Abstract
Femtosecond electron microscopy produces real-space images of matter in a series of ultrafast snapshots. Pulses of electrons self-disperse under space-charge broadening, so without compression, the ideal operation mode is a single electron per pulse. Here, we demonstrate femtosecond single-electron point projection microscopy (fs-ePPM) in a laser-pump fs-e-probe configuration. The electrons have an energy of only 150 eV and take tens of picoseconds to propagate to the object under study. Nonetheless, we achieve a temporal resolution with a standard deviation of 114 fs (equivalent to a full-width at half-maximum of 269 ± 40 fs) combined with a spatial resolution of 100 nm, applied to a localized region of charge at the apex of a nanoscale metal tip induced by 30 fs 800 nm laser pulses at 50 kHz. These observations demonstrate real-space imaging of reversible processes, such as tracking charge distributions, is feasible whilst maintaining femtosecond resolution. Our findings could find application as a characterization method, which, depending on geometry, could resolve tens of femtoseconds and tens of nanometres. Dynamically imaging electric and magnetic fields and charge distributions on sub-micron length scales opens new avenues of ultrafast dynamics. Furthermore, through the use of active compression, such pulses are an ideal seed for few-femtosecond to attosecond imaging applications which will access sub-optical cycle processes in nanoplasmonics.
Highlights
A variety of fundamental physical, chemical, and biological processes occur on ultrafast timescales and over length scales ranging from microns to Angstroms
Typical magnifications were in excess of 103, which should elucidate the features on the apex of NSMT2, are not sufficient to facilitate the observation of in-line holograms
Point projection electron microscopy of a tungsten nanotip has been performed with submicron spatial resolution using femtosecond laser emitted photoelectrons
Summary
A variety of fundamental physical, chemical, and biological processes occur on ultrafast timescales and over length scales ranging from microns to Angstroms. Rather than using the photoelectric effect, the small RoC causes a significant field enhancement when an electric field is applied to the NSMT This process has been studied for some time as DC field emission, whereby the electric field at the apex is sufficient to allow electrons near the Fermi level to tunnel directly into the continuum.. Generating the shortest pulses in fs-ePPM requires as few electrons per pulse as possible, to generate real-space images with sufficient signal to perform time resolved measurements requires a process that can be cycled repeatedly, preferably at high laser repetition rates. The present work aligns itself to these efforts by looking to better quantify the spatial and temporal characteristics of femtosecond laser-driven electron microscopy to demonstrate a potential characterization method and point to the potential of dynamic charge imaging Themes including optical near-field sensing, sub-cycle carrier-envelope phase phenomena, investigations into surface plasmons, and laser-driven electron accelerators have emerged. The present work aligns itself to these efforts by looking to better quantify the spatial and temporal characteristics of femtosecond laser-driven electron microscopy to demonstrate a potential characterization method and point to the potential of dynamic charge imaging
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