The FERMI beamline system: a unique tool for investigating matter at nano-scale

All natural sciences benefit from the use of electromagnetic radiation to probe the phenomena of nature. From the structure of atoms and molecules to their evolution in complex system for various energy conditions, photons have been proved to be an essential tool for exploring matter in all its form. The advent of Free Electron Lasers (FEL) has allowed to perform unprecedented experiments. The properties of the FEL radiation (transverse and longitudinal coherence, brightness, pulse length and energy bandwidth) allow to perform Coherent Diffraction Imaging at a nano-scale level, non-linear spectroscopy and to probe ultra-fast phenomena in a wide set of simple and complex systems. On the the hand the FEL radiation needs to be characterised, transported and focused in order to allow the users to perform the experiments. In this thesis the FERMI beamlines (the Italian FEL) are described in detail together with the outstanding technological and scientific experimental results

BGO relaxation dynamics probed with heterodyne detected optical transient gratings

We used optical laser pulses to create transient gratings (TGs) with sub-10 {\mu}m spatial periodicity in a Bismuth Germanate (310) (Bi4Ge3O12) single crystal at room temperature. The TG launches phonon modes, whose dynamics were revealed via forward diffraction of a third, time-delayed, heterodyne-detected optical pulse. Acoustic oscillations have been clearly identified in a time-frequency window not covered by previous spectroscopic studies and their characteristic dynamic parameters have been measured as a function of transferred momenta magnitude and direction.Comment: 6 pages, 4 figure

Transient grating spectroscopy on a DyCo5_5 thin film with femtosecond extreme ultraviolet pulses

Surface acoustic waves (SAWs) are excited by femtosecond extreme ultraviolet (EUV) transient gratings (TGs) in a room-temperature ferrimagnetic DyCo$_5$ alloy. TGs are generated by crossing a pair of EUV pulses from a free electron laser (FEL) with the wavelength of 20.8\,nm matching the Co $M$-edge, resulting in a SAW wavelength of $\Lambda=44$\,nm. Using the pump-probe transient grating scheme in a reflection geometry the excited SAWs could be followed in the time range of -10 to 100\,ps in the thin film. Coherent generation of TGs by ultrafast EUV pulses allows to excite SAW in any material and to investigate their couplings to other dynamics such as spin waves and orbital dynamics

FEL stochastic spectroscopy revealing silicon bond softening dynamics

Time-resolved X-ray Emission/Absorption Spectroscopy (Tr-XES/XAS) is an informative experimental tool sensitive to electronic dynamics in materials, widely exploited in diverse research fields. Typically, Tr-XES/XAS requires X-ray pulses with both a narrow bandwidth and sub-picosecond pulse duration, a combination that in principle finds its optimum with Fourier transform-limited pulses. In this work, we explore an alternative xperimental approach, capable of simultaneously retrieving information about unoccupied (XAS) and occupied (XES) states from the stochastic fluctuations of broadband extreme ultraviolet pulses of a free-electron laser. We used this method, in combination with singular value decomposition and Tikhonov regularization procedures, to determine the XAS/XES response from a crystalline silicon sample at the L2,3-edge, with an energy resolution of a few tens of meV. Finally, we combined this spectroscopic method with a pump-probe approach to measure structural and electronic dynamics of a silicon membrane. Tr-XAS/XES data obtained after photoexcitation with an optical laser pulse at 390 nm allowed us to observe perturbations of the band structure, which are compatible with the formation of the predicted precursor state of a non-thermal solid-liquid phase transition associated with a bond softening phenomenon

Widely tunable two-colour seeded free-electron laser source for resonant-pump resonant-probe magnetic scattering

International audienceThe advent of free-electron laser (FEL) sources delivering two synchronized pulses of different wavelengths (or colours) has made available a whole range of novel pump–probe experiments. This communication describes a major step forward using a new configuration of the FERMI FEL-seeded source to deliver two pulses with different wavelengths, each tunable independently over a broad spectral range with adjustable time delay. The FEL scheme makes use of two seed laser beams of different wavelengths and of a split radiator section to generate two extreme ultraviolet pulses from distinct portions of the same electron bunch. The tunability range of this new two-colour source meets the requirements of double-resonant FEL pump/FEL probe time-resolved studies. We demonstrate its performance in a proof-of-principle magnetic scattering experiment in Fe–Ni compounds, by tuning the FEL wavelengths to the Fe and Ni 3p resonances

The Low Density Matter (LDM) beamline at FERMI: Optical layout and first commissioning

The Low Density Matter (LDM) beamline has been built as part of the FERMI free-electron laser (FEL) facility to serve the atomic, molecular and cluster physics community. After the commissioning phase, it received the first external users at the end of 2012. The design and characterization of the LDM photon transport system is described, detailing the optical components of the beamline