Fakultät für Physik - Digitale Hochschulschriften der LMU - Teil 05/05 Archiv

Whether it is to form an optical cavity, to control dispersion, or merely to transport the laser beam, multilayer mirrors are fundamental components of every ultrafast laser system. The performance of current state of the art ultrafast high-power lasers in terms of pulse energy is often restrained by optical breakdown of multilayer coatings. One way to overcome this problem is to increase the size of the laser beam, but this is usually undesirable, as it rises the costs and the footprint of the laser system. Therefore, increasing the optical resistance of multilayer mirrors is essential to the development of cost- and space-efficient lasers. In turn, this requires a thorough understanding of the mechanisms behind optical damage. In this work, we have studied the ultrafast optical breakdown of dispersive mirrors, as well as that of other multilayer thin-films, in three different regimes: (i) at 500 Hz repetition rate with 30 fs pulses, at a central wavelength of 800nm; (ii) at 11:5MHz repetition rate with 1 ps pulses, at 1030 nm; (iii) at 5 kHz repetition rate with 1:4 ps pulses at 1030 nm. The results from (ii) and (iii) have been compared side by side. In addition, a novel technique for dispersion measurements has been developed. In the femstosecond regime, the samples have been: single layer coatings made of Au; Ag; Nb2O5; SiO2;Ta2O5 and mixtures of Ta2O5 with silica in different concentrations; and different dispersive coatings, consisting of SiO2 as the low-index material and different high-index materials (Nb2O5; Ta2O5; HfO2). We have also given a suggestion as to what is the best approach to increase the damage threshold of thin-film dielectric coatings. The ultrafast optical breakdown of multilayer thin-films has been investigated at MHz repetition rate and high average power. The optical breakdown threshold of three different types of coatings has been measured. All samples have been coated with either TiO2, Ta2O5, HfO2, or Al2O3 as high-index material and with SiO2 as low-index material. The same samples have been measured also at kHz repetition rate. The results obtained in both regimes have been compared. The band gap dependencies of damage threshold in both cases were linear. However, the one retrieved at kHz rate was steeper than its MHz counterpart. This is an interesting finding, which must be investigated further. The developed method for dispersion measurements has been based on the location of resonance peaks in a Fabry-Perot-type of interferometer. By simultaneously processing data obtained at different spacer thicknesses, we were able to obtain superior resolution compared to the conventional method.

Judging by the compelling number of innovations based on taming quantum mechanical effects, such as the development of transistors and lasers, further research in this field promises to tackle further technological challenges in the years to come. This statement gains even more importance in the information processing scenario. Here, the growing data generation and the correspondingly higher need for more efficient computational resources and secure high bandwidth networks are central problems which need to be tackled. In this sense, the required CPU minituarization makes the design of structures at atomic levels inevitable, as foreseen by Moore's law. From these perspectives, it is necessary to concentrate further research efforts into controlling and manipulating quantum mechanical systems. This enables for example to encode quantum superposition states to tackle problems which are computationally NP hard and which therefore cannot be solved efficiently by classical computers. The only limitation affecting these solutions is the low scalability of existing quantum systems. Similarly, quantum communication schemes are devised to certify the secure transmission of quantum information, but are still limited by a low transmission bandwidth. This thesis follows the guideline defined by these research projects and aims to further increase the scalability of the quantum mechanical systems required to perform these tasks. The method used here is to encode quantum states into photons generated by spontaneous parametric down-conversion (SPDC). An intrinsic limitation of photons is that the scalability of quantum information schemes employing them is limited by the low detection efficiency of commercial single photon detectors. This is addressed by encoding higher dimensional quantum states into two photons, increasing the scalability of the scheme in comparison to multi-photon states. Further on, the encoding of quantum information into the emission-time degree of freedom improves its applicability to long distance quantum communication schemes. By doing that, the intrinsic limitations of other schemes based on the encoding into the momentum and polarization degree of freedom are overcome. This work presents results on a scalable experimental implementation of time-energy encoded higher dimensional states, demonstrating the feasibility of the scheme. Further tools are defined and used to characterize the properties of the prepared quantum states, such as their entanglement, their dimension and their preparation fidelity. Finally, the method of quantum state tomography is used to fully determine the underlying quantum states at the cost of an increased measurement effort and thus operation time. It is at this point that results obtained from the research field of compressed sensing help to decrease the necessary number of measurements. This scheme is compared with an adaptive tomography scheme designed to offer an additional reconstruction speedup. These results display the scalability of the scheme to bipartite dimensions higher than 2x8, equivalent to the encoding of quantum information into more than 6 qubits.

Emerging from the cosmic web, galaxy clusters are the most massive gravitationally bound structures in the universe. Thought to have begun their assembly at 2 < z < 3, i.e. 10 to 11 billion years ago, clusters provide insights into the growth of large-scale structure as well as the physics that drives galaxy evolution. The redshift range 1 < z < 3 is a key epoch in their evolution. At z ∼ 1.5, elliptical galaxies start to become the dominant population in cluster cores, and star formation in spiral galaxies is being quenched. But there is also evidence for a progressive increase in the amount of star formation that occurs in galaxy cluster cores at z ≳ 1.5. To understand the dependence of the formation mechanisms of massive galaxies with environment, we must focus on clusters at relatively unexplored redshifts z > 1.5 where major assembly is in progress. The search for galaxy clusters at high redshift, so far, has been mildly successful and only a handful of clusters at z > 1.5 have been confirmed. Because this redshift range was essentially unreachable with previous instrumentation, it was dubbed a ‘redshift desert’. The work presented in this thesis has made a major contribution to this field. The Clusters Around Radio- Loud AGN (CARLA) survey, a 400 hr targeted Warm Spitzer program, observed 420 radio-loud AGN (active galactic nuclei) at 1.3 < z < 3.2 across the full sky. Extensive literature over several decades shows that powerful radio-loud AGN preferentially reside in overdense environments. From this survey, we have identified a sample of ∼ 200 galaxy cluster candidates by selecting strong overdensities of color-selected sources. By studying the luminosity function of the CARLA cluster candidates, we showed that quenching is happening much earlier in clusters around radio-loud AGN than in field galaxy samples. This suggests that our targets may well be the most massive and evolved structures known to date at z > 1.5. We also showed that radio-loud AGN reside in denser environments than similarly massive galaxies. This makes high-redshift clusters around radio-loud AGN particularly interesting as they can reveal how galaxies in the most massive dark matter halos assembled. A complementary project, HERGE (Herschel Radio Galaxy Evolution Project) observed a sample of 71 radio galaxies at 1 < z < 5 at far-IR wavelengths with the Herschel Space Observatory. Supporting data in the mid-IR, partially in the near-IR and at sub-mm wave- lengths allow to study cluster fields in more detail. A pilot project on a single field showed that we can identify cluster members and constrain their star-formation properties. These projects laid the foundation for future work, which will make a significant impact on understanding the formation of the most massive structures over several billion years.

In this thesis we study the low-energy effective dynamics emerging from a class of F-theory compactifications in four and six dimensions. We also investigate six-dimensional supersymmetric quantum field theories with self-dual tensors, motivated by the problem of describing the long-wavelength regime of a stack of M5-branes in M-theory. These setups share interesting common features. They both constitute examples of intrinsically non-perturbative physics. On the one hand, in the context of F-theory the non-perturbative character is encoded in the geometric formulation of this class of string vacua, which allows the complexified string coupling to vary in space. On the other hand, the dynamics of a stack of multiple M5-branes flows in the infrared to a novel kind of superconformal field theories in six dimensions - commonly referred to as (2,0) theories - that are expected to possess no perturbative weakly coupled regime and have resisted a complete understanding so far. In particular, no Lagrangian description is known for these models. The strategy we employ to address these two problems is also analogous. A recurring Leitmotif of our work is a transdimensional treatment of the system under examination: in order to extract information about dynamics in $d$ dimensions we consider a (d-1)-dimensional setup. As far as F-theory compactifications are concerned, this is a consequence of the duality between M-theory and F-theory, which constitutes our main tool in the derivation of the effective action of F-theory compactifications. We apply it to six-dimensional F-theory vacua, obtained by taking the internal space to be an elliptically fibered Calabi-Yau threefold, but we also employ it to explore a novel kind of F-theory constructions in four dimensions based on manifolds with Spin(7) holonomy. With reference to six-dimensional (2,0) theories, the transdimensional character of our approach relies in the idea of studying these theories in five dimensions. Indeed, we propose a Lagrangian that is formulated in five dimensions but has the potential to capture the six-dimensional interactions of (2,0) theories. This investigation leads us to explore in closer detail the relation between physics in five and in six dimensions. One of the outcomes of our exploration is a general result for one-loop corrections to Chern-Simons couplings in five dimensions.

The upcoming luminosity upgrades of the LHC accelerator at CERN demand several upgrades to the detectors of the ATLAS muon spectrometer, mainly due to the proportionally increasing rate of uncorrelated background irradiation. This concerns also the "Small Wheel" tracking stations of the ATLAS muon spectrometer, where precise muon track reconstruction will no longer be assured when around 2020 the LHC luminosity is expected to reach values 2 to 5 times the design luminosity of $1 \times 10^{34} \text{cm}^{-2}\text{s}^{-1}$, and when background hit rates will exceed 10 kHz/cm$^2$. This, together with the need of an additional triggering station in this area with an angular resolution of 1 mrad, requires the construction of "New Small Wheel" detectors for a complete replacement during the long maintenance period in 2018 and 2019. As possible technology for these New Small Wheels, high-rate capable sMDT drift tubes have been investigated, based on the ATLAS 30 mm Monitored Drift Tube technology, but with a smaller diameter of 15 mm. In this work, a prototype sMDT chamber has been tested under the influence of high-rate irradiation with protons, neutrons and photons at the Munich tandem accelerator, simulating the conditions within a high luminosity LHC experiment. Tracking resolution and detection efficiency for minimum ionizing muons are presented as a function of irradiation rate. The experimental muon trigger geometry allows to distinguish between efficiency degradation due to deadtime effects and space charge in the detectors. Using modified readout electronics the analog pulse shape of the detector has been investigated for gain reduction and potential irregularities due to the high irradiation rates and ionization doses. This study shows that the sMDT detectors would fulfill all requirements for successful use in the ATLAS New Small Wheel endcap detector array, with an average spatial resolution of 140 $\mu$m and a track reconstruction efficiency of around 72\% for a single tube layer at 10 kHz/cm$^2$ irradiation rate. A second proposal for a New Small Wheel detector technology are Micromegas detectors. These highly segmented planar gaseous detectors are capable of very high rate particle tracking with single plane angular resolution or track reconstruction. The ATLAS community has decided in 2013 in favor of this technology for precision tracking in the New Small Wheels. A prototype Micromegas detector will be installed in summer 2014 on the present ATLAS Small Wheel to serve as test case of the technology and as template for the necessary changes to the ATLAS hardware and software infrastructure. To fully profit from this installation, an ATLAS compatible Read Out Driver (ROD) had to be developed, that allows to completely integrate the prototype chamber into the ATLAS data acquisition chain. This device contains state-of-the-art FPGAs and is based on the Scalable Readout System (SRS) of the RD51 collaboration. The system design, its necessary functionalities and its interfaces to other systems are presented at use of APV25 frontend chips. Several initial issues with the system have been solved during the development. The new ROD was integrated into the ATLAS Monitored Drift Tube Readout and into a VME based readout system of the LMU Cosmic Ray Facility. Additional successful operation has been proven meanwhile in several test cases within the ATLAS infrastructure. The whole data acquisition chain is ready for productive use in the ATLAS environment.

Epithelzellen, die Modell-Zelllinien dieser Dissertation, sind für die Aufteilung und Abtrennung verschiedener Kompartimente eines Organismus zuständig, indem sie sich zu Grenzflächen zusammenschliessen, welche häufig hohen physikalischen Spannungen und Kräften ausgesetzt sind. Um diese physikalischen Kräfte zu verarbeiten oder sie selbst zu produzieren, verwenden Epithelzellen, wie alle anderen Zelltypen auch, das Zytoskelett, das sich im Allgemeinen aus den Komponenten Mikrotubuli, Intermediär-Filamenten und Aktin sowie den damit korrespondierenden Motorproteinen Dynein, Kinesin sowie Myosin zusammensetzt. In dieser Dissertation wird das Zusammenspiel von Aktin und Myosin auf der apikalen Seite von Epithelzellen untersucht. Im Falle von konfluenten Zellen mit vollständig ausgebildeten Zell-Zell-Kontakten sind auf der apikalen Seite der Zellen Mikrovilli zu finden, kleine, mit Aktin-Bündeln gefüllte Ausstülpungen aus der Zelloberfläche, welche für die optimierte Nahrungsaufnahme sowie als Antennen für Signalverarbeitung zuständig sind. Im Zuge der Arbeit konnten wir feststellen, dass sich der Aktin-Myosin-Aufbau auf der apikalen Seite von Einzelzellen ohne Zell-Zell-Kontakte, sogenannten nicht-konfluenten Zellen, grundsätzlich ändert. Mittels Fluoreszenz-Mikroskopie und anderen experimentellen Methoden zeigen wir, dass zwar ähnliche Ausstülpungen auf der apikalen Oberfläche von Einzelzellen zu finden, diese jedoch häufig verlängert, gebogen, hoch-dynamisch und oft parallel zur Zellmembran orientiert sind. Wir zeigen mittels molekularbiologischer Methoden, dass ein zusätzliches, innerhalb der apikalen Zellmembran liegendes isotropes Akto-Myosin-Netzwerk für die dynamische Reorganisation der Mikrovilli-Ausstülpungen verantwortlich ist. Der Identifzierung des isotropen Akto-Myosin-Netzwerkes, welches eine der Hauptaussagen dieser Dissertation ist, wird eine detaillierte Analyse der dynamischen Netzwerkreorganisation angefügt, die mittels temporaler und örtlicher Bild-Korrelationsanalysen charakteristische Zeiten und Längen der Dynamik definiert. Des Weiteren entwickeln wir mehrere Bild- Analyseverfahren, allen voran die Methode der iterativen temporalen Bildkorellation sowie des optischen Flusses, wodurch wir eine Oszillation der Netzwerk-Reorganisationsgeschwindigkeit identifizieren und parametrisieren können. Verschiedene, auf Fluoreszenzmikroskopie und automatisierter optischer Fluss-Bildanalyse basierende Experimente geben Hinweise auf zwei mögliche Erklärungen für die identifizierten Oszillationen. Sowohl Myosin aktivitätsregulierende Proteine als auch spontan auftretende Spannungsfluktuationen im unter Zugspannung liegenden Netzwerk können mögliche Ursachen für die identifizierten Netzwerkoszillationen sein. Obwohl eine eindeutige zelluläre Funktion des apikalen Akto-Myosin-Netzwerkes im Rahmen dieser Doktorarbeit noch nicht identifiziert werden konnte, so können wir aufgrund von verschiedenen Resultaten dennoch postulieren, dass das hier identifizierte Netzwerk eine entscheidende Rolle bei der Zellmigration und Signaltransduktion einnimmt. Unabhängig davon repräsentiert das hier gefundene Netzwerk die faszinierende Möglichkeit, ein aktives, zweidimensionales Akto-Myosin-Netzwerk nicht nur in vitro, sondern in seiner natürlichen Umgebung studieren und biophysikalische Eigenschaften analysieren zu können.