Novel coherent ultrafast Transmission Electron Microscope for innovative nanospectroscopies and time-resolved electron interferometry – FEMTOTEM

Today, it is not possible to perform COHERENT Transmission Electron Microscopy with subpicosecond temporal resolution. Indeed, the ultrafast Transmission Electron Microscopes (UTEM) developed so far are all based on flat photocathodes. The poor spatial coherence, brightness and spectral resolution of these electron sources impede their use for the most demanding time-resolved TEM applications.

The FemTOTEM project is radically different from the other UTEM projects : its aims at developing the first UTEM based on a high brightness laser-driven field emission electron source. This will be achieved by bringing together a femtosecond laser source and a customized cold-field emission Transmission Electron Microscope (CFEG-TEM). This unique combination of femtosecond time resolution, high brightness (allowing high spatial resolution and coherence of electrons) and energy resolution will have an unrivalled potential for frontier research in nanophysics and materials science.

The first part of this project involves instrumental developments. First, we will demonstrate and characterize laser driven field emission from the customized electron source of a commercial 200kV TEM. The femtosecond cold field emission gun that we have already mounted in a dedicated Ultra High Vacuum (UHV) bench will be completely characterized in terms of emitted current and energy spectrum as a function of laser parameters and extraction voltage. Then, it will be transferred on a TEM column. Its potential for electron microscopy experiments will be thoroughly investigated first on routine TEM applications and later on more demanding experiments such as electron holography. A new attachment allowing for light injection and collection on the TEM sample, already designed, will enable us to perform original experiments involving electrons and photons.

The second part of the project will demonstrate the potential of the new ultrafast coherent TEM on three cutting edge applications. These are nanometer scale, picosecond-resolved Cathodoluminescence (pTRCL) experiments, nanometer scale, high spectral resolution Electron Energy Gain Spectroscopy (EEGS) and ultrafast electron holography. We will demonstrate the measurement of the picosecond luminescence dynamics of individual quantum emitters even in complex or closely packed environments, which is out of reach of the best available pTRCL set-ups, thanks to an expected 6 orders of magnitude increase in brightness. This should allow deepening our understanding of exciton physics in confined systems with numerous applications in photodetection, light emission, single photon sources... The EEGS experiments that we propose are also disruptive experiments going beyond current state of the art in plasmonics and nano-optics. Finally, we will perform the first time-resolved coherent electron microscopy experiments. The fundamental aspects of electron interferometry with non-stochastic electron emission will be investigated first and followed by time-resolved electron holography with applications in nanomagnetism and nanomechanics.

Our consortium brings together the complementary skills and expertise to overcome well-identified challenges. This project will provide a unique tool enabling the time resolved investigation of dynamical processes in many fields of physics such as nano-optics, mechanics or magnetism with nanometer spatial resolution. Such a breakthrough in the field of experimental physics will open a wide range of unexplored routes in nanoscience, chemistry and fundamental physics.