Energetic keV electron scattering is a sensitive probe of microscopic material structure. The ultrafast electron diffractometer at LACUS expands this microscopic sensitivity to the lattice atomic position to a femtosecond time resolution, enabling for real time movies of atomic motion. This bring new insights on the fundamental couplings in a material and opens new route to control matter with light.

Fig 1: UED with and without a radio-frequency (RF) compressor. From https://infoscience.epfl.ch/record/205080

An UED diffractometer consists of an ultrafast photocatode, electronic lens and suitable electron detector. This basic design (Fig 1 b), does not come with a femtosecond time resolution due to unavoidable electronic wavepacket spread upon propagation form the photocatode to the sample. The UED machine at LACUS is equipped with a electronic pulse compressor, which reduces the electronic pulse duration to a few 100s fs pulse duration, which is comparable to the intrinsic time scale of lattice ionic motion.

Two forms of diffraction geometries can be performed in our machine, transmission electron diffraction and reflection high energy electron diffraction (RHEED).

The latter is sensitive to the topmost atomic layers of a materials and to adsorbed overlayers but requires special pulse front tilting schemes to preserve a good temporal resolution, due to the group velocity mismatch between light and non-relativistic electrons.

Pulse front tilt in ultrafast RHEED. From https://doi.org/10.1063/1.4991483

This is implemented in the LACUS machine, and we obtain a few 100s of fs temporal resolution in reflection geometry.

RHEED time resolution. From https://doi.org/10.1063/1.4991483

Learn more about the UED machinee on its webpage and on the following publications:

Design and implementation of a flexible beamline for fs electron diffraction experiments

G. F. Mancini; B. Mansart; S. Pagano; B. Van Der Geer; M. De Loos et al. 

Nuclear Instruments & Methods In Physics Research Section A-Accelerators Spectrometers Detectors And Associated Equipment. 2012. Vol. 691, p. 113-122. DOI : 10.1016/j.nima.2012.06.057.

Design and implementation of an optimal laser pulse front tilting scheme for ultrafast electron diffraction in reflection geometry with high temporal resolution

F. Pennacchio; G. M. Vanacore; G. F. Mancini; M. Oppermann; R. Jayaraman et al. 

Structural Dynamics. 2017. Vol. 4, num. 4, p. 044032. DOI : 10.1063/1.4991483.

Femtosecond diffractive imaging of structures, charge and spin textures

G. F. Mancini / F. Carbone (Dir.)  

Lausanne, EPFL, 2015.