The Elastic and Inelastic Scattering (EIS) beamline consists of two separate end-stations (EIS-TIMEX and EIS-TIMER), dedicated to two different research projects. EIS-TIMEX is fully operational since July 2013 and it is devoted to study matter under extreme thermodynamic conditions on ultrafast timescales. EIS-TIMER is dedicated to wave-mixing experiments. It was fully commissioned in 2017, when we started accepting user proposals. The common aim of the two instruments is to exploit the time structure of the FERMI FEL source for performing time-resolved experiments through the pump-probe approach. Each end-station will exploit different key properties of the source, that operates in the 20-300 eV spectral range at the fundamental harmonics (up to 900 eV in the 3rd harmonics), delivering photon pulses of ~ 40-80 fs duration and ~100 uJ energy, with selectable photon wavelenght and polarization. Different types of multi-colour FEL emission schemes are also available.


Basic principles

EIS-TIMER end-station is a FEL-based four-wave-mixing instrument that will exploit the time structure, harmonic content and coherence properties of the FERMI source. EIS-TIMER is specially designed to exploit the transient grating (TG) approach. In TG-based experiments two non-collinear coherent FEL pulses (pump) are overlapped, in time and space, at the sample. Their interference originates a transient standing electromagnetic wave (i.e. the TG) with a spatial periodicity in the 1-100 nm range. The TG imposes a nanoscale modulation of sample parameters, whose time evolution can be monitored by measuring the diffraction of a third time-delayed coherent pulse (probe), which inpinges into the sample at the Bragg angle. The time-dependent diffracted signal encodes relevant information on several kinds of dynamics, ranging from slow (>ns scale) diffusion processes to fast (sub-fs scale) electron dynamics. The implementation of this experimental scheme, nowadays used only with optical lasers, to the EUV/soft X-ray range would be of relevance, e.g., for the physics of disordered systems, since it will make accessible the mesoscopic kinematic region (wavevectors in the 0.1-1 nm-1 range) that cannot be explored by available instruments. Nanoscale TG experiments could also allow sensitive probing of thin films/interfaces, transport properties and correlations in nanostructured materials.

The combination of the TG method and the multi-colour seeded FEL emission available at FERMI also allows for coherent Raman scattering (CRS) experiments. EUV/soft X-ray CRS can be a unique ultrafast probe for high-energy excitations, such as high-frequency vibrations or electronic excitations. A succesful test of FEL stimulated CRS has been recently carried out using the mini-TIMER setup and a "phase-matched" optical probe; CRS will be also available at EIS-TIMER. We finally mention that the wavelength tunability of FERMI enables the exploitation of EUV/soft X-ray core-resonances of selected atoms, which may be used to add atomic selectivity to both TG and CRS experiments.


The mini-TIMER setup is also available for users at the DiProI end station (see here for further details). The main differences between EIS-TIMER and mini-TIMER are:

  • the range in the crossing angle (~ 2o-12o, continuously variable, at mini-TIMER and 18o, 27o90o or 105o at EIS-TIMER); 
  • the range in the time delay between the two FEL "pump" pulses (< 0.5 ps at mini-TIMER, up to ~7 ps at EIS-TIMER);
  • the possibility to use an EUV/soft X-ray probe at selected wavelength (17.8, 13.3, 6.7 and 3.2 nm; other wavelength may be possible upon request). In this case the pump wavelength has to be 3 times larger than the probe one.


The TIMER project has been partially financed by the ERC Grant N.202804-TIMER

Top panel: a simple sketch of a TG experiment. Bottom panel: the region of the energy-wavevector (ω-k) plane plane accessible by FEL-based TG (orange area) and CRS (cyan area) along with the typical range of sample excitations; the range probed by optical optical four-wave-mixing (yellow area) is limited to the low-k region by the long wavelength of optical photons. The main aim of TIMER is to use FEL-based TG to probe low-energy excitations (ω < 0.1 eV: phonons, thermal modes, etc.) in the 0.1-1 nm-1 k-range and FEL-based CRS to probe high-energy excitations (ω up to a few eV: high-frequency vibrations, excitons, etc.), and, more in general, to experimentally develop the wave-mixing approach in theEUV and soft X-ray range.


Basic principles

EIS-TIMEX end-station will exploit the high intensity, energy domain and time structure of the FEL to probe fundamental properties of dense matter under metastable and/or extreme thermodynamic conditions with sub-ps time resolution. The basic idea is to exploit the high peak power of the Fermi FEL source of a fs optical laser  to induce an efficient ultrafast (< ps) and almost isochoric heating (up to the 10's eV (~106 K) range) of bulk-like dense samples followed by a slower (~10 ps) isoentropic expansion. The energy deposited in the sample is large enough to stimulate a nonequilibrium condition characterized by hot electron temperatures and cold lattice followed by structural changes including phase transitions. Under specific conditions optically excited condensed matter can reach a Warm Dense Matter (WDM) thermodynamic regime. WDM is a poorly understood state of matter located "in between" the classical plasma and the condensed matter (see phase diagram on the right), where the atoms/ions behaviour is strongly coupled to the dense electron plasma despite the high temperature. The understanding of the WDM state represents a real challenge for researchers, and it would be of great relevance for several applications, such as: inertial fusion, extreme-state chemistry, high-pressure research, etc. The study of WDM properties and/or phase transitions dynamics in the 0.1-100 ps range will be done through a pump-probe (time-resolved) scheme, which can exploit either an optical (i.e., a fs laser) and/or a EUV probe (i.e., a second FEL pulse).


Soft X-Ray SHG from graphite

We report the observation of soft x-ray second harmonic generation near the carbon K edge (∼284 eV) in graphite thin films generated by high intensity, coherent soft x-ray pulses at the FERMI free electron laser. Our experimental results and accompanying first-principles theoretical analysis highlight the effect of resonant enhancement above the carbon K edge and show the technique to be interfacially sensitive in a centrosymmetric sample with second harmonic intensity arising primarily from the first atomic layer at the open surface. This technique and the associated theoretical framework demonstrate the ability to selectively probe interfaces, including those that are buried, with elemental specificity, providing a new tool for a range of scientific problems. For more details: Lam et al. PHYSICAL REVIEW LETTERS 120, 023901 (2018)


Generation of coherent magnons in NiO

We report the observation of coherent collective modes in the antiferromagnetic insulator nickel oxide (NiO) identified by a frequency of 0.86 THz, which matches the expected out-of-plane single-mode magnon resonance. Such collective excitations are inelastically stimulated by extreme ultraviolet (EUV) pulses delivered by a seeded free-electron laser (FEL) and subsequently revealed probing the transient optical activity of NiO looking at the Faraday effect. Moreover, the unique capability of the employed FEL source to deliver circularly polarized pulses allows us to demonstrate optomagnetic control of such collective modes at EUV photon energies. These results may set a starting point for future investigations of magnetic materials at time scales comparable or faster than those typical of exchange interactions. For more details: Simoncig et al. PHYSICAL REVIEW MATERIALS 1, 073802 (2017)


Extreme-Ultraviolet Vortices from a FEL

We present an in-situ and an ex-situ technique for generating intense, femtosecond, coherent optical vortices at a free-electron laser in the extreme ultraviolet. The first method takes advantage of nonlinear harmonic generation in a helical undulator, producing vortex beams at the second harmonic without the need for additional optical elements, while the latter one relies on the use of a spiral zone plate to generate a focused, micron-size optical vortex with a peak intensity approaching 10^14 W/cm2, paving the way to nonlinear optical experiments with vortex beams at short wavelengths. More details: Ribic et al. PHYSICAL REVIEW X 7, 031036 (2017)

RIXS experiments at EIS-TIMEX

We have designed and constructed a RIXS experimental endstation that allowed us to successfully measure the d-d excitations in KCoF3 single crystals at the cobalt M2,3-edge. The FEL-RIXS spectra show an excellent agreement with the ones obtained from the same samples at the MERIXS endstation of the MERLIN beamline at the Advanced Light Source storage ring (Berkeley, USA). For more details: Dell'Angela et al. Scientific Reports 6, 38796 (2016)

Long-lived hot electrons in Al 

We report a time-resolved study of the relaxation dynamics of Al films excited by ultrashort intense free-electron laser (FEL) extreme-ultraviolet pulses. The system response was measured through a pump-probe detection scheme, in which an intense FEL pulse tuned around the Al L2,3 edge (72.5 eV) acted as the pump, while a time-delayed ultrafast pulse probed the near-infrared (NIR) reflectivity of the Al film. Remarkably, following the intense FEL excitation, the reflectivity of the film exhibited no detectable variation for hundreds of fs. For more details: F. Bisio, E. Principi et al. PRB 96, 081119(R) (2017)

EUV stimulated emission from MgO

We give evidence for soft-x-ray stimulated emission from a magnesium oxide solid target pumped by EUV FEL pulses formed in the regime of travelling-wave amplified spontaneous emission in backward geometry. Our results combine two effects separately reported in previous works: emission in a privileged direction and existence of a material-dependent threshold for the stimulated emission. Our model accounts for both observed mechanisms that are the privileged direction for the stimulated emission of the Mg L2,3 characteristic emission and the pumping threshold. More details:  P. Jonnard et al. Structural Dynamics 4, 054306 (2017)




Last Updated on Monday, 14 October 2019 14:01