Fakultät für Physik - Digitale Hochschulschriften der LMU - Teil 04/05

This work focuses on the interaction of few-cycle laser pulses with nanosystems. Special emphasis is placed on the spatio-temporal evolution of the induced near-fields. Measurements on carrier-envelope-phase (CEP) controlled photoemission from isolated SiO2 nanospheres are taken by single-shot velocity map imaging (VMI) combined with CEP tagging. The obtained photoelectron spectra show a pronounced dependence on the CEP and extend to unexpectedly high energies. Comparison with numerical simulations identify the additional Coulomb forces of the liberated electron cloud as an effective additional acceleration mechanism for distinct trajectories. For larger spheres, an asymmetry in the field distribution is classically predicted. This asymmetry is also observed in the photoelectron momentum distributions. The mapping between position and momentum space in the VMI approach are investigated by analyzing the correlation of the photoelectron's birth and detection position.

In a second set of experiments, photoemission at intensities exceeding 10^14 W/cm^2 from isolated nanospheres of different composition (SiO2, ZrO2, TiO2, Si, Au) is examined by stereo time-of-flight spectroscopy. It is found that the measured cutoff energies scale non-linearly with laser intensity depending on the material properties of the nanosystem. A trend towards a unified behavior for high intensities is observed indicating a drastic change in optical properties within the duration of the few-cycle laser pulse. The charge carrier generation mechanisms that could lead to such a transient effect are discussed.

For a better understanding of the interaction of few-cycle fields with nanosystems, a direct access to the temporal evolution of (plasmonic) near-fields is highly desirable. The efforts on the realization of nanoplasmonic attosecond streaking spectroscopy are presented. Numerical simulations are used to identify the influence of the inhomogeneous near-field distributions on the streaking process. First experimental results obtained from Au nanotips show clear streaking features of sub-micron localized near-fields. The near-field oscillations are found to be phase offset as compared to reference measurements. The exact origin of the streaking features of the Au tip and possible improvements of the experimental approach are discussed.

Massive gravity is a particular theoretical model that modifies gravity on cosmological scales and therefore could provide a dynamical explanation for the observed accelerated expansion of our Universe. In this thesis we investigate various theoretical problems of massive gravity, important for its consistency and phenomenological viability. It is known that the predictions from the linearized massive gravity contradict the predictions of General Relativity. It is, however, an artifact due to the breakdown of the perturbative expansion in the massless limit. In our work we investigate this problem in the diffeomorphism invariant formulation of massive gravity in which the graviton mass term is written in terms of four scalar fields. We determine the so-called Vainshtein scale below which the scalar modes of the massive graviton enter the non-perturbative regime for a wide class of non-linear mass terms. We find the asymptotic solutions of the spherically symmetric gravitational field below and above the Vainshtein radius, and show that massive gravity goes smoothly to the General Relativity below this scale. We also determine the corresponding corrections to the Newton potential. In general, any non-linear extension of the quadratic graviton mass term propagates the Boulware-Deser ghost. The only theory in which the ghost is not propagating in the high energy decoupling limit, is the de Rham-Gabadadze-Tolley theory. Here we show that the ghost arises in the fourth order of perturbations in this theory away from the decoupling limit. However, we further argue that the ghost can be avoided in the full non-linear theory if not all four scalar fields propagate independent degrees of freedom. In particular, we investigate the simple example of (1+1)-dimensional massive gravity and find that the theory exhibits a gauge symmetry, which reduces the number of degrees of freedom. We also generalize the diffeomorphism invariant formalism of massive gravity to arbitrary curved backgrounds. We find that, given a specific background metric, the resulting generally covariant massive gravity exhibits an internal symmetry in the configuration space of the scalar fields. The symmetry transformations of the scalar fields are given by the isometries of the reference metric. In particular, we investigate massive gravity on de Sitter space in this formalism. We confirm the known result that, in the case when the graviton mass is related to the cosmological constant as m^2=2\Lambda/3, the theory is partially massless and propagates only four degrees of freedom.

The petawatt field synthesizer (PFS) is a high-power optical parametric chirped-pulse

amplification (OPCPA) system under development, which aims at generating fewcycle

pulses with high energies of several Joule. The availability of light pulses with

these unique parameters will enable an efficient generation of even shorter attosecond

pulses with significantly higher photon flux than achievable today [1]. Not

only the real-time observation, but also the control of charge transfer in molecular

systems will become feasible for the first time [2].

The technique for realizing the ambitious PFS specifications is short-pulse pumped

OPCPA in mm-thin crystals. The reduced crystal thickness allows for ultra-broadband

amplification. The pump-pulse duration is reduced to a picosecond—compared to

100 ps to nanosecond pump-pulse duration in conventional high power OPCPA systems.

The shortened pulse duration facilitates higher pump intensities whereby an

efficient amplification in the mm-thin crystals is achieved.

The demonstration of this novel scheme in the PFS project will allow its use in

the extreme light infrastructure (ELI)[3]—a pan-European high-power laser project.

Based on the PFS technology for the front end, the ELI will generate exawatt peakpower

pulses and therefore facilitate the study of laser-matter interaction in an unprecedented

intensity range [4].

This work describes the CPA-aspects of a suitable chirped pulse amplification (CPA)

pump laser for the PFS OPCPA system. The diode-pumped Yb:YAG amplifiers up to

an energy of 300 mJ (at 1030 nm) are presented in combination with the dispersion

management. The application of spectral-amplitude shaping in conjunction with

an Yb:glass amplifier with broader bandwidth than Yb:YAG enables an unprecedented

bandwidth of 3.5nm in the Yb:YAG amplifier at this energy level. Simulations

show that a similar bandwidth can be maintained for the full amplifier system.

The pulses with 200 mJ could be compressed to 900 fs, close to the transform limit.

Later changes in the stretcher increase the bandwidth more and compression down

to 740 fs is demonstrated. To date, these are the highest peak power pulses generated

in Yb:YAG. For the application as OPCPA pump, the so generated pulses are

frequency doubled in a DKDP crystal.

Another key aspect of this work is the synchronization of the OPCPA pump and

signal pulses. In spite of optical synchronization of both pulses, a large timing fluctuation

between these pulses is measured at the first OPCPA stage. The high accuracy

jitter measurement setup and a series of measurements, which showed that the

stretcher/compressor setup is the main source of jitter, are presented. Theoretical

investigations yield that the optical delay in a compressor is orders of magnitude

more sensitive to angle changes compared to free space propagation. This makes the

stretcher and compressor extremely sensitive for timing jitter caused by turbulent

air or mechanical instabilities. This novel insight helped us to significantly reduce

the jitter to 100 fs and to demonstrate the feasibility of the PFS concept with first

broad-band OPCPA experiments.

Birds fly, cells crawl and bacteria swim. Each of these individuals has their own propulsion mechanism leading to a persistent motion, hence they all belong to the class referred to as propelled particle systems. Propelled particles in large number typically exhibit impressive self-organization processes such as the flocking motion of birds, the coherent motion of cell colonies and the swarming of bacteria. The emergence of collective motion in these non-equilibrium systems constitutes a ubiquitous phenomenon in nature—and perhaps one of the most fascinating. One reason for this might be the emergence of highly dynamic, coherently moving spatial patterns such as clusters, swirls or waves, and the fact that the patterns commonly extend over length scales much larger than the size of the individuals. To elucidate the physical principles underlying the collective motion of these particles, numerous theoretical studies have been devoted to model propelled particle systems by approaching the problem on all levels of description. These range from particle-based simulations to kinetic theory and hydrodynamic models. However, these models were typically inspired by the idea of universality and tended to analyze the generic non-equilibrium phenomena in propelled particle systems, thereby obscuring a one-to-one relation to experimental studies.

This thesis focusses on theoretical modeling approaches for propelled particle systems that are either in qualitative or quantitative agreement with recent observations and mea- surements in experimental propelled particle systems. Two experimental systems are specifically considered: The actin gliding assay, where molecular motors, immobilized on a substrate, propel actin filaments, and the polar vibrated disk assay, where disk-like gran- ular particles with a built-in polar asymmetry are driven by vertical vibrations. By means of various theoretical tools including rule-based automaton models, numerical solution of Newtonian equations of motion, and kinetic theory, the following central new findings and insights within the field of propelled particle systems were discovered:

Anomalously strong curvature fluctuations of single actin filaments moving in the glid- ing assay arise from two different interactions with the molecular motors. The motors either “Push” or “Hold” locally, giving rise to persistent movement or localized jams, which, in turn, lead to pronounced curvature kinks. Interestingly, both excitation and relaxation of curvature originates from these interactions, and it is shown that the impact of thermal fluctuations on the curvature distribution is negligible compared to these active fluctua- tions.

At high densities, filaments in the gliding assay form beautiful patterns of coherent motion such as clusters, swirls and waves. These patterns were shown to be triggered by local “ferromagnetic”-like alignment interactions between the filaments. In the later stages of pattern formation, hydrodynamic interactions become relevant. Coherently moving clus- ters induce a back-flow in the overlying fluid, mediating a ‘repulsive’ cluster–cluster or cluster–boundary interaction.

With the addition of crosslinking molecules to the gliding assay at high filament densities, an absorbing state comprised of open and closed rings can form. The assembly dynamics is fully understood in terms of a competition between merging events of filaments and filament growth that freezes the curvature. Specifically, by means of an appropriate particle-based model, the statistical properties of the system, such as the characteristics of the ring radii distribution and the ratio of open to closed rings as a function of the system’s noise level, was qualitatively reproduced.

The vibrated polar disk assay has a size of only about 20 particle diameters —a fact which precludes definitive conclusions on the nature of the underlying phase transition to a polarized state of coherent motion. By means of a microscopic

In der vorliegenden Arbeit wird mit Hilfe der verallgemeinerten

Eichtheorie/Gravitations-Dualität, welche stark gekoppelte Eichtheorien mit

schwach gekrümmten gravitativen Theorien verbindet, stark korrelierte

Quantenzustände der Materie untersucht. Der Schwerpunkt liegt dabei in

Anwendungen auf Systeme der kondensierten Materie, insbesondere

Hochtemperatur-Supraleitung und kritische Quantenzustände bei verschwindender

Temperatur. Die Eichtheorie/Gravitations-Dualität entstammt der Stringtheorie

und erlaubt eine Umsetzung des holographischen Prinzips. Aus diesem Grund

wird eine kurze Einführung in die Konzepte der Stringtheorie und ihre

Auswirkungen auf das holographische Prinzip gegeben. Für das tiefere Verständnis der effektiven Niederenergie-Feldtheorien wird zusätzlich die Supersymmetrie benötigt. Ausgestattet mit einem robusten Stringtheorie-Hintergrund wird die unterschiedliche Interpretation der Dirichlet- oder D-Branen, ausgedehnte Objekte auf denen offene Strings/Fäden enden können, diskutiert: Zum einen als massive solitonische Lösungen der Typ II Supergravitation und auf der anderen Seite, ihre Rolle als Quelle für supersymmetrische Yang-Mills Theorien. Die Verbindung dieser unterschiedlichen Betrachtungsweise der D-Branen liefert eine

explizite Konstruktion der Eichtheorie/Gravitations-Dualität, genauer

der AdS_5/CFT_4 Korrespondenz zwischen der N=4 supersymmetrischen SU(N_c) Yang-Mills Theorie in vier Dimensionen mit verschwindender beta-Funktion in allen Ordnungen, also eine echte konforme Theorie, und Type IIB Supergravitation in der zehn dimensionalen AdS_5 X S^5 Raumzeit. Darüber hinaus wird das Wörterbuch, das zwischen den Operatoren der konformen Feldtheorie und den gravitativen Feldern übersetzt, im Detail eingeführt. Genauer gesagt, die Zustandssumme der stark gekoppelten N=4 supersymmetrischen Yang-Mills Theorie im Grenzwert großer N_c, ist identisch mit der Zustandssumme der Supergravitation unter Berücksichtigung der zugehörigen Lösungen der Bewegungsgleichungen, ausgewertet am Rand des AdS-Raumes. Die Anwendung der perturbativen Quantenfeldtheorie und die Verbindungen zur quantenstatistischen Zustandssumme erlaubt die Erweiterung des holographischen Wörterbuchs auf Systeme mit endlichen Dichten und endlicher Temperatur. Aus diesem Grund werden alle Aspekte der Quantenfeldtheorie behandelt, die für die Anwendung der ``Linear-Response''-Theorie, der Berechnung von Korrelationsfunktionen und die Beschreibung von kritischen Phänomenen benötigt werden, wobei die Betonung auf allgemeine Zusammenhänge zwischen Thermodynamik, statistischer Physik bzw. statistischer Feldtheorie und Quantenfeldtheorie liegt. Des Weiteren wird der Renormierungsgruppen-Formalismus zur Beschreibung von effektiven Feldtheorien

und kritischen Phänomene im Kontext der verallgemeinerten Eichtheorie/Gravitations-Dualität ausführlich dargelegt. Folgende Hauptthemen werden in dieser Arbeit behandelt: Die Untersuchung der optischen Eigenschaften von holographischen Metallen und ihre Beschreibung durch das Drude-Sommerfeld

Modell, ein Versuch das Homes'sche Gesetz in Hochtemperatur-Supraleitern

holographisch zu beschreiben indem verschiedene Diffusionskonstanten und

zugehörige Zeitskalen berechnet werden, das mesonische Spektrum bei verschwindender Temperatur und schlussendlich holographische Quantenzustände bei endlichen Dichten. Entscheidend für die Anwendung dieses Rahmenprogramms auf stark korrelierte Systeme der kondensierten Materie ist die Renormierungsgruppenfluss-Interpretation der AdS_5/CFT_4 Korrespondenz und die daraus resultierenden emergenten, holographischen Duale, welche die meisten Beschränkungen der ursprünglichen Theorie aufheben. Diese sogenannten ``Bottom-Up'' Zugänge sind besonders geeignet für Anwendungen auf Fragestellungen in der Theorie der kondensierten Materie und der ``Linear-Response''-Theorie, mittels des holographischen Fluktuations-Dissipations-Theorem. Die Hauptergebnisse der vorliegenden Arb

Self-assembled InGaAs quantum dots are nano-objects embedded in the solid-state matrix

of GaAs. They act as natural potential traps for charge carriers and feature a number

of quantized states due to the quantum confinement. When incorporated in a field effect

structure the quantum dot states can be conveniently manipulated with an electric field

and probed by resonant laser spectroscopy. In this thesis self-assembled quantum dots were

investigated with an emphasis on the study of interactions between localized quantum dot

states and charge or spin reservoirs in the environment. Experimentally the quantum dots

were addressed in distinct regimes where the quantum dot spectrum was sensitive to individual charge fluctuations or mesoscopic reservoirs.

The fundamental transition of a neutral quantum dot was found to exhibit a number of

discontinuities in the usually linear dispersion of the exciton energy in external electrostatic fields. The discontinuities were identified to arise from charge fluctuations in the

surrounding crystalline matrix in which impurity atoms can capture or release electrons.

At characteristic conditions charging and discharging events lead to discrete changes of

the electrostatic environment which in turn gives rise to an energy shift of the optical

resonance condition. An electrostatic model was developed for a quantitative analysis of

charging events and their signatures. On the basis of the model a comprehensive study of

nearby quantum dots allowed to map out the relative spatial positions of quantum dots and

impurities. In contrast to previous reports our results provide evidence for bulk impurities

as the main source of charge fluctuations.

By means of resonant laser spectroscopy in the energy dispersion of the neutral exciton a

kink with a continuous energy shift has been observed which only occurs close to the regime

where an electron is tunneling between the quantum dot and a 2D electron reservoir. The

tunneling induces a weak coupling between the localized electron state of the quantum dot

and the continuum of states in the reservoir. The tunnel coupling between the interacting

states leads to hybridization into a new superposition state. In consequence the energy

of the transition is renormalized which explains the kink in the energy dispersion. The

hybridization model based on an Anderson-Fano approach quantitatively agrees with the

experimental data and allows to extract the coupling strength between the reservoir and

the localized state. In addition to the neutral exciton hybridization effects were also ob-served on the charged exciton.

To study optical signatures of many-body effects sub-K laser spectroscopy was established

and the setup performance was characterized with optical studies of a quantum dot in the

Pauli-blockade regime. The electron bath temperature was determined using experimental

and calculated electron spin populations as a function of magnetic field and temperature.

The experiment provided quantitative access to all parameters except the electron bath

temperature. With the optical Bloch equations the electron spin populations were modeled

taking into account all relevant external parameters. An analysis of the evolution of the

spin population in magnetic fields with the electron bath temperature as the only free fitting parameter was performed. An electron bath temperature of 380 mK was derived being

slightly offset to the nominal base temperature of 250 mK. This proves the successful

implementation of the sub-K laser spectroscopy setup.

The centers of galaxies are their own ultimate gravitational sinks.

Massive black holes and star clusters as well as gas are especially likely to fall into the centers of galaxies by

dynamical friction or dissipation. Many galactic centers harbor supermassive black holes (SMBH)

and dense nuclear (star) clusters which possibly arrived there by these processes.

Nuclear clusters can be formed in situ from gas, or from smaller star clusters which fall to the center.

Since the Milky Way harbors both an SMBH and a nuclear cluster, both can be studied best in the Galactic Center (GC),

which is the closest galactic nucleus to us.

In Chapter 1, I introduce the different components of the Milky Way, and put these into the context of the GC.

I then give an overview of relevant properties (e.g. star content and distribution) of the GC.

Afterwards, I report the results of four different studies about the GC.

In Chapter 2, I analyze the limitations of astrometry, one of the most useful methods for the study of the GC. Thanks to the

high density of stars and its relatively small distance from us it is possible to measure the motions of thousands of stars in the GC with images,

separated by few years only. I find two main limitations to this method: (1) for bright stars the not perfectly correctable distortion of the

camera limits the accuracy, and (2)

for the majority of the fainter stars, the main limitation is crowding from the other stars in the GC. The position uncertainty of faint stars is mainly

caused by the seeing halos of bright stars. In the very center faint unresolvable stars are also important for the position uncertainty.

In Chapter 3, I evaluate the evidence for an intermediate mass black hole in the small candidate cluster IRS13E within the GC.

Intermediate mass black holes (IMBHs) have a mass between the two types of confirmed black hole: the stellar remnants and the

supermassive black holes in the centers of galaxies.

One possibility for their formation is the collision of stars in a dense young star cluster. Such a cluster

could sink to the GC by dynamical friction. There it would consist of few bright stars like IRS13E.

Firstly, I analyze the SEDs of the objects in IRS13E. The SEDs of most objects can be explained by pure dust emission.

Thus, most objects in IRS13E are pure dust clumps and only three young stars.

This reduces the significance of the 'cluster' IRS13E compared to the stellar background.

Secondly, I obtain acceleration limits for these three stars. The non-detection of accelerations makes an IMBH an

unlikely scenario in IRS13E. However, since its three stars

form a comoving association, which is unlikely to form by chance, the nature of IRS13E is not yet settled.

In the third study (Chapter 4) I measure and analyze the extinction curve toward the GC. The extinction is a contaminant for GC

observations and therefore it is necessary to know the extinction toward the GC to determine the luminosity properties of its stars.

I obtain the extinction curve by measuring

the flux of the HII region in the GC in several infrared HII lines and in the unextincted radio continuum.

I compare these ratios with the ratios expected from recombination

physics and obtain extinctions at 22 different lines between 1 and 19 micron. For the K-band I derive A_Ks=2.62+/-0.11.

The extinction curve follows a power law with a steep slope of -2.11+/-0.06 shortward of 2.8 micron. At longer wavelengths the extinction is grayer and

there are absorption features from ices. The extinction curve is a tool to constrain the properties of cosmic dust between the sun

and the GC. The extinction curve cannot be explained by dust grains consisting of carbonaceous and silicate grains only. In addition

composite particles, which also contain ices are necessary to fit the extinction curve.

In the final part of this thesis (Chapter 5) I look at the properties of most of the stars in

Der Compact Linear Collider (CLIC) ist ein Konzept eines 48.3km langen e+ e- Beschleunigers mit einer Schwerpunktsenergie von 3TeV. Sein Ziel ist sowohl die Präzisionsvermessung bereits bekannter, als auch die Entdeckung bislang unbekannter Teilchen. Der International Large Detector (ILD) ist eines der Detektorkonzepte, die speziell für die Anwendung des Particle Flow Algorithmus entworfen wurde. Der Inhalt dieser Arbeit gliedert sich in zwei Teilbereiche, die beide im Kontext von CLIC stehen.

Im ersten Teil dieser Arbeit wird die Messung der Zeitstruktur hadronischer Schauer in Kalorimetern mit Wolframabsorber, wie es auch im ILD Konzept für CLIC benutzt wird, präsentiert. Das beinhaltet die Entwicklung und Konstruktion eines kleinen Testbeam-Experimentes namens Tungsten Timing Testbeam (T3B), welches aus lediglich 15 Szintillator Kacheln der Dimension 30mm x 30mm x 5mm besteht. Diese werden mit Silicon Photomultipliern ausgelesen, welche wiederum mit USB Oszilloskopen verbunden sind. T3B wurde während der Testreihen am CERN in den Jahren 2010 und 2011 hinter dem Prototypen des analogen hadronischen Kalorimeters (W-AHCal) der CALICE Kollaboration platziert. Die gewonnenen Daten wurden mit Simulationsergebnissen verglichen, die mit den drei verschiedenen Modellen hadronischer Schauer der Geant4 Simulation gewonnen wurden: QGSP_BERT, QGSP_BERT_HP und QBBC. Die Ergebnisse der 60GeV Datennahme sind zumeist mit QBBC und QGSP_BERT_HP konsistent. Hingegen überschätzt QGSP_BERT wegen der fehlenden Präzisionsverfolgung von Neutronen die Häufigkeit später Energiedepositionen.

Im zweiten Teil wird einer der sechs Benchmark-Prozesse gezeigt, die im Rahmen des CLIC Conceptual Design Report die Detektor Leistungsfähigkeit am CLIC Beschleuniger gezeigt haben. Der vorgestellte Benchmark Prozess behandelt die Messung der Masse und des Wirkungsquerschnitts der Paarerzeugung von supersymmetrischen rechtshändigen skalaren Quarks (squarks). Im zugrundeliegenden SUSY Modell zerfallen diese fast ausschließlich in das leichteste Neutralino (fehlende Energie) und das zugehörige Standardmodell Quark (Jet). In der Analyse wird der Beam-generierte pile-up Untergrund von gamma gamma -> Hadronen durch die Anwendung der Hadronen Variante des kt-Algorithmus der FastJet-Bibliothek unterdrückt. Standardmodell Prozesse, die der Ereignisstopologie entsprechen, werden durch die Anwendung von Boosted Decision Trees, implementiert im Toolkit for Multivariate Analysis (TMVA), zurückgewiesen. Die Squark-Masse wird durch die Konstruktion der MC-Verteilung und dem folgenden Fit an die mit verschiedenen Squark-Massen generierten Templates extrahiert. Die Ergebnisse sind konsistent mit ihren Eingangswerten und zeigen, dass Massenmessungen von schweren, stark wechselwirkenden Teilchen bei CLIC mit Genauigkeiten von unter 1% möglich sind.