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

Die Masse des Top Quarks ist ein fundamentaler Parameter des Standardmodells und ihre präzise Bestimmung ist von großer Bedeutung für die Teilchenphysik. In dieser Dissertation werden Messungen der Top Quark Masse im dileptonischen Zerfallskanal von Top Quark Paaren präsentiert und experimentelle und theoretische Aspekte der Präzisionsmessung untersucht. Neben technischen Maßnahmen zur Gewährleistung optimaler Nachweiskapazität für zukünftige Datennahme werden Messungen der Top Quark Masse mit den Daten der Jahre 2011 und 2012 des ATLAS Detektors durchgeführt, basierend auf Proton-Proton Kollisionen mit einer Schwerpunktsenergie von √s = 7 und 8 TeV. Verschiedene Techniken zur Reduzierung der statistischen und wichtigsten systematischen Unsicherheiten werden angewandt, was zur bisher genauesten Einzelmessung der Top Quark Masse im dileptonischen Top Quark Paar Zerfallskanal weltweit führt. Durch eine Kombination mit ATLAS Messungen unter Berücksichtigung der Korrelationen wird die Präzision weiter erhöht. Die in einer Blindstudie ermittelte Masse des Top Quarks ist mtop = 172.40 ± 0.31 (stat) ± 0.62 (syst) GeV/c2 = 172.40 ± 0.70 GeV/c2, wobei die Unsicherheit von der begrenzten Auflösung der Jetenergiemessungen dominiert wird. Außerdem werden mit einer Entfaltungsmethode die Daten von Detektoreffekten bereinigt und die ersten Schritte zu einer Messung der Top Quark Masse auf dem Niveau stabiler Teilchen durchgeführt. Anschließend werden die Auswirkungen von vollen QCD Rechnungen in zweiter Ordnung Störungstheorie auf Top Quark Massenmessungen untersucht.

This thesis focuses on the development and application of Bayesian inference techniques for early-Universe signals and on the advancement of mathematical tools for information retrieval. A crucial quantity required to gain information from the early Universe is the primordial scalar potential and its statistics. We reconstruct this scalar potential from cosmic microwave background data. Technically, the inference is done by splitting the large inverse problem of such a reconstruction into many, each of them solved by an optimal linear filter. Once the primordial scalar potential and its correlation structure have been obtained the underlying physics can be directly inferred from it. Small deviations of the scalar potential from Gaussianity, for instance, can be used to study parameters of inflationary models. A method to infer such parameters from non-Gaussianity is presented. To avoid expensive numerical techniques the method is kept analytical as far as possible. This is achieved by introducing an approximation of the desired posterior probability including a Taylor expansion of a matrix determinant. The calculation of a determinant is also essential in many other Bayesian approaches, both apart from and within cosmology. In cases where a Taylor approximation fails, its evaluation is usually challenging. The evaluation is in particular difficult, when dealing with big data, where matrices are to huge to be accessible directly, but need to be represented indirectly by a computer routine implementing the action of the matrix. To solve this problem, we develop a method to calculate the determinant of a matrix by using well-known sampling techniques and an integral representation of the log-determinant. The prerequisite for the presented methods as well as for every data analysis of scientific experiments is a proper calibration of the measurement device. Therefore we advance the theory of self-calibration at the beginning of the thesis to infer signal and calibration simultaneously from data. This is achieved by successively absorbing more and more portions of calibration uncertainty into the signal inference equations. The result, the Calibration-Uncertainty Renormalized Estimator, follows from the solution of a coupled differential equation.

Der anthropogene Klimawandel wird vor allem durch die Emissionen von Treibhausgasen (GHG) verursacht, welche den Energiehaushalt der Erde ändern. Der Anstieg in GHG- Konzentrationen verstärkt nicht nur den strahlungsgetriebenen Treibhauseffekt, sondern beeinflusst auch die atmosphärische Zirkulation sowie biogeochemische Kreisläufe. Rückkopplungsprozesse von Biogeochemischen Kreisläufen können dabei die Klimaerwärmung verstärken oder abschwächen. Aktuelle Erdsystemmodelle (ESMs) aus der fünften Phase des Coupled Model Intercomparison Project (CMIP5), beinhalten solche biogeochemische Prozesse. Diese ermöglichen die Untersuchung von biogeochemischen und Klima Feedbacks des Erdsystems. Diese Feedbacks in Klimaprojektionen unterliegen jedoch großen Unsicherheiten, da das Verständnis der zugrundeliegenden Prozessen und deren Repräsentation in ESMs oft noch unzureichend ist. Das Ziel dieser Arbeit ist zu untersuchen wie beobachtbare Eigenschaften des aktuellen Klimas genutzt werden können, um Unsicherheiten in ausgesuchten Rückkopplungsprozessen zu reduzieren. Um den Zusammenhang zwischen der Klimasensitivität auf anthropogen verursachte Klimaänderungen und beobachtbare Eigenschaften des globalen Klimasystems besser zu verstehen, wurde die relativ neue Methode der so genannten Emergent Constraints verwendet. Emergent Constraints beschreiben dabei Zusammenhänge zwischen einem Aspekt der simulierten Erdsystemsensitivität und einem beobachtbaren Trend oder Variation des aktuellen Klimas. Diese Methode wurde in dieser Arbeit verwendet um Feedbacks im Kohlenstoffkreislauf sowie Änderungen in der Position des Südhemisphären (SH) Jets auf anthropogene Klimaänderungen genauer zu bestimmen. Dafür wurden neue Diagnostiken entwickelt und in das Earth System Model Evaluation Tool (ESMValTool) implementiert. Diese erste Studie nutzt Beobachtungsdaten, um den Kohlenstofkreislauf-Klima- Feedback genauer zu bestimmen und wurde in Journal of Geophysical Research 2014 publiziert. In den meisten Klimaprojektionen führt eine Erwärmung des Klimas zu einer geringeren Aufnahmefähigkeit von atmosphärischem Kohlenstoff Dioxid (CO2) durch die terrestrische Senke. Als Ergebnis bleibt mehr CO2 in der Atmosphäre zurück wo es als GHG klimawirksam ist. Dieser Effekt beschreibt einen positiven Rückkopplungsprozess des Kohlenstoffkreislaufes zur Klimaerwärmung (L) und wird durch den anteiligen Kohlenstoffverlust pro Kelvin Erwärmung quantifiziert, in Einheiten von GtC pro K. Dieser unterliegt jedoch starken Unsicherheiten in Klimaprojektionen des 21. Jahrhunderts. CMIP5 Modelle simulieren den Betrag der tropischen terrestrischen Kohlenstoffsenke, bei ausgeblendeten Klimaeinwirkungen auf den Kohlenstoffkreislauf, im Bereich von 252 ± 112 GtC für eine Verdopplung atmosphärischen CO2 Konzentrationen. Eine gute Korrelation zwischen dem Kohlenstoffkreislauf-Klima-Rückkopplungsfaktor und der beobachtbaren Sensitivität der interannualen CO2-Wachstumsrate auf Temperaturschwankungen ermöglicht es die Unsicherheiten in Klimaprojektionen mit Beobachtungen einzuschränken. Die beobachtete Sensitivität (-4.4 ± 0.9 GtC per year and K) reduziert dabei die Unsicherheiten zu -44 ± 14 GtC pro K um mehr als die Hälfte im Vergleich zum Multimodellmittelwert von 49 ± 40 GtC pro K. Die Ergebnisse der ersten Studie implizieren, dass mit einem Temperaturanstieg weniger Kohlenstoff in der terrestrischen Senke gespeichert wird. Dieser Effekt ist im Vergleich zum Multimodellmittel für den neu berechneten Wert geringer, was einen geringeren Anstieg der CO2 Konzentration durch Klimaerwärmung bedeutet. Die zweite Studie nutzt Beobachtungsdaten, um den Kohlestofkreislauf-CO2 Feedback genauer zu bestimmen und ist in der Begutachtung bei Nature. Unsicherheiten in der Sensitivität des Landökosystems auf erhöhte atmosphärische CO2 Konzentrationen tragen zusätzlich zu Unsicherheiten von Klimaprojektionen bei. CMIP5 Modelle mit interaktivem Kohlenstoffkreislauf simulieren für einen Anstieg der atmosphärischen CO2 Konzentration eine Erhöhung der terrestrischen Brutto Primärproduktion (GPP). Dieser Düngeeffekt wird jedoch von den CMIP5 Modellen unterschiedlich stark für eine aktuelle atmosphärische CO2 Konzentration (ca. 400 ppmv) simuliert und ist im Bereich von 7.5 ± 7 GtC relativ zu vorindustriellen Zeiten. In dieser Studie wurde eine starke Korrelation zwischen dem Düngeeffekt von CO2 auf GPP in höheren Breiten sowie den Extratropen und der beobachteten Änderung der CO2 Amplitude im Jahresgangs (0.05 ± 0.001 ppmv pro ppmv) festgestellt. Mithilfe der Beobachtungen konnte für eine Verdopplung der atmosphärischen CO2 Konzentrationen ein Düngeeffekt auf GPP in hohen Breiten von 0.14% pro ppmv und für GPP in den extratropischen Regionen von 0.12% pro ppmv ermittelt werden. Durch die Anwendung der beobachtungsbasierte Methode auf den Kohlestofkreislauf-CO2 Feedback konnte deutliche Verringerung der Unsicherheiten des Düngeeffekts erzielt werden. Die dritte Studie nutzt Beobachtungen um die Position des SH Jets in Klimaprojektionen genauer zu bestimmen und wurde im Journal of Climate 2016 publiziert. Die Zuname stratosphärischen Ozons und den Anstieg von GHG haben einen starken Einfluss auf die SH extratropische Zirkulation was eine Verlagerung der SH Jetposition zur Folge hat. Die mittlere SH Jetposition ist in CMIP Modellen in Bezug auf Beobachtungsdaten zum Äquartor verschoben und die Modelle simulieren eine Verteilung der Jetposition über 10 Grad in der historischen Klimatologie und in Klimaprojektionen. Die Multiple Diagnostik Ensemble Regression (MDER) Methode wurde verwendet um prozess-orientierte Diagnostiken des aktuellen Klimas mit Projektionen der SH Jetposition zu korrelieren. Die MDER Methode wurde auf den Zeitraum 2015 - 2034 angewendet, wo sie aus den 20 Diagnostiken die historische Jetposition als die wichtigste Größe aussucht. Die Methode detektiert den zum Äquator hin verschobenen Bias in der historischen Jetposition und berechnet eine Korrektur von 1.5 Grad südlich für die Vorhersage. Durch die Analyse konnte somit eine Verbesserung zum Ensemblemittelwert und dessen Unsicherheit erzielt werden. Emergent Constraints, wie sie in dieser Arbeit untersucht wurden, können helfen Modellentwicklungen und Beobachtungen auf Prozesse zu fokussieren, die zur Größenordnung und den Unsicherheiten zukünftiger Klimavorhersagen maßgeblich beitragen.

The ability to probe and manipulate electron dynamics and correlations on their characteristic time scales would open up many technological and scientific possibilities. While modern laser technology already allows to do that in principle, a lot of theoretical ground work is still missing. This thesis focuses on the elementary effect of laser strong field ionization of the two simplest systems: The Hydrogen and Helium atoms. To that end, the time-dependent Schroedinger equation is solved numerically, and photo-electron spectra are extracted using the highly efficient tSurff technique. We implemented both the one and two particle versions of tSurff together with several other numerical techniques in a new parallelizable C++ code. We provide details on the employed methods and algorithms, and study numerical efficiency properties of various approaches. We propose a description of the electric field interaction in a mixture of length and velocity gauge for the correct and most efficient implementation of a coupled channels approach, which can be used to compute accurate single ionization photo-electron spectra from true multi-electron systems, even molecules. We provide extensive numerical data for a detailed study of the Hydrogen atom in an Attoclock experimental setup, where it is found that the involved strong field tunnel ionization processes can be considered instantaneous. In particular, there appear no tunneling delays, which can be used as a calibration for experiments with more complicated targets. Similarly, it is investigated whether discrepancies between theory and experimental data for the longitudinal photo-electron momentum spread, resulting from photo-ionization of Helium in elliptically polarized laser pulses, can be explained by non-adiabatic effects, and a related consistency problem in current laser intensity calibration methods is pointed out. We further show that Fano resonance line shapes of doubly excited states in the Helium atom, prominently appearing in single ionization spectra generated by short wavelength laser pulses, can be controlled by an external long wavelength streaking field. The resulting line shapes are still characterized by the general Fano situation, but with a complex - rather than real - Fano parameter. We provide a theoretical description of this two color process and prove numerically that the entire doubly excited state series exhibits synchronized line shape modifications as the specifics of the involved states are unimportant. Finally, we compute fully differential double ionization spectra and suggest a measure of correlation that is directly applicable to experimental data. We confirm literature results at short wavelengths, and achieve to compute five-fold differential double ionization photo-electron spectra at infrared wavelengths from the Helium atom, thereby reproducing a characteristic several orders of magnitude enhancement of double emission due to correlation effects.

While quantum theory has been tested to an incredible degree on microscopic scales, quantum effects are seldom observed in our everyday macroscopic world. The curious results of applying quantum mechanics to macroscopic objects are perhaps best illustrated by Erwin Schrödinger's famous thought experiment, where a cat can be put into a superposition state of being both dead and alive. Obviously, these quantum predictions are in stark contradiction to our common experience. Even with plenty of theoretical explanations put forward to explain this discrepancy, a large number of questions about the frontier between the quantum and the classical world remain unanswered. To distinguish between classical and quantum behavior, two fundamental concepts inherent to classical physics have been established over the years: The world view of local realism limits the power of classical experiments to establish correlations over space, while the world view of macroscopic realism (or macrorealism) restricts temporal correlations. Necessary conditions for both world views have been formulated in the form of Bell and Leggett-Garg inequalities, and Bell inequalities have been shown to be violated by quantum mechanics through increasingly conclusive experiments. Furthermore, many challenging steps towards convincing violations of macrorealism have been taken in a number of recent experiments. In the first part of this thesis, conditions for macrorealism are analyzed in detail. Two necessary conditions for macrorealism, the original Leggett-Garg inequality and the recently proposed no-signaling in time condition, are presented. It is then shown that a combination of no-signaling in time conditions is not only necessary but also sufficient for the existence of a macrorealistic description. Finally, an operational formulation of no-signaling in time, in terms of positive-operator valued measurements and Hamiltonians, is derived. In the next part, we argue that these results lead to a suitable definition of classical behavior. In particular, we provide a formalism to judge the classicality of measurements and time evolutions. We then proceed to apply it to a number of exemplary measurement operators and Hamiltonians. Finally, we argue for the importance of spontaneously realized Hamiltonians in our intuition of classical behavior. Next, differences between local realism and macrorealism are analyzed. For this purpose, the probability polytopes for spatially and temporally separated experiments are compared, and a fundamental difference in the power of quantum mechanics to build both types of correlations is discovered. This result shows that Fine's theorem, which states that a set of Bell inequalities is necessary and sufficient for local realism, is not transferable to macrorealism. Thus, (Leggett-Garg) inequalities are in principle not well-suited for tests of macrorealism, as they can never form a necessary and sufficient condition, and unnecessarily restrict the violating parameter space. No-signaling in time is both better suited and more strongly motivated from the underlying physical theory. In the final part of this thesis, a concrete experimental setup for implementing quantum experiments with macroscopic objects is proposed. It consists of a superconducting micro-sphere in the Meißner state, which is levitated by magnetic fields. Through its expelled magnetic field, the sphere's center-of-mass motion couples to a superconducting quantum circuit. Properly tuned, ground state cooling can be realized, since the sphere's motion is extremely well isolated from the surrounding environment. This setup therefore is a promising candidate for the observation of quantum effects in macroscopic systems.

This thesis aims to answer the question if 3D effects of thermal radiative transfer need to be considered in cloud resolving simulations and if an influence of 3D thermal heating and cooling rates exists in contrast to common 1D approximations. To study this question with the help of a cloud resolving model, an accurate, yet fast parameterization of 3D radiative transfer is needed. First, an accurate 3D Monte Carlo model was developed which was used as benchmark for developing the fast `Neighboring Column approximation' (NCA), which was then coupled to the UCLA-LES to study the effects of 3D thermal heating and cooling rates in comparison to common 1D radiative transfer approximations. First, differences between common 1D radiative transfer approximations and a correct 3D radiative transfer model were analyzed. For this, efficient Monte Carlo variance reduction methods have been developed and implemented in MYSTIC, a Monte Carlo radiative transfer model. The dependence of 1D and 3D heating and cooling rates on cloud geometry has been investigated by analyzing idealized clouds such as cubes or half spheres. Further more, 1D and 3D heating and cooling rates in realistic cloud fields were simulated and compared. It could be shown that cooling rates reach maximum values of several 100 K/d at cloud tops if the model resolution was between 50 m to 200 m. Additional cloud side cooling of several 10 to 100 K/d was found in 3D heating and cooling rate simulations. At the cloud bottom, modest warming of a few 10 K/d occurs. Heating and cooling rates depend on the vertical location of the cloud in the atmosphere, the liquid water content of the cloud, the shape of the cloud and the geometry of the cloud field (for example the distance between clouds). Based on the results of a detailed analysis of exact simulations of 3D thermal heating and cooling rates, a fast, but still accurate 3D parameterization for thermal heating and cooling rates has been developed. This parameterization, the `Neighboring Column Approximation' (NCA), is based on a 1D radiative transfer solution and uses the next neighboring columns of a column to estimate the 3D heating or cooling rate. The method can be used in parallelized models. With the NCA, it is possible to simulate 3D cloud side cooling and warming. It was shown that the NCA is a factor of 1.5 to 2 more expensive in terms of computational time when used in a cloud resolving model, compared to a 1D radiative transfer approximation. The NCA was implemented in UCLA-LES, a cloud resolving, large-eddy simulation model. With the UCLA-LES and the NCA it was possible for the first time to study the effects of 3D interactive thermal radiation on cloud development. Simulations without radiation, with 1D thermal radiation and 3D thermal (NCA) radiation have been performed and differences have been analyzed. First, single, isolated clouds were investigated. Depending on the cloud shape, 3D thermal radiation changes cloud development in comparison to 1D thermal radiation. Overall it could be shown that a thermal radiation effect on cloud development exists in general. Whether there is a differences between 1D and 3D thermal radiation on cloud development seems to depend on the specific situation. One of the main features of thermal radiation affecting a single cloud is a change in the cloud circulation. Stronger updrafts in the cloud core and stronger downdrafts at the cloud sides were found, causing an enhanced cloud development at first, but a faster decay of the cloud in the end. Second, large scale simulations of a shallow cumulus cloud field in a 25 x 25 km^2 domain with 100~m horizontal resolution were analyzed. To the authors knowledge, this is the first time that a cloud field of this size and resolution was simulated including 3D interactive thermal radiation. It was shown that on average, updrafts, downdrafts and liquid water increases if thermal radiation is accounted for. While most variables (for example liquid water mixing ratio or cloud cover) did not show significant systematic difference between no-radiation simulation and the simulations with 1D and 3D thermal radiation, the cloud size (or horizontal extent) was larger in the simulations with interactive 3D thermal radiation. Convective organization set in after a few hours already. This is a clear indication that 3D thermal radiation could trigger convective organization.

The interaction of photons with individual quantum systems is a very fundamental process in physics. Thereby, the emission rate as well as the angular emission pattern of a quantum emitter are not only a function of intrinsic properties of the emitter itself, but are also strongly modified by its surrounding. For instance, by restricting the optical modes which are allowed at the position of the dipole, the emission rate can be strongly modified and the emitted photons can be directed into specific optical modes. This effect can be demonstrated by the interaction of a single optically active quantum emitter with the strongly confined optical mode of a single-mode dielectric waveguide. Efficient coupling of the emitter to the dielectric structure can be achieved by placing the quantum emitter inside the evanescent field of the guided mode. This evanescent field coupling mechanism is discussed and demonstrated experimentally. A single nitrogen-vacancy center (NV-center), hosted in a nanodiamond is deterministically coupled to a tapered optical fiber (TOF) via the evanescent field of its guided mode (coupling efficiencies exceeding 30% are predicted). By employing an AFM-based nanomanipulation technique, the diamond nanocrystal is placed on the nanofiber waist of the TOF. Beforehand, the diamond nanocrystal has been characterized to guarantee that it hosts only one fluorescing NV-center. While the diamond nanocrystal is optically exited, single photon fluorescence of the NV-center is detected at both outputs of the tapered optical fiber. This verifies the evanescent coupling of the emitter to the guided mode. In order to quantify the coupling, the comparison of the emission rate into free space with the rate into the fiber yields that 10.0(5) of the emitted photons are coupled into the tapered optical fiber. In the determination of this value, the orientation of the emitting dipoles and the emission pattern, which are modified by the TOF, have been considered. The NV-center features a broad emission spectrum which can be used to investigate the wavelength-dependence of the coupling. Comparing the spectra of the emission into the fiber mode with the emission into free space modes roughly resembles the expected wavelength dependency of the coupling efficiency. The evanescent coupling and the deterministic positioning of preselected fluorescing diamond nanocrystals, which has been demonstrated with the TOF, can be applied to other waveguide structures as well. Dielectric single-mode waveguides made of Ta2O5 on a SiO2 substrate promise similar coupling efficiencies to tapered optical fibers (above 30%). With the design of the on-chip wave-guiding structure being flexible, the combination with other optical on-chip elements is feasible, rendering it a promising platform for on-chip photonic experiments. Test structures of this waveguide design are realized using lithographic processes and are characterized. These waveguides are equipped with inverted taper structures to allow efficient off-chip coupling with butt-coupling to standard single-mode fibers. The evanescent coupling of a single quantum emitter to a singe optical mode can be used to efficiently collect emission of the quantum emitter. This can help building a compact single photon source and is beneficial for the optical read-out of the quantum emitter's internal degree of freedom, which can be either used as probe (sensing) or as information-storage. Utilizing the high coupling efficiency, for instance, the non-linearities of the quantum system can be exploited to build a single photon transistor. The evanescent coupling is very broadband (about hundred nanometers), allowing to efficiently collect emission from broadband emitters like the NV-center, but it can also be used for multi-wavelength manipulation schemes.

Active Galactic Nuclei (AGN) are the accreting super massive black hole (SMBH) at the center of massive galaxies. The tight M-σ and M_BH-M_bulge correlations reveal that the host galaxies are affected by the effects of the SMBHs. In addition, many works studying on the rest-frame color-magnitude relation have shown that AGN host galaxies have intermediate colors, which are considered as a transition from the blue cloud to red sequence in host-galaxy color evolution. Some works interpreted this result as an evidence for the AGN feedback, in the sense that the accretion process not only builds up the massive black holes, but also has a powerful influence on the surrounding environment, triggering or suppressing the star-forming activity in the host galaxy. These correlations make obvious the need to investigate AGN-host coevolution. One way to understand this coevolution is to study the AGN duty cycle (i.e., the time scale that the SMBH is active), which can be obtained by estimating AGN population among all the galaxies through cosmic times. Such demographic studies require a well-sampled census and accurate redshift information. In particular at high redshift, objects are extremely faint and sample numbers are very small. This could cause large statistical errors. For example, Aird et al. (2010) argued that luminosity-dependent density evolution with a flattening faint-end slope of the X-ray luminosity function (XLF) at z > 1.2 may result from catastrophic photo-z failures caused by observational limitations and improper templates used for photo-z computation. However, spectroscopic redshifts are time-consuming and difficult to be obtained for faint sources at high redshifts. Therefore we have to rely on photometric redshifts (photo-z) techniques which need to be tuned specifically to be reliable for AGNs (i.e., proper magnitude priors, appropriate AGN-galaxy hybrid template for SED-fitting, and correct multi-wavelength counterparts). In recent years, many deep and high-resolution observations become available in multiple wavebands, specially at near/mid-infrared. This allows us to reach higher redshift, and make more accurate analysis on the multi-wavelength properties of AGNs. In this thesis, we focus on the ECDFS area which comprise also the GOODS-S and CDFS regions. This is the portion of the sky with the deepest and most complete photometric information from X-ray to radio, including intermediate bands from the Subaru telescope, and optical/near-infrared data from the Hubble space telescope. To compute accurate photo-z using these data, first we combined multi-wavelength catalogs from UV to infrared after the astrometric calibration and correction for the different methods of flux extraction (e.g., total fluxes, flux apertures and PSF- fitted photometry). Second we identified the best multi-wavelength counterparts for X-ray sources from the 4Ms-CDFS and 250ks-ECDFS surveys, taking into account the positional errors and multiple magnitude distributions as priors simultaneously. We found that more then 96% of X-ray sources have multi-wavelength counterparts. Thirdly we built a new library of active galactic nuclei/galaxy hybrid templates appropriate for the faint X-ray population in the CDFS to simulate the AGN spectral energy distribution from low to high redshift. For X-ray-selected AGNs, we achieved a photo-z accuracy of 0.013 with an outliers fraction of 5.3%, while for non-X-ray galaxies, the photo-z accuracy is 0.010 with an outlier fraction of 4.6%. With the SED-fitting results of our well-trained AGN-galaxy hybrids, we further studied the galaxy and AGN host properties via the rest-frame color-magnitude diagram (CMD) which is an useful probe to trace the stellar populations. We made corrections for dust extinction and/or AGN contamination for the galaxy/AGN host colors in the CANDELS/GOODS-S region. We found that the AGN host colors also present bimodality in the CMD up to z~2.5 as found in normal galaxies, and the position of the blue peaks in the AGN samples are almost constant with cosmic time. This implies a weak connection between AGN activity and star formation in the host galaxy. For the X-ray sources in the 4Ms-CDFS survey, we found that for most of the sources, the correction for dust extinction is larger than the correction for the AGN contribution. This is because the AGN population in this field is dominated by low-luminosity AGNs which have host-dominated SEDs. However for few bright sources, their host colors are strongly effected by AGN contribution rather than by the dust extinction. For these sources, the correction for AGN contribution is about two times larger than the correction for dust extinction in general. Therefore AGN/galaxy decomposition becomes more important in a shallower and wider X-ray surveys, e.g., XMM-COSMOS and eROSITA, which contains a larger fraction of bright AGNs. Furthermore, with our accurate redshifts for galaxies and AGNs, we defined a high-redshift (high-z) sample using the redshift probability distribution function P(z) rather than relying on the best-fit value of photo-z. We integrated P(z) within a given redshift range to obtain the photo-z probability in that range and selected high-z sources above a given threshold. When computing the number of sources in a given redshift range, each source will not be counted as "1" but as the proportion of it. We compared this P(z) technique with traditional color techniques adopted for galaxy evolutionary stages, like the Lyman break galaxy and the BzK color-color selection via sample completeness and purity. We found that the P(z) technique is the most efficient and reliable method for selecting high-z sources. This is not surprising as it makes use of photometric information from the entire SED rather than using only three photometric points. Lastly, we built a high-z (z > 3) sources list for X-ray sources in ECDFS region, and compared our list with previous work. In our work, we made better X-ray-to-optical/NIR associations considering the positional errors and magnitude distribution. In addition, we obtained accurate photo-z using well-established AGN-galaxy hybrids for X-ray selected AGNs and applied P(z) for each source. These procedures help improving on our high-z sample selection.

The work presented in this thesis was aimed at developing a high-repetition rate source of coherent radiation in the extreme ultra-violet (XUV) spectral region, envisaging applications in attosecond physics or precision metrology in the XUV. Due to the lack of laser oscillators operating in the XUV, the method of choice was the frequency upconversion of a near-infrared laser via the nonlinear process of high-order harmonic generation. Obtaining sufficient XUV photon flux per pulse at repetition rates of several tens of MHz, despite the inherently low conversion efficiency, requires a powerful driving source. To date, passive enhancement of ultrashort pulses in an external resonator has been the most successful strategy in meeting this demand. In this thesis four main achievements towards extending this technique and understanding its limitations are presented. A first experiment was dedicated to obtaining shorter intracavity pulses without compromising the high average power available from Yb-based laser technology. To this end, we spectrally broadened and temporally compressed the pulses prior to the enhancement in a broadband resonator. Aside from being a prerequisite for time-domain applications, shorter intracavity pulses led to improved conditions for the harmonic generation process. Furthermore, we addressed the task of extracting the intracavity generated XUV light. We established two methods for geometrical XUV output coupling, one employing the fundamental mode of the cavity, and the other a tailored transverse mode, which offers additional degrees of freedom to shape the harmonic emission. Both techniques are particularly suited for the intracavity generation of attosecond pulses, because they afford an unparalleled flexibility for the resonator design, and exhibit a broadband output coupling efficiency approaching unity for short-wavelength radiation. This enabled a significant improvement of the crucial parameters, photon flux and photon energy. In a combined experimental and theoretical study, we investigated the ionization-related intensity limitations observed in state-of-the-art enhancement cavities. The quantitative modeling of the nonlinear interaction allows for an estimation of the achievable intracavity parameters and for a global optimization of the XUV photon flux. Based on this model, we proposed a strategy to mitigate this limitation by using the nonlinearity in combination with customized cavity optics for a further spectral broadening and temporal compression of the pulse in the resonator. More importantly, this work establishes enhancement cavities as a tool to investigate nonlinear light-matter interactions with the increased sensitivity provided by the resonator. The last study was dedicated to the technological challenge of building a resonator in which the electric field of the circulating pulse is reproduced at each round-trip. This is an essential prerequisite to generate identical XUV emission with each driving pulse. By tailoring the spectral phase of the cavity mirrors we succeeded in enhancing pulses of less than 30 fs (less than nine cycles of the driving field) to a few kilowatts of average power with zero pulse-to-pulse carrier-to-envelope phase slip. At similar pulse durations, the generation of isolated attosecond pulses has already been demonstrated in single-pass geometries. In conclusion, the results presented in this thesis are milestones on the way to a powerful, compact and coherent source of ultrashort XUV radiation. The unique property of the source, that is, its high repetition rate lays the foundation for advancing attosecond physics and precision spectroscopy in the XUV region