Abstract
Tip-based photoemission electron sources offer unique properties for ultrafast imaging, diffraction, and spectroscopy experiments with highly coherent few-electron pulses. Extending this approach to increased bunch-charges requires a comprehensive experimental study on Coulomb interactions in nanoscale electron pulses and their impact on beam quality. For a laser-driven Schottky field emitter, we assess the transverse and longitudinal electron pulse properties in an ultrafast transmission electron microscope at a high photoemission current density. A quantitative characterization of electron beam emittance, pulse duration, spectral bandwidth, and chirp is performed. Due to the cathode geometry, Coulomb interactions in the pulse predominantly occur in the direct vicinity to the tip apex, resulting in a well-defined pulse chirp and limited emittance growth. Strategies for optimizing electron source parameters are identified, enabling advanced ultrafast transmission electron microscopy approaches, such as phase-resolved imaging and holography.
Highlights
The observation of ultrafast nanoscale dynamics promises a profound understanding of the spatio-temporal dynamics of elementary excitations in solids and new avenues for tailoring and controlling the flow of energy, charges, and spins in nanostructured materials
The spatio-temporal resolution in these approaches crucially depends on the transverse and longitudinal electron pulse characteristics, including pulse duration, spatial coherence, and bunch charge
The Go€ttingen ultrafast transmission electron microscope (UTEM) is based on a Schottky field-emission JEOL JEM-2100F TEM instrument, which we modified to allow for a lasertriggered photoelectron mode and for optical sample excitation.[16]
Summary
The observation of ultrafast nanoscale dynamics promises a profound understanding of the spatio-temporal dynamics of elementary excitations in solids and new avenues for tailoring and controlling the flow of energy, charges, and spins in nanostructured materials. The mean Coulomb field induces a reversible deformation of the phase space distribution, whereas stochastic (pulse-to-pulse) local charge fluctuations irreversibly spread the ensemble-averaged phase space distribution This phenomenon is described as stochastic trajectory displacements[51] for the transverse direction and termed the Boersch effect[52] for the energy broadening corresponding to the longitudinal direction. The Go€ttingen ultrafast transmission electron microscope (UTEM) is based on a Schottky field-emission JEOL JEM-2100F TEM instrument, which we modified to allow for a lasertriggered photoelectron mode and for optical sample excitation.[16] Femtosecond electron pulses are generated by employing localized single-photon photoemission from the apex of a Schottky-type ZrO/W field emission tip which is placed into an electrostatic electrode assembly [Fig. 1(a)]. For the temporal characterization of ultrashort electron pulses, we perform electron-light cross-correlation measurements utilizing the inelastic scattering of free electrons at momentum-broadened femtosecond light fields.[69,70,71] From the dependence of the electron energy spectra on the electronlight delay, we extract the electron pulse duration and chirp
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