Putting Few-Electron Pulses to Work
An electron microscope can be made to emit ultrashort bunches of electrons by illuminating its electron source, a sharp metal tip, with femtosecond laser pulses. Being negatively charged, the expelled electrons repel each other, spreading out their energies and degrading the microscope’s contrast and resolution. In 2023, Rudolf Haindl and his collaborators at the Max Planck Institute for Multidisciplinary Sciences in Germany showed that, for a handful of electrons, each one has an energy that is clearly distinguishable from the others. Now the same team has gone further, mapping the effect of the interaction in both energy and time, and even controlling it with light down to the attosecond scale [1].
The time and energy difference between two electrons measured over many pairs can be plotted to form a 2D phase space. To map it, Haindl and his collaborators installed a silicon membrane inside their electron microscope’s chamber. Zapping the membrane with pulses of laser light created an optical near field that scattered the electrons when they passed through. Measuring the electrons with an energy filter revealed which ones were hit by the laser. The researchers varied the time between the pulse that generated the electrons and the pulse that scattered them to measure their combined energetic-temporal structure. As a result, they could, for each pair, measure the delay (in femtoseconds) between the slower electron and the faster one.
Next, by using a longer laser pulse that encompassed both electrons, the researchers imprinted a fixed optical phase onto both electrons. This tactic synchronized pairs of electrons on attosecond timescales, marking a major step toward controlling free-electron dynamics. Such control could spur advances in electron microscopy, quantum technologies, and ultrafast electronics.
–Charles Day
Charles Day is a Senior Editor for Physics Magazine.
References
- R. Haindl et al., “Femtosecond and attosecond phase-space correlations in few-particle photoelectron pulses,” Phys. Rev. Lett. 135, 165002 (2025).