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Plasma optics, promising for shaping and amplifying ultra-high-power laser pulses, are subject to the huge modulations and fluctuations inherent in plasma excitation at high intensities. Understanding the impact of plasma-optic-induced modulations on the spatio-temporal structure of the resulting pulses demands multidimensional characterization of relativistic plasma dynamics, an extremely difficult task, particularly at the low repetition rates typical of such lasers. Here, we present three-dimensional (3D) spatio-temporal measurements of such pulses based on spectral interferometry. We measure the complex space-time distortions induced in the laser pulses by relativistic plasma while simultaneously capturing the underlying plasma dynamics, all in a single shot. This all-optical technique can capture 3D spatio-temporal couplings within pulses at ultra-high peak powers, enabling further progress in ultra-high-intensity laser and plasma technologies.

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.

In the past 20 years, we have reached a broad understanding of many light-driven phenomena in nanoscale systems. The temporal dynamics of the excited states are instead quite challenging to explore, and, at the same time, crucial to study for understanding the origin of fundamental physical and chemical processes. In this review, we examine the current state and prospects of ultrafast phenomena driven by plasmons both from a fundamental and applied point of view. This research area is referred to as ultrafast plasmonics and represents an outstanding playground to tailor and control fast optical and electronic processes at the nanoscale, such as ultrafast optical switching, single photon emission, and strong coupling interactions to tailor photochemical reactions. Here, we provide an overview of the field and describe the methodologies to monitor and control nanoscale phenomena with plasmons at ultrafast timescales in terms of both modeling and experimental characterization. Various directions are showcased, among others recent advances in ultrafast plasmon-driven chemistry and multi-functional plasmonics, in which charge, spin, and lattice degrees of freedom are exploited to provide active control of the optical and electronic properties of nanoscale materials. As the focus shifts to the development of practical devices, such as all-optical transistors, we also emphasize new materials and applications in ultrafast plasmonics and highlight recent development in the relativistic realm. The latter is a promising research field with potential applications in fusion research or particle and light sources providing properties such as attosecond duration.

Vacuum laser acceleration (VLA) of electrons has been an intense field of research for a long time due to the extremely high (>1 TV/m) accelerating fields. However, it is very challenging to realize and only a few promising experiments have been performed which have demonstrated the principle. Here, we report on the interaction of relativistic intensity (10-20 Wcm-2) sub-two optical cycle (<5 fs) laser pulses with nanotips to realize and optimize VLA. Various properties of accelerated electrons (angular distribution, charge, and electron spectrum) are measured with different intensities and carrier envelope phases of the laser pulse. Among others, waveform dependence of the electron propagation direction is observed. Furthermore, comparable or even higher electron energies beyond 10 MeV are detected with lower laser intensity, i.e., longer focusing, than with high intensity. These surprising results are reproduced using Particle-In-Cell simulations, which indicate a nanophotonics electron emission from the nanotip followed by VLA. In fact, the unexpected observations are a direct proof of the VLA process and provide a way to optimize it towards higher energy, isolated, attosecond electron bunch generation.

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.

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.

Harmonics from relativistic laser driven plasma surfaces is a prospective high energy attosecond light source infuture XUV pump-probe experiments. Among all the schemes, the most ecient and direct way to realize anisolated attosecond pulse is through using a few-cycle laser as the driving pulse. The two goodness criteria: thelaser to harmonics energy conversion eciency and the \purity" of an isolated attosecond pulse are generallydetermined by a combination of interaction parameters. Through using particle-in-cell simulations and relativisticelectron dynamics model analyses, we explain how these two criteria are a ected by the laser intensity, incidenceangle, carrier-envelope phase, and the plasma scale length. We found that, there exist an optimal plasma scalelength and an optimal incidence angle to eciently generate harmonics and intense attosecond light pulses.When other parameters are  xed, using a moderately intense relativistic laser or using a large incidence anglecould result in a higher isolation degree as well as a broader range of controlling parameters to realize an isolatedattosecond light pulse.