Free-electron lasers provide high intensity pulses with femtosecond duration and are ideal tools in the investigation of ultrafast processes in materials. Illumination of any material with such pulses creates extreme conditions that drive the sample far from equilibrium and rapidly convert it into high temperature plasma. The dynamics of this transition is not fully understood and the main goal of this thesis is to further our knowledge in this area. We exposed a variety of materials to X-ray pulses of intensities from 1013 to above 1017 W/cm2. We found that the temporal evolution of the resulting plasmas depends strongly on the wavelength and pulse intensity, as well as on material related parameters, such as size, density, and composition. In experiments on atomic and molecular clusters, we find that cluster size and sample composition influence the destruction pathway. In small clusters a rapid Coulomb explosion takes place while larger clusters undergo a hydrodynamic expansion. We have characterized this transition in methane clusters and discovered a strong isotope effect that promotes the acceleration of deuterium ions relative to hydrogen. Our results also show that ions escaping from exploding xenon clusters are accelerated to several keV energies. Virus particles represent a transition between hetero-nuclear clusters and complex biological materials. We injected single mimivirus particles into the pulse train of an X-ray laser, and recorded coherent diffraction images simultaneously with the fragmentation patterns of the individual particles. We used these results to test theoretical damage models. Correlation between the diffraction patterns and sample fragmentation shows how damage develops after the intense pulse has left the sample. Moving from sub-micron objects to bulk materials gave rise to new phenomena. Our experiments with high-intensity X-ray pulses on bulk, metallic samples show the development of a transient X-ray transparency. We also describe the saturation of photoabsorption during ablation of vanadium and niobium samples. Photon science with extremely strong X-ray pulses is in its infancy today and will require much more effort to gain more knowledge. The work described in this thesis represents some of the first results in this area.

O free-electron laser (FEL) surgiu como uma nova fonte de radiação eletromagnética. O presente trabalho trata do efeito de carga em um FEL específico: de passagem única; e, sendo que as interações relevantes são entre o feixe e o campo magnético e entre os elétrons (não são estudados os campos auto consistentes do laser). Encontram-se as equações de movimento de cada partícula dentro do poço e comparam-se os resultados analíticos com os resultados obtidos por simulação computacional, para os limites de povoamento dos elétrons em uma situação de equilíbrio. Posteriormente, se analisa o efeito de carga através dos mapas de Poincaré, introduzindo partículas com uma determinada distribuição inicial, dentro do potencial aprisionador. Conclui-se que a introdução de cargas aumenta o número de graus de liberdade do sistema, fazendo com que os mapas de Poincaré não sejam mais uma boa ferramenta para analisar a dinâmica do sistema. Observa-se, também, o fenômeno de quebra de onda no FEL através do efeito do balanço do potencial e de uma distribuição inicial de elétrons diferente da distribuição de equilíbrio. / Free-electron laser (FEL) was first created as a new source of electromagnetic radiation. The present work is about the charge effect on a specific FEL: single pass; and, with interactions between the beam and the magnetic field and between electrons (laser self consistent fields are not studied). Motion equations of each particle inside ponderomotive well are discovered, and then analytic results for the limits of electronic population are compared with simulated ones, in the situation of equilibrium. Afterwards, particles are introduced in the trapping potential, respecting a defined initial distribution, so the charge effect is analyzed through Poincaré maps. In conclusion, the introduction of charges raises the number of freedom degrees of the system. This makes the Poincaré maps not such a good tool to analyze the system dynamics. The wave breaking phenomenon is also observed in FEL through oscillation balance effect and an initial electronic distribution distinct of the equilibrium one.

abstract: Scientists have used X-rays to study biological molecules for nearly a century. Now with the X-ray free electron laser (XFEL), new methods have been developed to advance structural biology. These new methods include serial femtosecond crystallography, single particle imaging, solution scattering, and time resolved techniques. The XFEL is characterized by high intensity pulses, which are only about 50 femtoseconds in duration. The intensity allows for scattering from microscopic particles, while the short pulses offer a way to outrun radiation damage. XFELs are powerful enough to obliterate most samples in a single pulse. While this allows for a “diffract and destroy” methodology, it also requires instrumentation that can position microscopic particles into the X-ray beam (which may also be microscopic), continuously renew the sample after each pulse, and maintain sample viability during data collection. Typically these experiments have used liquid microjets to continuously renew sample. The high flow rate associated with liquid microjets requires large amounts of sample, most of which runs to waste between pulses. An injector designed to stream a viscous gel-like material called lipidic cubic phase (LCP) was developed to address this problem. LCP, commonly used as a growth medium for membrane protein crystals, lends itself to low flow rate jetting and so reduces the amount of sample wasted significantly. This work discusses sample delivery and injection for XFEL experiments. It reviews the liquid microjet method extensively, and presents the LCP injector as a novel device for serial crystallography, including detailed protocols for the LCP injector and anti-settler operation. / Dissertation/Thesis / Doctoral Dissertation Physics 2015

O free-electron laser (FEL) surgiu como uma nova fonte de radiação eletromagnética. O presente trabalho trata do efeito de carga em um FEL específico: de passagem única; e, sendo que as interações relevantes são entre o feixe e o campo magnético e entre os elétrons (não são estudados os campos auto consistentes do laser). Encontram-se as equações de movimento de cada partícula dentro do poço e comparam-se os resultados analíticos com os resultados obtidos por simulação computacional, para os limites de povoamento dos elétrons em uma situação de equilíbrio. Posteriormente, se analisa o efeito de carga através dos mapas de Poincaré, introduzindo partículas com uma determinada distribuição inicial, dentro do potencial aprisionador. Conclui-se que a introdução de cargas aumenta o número de graus de liberdade do sistema, fazendo com que os mapas de Poincaré não sejam mais uma boa ferramenta para analisar a dinâmica do sistema. Observa-se, também, o fenômeno de quebra de onda no FEL através do efeito do balanço do potencial e de uma distribuição inicial de elétrons diferente da distribuição de equilíbrio. / Free-electron laser (FEL) was first created as a new source of electromagnetic radiation. The present work is about the charge effect on a specific FEL: single pass; and, with interactions between the beam and the magnetic field and between electrons (laser self consistent fields are not studied). Motion equations of each particle inside ponderomotive well are discovered, and then analytic results for the limits of electronic population are compared with simulated ones, in the situation of equilibrium. Afterwards, particles are introduced in the trapping potential, respecting a defined initial distribution, so the charge effect is analyzed through Poincaré maps. In conclusion, the introduction of charges raises the number of freedom degrees of the system. This makes the Poincaré maps not such a good tool to analyze the system dynamics. The wave breaking phenomenon is also observed in FEL through oscillation balance effect and an initial electronic distribution distinct of the equilibrium one.

O free-electron laser (FEL) surgiu como uma nova fonte de radiação eletromagnética. O presente trabalho trata do efeito de carga em um FEL específico: de passagem única; e, sendo que as interações relevantes são entre o feixe e o campo magnético e entre os elétrons (não são estudados os campos auto consistentes do laser). Encontram-se as equações de movimento de cada partícula dentro do poço e comparam-se os resultados analíticos com os resultados obtidos por simulação computacional, para os limites de povoamento dos elétrons em uma situação de equilíbrio. Posteriormente, se analisa o efeito de carga através dos mapas de Poincaré, introduzindo partículas com uma determinada distribuição inicial, dentro do potencial aprisionador. Conclui-se que a introdução de cargas aumenta o número de graus de liberdade do sistema, fazendo com que os mapas de Poincaré não sejam mais uma boa ferramenta para analisar a dinâmica do sistema. Observa-se, também, o fenômeno de quebra de onda no FEL através do efeito do balanço do potencial e de uma distribuição inicial de elétrons diferente da distribuição de equilíbrio. / Free-electron laser (FEL) was first created as a new source of electromagnetic radiation. The present work is about the charge effect on a specific FEL: single pass; and, with interactions between the beam and the magnetic field and between electrons (laser self consistent fields are not studied). Motion equations of each particle inside ponderomotive well are discovered, and then analytic results for the limits of electronic population are compared with simulated ones, in the situation of equilibrium. Afterwards, particles are introduced in the trapping potential, respecting a defined initial distribution, so the charge effect is analyzed through Poincaré maps. In conclusion, the introduction of charges raises the number of freedom degrees of the system. This makes the Poincaré maps not such a good tool to analyze the system dynamics. The wave breaking phenomenon is also observed in FEL through oscillation balance effect and an initial electronic distribution distinct of the equilibrium one.

All across physics research, incoherent and coherent light sources are extensively utilized. Especially highly brilliant X-ray sources such as third generation synchrotrons or free-electron lasers have become an invaluable tool enabling experimental techniques that are unique to these kinds of light sources. But these sources have developed to large scale facilities and a demand in compact laboratory scale sources providing radiation of similar quality arises nowadays. This thesis focuses on Traveling-Wave Thomson-Scattering (TWTS) which allows for the realization of ultra-compact, inherently synchronized and highly brilliant light sources. The TWTS geometry provides optical undulators, through which electrons pass and thereby emit radiation, with hundreds to thousands of undulator periods by utilizing pulse-front tilted lasers pulses from high peak-power laser systems. TWTS can realize incoherent radiation sources with orders of magnitude higher photon yield than established head-on Thomson sources. Moreover, optical free-electron lasers (OFELs) can be realized with TWTS if state-of-the-art technology in electron accelerators and laser systems is utilized. Tilting the laser pulse front with respect to the wavefront by half of this interaction angle optimizes electron and laser pulse overlap by compensating the spatial offset between electrons and the laser pulse-front at the beginning of the interaction when the electrons are far from the laser pulse axis. The laser pulse-front tilt ensures continuous overlap between electrons and laser pulse while the electrons cross the laser pulse cross-sectional area. Thus the interaction distance can be controlled in TWTS by the laser pulse width rather than laser pulse duration. Utilizing wide, petawatt class laser pulses allows realizing thousands of optical undulator periods. This thesis will show that TWTS OFELs emitting ultraviolet radiation are realizable today with existing technology for electron accelerators and laser systems. The requirements on electron bunch and laser pulse quality of these ultraviolet TWTS OFELs are discussed in detail as well as the corresponding requirements of TWTS OFELs emitting in the soft and hard X-ray range. These requirements are derived from scaling laws which stem from a self-consistent analytic description of the electron bunch and radiation field dynamics in TWTS OFELs presented within this thesis. It is shown that these dynamics in TWTS OFELs are qualitatively equivalent to the electron bunch and radiation field dynamics of standard free-electron lasers which analytically proves the applicability of TWTS for the realization of an optical free-electron laser. Furthermore, experimental setup strategies to generate the pulse-front tilted TWTS laser pulses are presented and designs of experimental setups for the above examples are discussed. The presented setup strategies provide dispersion compensation, required due to angular dispersion of the laser pulse, which is especially relevant when building compact, high-yield hard X-ray TWTS sources in large interaction angle setups. An example of such an enhanced Thomson source by TWTS, which provides orders of magnitude higher spectral photon density than a comparable head-on interaction geometry, is presented, too. / Inkohärente und kohärente Lichtquellen werden in allen Feldern der physikalischen Forschung intensiv eingesetzt. Im Besonderen ermöglichen hoch-brilliante Röntgenquellen, wie Synchrotrone der dritten Generation und Freie-Elektronen Laser, einzigartige Experimentiertechniken wodurch diese zu unverzichtbaren Werkzeugen wurden. Sie sind allerdings auch im Umfang zu Großforschungseinrichtungen herangewachsen. Um den hohen Bedarf an hoch-brillianten Lichtquellen zu decken, besteht daher die Notwendigkeit neuartige und kompakte Quellen zu entwickeln welche auf dem Maßstab eines Labors realisierbar sind. Diese Dissertation widmet sich der Traveling-Wave Thomsonstreuung (TWTS) welche die Realisierung ultra-kompakter, intrinsisch synchronisierbarer und hoch-brillianter Röntgenquellen ermöglicht. TWTS ist eine Methode der Streuung von Laserpulsen an relativistischen Elektronen. Dabei durchquert ein Elektronenpuls mit nahezu Lichtgeschwindigkeit einen Laserpuls. Während der Durchquerung beginnen die Elektronen im Feld des Laserpulses zu oszillieren wobei sie Strahlung emittieren. Die ausgesandte Strahlung besitzt eine deutlich kürzere Wellenlänge als das Laserfeld aufgrund der hohen Elektronengeschwindigkeit und der damit verbundenen großen Dopplerverschiebung. Das besondere an TWTS ist, dass Elektronen- und Laserpropagationsrichtung einen Winkel einschließen sowie pulsfrontverkippte Hochleistungslaserpulse eingesetzt werden. Dadurch können um Größenordnungen längere Interaktionsdistanzen als in herkömmlichen frontalen Thomsonstreuungsanordnungen erreicht werden. TWTS ermöglicht dadurch die Realisierung optischer Freie-Elektronen Laser (OFEL) und inkohärenter Strahlungsquellen mit einer um Größenordnungen erhöhten Photonenausbeute gegenüber Thomsonstreuungsquellen in frontalen Interaktionsanordungen. Werden modernste Elektronenbeschleuniger und Lasersysteme genutzt, dann ist der Betrieb optischer Freie-Elektronen Laser bereits heute mit TWTS möglich. Das wird in der Dissertation am Beispiel eines Vakuumultraviolettstrahlung emittierenden TWTS OFEL gezeigt. Dessen Anforderungen an die Qualität der Elektronen- und Laserpulse werden im Detail in der Arbeit besprochen sowie weitere Beispiele weicher und harter Röntgenstrahlung emittierender TWTS OFEL präsentiert. Diese Anforderungen werden anhand von Skalierungsvorschriften ermittelt welche aus einer selbstkonsistenten, 1.5 dimensionalen Theorie zur Wechselwirkung zwischen Elektronen und Laserfeld in TWTS abgeleitet sind. Sowohl die Theorie zur Wechselwirkung als auch die Ableitung der Skalierungsvorschriften sind Teile dieser Dissertation. Eine wichtige Erkenntnis der Theorie ist die qualitative Äquivalenz von Elektronen- und Strahlungsfeldbewegungsgleichungen in TWTS zu denen herkömmlicher Freie-Elektronen Laser. Das beweist analytisch die Möglichkeit zur Realisierung eines OFEL mit TWTS. Einen weiteren wichtigen Teil dieser Dissertation bildet die Arbeit zur Generierung der Laserpulse mit verkippter Pulsfront. Optische Aufbauten zur Verkippung der Laserpulsfront werden vorgestellt und für einige der präsentierten TWTS OFEL ausführlich dargelegt. Die Aufbauten verkippen nicht nur die Laserpulsfront sondern gewähren gleichzeitig Kontrolle über die Laserpulsdispersionen. Dadurch kann während der gesamten Interaktionen eine ausreichend hohe Qualität des Laserfeldes sichergestellt werden, was für TWTS OFEL und inkohärente TWTS Lichtquellen mit großem Interaktionswinkel unbedingt notwendig ist. Ein Beispiel einer inkohärenten TWTS Lichtquelle wird ebenfalls präsentiert. Diese emittiert Strahlung mit einer um Größenordnungen höheren spektrale Photonendichte als eine herkömmliche Thomsonquelle in einer frontalen Streuanordnung mit vergleichbaren Laser- und Elektronenpulsen.

Higher order modes (HOM) are electromagnetic resonant fields. They can be excited by an electron beam entering an accelerating cavity, and constitute a component of the wakefield. This wakefield has the potential to dilute the beam quality and, in the worst case, result in a beam-break-up instability. It is therefore important to ensure that these fields are well suppressed by extracting energy through special couplers. In addition, the effect of the transverse wakefield can be reduced by aligning the beam on the cavity axis. This is due to their strength depending on the transverse offset of the excitation beam. For suitably small offsets the dominant components of the transverse wakefield are dipole modes, with a linear dependence on the transverse offset of the excitation bunch. This fact enables the transverse beam position inside the cavity to be determined by measuring the dipole modes extracted from the couplers, similar to a cavity beam position monitor (BPM), but requires no additional vacuum instrumentation.At the FLASH facility in DESY, 1.3 GHz (known as TESLA) and 3.9 GHz (third harmonic) cavities are installed. Wakefields in 3.9 GHz cavities are significantly larger than in the 1.3 GHz cavities. It is therefore important to mitigate the adverse effects of HOMs to the beam by aligning the beam on the electric axis of the cavities. This alignment requires an accurate beam position diagnostics inside the 3.9 GHz cavities. It is this aspect that is focused on in this thesis. Although the principle of beam diagnostics with HOM has been demonstrated on 1.3 GHz cavities, the realization in 3.9 GHz cavities is considerably more challenging. This is due to the dense HOM spectrum and the relatively strong coupling of most HOMs amongst the four cavities in the third harmonic cryo-module. A comprehensive series of simulations and HOM spectra measurements have been performed in order to study the modal band structure of the 3.9 GHz cavities. The dependencies of various dipole modes on the offset of the excitation beam were subsequently studied using a spectrum analyzer. Various data analysis methods were used: modal identification, direct linear regression, singular value decomposition and k-means clustering. These studies lead to three modal options promising for beam position diagnostics, upon which a set of test electronics has been built. The experiments with these electronics suggest a resolution of 50 micron accuracy in predicting local beam position in the cavity and a global resolution of 20 micron over the complete module. This constitutes the first demonstration of HOM-based beam diagnostics in a third harmonic 3.9 GHz superconducting cavity module. These studies have finalized the design of the online HOM-BPM for 3.9 GHz cavities at FLASH.

Die vorliegende Dissertation befasst sich mit der streuenden, infraroten Rasternahfeldmikroskopie (engl. s-SNIM) in Kombination mit dem Freie-Elektronen Laser (FEL) am Helmholtz-Zentrum Dresden-Rossendorf. Der FEL ist eine intensive,schmalbandige Strahlungsquelle, welche vom mittleren bis ferninfraroten Spektralbereich durchstimmbar ist (5 meV bis 250 meV). Die s-SNIM Technik ermöglicht Infrarotmikroskopie- und spektroskopie mit einer wellenlängenunabhängigen räumlichen Auflösung von etwa 10nm. Der erste Ergebnisteil demonstriert die Erweiterung eines FEL-basierten s- SNIM hinsichtlich der Möglichkeit, bei tiefen Temperaturen bis 5K messen zu können. So verdeutlichen wir die Funktionalität unseres Tieftemperatur-s-SNIM anhand verschiedener Proben wie Au, strukturiertem Si/SiO2 sowie Gallium-Vanadium-Sulfid (GaV4S8). Das letztgenannte Material erregt momentan ein hohes wissenschaftliches Interesse, da es sogenannte Skyrmionen des Néel-Typs – periodische angeordnete Spinwirbel – enthält. GaV4S8 hat einen strukturellen Phasenübergang bei T = 42K und beinhaltet bei niedrigeren Temperaturen ferroelektrische Domänen, die wir unter anderem mittels s-SNIM abbilden können. Hierbei beobachten wir einen beträchtlichen Einfluss der Infrarotstrahlung auf die Domänenstruktur. Dies nutzen wir, um den lokalen Hitzeeintrag der Infrarotstrahlung lokal unter der s-SNIM Sonde zu quantifizieren. Der zweite Teil der Ergebnisse beinhaltet s-SNIM Messungen an hochwertigen Halbleiter-Nanodrähten (ND), welche mittels Molekularstrahlepitaxie gewachsen wurden. Derartige ND sind, unter anderem aufgrund ihrer hohen Ladungsträgermobilität, vielversprechende Komponenten für schnelle optoelektronische Nanoelemente der Zukunft. So untersuchen wir beispielsweise hochdotierte GaAs/InGaAs Kern/Schale ND, bei denen wir – unter Verwendung eines Dauerstrich CO2 Lasers – eine spektral scharfe plasmonische Resonanz bei etwa 125 meV beobachten. Betrachten wir selbige ND mittels intensiver, gepulster FEL-Strahlung, ist eine signifikante Rotverschiebung zu Energien kleiner als 100 meV sowie eine Verbreiterung der Resonanz festzustellen. Dieses nichtlineare Verhalten wird zurückgeführt auf eine starke Erhitzung des Elektronengases unter dem Einfluss der intensiven FEL-Pulse. Unsere Erkenntnisse zeigen dahingehend die Möglichkeiten auf, Nichtgleichgewichtszustände im s-SNIM gezielt zu induzieren und zu beinflussen. Abgesehen von den Messungen der Nichtlinearität ist die Herstellung und Charakterisierung von ND-Querschnitten – sowohl der genannten homogen dotierten, als auch modulationsdotierten– Gegenstand des zweiten Ergebniskapitels.:Abstract iii Zusammenfassung v 1 Introduction 1 2 Fundamentals 3 2.1 Scanning probe techniques 3 2.1.1 Atomic force microscopy 4 2.1.2 Piezoresponse force microscopy 8 2.1.3 Kelvin-probe force microscopy 9 2.2 Infrared nanospectroscopy 10 2.2.1 The diffraction limit 10 2.2.2 Scattering scanning near-field infrared microscopy 11 2.2.3 Point-dipole model 12 2.2.4 Signal detection 17 2.2.5 Higher harmonic demodulation and contrast 19 2.2.6 Advantages and limitations of s-SNIM 22 2.3 Infrared light sources 24 2.3.1 Carbon dioxide laser 24 2.3.2 Free-electron laser 26 3 Infrared nanospectroscopy at cryogenic temperatures 31 3.1 Introduction 31 3.2 Samples 33 3.3 Experimental details 36 3.3.1 Low-temperature atomic force microscopy 36 3.3.2 Optical setup 38 3.3.3 Low-temperature scattering scanning near-field infrared microscopy 39 3.3.4 Measurement modes and data acquisition 42 3.4 Results and discussion 44 3.4.1 Performance and IR heating calibration 44 3.4.2 s-SNIM study of gallium vanadium sulfide 49 3.5 Conclusion 51 4 Infrared nanospectroscopy on semiconductor nanowires 53 4.1 Introduction 53 4.2 Samples 55 4.2.1 GaAs/InGaAs core/shell nanowires 55 4.2.2 Modulation doped nanowires 56 4.2.3 Nanowire cross sections 57 4.2.4 Infrared response of doped nanowires 59 4.3 Experimental details 61 4.3.1 Room-temperature atomic force microscopy 61 4.3.2 Room-temperature scattering scanning near-field infrared microscopy 63 4.3.3 Properties of the free-electron laser pulses 65 4.4 Results and discussion 68 4.4.1 GaAs/InGaAs core/shell nanowires 68 4.4.2 Nanowire cross sections 75 4.5 Conclusion 79 5 Summary and outlook 81 A Citation metrics 85 B Additional nanospectroscopic studies 87 B.1 Silicon carbide nanoparticle probes 87 B.2 Individual impurities in Si 91 B.3 Surface phonon polaritons in moybdenum disulfide 96 C Derivation of the nonparabolic effective mass and density of states 99 C.1 Effective mass 99 C.2 Density of states 100 D Comparison of self-homodyne and pseudo-heterodyne detection 103 Bibliography 105 List of Abbreviations 125 List of Symbols 132 List of Publications 133 Acknowledgments 137 Versicherung 139 / This PhD thesis concentrates on scattering scanning near-field infrared microscopy (s-SNIM) which utilizes the radiation from the free-electron laser (FEL) at the Helmholtz-Zentrum Dresden-Rossendorf. The FEL is an intense, narrow-band radiation source, tunable from the mid- to far-infrared spectral range (5 meV to 250 meV). The s-SNIM technique enables infrared microscopy and spectroscopy with a wavelength-independent spatial resolution of about 10nm. The first part demonstrates the extension of s-SNIM at the FEL towards cryogenic temperatures as low as 5K. To this end, we show the functionality of our low-temperature s-SNIM apparatus on different samples such as Au, structured Si/SiO2, as well as the multiferroic material gallium vanadium sulfide (GaV4S8). The latter material recently attracted a lot of interest since it hosts a Néel-type skyrmion lattice – a periodic array of spin vortices. Below T = 42K, GaV4S8 undergoes a structural phase transition and then forms ferroelectric domains, which we can map out by low-tempererature s-SNIM. Notably, we found a strong impact on the ferroelectric domains upon infrared irradiation, which we further utilize to calibrate the local heat contribution of the focused infrared beam beneath the s-SNIM probe. The second part of this thesis contains comprehensive s-SNIM investigations of high-quality semiconductor nanowires (NWs) grown by molecular beam epitaxy. Such NWs are promising building blocks for fast opto-)electronic nanodevices, amongst others due to their high carrier mobility. We have examined highly doped GaAs/InGaAs core/shell NWs and observed a strong and spectrally sharp plasmonic resonance at about 125 meV, using a continuous wave CO2 laser for probing. If we probe the same NWs utilizing the intense, pulsed FEL radiation, we observe a pronounced redshift to energies less than 100 meV and a broading of the plasmonic response. This nonlinear response is most likely induced by heating of the electron gas upon irradiation by the strong FEL pulses. Our observations open up the possibility to actively induce and observe non-equilibrium states in s-SNIM directly by the mid-infrared beam. Beside the nonlinear effect, we prepared and measured cross sections of both homogeneously-doped and modulation-doped core/shell NWs.:Abstract iii Zusammenfassung v 1 Introduction 1 2 Fundamentals 3 2.1 Scanning probe techniques 3 2.1.1 Atomic force microscopy 4 2.1.2 Piezoresponse force microscopy 8 2.1.3 Kelvin-probe force microscopy 9 2.2 Infrared nanospectroscopy 10 2.2.1 The diffraction limit 10 2.2.2 Scattering scanning near-field infrared microscopy 11 2.2.3 Point-dipole model 12 2.2.4 Signal detection 17 2.2.5 Higher harmonic demodulation and contrast 19 2.2.6 Advantages and limitations of s-SNIM 22 2.3 Infrared light sources 24 2.3.1 Carbon dioxide laser 24 2.3.2 Free-electron laser 26 3 Infrared nanospectroscopy at cryogenic temperatures 31 3.1 Introduction 31 3.2 Samples 33 3.3 Experimental details 36 3.3.1 Low-temperature atomic force microscopy 36 3.3.2 Optical setup 38 3.3.3 Low-temperature scattering scanning near-field infrared microscopy 39 3.3.4 Measurement modes and data acquisition 42 3.4 Results and discussion 44 3.4.1 Performance and IR heating calibration 44 3.4.2 s-SNIM study of gallium vanadium sulfide 49 3.5 Conclusion 51 4 Infrared nanospectroscopy on semiconductor nanowires 53 4.1 Introduction 53 4.2 Samples 55 4.2.1 GaAs/InGaAs core/shell nanowires 55 4.2.2 Modulation doped nanowires 56 4.2.3 Nanowire cross sections 57 4.2.4 Infrared response of doped nanowires 59 4.3 Experimental details 61 4.3.1 Room-temperature atomic force microscopy 61 4.3.2 Room-temperature scattering scanning near-field infrared microscopy 63 4.3.3 Properties of the free-electron laser pulses 65 4.4 Results and discussion 68 4.4.1 GaAs/InGaAs core/shell nanowires 68 4.4.2 Nanowire cross sections 75 4.5 Conclusion 79 5 Summary and outlook 81 A Citation metrics 85 B Additional nanospectroscopic studies 87 B.1 Silicon carbide nanoparticle probes 87 B.2 Individual impurities in Si 91 B.3 Surface phonon polaritons in moybdenum disulfide 96 C Derivation of the nonparabolic effective mass and density of states 99 C.1 Effective mass 99 C.2 Density of states 100 D Comparison of self-homodyne and pseudo-heterodyne detection 103 Bibliography 105 List of Abbreviations 125 List of Symbols 132 List of Publications 133 Acknowledgments 137 Versicherung 139

All across physics research, incoherent and coherent light sources are extensively utilized. Especially highly brilliant X-ray sources such as third generation synchrotrons or free-electron lasers have become an invaluable tool enabling experimental techniques that are unique to these kinds of light sources. But these sources have developed to large scale facilities and a demand in compact laboratory scale sources providing radiation of similar quality arises nowadays. This thesis focuses on Traveling-Wave Thomson-Scattering (TWTS) which allows for the realization of ultra-compact, inherently synchronized and highly brilliant light sources. The TWTS geometry provides optical undulators, through which electrons pass and thereby emit radiation, with hundreds to thousands of undulator periods by utilizing pulse-front tilted lasers pulses from high peak-power laser systems. TWTS can realize incoherent radiation sources with orders of magnitude higher photon yield than established head-on Thomson sources. Moreover, optical free-electron lasers (OFELs) can be realized with TWTS if state-of-the-art technology in electron accelerators and laser systems is utilized. Tilting the laser pulse front with respect to the wavefront by half of this interaction angle optimizes electron and laser pulse overlap by compensating the spatial offset between electrons and the laser pulse-front at the beginning of the interaction when the electrons are far from the laser pulse axis. The laser pulse-front tilt ensures continuous overlap between electrons and laser pulse while the electrons cross the laser pulse cross-sectional area. Thus the interaction distance can be controlled in TWTS by the laser pulse width rather than laser pulse duration. Utilizing wide, petawatt class laser pulses allows realizing thousands of optical undulator periods. This thesis will show that TWTS OFELs emitting ultraviolet radiation are realizable today with existing technology for electron accelerators and laser systems. The requirements on electron bunch and laser pulse quality of these ultraviolet TWTS OFELs are discussed in detail as well as the corresponding requirements of TWTS OFELs emitting in the soft and hard X-ray range. These requirements are derived from scaling laws which stem from a self-consistent analytic description of the electron bunch and radiation field dynamics in TWTS OFELs presented within this thesis. It is shown that these dynamics in TWTS OFELs are qualitatively equivalent to the electron bunch and radiation field dynamics of standard free-electron lasers which analytically proves the applicability of TWTS for the realization of an optical free-electron laser. Furthermore, experimental setup strategies to generate the pulse-front tilted TWTS laser pulses are presented and designs of experimental setups for the above examples are discussed. The presented setup strategies provide dispersion compensation, required due to angular dispersion of the laser pulse, which is especially relevant when building compact, high-yield hard X-ray TWTS sources in large interaction angle setups. An example of such an enhanced Thomson source by TWTS, which provides orders of magnitude higher spectral photon density than a comparable head-on interaction geometry, is presented, too. / Inkohärente und kohärente Lichtquellen werden in allen Feldern der physikalischen Forschung intensiv eingesetzt. Im Besonderen ermöglichen hoch-brilliante Röntgenquellen, wie Synchrotrone der dritten Generation und Freie-Elektronen Laser, einzigartige Experimentiertechniken wodurch diese zu unverzichtbaren Werkzeugen wurden. Sie sind allerdings auch im Umfang zu Großforschungseinrichtungen herangewachsen. Um den hohen Bedarf an hoch-brillianten Lichtquellen zu decken, besteht daher die Notwendigkeit neuartige und kompakte Quellen zu entwickeln welche auf dem Maßstab eines Labors realisierbar sind. Diese Dissertation widmet sich der Traveling-Wave Thomsonstreuung (TWTS) welche die Realisierung ultra-kompakter, intrinsisch synchronisierbarer und hoch-brillianter Röntgenquellen ermöglicht. TWTS ist eine Methode der Streuung von Laserpulsen an relativistischen Elektronen. Dabei durchquert ein Elektronenpuls mit nahezu Lichtgeschwindigkeit einen Laserpuls. Während der Durchquerung beginnen die Elektronen im Feld des Laserpulses zu oszillieren wobei sie Strahlung emittieren. Die ausgesandte Strahlung besitzt eine deutlich kürzere Wellenlänge als das Laserfeld aufgrund der hohen Elektronengeschwindigkeit und der damit verbundenen großen Dopplerverschiebung. Das besondere an TWTS ist, dass Elektronen- und Laserpropagationsrichtung einen Winkel einschließen sowie pulsfrontverkippte Hochleistungslaserpulse eingesetzt werden. Dadurch können um Größenordnungen längere Interaktionsdistanzen als in herkömmlichen frontalen Thomsonstreuungsanordnungen erreicht werden. TWTS ermöglicht dadurch die Realisierung optischer Freie-Elektronen Laser (OFEL) und inkohärenter Strahlungsquellen mit einer um Größenordnungen erhöhten Photonenausbeute gegenüber Thomsonstreuungsquellen in frontalen Interaktionsanordungen. Werden modernste Elektronenbeschleuniger und Lasersysteme genutzt, dann ist der Betrieb optischer Freie-Elektronen Laser bereits heute mit TWTS möglich. Das wird in der Dissertation am Beispiel eines Vakuumultraviolettstrahlung emittierenden TWTS OFEL gezeigt. Dessen Anforderungen an die Qualität der Elektronen- und Laserpulse werden im Detail in der Arbeit besprochen sowie weitere Beispiele weicher und harter Röntgenstrahlung emittierender TWTS OFEL präsentiert. Diese Anforderungen werden anhand von Skalierungsvorschriften ermittelt welche aus einer selbstkonsistenten, 1.5 dimensionalen Theorie zur Wechselwirkung zwischen Elektronen und Laserfeld in TWTS abgeleitet sind. Sowohl die Theorie zur Wechselwirkung als auch die Ableitung der Skalierungsvorschriften sind Teile dieser Dissertation. Eine wichtige Erkenntnis der Theorie ist die qualitative Äquivalenz von Elektronen- und Strahlungsfeldbewegungsgleichungen in TWTS zu denen herkömmlicher Freie-Elektronen Laser. Das beweist analytisch die Möglichkeit zur Realisierung eines OFEL mit TWTS. Einen weiteren wichtigen Teil dieser Dissertation bildet die Arbeit zur Generierung der Laserpulse mit verkippter Pulsfront. Optische Aufbauten zur Verkippung der Laserpulsfront werden vorgestellt und für einige der präsentierten TWTS OFEL ausführlich dargelegt. Die Aufbauten verkippen nicht nur die Laserpulsfront sondern gewähren gleichzeitig Kontrolle über die Laserpulsdispersionen. Dadurch kann während der gesamten Interaktionen eine ausreichend hohe Qualität des Laserfeldes sichergestellt werden, was für TWTS OFEL und inkohärente TWTS Lichtquellen mit großem Interaktionswinkel unbedingt notwendig ist. Ein Beispiel einer inkohärenten TWTS Lichtquelle wird ebenfalls präsentiert. Diese emittiert Strahlung mit einer um Größenordnungen höheren spektrale Photonendichte als eine herkömmliche Thomsonquelle in einer frontalen Streuanordnung mit vergleichbaren Laser- und Elektronenpulsen.

In this thesis we investigate two potential applications of machine learning in the context of X-ray imaging and spectroscopy of biological samples, particularly such using X-ray free electron lasers (XFEL). We first investigate the possibility of using an emission spectrum, recorded from a sample after being probed by an incident X-ray, as a diagnostic tool. We produced a training dataset of simulated emission spectra, where the incident X-ray energy and fluence was varied as well as the sample density. The simulations were implemented using Cretin which is a radiation transfer code which model the behaviour of plasma. We then trained a dense neural network to predict the three above named features given an emission spectrum. The dependency between input and output is inherently non-linear, making neural networks a suitable method for these predictions. Our results show a mean prediction error of below 6% of the entire range of all three features. If a similar tool was to be implemented in real life XFEL experiments, it could provide useful information in the data analysis pipeline.   As a second focus of this thesis we aim to produce an application to be used by researchers in XFEL experiments. Given a set of input parameters, including the incident X-ray energy and fluence along with atomic content and density of the sample, our application generates an emission spectrum for the user. The application is based on a neural network trained on Cretin simulations. When evaluated by comparing the final model to simulations, our model was found to have a mean absolute percentage prediction error of 1.77%. In addition to this we include similar models that generate the time development of the electron temperature and mean ionization of the sample, since these properties are highly associated with the emission processes of plasma. We did this by training dense neural networks on a dataset consisting of simulations of the corresponding property. Finally we integrated our models in a graphical user interface web application, accessible via the QR code. With this approach, the desired data can be plotted in real-time in a user-friendly manner, without having to run complicated and time-consuming simulations. Our model is focused on biological samples and could be used as a reference tool in structural biology.