Accurate quantification of lattice temperature dynamics from ultrafast electron diffraction of single-crystal films using dynamical scattering simulations

In ultrafast electron diffraction (UED) experiments, accurate retrieval of time-resolved structural parameters such as atomic coordinates and thermal displacement parameters requires an accurate scattering model. Unfortunately, kinematical models are often inaccurate even for relativistic electron probes, especially for dense, oriented single crystals where strong channeling and multiple scattering effects are present. This article introduces and demonstrates dynamical scattering models tailored for quantitative analysis of UED experiments performed on single-crystal films. As a case study, we examine ultrafast laser heating of single-crystal gold films. Comparison of kinematical and dynamical models reveals the strong effects of dynamical scattering within nm-scale films and their dependence on sample topography and probe kinetic energy. Applied to UED experiments on an 11 nm thick film using 750 keV electron probe pulses, the dynamical models provide a tenfold improvement over a comparable kinematical model in matching the measured UED patterns. Also, the retrieved lattice temperature rise is in very good agreement with predictions based on previously measured optical constants of gold, whereas fitting the Debye-Waller factor retrieves values that are more than three times lower. Altogether, these results show the importance of dynamical scattering theory for quantitative analysis of UED, and demonstrate models that can be practically applied to single-crystal materials and heterostructures.Comment: 12 pages, 7 figure

Femtosecond x rays from laser-plasma accelerators

Relativistic interaction of short-pulse lasers with underdense plasmas has recently led to the emergence of a novel generation of femtosecond x-ray sources. Based on radiation from electrons accelerated in plasma, these sources have the common properties to be compact and to deliver collimated, incoherent and femtosecond radiation. In this article we review, within a unified formalism, the betatron radiation of trapped and accelerated electrons in the so-called bubble regime, the synchrotron radiation of laser-accelerated electrons in usual meter-scale undulators, the nonlinear Thomson scattering from relativistic electrons oscillating in an intense laser field, and the Thomson backscattered radiation of a laser beam by laser-accelerated electrons. The underlying physics is presented using ideal models, the relevant parameters are defined, and analytical expressions providing the features of the sources are given. Numerical simulations and a summary of recent experimental results on the different mechanisms are also presented. Each section ends with the foreseen development of each scheme. Finally, one of the most promising applications of laser-plasma accelerators is discussed: the realization of a compact free-electron laser in the x-ray range of the spectrum. In the conclusion, the relevant parameters characterizing each sources are summarized. Considering typical laser-plasma interaction parameters obtained with currently available lasers, examples of the source features are given. The sources are then compared to each other in order to define their field of applications.Comment: 58 pages, 41 figure

Towards attosecond 4D imaging of atomic-scale dynamics by single-electron diffraction

Many physical and chemical processes which define our daily life take place on atomic scales in space and time. Time-resolved electron diffraction is an excellent tool for investigation of atomic-scale structural dynamics (4D imaging) due to the short de Broglie wavelength of fast electrons. This requires electron pulses with durations on the order of femtoseconds or below. Challenges arise from Coulomb repulsion and dispersion of non-relativistic electron wave packets in vacuum, which currently limits the temporal resolution of diffraction experiments to some hundreds of femtoseconds. In order to eventually advance the temporal resolution of electron diffraction into the few-femtosecond range or below, four new concepts are investigated and combined in this work: First, Coulomb repulsion is avoided by using only a single electron per pulse, which does not repel itself but interferes with itself when being diffracted from atoms. Secondly, dispersion control for electron pulses is implemented with time-dependent electric fields at microwave frequencies, compressing the duration of single-electron pulses at the expense of simultaneous energy broadening. Thirdly, a microwave signal used for electron pulse compression is derived from an ultrashort laser pulse train. Optical enhancement allows a temporal synchronization between the microwave field and the laser pulses with a precision below one femtosecond. Fourthly, a cross-correlation between laser and electron pulses is measured in this work with the purpose of determining the possible temporal resolution of diffraction experiments employing compressed single-electron pulses. This novel characterization method uses the principles of a streak camera with optical fields and potentially offers attosecond temporal resolution. These four concepts show a clear path towards improving the temporal resolution of electron diffraction into the few-femtosecond domain or below, which opens the possibility of observing electron densities in motion. In this work, a compressed electron pulse's duration of 28±5 fs full width at half maximum (12±2 fs standard deviation) at a de Broglie wavelength of 0.08 Å is achieved. Currently, this constitutes the shortest electron pulses suitable for diffraction, about sixfold shorter than in previous work. Ultrafast electron diffraction now meets the requirements for investigating the fastest primary processes in molecules and solids with atomic resolution in space and time

Experimental study of long timescale plasma wakefield evolution

Over the last century, particle accelerators have significantly enhanced our understanding of the fundamental nature of the universe and, as such, have become ubiquitous tools within society. The continued search for explanations of phenomena beyond our current best models motivates the proposal of ever-larger and more expensive particle accelerators. However, the economic impact of building and operating such machines potentially casts doubt on their delivery. The exploration of alternative acceleration concepts that can potentially provide a reduction in the size and cost of these machines has therefore seen a significant growth in interest over the past few decades. One such acceleration concept is that of plasma-based wakefield acceleration where high-intensity particle or laser beams strongly perturb a plasma and, in doing so, generate fields in their wake that can be used to accelerate charged particles. This is an attractive concept as plasmas can support accelerating fields multiple orders of magnitude larger than those provided in conventional accelerators, potentially reducing the accelerating length by similar scales. While rapid progress has recently been made with regards to high-quality acceleration in plasma, comparatively little effort has been applied to the study of the frequency at which this is possible. The measurements presented in this thesis study the evolution of a plasma as the energy imparted into it via the wakefield acceleration process dissipates. Such detailed measurements allow determination of the fundamental mechanisms that will limit high-repetition-rate operation of plasma-based accelerators. As such, these measurements represent a significant first step towards the demonstration of plasma-based acceleration at frequencies comparable to those provided by state-of-the-art conventional accelerators, helping to define the achievable luminosity of future facilities that rely on such technology

THz: Research Frontiers for New Sources, Imaging and Other Advanced Technologies

The THz region of the electromagnetic spectrum is a frontier research area involving application of many disciplines, from outdoor to indoor communications, security, drug detection, biometrics, food quality control, agriculture, medicine, semiconductors, and air pollution. THz research is highly demanding in term of sources with high power and time resolution, detectors, and new spectrometer systems. Many open questions still exist regarding working at THz frequencies; many materials exhibit unusual or exotic properties in the THz domain, and researchers need new methodologies to exploit these opportunities. This book contains original papers dealing with emerging applications, new devices, sources and detectors, and materials with advanced properties for applications in biomedicine, cultural heritage, technology, and space