Femtosecond electrons probing currents and atomic structure in nanomaterials

Abstract

The investigation of ultrafast electronic and structural dynamics in low-dimensional systems such as nanowires and two-dimensional materials requires femtosecond probes providing high spatial resolution and strong interaction with small volume samples. Low-energy electrons exhibit large scattering cross-sections and high sensitivity to electric fields, but their pronounced dispersion during propagation in vacuum so far prevented their use as femtosecond probe pulses in time-resolved experiments. Here, employing a laser-triggered point-like source of either divergent or collimated electron wave packets, we developed a hybrid approach for femtosecond point projection microscopy and femtosecond low-energy electron diffraction. We investigate ultrafast electric currents in nanowires with sub-100 femtosecond temporal and few 10 nm spatial resolutions, and demonstrate the potential of our approach for studying structural dynamics in crystalline single-layer materials.

Nanofocused Plasmon-Driven Sub-10 fs Electron Point Source

“…The tips are positioned by a 4-axis manipulator with nm-precision and are optionally mounted inside an electrostatic lens for focusing of the electron beam [3]. The corresponding electron optical system is very similar to that of a Schottky field emission gun in conventional high resolution electron microscopes using virtual point source cathodes [42].…”

“…The point-projection microscopy image is therefore primarily a measure of the electrostatic near-field rather than a shadow image of the geometric structure of the nanoobject. In particular, the point-projection image is sensitive to the doping profile in nanowires [3,50]. Figure 4 b) compares PPM images of a NW recorded in DC field emission mode without laser (top) and SPPdriven mode (bottom) with the same tip-sample distance d = 14 µm, corresponding to a geometric magnification of M ≈ 7000.…”

“…We find that multiphoton photoemission is the dominant emission process and that the nonlinear electron emission is triggered within a time window of approximately 5 femtoseconds by the nanofocused near-field. We demonstrate the application for fsPPM by imaging the nanoscale surface electric field of a single doped InP nanowire at a tip-sample distance of 3 µm, substantially shorter compared to previous fsPPM studies using direct illumination of the tip apex [3,17]. We estimate an electron pulse duration at the sample below 10 fs for future fsPPM experiments at such small distances.…”

“…Due to the spatially confined emission and the resulting large transverse coherence length of the photoelectrons [15], such laser-triggered nanotips proved to be ideal point-like sources of high-brightness coherent femtosecond electron wave packets [16,17]. Currently, fsPPM is realized with compact setups, where excitation of the sample and photoemission of probe electrons from a nanotip is achieved with two tightly focused laser pulses [3,5,17]. This allows for a minimal tip-sample distance in the range of 10-20 µm given by the spatial separation of the laser pulses, and a resulting spatio-temporal resolution of tens of nanometers and tens of femtoseconds, respectively [3,16].…”

Development of an Ultrafast Low-Energy Electron Diffraction Setup

“…An alternative approach is the lensless diffraction by a nanometric needle emitter directly in front of the sample [215]. The approach has shown to allow for about 250 fs temporal resolution at electron energies as low as 70 eV with excellent focusability.…”