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Ultrahigh peak-power laser systems with pulse durations of tens of femtoseconds are widely used as drivers for compact sources of particles and secondary radiation. Conversely, lasers with shorter (a few femtoseconds) pulse durations and lower peak powers enable the generation of isolated attosecond light pulses to study nature with unparalleled temporal resolution. Here we report an enhanced optical parametric chirped pulse amplifier system that produces light pulses with a peak power of about 100 TW and a pulse duration as short as 4.3 fs with full waveform control. Coherent field synthesis generates a broadband spectrum, spanning from the visible to the near infrared, through three cascaded amplification stages, each housing two optical parametric amplifiers that sequentially boost complementary spectral regions. The resulting light transients are waveform-stabilized to <300 mrad and focused to an intensity of 1021 W cm−2 and exhibit an outstanding high dynamic range in temporal contrast. Together, these characteristics render the system well suited for demanding relativistic laser–plasma experiments. Utilizing temporal super-resolution, the pulses are shortened to sub-4-fs duration. This platform is dedicated to advancing the frontiers of attosecond electron and X-ray sources.

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].

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. 

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.

Optimization and simulation of non-collinear ultra-broadband optical parametric chirped pulse amplification setups rely on exact knowledge of the phase matching conditions. We present a method for their accurate retrieval by deterministic angular jitter and Monte-Carlo simulations.

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.

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.