Femtosecond few- to single-electron point-projection microscopy for nanoscale dynamic imaging

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.

Ultrafast Field‐Emission Electron Sources Based on Nanomaterials

“…Due to the extremely high temporal resolution, metallic tips have been applied as ultrafast field‐emission electron sources in femtosecond point‐projection microscopy (fsPPM), ultrafast low‐energy electron diffraction (ULEED), and the combination of these two technologies . This development has extended the temporal resolution of electron microscopy to the picosecond and even to the femtosecond scale; however, the spatial resolution is limited to the order of a hundred micrometers.…”

Ultrafast Relativistic Electron Nanoprobes

“…We then show the potential for such beams in ultrafast nano-diffraction (relativistic nano-UED) and scanning transmission electron microscopy (relativistic USTEM) by recording high quality diffraction patterns and mapping grain orientation with sub-micrometer resolution, pinpointing a grain boundary with sub-spot size resolution. In this contest, we first characterize the ultrafast point-projection microscopy mode of the instrument (relativistic UPPM), obtaining a spatial resolution consistent with the knife-edge waist size measurements [16][17][18] , and then elucidate the critical role of this modality in the context of the nano-UED experiments. UPPM in fact, is shown to provide key information which can be used for correlating the ultrafast structural dynamics data from nano-UED with static information retrievable with other techniques such as conventional TEM.…”

Electron-light interactions beyond the adiabatic approximation: recoil engineering and spectral interferometry

“…PPM holds the advantage of a less complicated setup without the need for massive magnetic lenses in TEMs, though the spatial resolution has yet to be improved to compete with ultrafast TEMs. The key element behind the development of a PPM is the recent progress in controlling the ultrafast dynamics of photoelectrons emitted from sharp tips, either employing adiabatic nanofocusing [38][39][40] for transferring the grating-coupled light from the shaft at several micrometers away from the apex to the apex itself (refer Figures 8(a) and (b)), or by directly illuminating the tip apex [21,114,115] Figure 8. PPM with laser driven nanotips.…”