Probing Nanoscale Optical Fields and Phase-Shaping Electron Beams by Inelastic Electron-Light Scattering

Summary form only given. Ultrafast transmission electron microscopy (UTEM) is a powerful tool to study the dynamics of nanoscale systems, combining the versatile imaging, diffraction and spectroscopy capabilities of state-of-the-art TEM with femtosecond temporal resolution of a laser pump/electron probe scheme [1,2]. In particular, inelastic scattering between a free electron pulse and strong optical near fields [3,4] (Fig.1a) allows for a coherent manipulation of the electron quantum state. In this mechanism, the optical field imprints a sinusoidal phase modulation on the electron wave function [4], which manifests in a comb of photon sidebands in the kinetic energy distribution (Fig.1b) and - by energy-filtering - enables photon-induced near-field electron microscopy (PINEM) [3]. Here, we discuss a range of applications enabled by quantum-coherent electron-light interactions in ultrafast transmission electron microscopy. Specifically, we employ electron spectroscopy to map intense optical near fields by phase -resolved P1NEM, and utilize the interaction to manipulate the longitudinal and transverse free electron wave function. The Gottingen UTEM instrument features optical sample excitation and the generation of highly coherent electron pulses with a pulse duration down to 200 fs (full-width-at-half-maximum), a spectral width of 0.6 eV, and a sub-1 nm electron focal spot size [2]. Hereby, in a scanning variant of photon-induced near-field electron microscopy (S-P1NEM), we quantitatively map local plasmonic modes and their optical properties with nanometer spatial resolution. Specifically, tailoring the polarization state of the illuminating light gives access to the li near or chiral optical response of a nanostructure [2,5]. In a further application, we employ two subsequent near field interactions (Figid) to demonstrate the temporal structuring of free -electron beams on the attosecond timescale [6]. By dispersive propagation after the first interaction, the phase modulation of the initial electron wave function results in an electron density modulation. After the second interaction at varied propagation distances we record spectrograms with accurately controllable phase delay (Fig.le), which allows us to reconstruct the temporal shape of the electron density and to show the compression of electron pulses into trains of attosecond bursts. In conclusion, we demonstrated the quantitative nanoscale mapping of the local optical response in tailored nanostructures. Furthermore, the generation of attosecond electron pulse trains paves the way towards a new kind of optically phase -resolved electron microscopy.