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Acceleration of electrons in vacuum directly by intense laser fields holds great promise for the generation of high-charge, ultrashort, relativistic electron bunches. While the energy gain is expected to be higher with tighter focusing, this does not account for the reduced acceleration range, which is limited by diffraction. Here, we present the results of an experimental investigation that exposed nanotips to relativistic few-cycle laser pulses. We demonstrate the vacuum laser acceleration of electron beams with 100s pC charge and 15 MeV energy. Two different focusing geometries, with normalized vector potential a0 of 9.8 and 3.8, produced comparable overall charge and electron spectra, despite a factor of almost ten difference in peak intensity. Our results are in good agreement with 3D particle-in-cell simulations, which indicate the importance of dephasing.

Optical parametric amplification (OPA) is a powerful tool for the generation of ultrashort light pulses. However, under certain circumstances, it develops spatio-spectral couplings, color dependent aberrations that degrade the pulse properties. In this work, we present a spatio-spectral coupling generated by a non-collimated pump beam and resulting in the change of direction of the amplified signal with respect to the input seed. We experimentally characterize the effect, introduce a theoretical model to explain it as well as reproduce it through numerical simulations. It affects high-gain non-collinear OPA configurations and becomes especially relevant in sequential optical parametric synthesizers. In collinear configuration, however, beyond the direction change, also angular and spatial chirp is produced. We obtain with a synthesizer about 40% decrease in peak intensity in the experiments and local elongation of the pulse duration by more than 25% within the spatial full width at half maximum at the focus. Finally, we present strategies to correct or mitigate the coupling and demonstrate them in two different systems. Our work is important for the development of OPA-based systems as well as few-cycle sequential synthesizers.

Vacuum laser acceleration (VLA) is a paradigm that utilizes the strong fields of focused laser light to accelerate electrons in vacuum. Despite its conceptual simplicity and a large existing collection of theoretical studies, realizing VLA in practice has proven remarkably challenging due to the difficulties associated with efficient injection: the electrons to be accelerated must be pre-energized and temporally compressed below an optical half-cycle before timely entering the rapidly oscillating fields of the laser. Therefore, only a handful of experiments have been published up to date, and a knowledge gap remains [1-3].

For many applications of extreme ultraviolet (XUV) and X-ray pulses, a small focus size is crucial to reach the required intensity or spatial resolution. In this article, we present a simple way to characterize an XUV focus with a resolution of 1.85 µm. Furthermore, this technique was applied for the measurement and optimization of the focus of an ellipsoidal mirror for photon energies ranging from 18 to 150 eV generated by high-order harmonics. We envisage a broad range of applications of this approach with sub-micrometer resolution from high-harmonic sources via synchrotrons to free-electron lasers.

The spectral phase and amplitude of a multi-TW laser with a Fourier transform limit of 4.6 fs was optimized to obtain 3.9 fs pulses with >5TW, providing the most energetic sub-4-fs pulses in the world.

We report on the detection of spatio-temporal couplings in a 700-1000 nm NOPA using an optimized characterization method. The technique is performed during normal focus observation and requires little additional hardware.

The Fourier-transform limit achieved by a linear spectral phase is the typical optimum by the generation of ultrashort light pulses. It provides the highest possible intensity, however, not the shortest full width at half maximum of the pulse duration, which is relevant for many experiments. The approach for achieving shorter pulses than the original Fourier limit is termed temporal superresolution. We demonstrate this approach by shaping the spectral phase of light from an optical parametric chirped pulse amplifier and generate sub-Fourier limited pulses. We also realize it in a simpler way by controlling only the amplitude of the spectrum, producing a shorter Fourier-limited duration. Furthermore, we apply this technique to an optical parametric synthesizer and generate multi-TW sub-4-fs light pulses. This light source is a promising tool for generating intense and isolated attosecond light and electron pulses.

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.

Optical parametric chirped-pulse amplification (OPCPA) [1] is an established light amplification technique with many beneficial properties, like high single pass gain, scalability, large spectral bandwidth, tunability and good conversion efficiency. Different methods have been proposed for optimization of conversion [2] - [4] mainly altering the pump or the crystal properties. However, seed manipulation to increase the OPCPA conversion efficiency has been only described in a general spatiotemporal field optimization theory so far [5]. Here, we show numerical and experimental results of a novel method to improve the gain saturation in an ultra-broadband OPCPA, hence conversion efficiency, by applying an adaptive spectral filter function to the seed pulses.

When a pulsed, few-cycle electromagnetic wave is focused by optics with f-number smaller than two, the frequency components it contains are focused to different regions of space, building up a complex electromagnetic field structure. Accurate numerical computation of this structure is essential for many applications such as the analysis, diagnostics, and control of high-intensity laser-matter interactions. However, straightforward use of finite-difference methods can impose unacceptably high demands on computational resources, owing to the necessity of resolving far-field and near-field zones at sufficiently high resolution to overcome numerical dispersion effects. Here, we present a procedure for fast computation of tight focusing by mapping a spherically curved far-field region to periodic space, where the field can be advanced by a dispersion-free spectral solver. In many cases of interest, the mapping reduces both run time and memory requirements by a factor of order 10, making it possible to carry out simulations on a desktop machine or a single node of a supercomputer. We provide an open-source C++ implementation with Python bindings and demonstrate its use for a desktop machine, where the routine provides the opportunity to use the resolution sufficient for handling the pulses with spectra spanning over several octaves. The described approach can facilitate the stability analysis of theoretical proposals, the studies based on statistical inferences, as well as the overall development and analysis of experiments with tightly-focused short laser pulses.