Observing charge separation in nanoantennas via ultrafast point-projection electron microscopy

Abstract

Observing the motion of electrons on their natural nanometer length and femtosecond time scales is a fundamental goal of and an open challenge for contemporary ultrafast science1–5. At present, optical techniques and electron microscopy mostly provide either ultrahigh temporal or spatial resolution, and microscopy techniques with combined space-time resolution require further development6–11. In this study, we create an ultrafast electron source via plasmon nanofocusing on a sharp gold taper and implement this source in an ultrafast point-projection electron microscope. This source is used in an optical pump—electron probe experiment to study ultrafast photoemissions from a nanometer-sized plasmonic antenna12–15. We probe the real space motion of the photoemitted electrons with a 20-nm spatial resolution and a 25-fs time resolution and reveal the deflection of probe electrons by residual holes in the metal. This is a step toward time-resolved microscopy of electronic motion in nanostructures.

Ultrashort field emission in metallic nanostructures and low-dimensional carbon materials

“…The resultant electric field, due to nanofocusing at the apex, was estimated to be~6 V•nm −1 , an order of magnitude higher than the conventional case. Using this nanoprobe electron source, the real-time imaging of charge separation in a nanoantenna system has been successfully demonstrated with a time resolution of~25 fs, resulting in a new application for ultrafast nanostructure microscopy [88]. In addition, a suppressor and an extractor which surround the tips (with a grating) have been introduced to remove the unexpected photoelectrons emitted outside the tip apex area, as shown in Figure 3(b) [89].…”

Perspective on Petahertz Electronics and Attosecond Nanoscopy

“…Practically, in order to achieve a short electron pulse duration at the sample, two different approaches have been adopted. The first approach is to minimize the distance between the source -in this case, nanotips -and the sample, so that there is a minimal increase in the electron pulse duration from the generation target to the sample; source-to-sample distances as short as 2.7 µm have been reported [164]. In this scheme, it is important to avoid sample excitation caused by the generating laser pulse, and this can be done by nanofocusing of propagating plasmons at the tip apex, as discussed above, with the laser driver coupled to the nanotip via a grating at the shank.…”

Probing plasmonic excitation mechanisms and far-field radiation of single-crystalline gold tapers with electrons

“…One of the most established and widely used nanostructures are conically shaped metallic tapers, due to their practical application as waveguides or nanoantennas in scanning near-field optical microscopy (SNOM) [7,[15][16][17][18][19][20][21][22][23] or as an ultrafast photoemission source in point-projection microscopes [24][25][26][27][28][29][30][31][32][33].…”