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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. 

Throughout the history of laser development, the generation of ultra-short pulses has always been one of the most active frontiers. The generation of visible light pulses that contain only a few optical cy-cles is therefore a major achievement of modern physics. Femtosec-ond laser pulses now find applications across science, medicine, industrial manufacturing, and telecommunications, impacting many aspects of modern life.

This thesis presents the generation, characterization, and application of few-cycle light pulses. It includes the development of the Light Wave Synthesizer 100, one of the pioneering systems for waveform-controlled ultrafast optics. Throughout my thesis, this system was up-graded to facilitate higher peak intensities, shorter pulses and carrier-envelope phase (CEP) stabilization. A key contribution of this thesis is the the development of a new compression chamber layout.

The broad spectra required for few-femtosecond pulses can give rise to spatio-spectral couplings. To characterize and control these effects, reliable measurement techniques are necessary. The sim-plified INSIGHT method is presented and evaluated against HASO multispectral, a commercially available device to whose development our research group, the RElativistic Attosecond physics Laboratory (REAL), contributed.

There are fundamental limits to how short a pulse of a given spectrum can be, commonly referred to as the Fourier transform limit. Different approaches are demonstrated how this limitation can be circumvented, supported by both simulations and experimental results.

Finally, one of the main applications of ultrashort pulses is the field of ultrafast spectroscopy. A central component of this work is the development of a degenerate pump-probe setup with excep-tional sub-5-fs time resolution. This experimental platform is applied to a strongly coupled exciton-cavity system to observe the ultrafast generation and decay of polaritons, as well as excitons in WS2, a semiconducting transition metal dichalcogenide.