie ist es von entscheidender Bedeutung, den Einfluss dieser Prozesse auf die CEP gering zu halten. Die Verwendung eines mit der alternativen CEP-Regelung ausgestatteten Oszillators zur zeitlich gestreckten Verstärkung wird beschrieben, was in hochenergetischen Pulsen mit über 24 Stunden konstanter Wellenform resultiert. Alsdann wird ein neuartiger CEP-Aktuator beschrieben, der in Kombination mit einer schnellen Messmethode die CEP-Korrektur eines jeden Pulses bei einer Bandbreite von mehreren Kilohertz leistet. Das Resultat ist ein Pulszug auf Millijoule-Niveau, dessen CEP-Rauschen mit dem eines Nanojoule-Oszillators vergleichbar ist. Abschließend wird ein Experiment vorgestellt, mit dem der Einfluss von Hohlfaser-Kompression auf die CEP untersucht wird. Die Ergebnisse werfen neues Licht auf den Ursprung zusätzlichen Rauschens in solchen Aufbauten, und zeigen Wege zu dessen Vermeidung auf.

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In kühlen Sternen wie der Sonne wird die nuklear erzeugte Energie aus dem Inneren an die Oberfläche transportiert. Diese kann dann in den freien Weltraum entfliehen, und so können wir das Sternenlicht letztlich beobachten. Die theoretische Modellierung des photosphärischen Übergangsbereiches – vom konvektiven zum radiativen Energietransport – ist aufgrund der inhärenten dreidimensionalen (3D) Natur der Konvektion und der komplexen, nicht-linearen und nicht-lokalen Interaktionen des Strahlungsfelds mit dem stellaren Plasma sehr anspruchsvoll. Theoretische Atmosphärenmodelle stellen die fundamentale Basis für die Untersuchung von Sternen dar, daher sind Astronomen für ihr Verständnis der Sterne auf diese letztlich angewiesen. Die üblicherweise verwendeten eindimensionalen (1D) Atmosphärenmodelle beinhalten verschiedene Vereinfachungen. Insbesondere wird die Konvektion für gewöhnlich mit der Mischungswegtheorie berechnet, trotz ihrer wohlbekannten Fehler, da derzeit keine deutlich besseren Alternativen vorhanden sind. Der einzige Ausweg, um dieses Problem zu überwinden ist, die zeitabhängigen, dreidimensionalen, hydrodynamischen Gleichungen, welche mit einem realistischen Strahlungstransport gekoppelt sind, zu lösen. Aufgrund der in den vergangenen Jahrzehnten rasch gestiegenen Rechenleistung wurden bedeutende Fortschritte mit Simulationen für 3D Strahlungshydrodynamik (RHD) von Atmosphären erzielt. Diese Modelle sind außerordentlich leistungsfähig, und besitzen eine enorme Vorhersagekraft, wie präzise Vergleiche mit Sonnenbeobachtungen wiederholt beweisen konnten. Ausgestattet mit diesen ausgereiften Methoden möchte ich als Ziel meiner Dissertation die drei folgenden Fragen näher untersuchen: Was sind die Eigenschaften der Atmosphären von kühlen Sternen? Welche Unterschiede sind zwischen den 1D und 3D Modellen vorhanden? Wie verändern sich die Vorhersagen für die Sternstrukturen und Spektrallinien? Um mich dieser Aufgabenstellung systematisch anzunehmen, habe ich das Stagger-Gitter berechnet. Das Stagger-Gitter ist ein umfangreiches Gitter aus 3D RHD Atmosphärenmodellen von kühlen Sternen, welches einen großen stellaren Parameterbereich abdeckt. In der vorliegenden Dissertation beschreibe ich die Methoden, welche angewendet wurden, um die vielen Atmosphärenmodelle zu berechnen. Zudem werden die allgemeinen Eigenschaften der resultierenden 3D Modelle, auch deren zeitliche und räumliche Mittelwerte detailliert dargestellt und diskutiert. Die Unterschiede zwischen den 1D und 3D Schichtungen, sowie die Details der stellaren Granulation (die Manifestation der Konvektion unterhalb der Sternoberfläche) werden ebenfalls umfangreich erläutert und beschrieben. Des Weiteren habe ich folgende Anwendungen für die 3D Atmosphärenmodelle untersucht: Berechnung theoretischer Spektrallinien, wichtig für die Bestimmung von Sternparametern, Spektroskopie und Häufigkeiten-Analyse; die sogenannte Randverdunkelung, notwendig für die Analyse interferometrischer Beobachtungen und Suche nach extrasolaren Planeten; und die Kalibrierung der Mischungsweglänge, womit 1D-Sternmodelle verbessert werden können. Die Qualität der hochauflösenden Beobachtungen hat inzwischen die der theoretischen 1D Atmosphärenmodelle aufgrund der technischen Entwicklungen in den vergangenen Jahren überschritten. Daher hat sich der Bedarf an besseren Simulationen für Atmosphärenmodelle erhöht. Durch die Bereitstellung eines umfangreichen Gitters aus 3D RHD Atmosphärenmodellen wurde dazu ein erheblicher Beitrag geleistet. Damit werden wir den Anforderungen an die Theorie für die derzeitigen und zukünftigen Beobachtungen gerecht werden, und können somit zu einem besseren Verständnis der kühlen Sterne beitragen. / In cool stars, like the Sun, energy from the inside is transported to its surface by convection, which then can escape into space as radiation that we can observe. Modeling this photospheric transition region – from convective to radiative energy transport – is notoriously challenging due to the inherent three-dimensional (3D) nature of convection itself and the complex non-linear and non-local interaction of the radiation field with the stellar plasma. Astronomers rely on theoretical atmosphere models, which provide the fundamental basis to study and understand stars. The commonly employed one-dimensional (1D) atmosphere models make use of several simplifications, in particular, convection is usually treated with the mixing-length theory (MLT), despite its well-known wrongness simply due to the lack of a considerably improved alternative. Therefore, the only appropriate approach to overcome this issue, is to solve the time-dependent, 3D, hydrodynamic equations coupled a with the realistic treatment of radiative transfer. Due to the soaring computational power in the recent decades, significant progress has been established with the advent of 3D radiative hydrodynamic (RHD) atmosphere simulations. Nowadays, these perform exceedingly well and offer exceptional predictive potential as detailed comparisons with the Sun have repeatedly revealed. Equipped with this matured, powerful tool, I want to address the following three main questions as the aim of my thesis: What are the atmospheric properties of cool stars besides the Sun? Which differences are given between 1D and 3D models? How do the application-based predictions change? To attend to this matter in a systematical approach, I have computed the Stagger-grid, a comprehensive grid of 3D RHD model atmospheres of cool stars covering a wide range in stellar parameters. In this thesis I describe the methods I have applied for the computation of the grid models, and the general properties of the 3D models, as well as their temporal and spatial averages are presented and discussed in detail. Also, the differences between 1D and 3D stratifications are determined, and the details of stellar granulation, the manifestation of subsurface convection, is extensively depicted. Furthermore, I investigated with the Stagger-grid several applications for 3D atmosphere simulations including: spectral line profiles, important for stellar parameter determination, stellar spectroscopy and abundance analysis; limb darkening, necessary for interferometry and extrasolar planet search; and the calibration of the mixing length, which will improve stellar evolution models. The cumulative technical developments of high-resolution observations in the recent years have surpassed the standards of theoretical 1D atmosphere models, thereby, it has given rise to the enhanced demand of improved atmosphere simulations. By providing a comprehensive grid of 3D RHD atmosphere models to the astronomical community, a major contribution has been achieved to live up to the current and future high-precision observations, which ultimately will lead to a better understanding of cool stars.