A layperson’s introduction to SE

For a long time, progress in Electron Microscopy (EM) has been equated with the art of obtaining images of increasingly higher resolution: instrumental research was mainly aimed at obtaining a very small probe and aberration-free images.

A swift change of paradigm has occurred during recent years, when the first ideas of electron-beam shaping were introduced. The main idea is to use properly designed holograms and electrostatic and magnetostatic phase elements to manipulate both phase and amplitude of the free-electron in a nearly arbitrary manner. A new way of conceiving EM is thus underway, with the potential to achieve enhanced performance and to measure new physical quantities.

The field of electron wavefunction engineering has mainly originated from light optics due to the strong analogy between electron and photon wave optics. For a variety of reasons, in this field experiments with photons are easier and cheaper to realise than the corresponding ones with electrons: it is therefore natural to wonder whether electron manipulation could benefit even further from such a precious connection with light.

The first idea in this sense has been the development of photon-assisted electron emission in a transmission electron microscope (TEM). This has prompted the creation of Ultrafast Electron Microscopy (UEM), which allows dynamical investigation of materials on the femtosecond time scale, such as lattice vibrations, charge dynamics, and plasmonics. This technique is based on the so-called pump-and-probe scheme, whereby a laser excites the sample and a carefully-timed ultrashort electron pulse then probes its non-equilibrium status.

One recent evolution of such an approach is the Photon-Induced Near-field Electron Microscopy (PINEM) method, which is able to transiently probe near-fields at the nanoscale, using an electron microscope. These experiments showed that nanometer-sized structures can be employed to produce uniquely enhanced interactions between electrons and light. For instance, because light can be rapidly modulated at the femtosecond timescale, PINEM represents an ideal way to induce an ultrafast phase and/or amplitude change on the electron wave function.

This is the key concept behind a new exciting paradigm for electron engineering, i.e. a special form of electron-beam shaping mediated by light: at its heart is the interaction between the microscope’s electron pulse and a properly-designed time-varying electromagnetic field. Achieving this form of ultrafast modulation of the electron is challenging because the electron wave and the modulating photon wave need to be not only carefully timed, but also sufficiently coherent, i.e. well shaped in space. On the other hand, this opens new exciting routes for the dynamical investigation of matter, endowing scientists with new superpowers to probe quantities of interest with unprecedented sensitivities.

The first steps in this line of research have been taken by the scientists involved in the SMART-electron project, who have experimentally demonstrated spatio-temporal shaping of a free-electron wave function using light. In particular, they showed the quantised light-induced modulation of electron states in their energy-momentum phase space, as well as attosecond electron modulation, and orbital angular momentum transfer from photons to electrons.

In the latter experiment, they showed for the first time the generation and sub-femtosecond control of ultrashort electron vortex beams, which is crucial, for instance, for the dynamic investigation of magnetic nanomaterials.

These experiments can be viewed as the first examples of a more general concept, whereby an electron beam is shaped arbitrarily by an appropriate electromagnetic field due to a light beam. This is one of the main concepts of the EU-funded FET-Open project SMART-electron, which aims to create a photonic-based free-electron modulator for dynamic electron phase manipulation.