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Non-linear interactions between light and matter have nowadays a broad range of applications. They are used for frequency doubling in simple laser pointers as well as for a variety of purposes in complex laser systems like the one presented in this thesis. For the study of ultrafast phenomena, those non-linear interactions are crucial to trigger and observe events at the fastest timescale, which is currently the attosecond regime (10^-15 – 10^-18 s). As the duration of a single optical cycle of a visible light wave is longer than this timescale, these investigations necessitate the application of XUV and X-ray pulses. However, the generation of isolated attosecond light pulses sufficiently intense to initiate non-linear interactions with matter is restricted to photon energies below 50 eV. The aim of this thesis is to establish a new light source, which pushes this boundary further and thereby enables the observation of up to now unrevealed electron dynamics.

The presented new light source provides attosecond pulses with approximately hundred times more pulse energy than typical systems (up to 55 nJ in the spectral range from approximately 65 eV to 140 eV). This facilitates non-linear measurements at these photon energies. The achieved high energy stability (5 %) of this light source allows precise and time efficient measurements. These parameters are obtained via energy-upscaling of high-harmonic generation in gas medium. For the generation of well isolated attosecond pulses a unique laser, like the Light Wave Synthesizer 20, is necessary. This laser uses optical parametric synthesis to produce the most intense sub-5 fs, sub two-cycle laser pulses in the world (80 mJ, 4.5 fs).

Furthermore, an optimal focus of the XUV pulses is crucial to provide the necessary intensity for non-linear interactions. Therefore, different methods for focusing the XUV pulses are investigated. Moreover, the construction and characterization of a robust split and delay stage is presented, which is essential for time resolved measurements.

The detection of the non-linear interaction is realized via a spatially resolved ion time-of-flight detector, the ion microscope. This allows for a quantitative measurement of different ionization states. With the combination of this detector and the new light source the non-linear generation of Xe⁴⁺ and Xe⁵⁺ at photon energies around 100 eV is demonstrated. This enables the determination of the two-photon ionization cross-sections, which could up to now only be measured with much longer pulses at large scientific infrastructures. This paves the way towards time-resolved XUV pump – XUV probe measurements at 100 eV.

Icke linjära interaktioner mellan ljus och materia har idag ett brett användningsområde. De kan användas för frekvensdubbling i enkla laserpekare och för en mängd olika syften i komplexa lasersystem som det som presenteras i denna avhandling. För forskning om ultrasnabba fenomen är dessa icke-linjära interaktioner avgörande för att utlösa och observera händelser i den snabbaste tidsskalan, vilket för närvarande är den så kallade attosekundregimen (10^-15 - 10^-18 s). Eftersom varaktigheten av en optisk cykel, i det synliga spektrumet är längre än denna tidsskala, kräver dessa undersökningar användningen av XUV och röntgenpulser. Genereringen av isolerade attosekundljuspulser som är tillräckligt intensiva för att ge upphov till icke-linjära interaktioner med materia är dock begränsad till fotonenergier under 50 eV. Syftet med denna avhandling är att etablera en ny ljuskälla som flyttar denna gräns ytterligare framåt och därmed möjliggör observation av hittills outforskad elektrondynamik.

Den nya ljuskällan ger attosekundpulser med ungefär hundra gånger mer pulsenergi än för typiska system (upp till 55 nJ i spektralområdet från cirka 65 eV till 140 eV). Detta underlättar icke-linjära mätningar vid dessa fotonenergier. Den höga energistabiliteten (5%) som uppnås med denna ljuskälla möjliggör exakta och tidseffektiva mätningar. Dessa egenskaper erhålls via energiuppskalning av övertonsgenerering (high-harmonic generation på engelska) i gasmedium. För att generera välisolerade attosekundpulser är en specialbyggd laser, som Light Wave Synthesizer 20, nödvändig. Denna laser använder optisk parametrisk syntes för att producera de mest intensiva sub-5 femtosekunder, sub-två cykel laserpulsarna i världen (80 mJ, 4,5 fs).

Vidare, är ett optimalt fokus för XUV-pulserna avgörande för att ge den nödvändiga intensiteten för icke-linjära interaktioner. Därför undersöks olika metoder för att fokusera XUV-pulserna. Dessutom så presenteras konstruktionen och karaktäriseringen av ett robust split- och fördröjningssteg (split and delay stage på engelska), vilket är viktigt för tidsupplösta mätningar.

Detekteringen av den icke-linjära interaktionen görs med en rumsligt upplöst jondetektor som mäter propageringstid genom detektorn, det så kallade jonmikroskopet. Detta möjliggör en kvantitativ mätning av olika joniseringstillstånd. Genom att kombinera denna detektor med den nya ljuskällan kan vi påvisa icke-linjär generation av Xe⁴⁺ och Xe⁵⁺ vid fotonenergier runt 100 eV. Detta möjliggör bestämning av de två-fotonjoniseringstvärsnitten, som hittills bara kunde mätas med mycket längre pulser vid stora forskningsanläggningar. Detta lägger grunden för tidsupplöst XUV pump – XUV probe mätningar vid 100 eV.

State-of-the-art ultrashort light sources in the visible and near-infrared spectral regions provide direct access to the femtosecond realm, thereby enabling understanding and control of electronic processes within matter. On the other hand, ultra-intense light pulses lead to the emergence of relativistic electron motion and many related phenomena, such as electron & ion acceleration and high-order harmonic generation in plasmas. The generation and amplification techniques for those intense short light pulses were developed over the last 60 years. Nowadays, they are unique scientific research tools and the basis of commercial applications. The driving forces behind many of these new optical technologies are second and third order nonlinear ultrashort processes. Optical parametric chirped pulse amplification (OPCPA) is currently the most interesting of these techniques and promises in particular high single-pass gain, broad gain bandwidth, scalability, good high-dynamic range temporal contrast, and tunability. However, OPCPA comes also with a bundle of challenges. The aim of this thesis, by utilizing the advantages and facing these challenges, is to boost a sub-two cycle optical parametric synthesizer (OPS), a two-color-pumped OPCPA, to an unprecedented parameter regime in respect of energy, intensity, contrast and stability.

The presented sub-2-optical cycle OPS – the light wave synthesizer (LWS) - is a worldwide unique system, amplifying a spectral bandwidth in three pairs of OPCPA stages. One pair of these stages sequentially amplifies and coherently combines two complementary spectral ranges to an almost octave spanning bandwidth. The amplified spectrum ranges from 580 nm to 1000 nm, which makes Fourier limited pulses with 4.6 fs possible. The present system is a fundamental reconstruction and extension of a former version of LWS that provided peak powers of up to 16 TW. By carefully redesigning of the former OPCPA stages, implementing a new front end and adding two nominally 2.3 J Nd:YAG amplifiers, harmonic generation setups and a third pair of OPCPA stages, the pulse energy has been raised up to 450-500 mJ while keeping the spectral bandwidth. After compression, this corresponds to about the aspired 100 TW peak power.

Focus was also laid on various important parameters for such ultra-short and ultra-intense light pulses, such as the temporal contrast, the carrier-envelope phase (CEP) and energy stability. Analysis and optimization of the 16 TW LWS version operation parameters made it possible to optimize the LWS-100 root mean square energy stabilities down to 0.3-0.5% over 100 s, which is significantly lower than previously reported for the former version. For the first time, the CEP-stability for this full system has been demonstrated. Currently, it is limited by slow drifts, but an active feedback system could suppress this to 400 mrad. The influences on the temporal contrast were investigated and prepulses identified and eliminated. Furthermore, hardware and software control for easy handling and reliable operation have been implemented.

The LWS-100 pushes the limits for few-cycle laser technology even further. It enables the generation of intense and isolated attosecond pulses beyond 100 eV photon energy, acceleration of attosecond electron bunches to relativistic energies, measurement of nonlinear processes of inner shell electrons via XUV pump-probe experiments and generation of isolated attosecond pulses on plasma mirrors. 

Pulses of light that are both ultrashort and ultraintense are often generated using optical parametric amplifiers (OPA). These are capable of driving highly non-linear interactions with matter, which are interesting when studying the fundamental laws of our universe. Furthermore, they are also used in many scientific and industrial applications, such as particle accelerators, inertial-confinement nuclear fusion, and medical diagnostics and treatment. This thesis explores the diagnostic and optimization of pulses of light with extreme properties and utilizes them to drive electron acceleration.

The applied light pulses with very short duration (<5 fs) and high peak power (>10 TW) are sensitive to develop spatio-temporal aberrations. These are color-dependent distortions that can significantly degrade the pulse properties, like peak-intensity, and affect their applicability. Furthermore, in most cases they are not easy to correctly diagnose, with current tools failing to provide widely applicable solutions. In this thesis, we describe a new type of spatio-temporal coupling that is especially relevant for optical parametric synthesizers (OPS), systems that coherently combine multiple OPA stages. To do this, we have contributed to the development of two methods for the characterization of such aberrations, the so-called simplified-INSIGHT and HASO multispectral. These enabled us to further improve the structure of our OPS and laser systems.

We also explored the applicability of light pulses to drive relativistic electron acceleration in vacuum. To this end, an injection system using nanotips is presented, capable of inserting electrons spatially in the focus and temporally in the most intense light-cycle. This way, vacuum laser accelerated electrons of up to 14 MeV were detected using a tight focusing configuration (f#1) and their properties characterized. Furthermore, we investigated the dependence of the acceleration process when the focusing geometry is relaxed (f#3). This resulted in the unexpected outcome of similar electron energies in both cases, although the intensity was ten times reduced. This indicates that the decrease in accelerating field strength is compensated by longer acceleration lengths, which is not predicted by currently existing analytical models.