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Research

EIS-TIMER: results

The main goal of EIS-TIMER is to realize an FEL-based four-wave-mixing (FWM) instrument, which exploits the transient grating (TG) approach to measure thermal excitations (phonons, diffusion and transport processes, relaxations, etc.) in the 0.1-1 nm-1 wavevector (k) range. The recent development of the multi-color seeded FEL emission called for the development of FEL-based coherent Raman scattering (CRS), which can be used to determine the ultrafast dynamics of high-energy excitations (high-frequency molecular vibrations, excitons, etc.) with the unique advantage of atomic-selectivity.


Layout of the instrument
The main experimental hurdle for FEL-based TG is to transpose the conventional optical layout into the EUV/soft X-ray range. The lack of reliably transmission optics in this wavelength range (1-100 nm) prevents the use of diffractive optical elements (phase masks) and achromatic doublets, that are at the basis of optical setups. Moreover, the lack of phase masks makes quite challenging the implementation of heterodyne detection, which greatly improve many aspects of the experiments
We tested various options at the EIS-TIMER laser laboratory and we evaluated that an "all-reflective" optical layout (top panel) is the best compromise for an FEL-based instrument; please note that the trasmission optics used for second harmonic (SH) generation are is not needed with the FEL. This led to some revisions of the original layout (F. Bencivenga and C. Masciovecchio, NIMA 606, 785 (2009) and F. Bencivenga et al., J. Sync. Rad. 23, 132 (2016)). More complex solutions, able to exploit multi-colour FEL emission (not foreseen at the beginning of the TIMER project), are under evaluation for the eventual and more advanced FWM applications (see. e.g., F. Bencivenga et al., New J. Phys. 15, 123023 (2013)). 
Bottom panel reports TG data collected with our "all-reflective" optical TG setup, the oscillations are related with phonons propagation (in vitreous SiO2, bottom-left panel, and liquid dymethilsolfossyl, bottom-right panel). The phonon lifetime is not appreciable in this time-scale window because of the low-k value (~0.006 nm-1). In FEL-based experiments the phonon time decay is shortened by 2-3 orders of magnitude, because of the much larger k-range, and, consequently, all relevant dynamics will fall into the ~4 ns time-delay window provided by the 600 mm long delay-line of EIS-TIMER (further information in R. Cucini et al., Opt. Lett. 36, 1032 (2011)). We also investigated on the possibility to determine the translational and rotational contributions to the sample dynamics by playing with the polarization of the input fields (possible at FERMI) without using heterodyne detection (further information in R. Cucini et al., Opt. Lett. 39, 5110 (2014)).

Phys. Rev. E 92, 011101(R) – Published 31 July 2015


 
Demonstration of FEL-stimulated four-wave-mixing
We have built a special setup (mini-TIMER, click here for further details) to perform FEL-based FWM experiments stimulated by EUV transient gratings and probed by an optical pulse; for more information see F. Bencivenga et al., Nature 520, 205 (2015); Proc. SPIE 9512, 951212 (2015) and the Elettra top-story. A sketch of the experiment is reported in the picture on the right. The CCD image is the first FEL-stimulated FWM signal, that propagated downstream the sample along the "phase-matched" direction (kFWM, see the small panel). Such a setup is available for users at the DiProI end-station and  was recently used to observe the first FEL-based CRS signal stimulated by the interaction of two FEL pulses at different photon frequencies and an optical pulse.

Long-lived hot electrons in Al 

We report a time


 
Two-colors EUV-FWM experiments
The recent demonstration of multi-colour seeded FEL emission (see Allaria et al., Nat. Commun. 4, 2476 (2013)) allows for the realization of CRS applications, which may benefit from the TG setups (mini-TIMER and EIS-TIMER) developed within the TIMER project. EUV /soft X-ray CRS could be an unvaluable tool to probe ultrafast dynamics of high-energy excitation, such as excitons, with atomic selectivity (see, e.g., S. Tanaka and S. Mukamel, Phys. Rev. Lett. 89, 043001 (2002)); information basically unacessible by both optical and linear X-ray methods. A possible approach to carry out FEL-based CRS using the two-colour FEL emission scheme and the all-reflective FEL setup developed at FERMI has been proposed (the idea is sketched in the picture on the right, further details can be found in F. Bencivenga et al., Faraday Discuss. 171, 487 (2014) and note that in this "exotic" setup we have an FEL pulse ~0.4 ps before and another ~0.4 ps after the interfeering two-colors FEL pulses) and a succesful test experiment has been recently carried out (October 2015, unpublished) at mini-TIMER; user proposals on FEL-based CRS are very welcome!  


 

EIS-TIMEX: results


 

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)

 

 

High-energy density extreme ultraviolet radiation delivered by the FERMI seeded free-electron laser has been used to create an exotic nonequilibrium state of matter in a titanium sample characterized by a highly excited electron subsystem at temperatures in excess of 10 eV and a cold solid-density ion lattice. The obtained transient state has been investigated through ultrafast absorption spectroscopy across the Ti M2,3-edge revealing a drastic rearrangement of the sample electronic structure around the Fermi level occurring on a time scale of about 100 fs.

Right figure: Absorption spectrum of Ti at ambient conditions in the region of the M2;3-edge (32.6 eV). The spectrum has been recorded at the Elettra synchrotron (BEAR beamline) and at the FERMI FEL (EIS-TIMEX beamline). In the latter case, the FEL fluence at the sample was highly attenuated (F < 1 mJ cm2 ). Red arrows depict the trend of the ultrafast variation of the absorption coefficient induced by exposure to FEL pulses with fluences in the order of 1 J cm2 .

More info: E. Principi et al. Struct Dyn 3, 023604 (2016) 




 

A composite metallic foil (Al/Mg/Al) has been exposed to intense sub-100 fs free electron laser (FEL) pulses and driven to ultrafast massive photoionization. The resulting nonequilibrium state of matter has been monitored through absorption spectroscopy across the edge of Mg as a function of the FEL fluence. The raw spectroscopic data indicate that at about the main absorption channels of the sample, i.e., Mg (2p--->free)  and oxidized Al (valence ---> free), are almost saturated. The spectral behavior of the induced transparency has been interpreted with an analytical approach based on an effective ionization potential of the generated solid-density plasma.

Right figure: Comparison of experimental low-F (squares) and high-F (circles) transmission data with theoretical curves. Low-F data are compared with the transmission spectrum of the sample at ambient conditions (solid curve). Dashed-dotted and dashed curves are the sample transmission at fluences of 36 and 140 J/cm2, respectively, calculated by an analytical model.

More info: R. Mincigrucci et al. Phys. Rev. E 92, 011101(R) – Published 31 July 2015



 
EUV reflectivity experiment from a titanium (Ti) sample irradiated with ultrafast seeded FEL pulses at variable incident photon fluence and frequency has been carried out. Using a Drude formalism we relate the observed increase in reflectivity as a function of the excitation fluence to an increase in the plasma frequency, which allows us to estimate the free electron density in the excited sample. The extreme simplicity of the experimental setup makes the present approach potentially a valuable complementary tool to determine the average ionization state of the excited sample, information of primary relevance for understanding the physics of matter under
extreme conditions.

Right figure: Fluence dependence of normalized reflectivity R(F)/R(0) for some selected hv-values, indicated in the individual panels. Each data corresponds to a single shot measurement taken in a fresh portion of the sample. Red full lines are the R(F)/R(0) trends calculated analitically; dashed lines are the estimate of the confidence interval.

More details in F. Bencivenga et al. Sci. Reports 4, 4952 (2014).



 
In August 2013 the EIS-TIMEX end-station was equipped with an ellipsoidal mirror thus allowing FEL focal spot in the order of  5 um. This will allow achievement of irradiances greater than 1015 W/cm2. Under these conditions the FEL beam is expected to isochorically heat specimens and give access to unexplored transient states of matter, such as the warm dense matter (WDM).



The advent of 4th generation light sources gives the unique opportunity to generate isochoric uniform heating of specimens thus reaching the WDM transient regime (lifetime of a few ps). The knowledge of the temperature of the WDM state is a crucial issue that must be tackled in this class of experiments.
The EIS-TIMEX team has proposed and it is developing a concrete and original approach to 'measure' the ion temperature of WDM based on the heat diffusion law. The basic idea is reported in the side picture: the ion temperature reached in the interaction region (r<rc) can be determined by measuring the time variation of the temperature at a given radial distance
(r=r*) from the FEL beam center (further details are reported in E. Principi et al., NIMA 621, 643 (2010) while the experimental demosntration of this approach can be found in E. Principi et al., Phys. Rev. Lett. 109025005 (2012)).
The research activity includes design and construction of a Cassegrain infrared pyrometer capable to monitor the temperature of the sample close to the FEL beam focus. Typical spot size of the pyrometer is below 250 um. 


Last Updated on Thursday, 22 February 2018 17:24