Development of a second generation coherent ultrafast TEM – ULTEM

During the now ended ANR project FemtoTEM (2014-2019), the first Ultrafast Transmission Electron Microscope (UTEM) based on a laser-driven Cold-Field Emission Gun (CFEG) has been developed. This prototype was based on a 30 year-old TEM with limited functionalities. Nevertheless, it demonstrated the highest brightness of all UTEMs and yielded several scientific firsts. On a new modern TEM, ULTEM proposes to bring this technology to the next level with three ambitious objectives : a sensitivity boosted compared to the first generation coherent UTEM (objective 1), the possibility of controlling the dynamics of the sample inside the electron microscope (obj. 2) and the demonstration of a new technology for on-demand electron beam phase shaping that will expand the possibilities of electron microscopies (obj. 3). Objective 1 will be met by combining advanced instrumentation (direct electron detection, real-time microscope optimization) and methodological innovations (acquisition of data stacks, drift correction). Objective 2 will be achieved thanks to the development of a secondary laser source combined with an interferometer that will allow to tailor the excitation laser pulses and implement optical control schemes of the vibrational dynamics of the sample in the TEM. Objective 3 will explore two possibilities for transferring the phase of a laser pulse shaped by a spatial light modulator onto the wave function of the electron: (i) the inelastic interactions between the electrons and the electric field reflected on a membrane and (ii) the elastic interaction between fast electrons and the ponderomotive potential of light in vacuum. Complex shaping of the electron beam will be achieved using evolutionary optimization algorithms. The new capabilities will be demonstrated on cutting-edge applications in materials science: ultrafast electron holography and diffraction, time-resolved cathodoluminescence (TR-CL) correlated with TR-EELS and electron beam synthetizing. ULTEM will yield a unique user-friendly instrument available for materials science at ultimate spatio-temporal resolution.