Phase-locked interactions enhance the time resolution of electron microscopes

December 18, 2024

In a series of works, we have developed different techniques harnessing phase-locking in electron-laser interaction to push ultrafast transmission electron microscopes’ resolution – so far limited to 10^-13s – to the attosecond regime (10^-18s). These five orders of magnitude improvement enables the recording of attosecond movies showing the oscillation of nano-optical excitations (e.g. surface plasmon polaritons) at the nanometer scale (see figure).

Observing and manipulating the dynamics of nano-optical excitations requires imaging techniques combining sub-wavelength spatial resolution and sub-cycle temporal resolution. In that aim, the last decade has seen the development of ultrafast transmission electron microscopy (UTEM) which combines the temporal resolution of ultrashort laser sources (few hundreds of femtoseconds) with the spatial resolution of electron microscopes (sub-nanometer) in a pump-probe scheme: a first laser pulse (pump) excites the sample, which is then probed by a pulsed electron beam (probe). This technique has been able to resolve a plethora of phenomena but remains too slow for nano-optical excitations which generally oscillate with a period of few attoseconds.

In this series of works, we have developed different techniques able to push the resolution of UTEM to the attosecond regime.
All of them are based on the same idea: sequential phase-locked interactions. Indeed, while in a conventional UTEM the electron beam measures a laser-driven sample, here we have added a second interaction stage with a fixed laser field to imprint a reference phase on the electron beam. This pre-modulation leads to interferences in the electron-sample interaction, thus revealing the sub-cycle temporal dynamics of the target. Our works explore different variations of this technique in which the reference interaction can occur either with another sample, a different optical excitation within the same sample or different scattering orders.

All these techniques have been experimentally demonstrated on the UTEM of the Göttingen university. For example, one can observe the oscillations of a localized surface plasmon mode in a gold nano-triangle on the figure above. The colormap represents the plasmonic electric field (negative values in blue, positive values in red). You can watch the full attosecond movies by clicking on this link or this one.

Contact:
Hugo Lourenço-Martins | hugo.lourenco-martins[at]cemes.fr

Publications:
Attosecond electron microscopy by free-electron homodyne detection
H. Gaida, H. Lourenço-Martins, M. Sivis, T. Rittmann, A. Feist, F. J. García de Abajo, and C. Ropers
Nature Photonics 18, 509-515 (2024)
DOI : https://doi.org/10.1038/s41566-024-01380-8

Lorentz microscopy of optical fields
H. Gaida, H. Lourenço-Martins, S. V. Yalunin, A. Feist, M. Sivis, T. Hohage, F. J. García de Abajo, and C. Ropers
Nature Communications 14, 6545 (2023)
DOI : https://doi.org/10.1038/s41566-024-01380-8