L'impact du glissement en fréquence lors de l'accélération directe d'électrons par le faisceau laser

L’accélération directe d’électrons par des impulsions ultrabrèves de polarisation radiale fortement focalisées démontre un grand potentiel, notamment, pour la production de paquets d’électrons ultrabrefs. Plusieurs aspects de ce schéma d’accélération restent toutefois à être explorés pour en permettre une maîtrise approfondie. Dans le cadre du présent mémoire, on s’intéresse à l’ajout d’une dérive de fréquence au champ de l’impulsion TM01 utilisée. Les expressions exactes des composantes du champ électromagnétique de l’impulsion TM01 sont établies à partir d’une généralisation du spectre de Poisson. Il s’agit, à notre connaissance, du premier modèle analytique exact pour la description d’une impulsion avec une dérive de fréquence. Ce modèle est utilisé pour étudier l’impact du glissement en fréquence sur le schéma d’accélération, grâce à des simulations “particule test” unidimensionnelles, considérant en premier lieu une énergie constante par impulsion, puis un champ maximum constant. Les résultats révèlent que le glissement en fréquence diminue le gain en énergie maximum atteignable dans le cadre du schéma d’accélération à l’étude ; une baisse d’efficacité de plusieurs dizaines de pourcents peut survenir. De plus, les simulations mettent en évidence certaines différences reliées à l’utilisation d’impulsions avec une dérive vers les basses fréquences ou avec une dérive vers les hautes fréquences : il se trouve que, pour un glissement en fréquence de même grandeur, l’impulsion avec une dérive vers les basses fréquences conduit à un gain en énergie cinétique maximum plus élevé pour l’électron que l’impulsion avec une dérive vers les hautes fréquences.Direct electron acceleration using ultrashort radially polarized laser pulses in strong focusing conditions exhibits unique properties for the production of ultrashort electron bunches. However, several aspects of this acceleration scheme are still left to investigate in order to ensure our complete understanding of the processes taking place. The present master’s thesis studies the use of TM01 chirped pulses in this acceleration scheme. Exact closed-form expressions for the description of all the components of the electromagnetic field of the TM01 chirped pulse are established by the generalization of the Poisson-like spectrum. It is, to the best of our knowledge, the first exact analytical model of an ultrashort non paraxial chirped pulse. These expressions are then used for on-axis “test particle” simulations, considering, at first, a constant pulse energy and, as a second case, a constant field maximum. Results reveal that chirp pulses seem to cause a decrease of the maximum energy gain achievable using this acceleration scheme; a decrease of the efficiency of tens of percents was observed. Additionnaly, numerical simulations show that down-chirp and up-chirp do not lead to the same results: we find that down-chirped pulses produce a slightly larger energy gain than up-chirped pulses, while still being less efficient than Fourier-transform-limited laser pulse

STEM serial electron diffraction data

<p>Serial electron diffraction (SerialED) data of Zeolite Y and ZSM-25 (electron-beam sensitive). The data were collected under STEM mode on a Thermo Fisher Scientific Themis microscope (300kV) using a gatan oneview camera. The cRED data of FAU type zeolite was collected using a JEOL 2100 LaB6 TEM using an ASI Timepix hybrid dector. </p&gt

STEM SerialED: achieving high-resolution data for ab initio structure determination of beam-sensitive nanocrystalline materials

Serial electron diffraction (SerialED), which applies a snapshot data acquisition strategy on each crystal, was introduced to tackle the problem of radiation damage in the structure determination of beam-sensitive materials by three-dimensional electron diffraction (3D ED). The snapshot data acquisition in SerialED can be realized both in transmission and scanning transmission electron microscopes (TEM/STEM). However, the current SerialED workflow based on STEM setups requires special external devices and software, which brings challenges for its broader adoption. Here, we present a simplified experimental implementation of STEM-based SerialED on Thermo Fisher Scientific STEMs using common proprietary software interfaced through Python scripts to automate data collection. Specifically, we utilize TEM Imaging and Analysis (TIA) scripting and TEM scripting to access the STEM functionalities of the microscope, and DigitalMicrograph (DM) scripting to control the camera for snapshot data acquisition. Data analysis adapts the existing workflow using the software CrystFEL developed for serial X-ray crystallography. Our workflow for SerialED can be used on any Gatan or Thermo Fisher Scientific camera. We apply this workflow to collect high-resolution STEM SerialED data from two aluminosilicate zeolites, Zeolite Y and ZSM-25, and demonstrate, for the first time, ab initio structure determination through direct methods using the STEM SerialED data. Zeolite Y is relatively stable under the electron beam, and SerialED data extend to 0.60 Å. We show that the structural model obtained using SerialED data merged from 358 crystals is nearly identical to that using continuous rotation electron diffraction (cRED) data from one crystal. This demonstrates that accurate structures can be obtained from SerialED. Zeolite ZSM-25 is very beam-sensitive and has a complex structure. We show that SerialED greatly improves data resolution of ZSM-25, compared to serial rotation electron diffraction (SerialRED), from 1.50 Å to 0.90 Å. This allows for the first time the use of standard phasing methods such as direct methods for ab initio structure determination of ZSM-25

Serial protein crystallography in an electron microscope

Serial X-ray crystallography at free-electron lasers allows to solve biomolecular structures from sub-micron-sized crystals. However, beam time at these facilities is scarce, and involved sample delivery techniques are required. On the other hand, rotation electron diffraction (MicroED) has shown great potential as an alternative means for protein nanocrystallography. Here, we present a method for serial electron diffraction of protein nanocrystals combining the benefits of both approaches. In a scanning transmission electron microscope, crystals randomly dispersed on a sample grid are automatically mapped, and a diffraction pattern at fixed orientation is recorded from each at a high acquisition rate. Dose fractionation ensures minimal radiation damage effects. We demonstrate the method by solving the structure of granulovirus occlusion bodies and lysozyme to resolutions of 1.55 Å and 1.80 Å, respectively. Our method promises to provide rapid structure determination for many classes of materials with minimal sample consumption, using readily available instrumentation.Sprovided by the Max Planck Society, Deutsches Elektronen-Synchrotron (DESY), the excellence cluster “The Hamburg Center for Ultrafast Imaging” of the Deutsche Forschungsgemeinschaft (EXC 1074 project ID 194651731), the European Research Council project “Attosecond X-ray Science: Imaging and Spectroscopy” (Award/Contract Number ERC-2013-SyG 609920), and the Joachim Herz Foundation (Biomedical Physics of Infection). P.H. acknowledges support by the Natural Sciences and Engineering Research Council of Canada. P.M. was supported by the Alexander von Humboldt-Stiftung for postdoctoral researchers