Among the fourth-generation light sources, the Italian free-electron laser (FEL) FERMI is the only one operating in the high-gain harmonic generation (HGHG) seeding mode. FERMI delivers pulses characterized by a quasi transform limited temporal structure, photon energies lying in the extreme ultra-violet (EUV) region, supreme transversal and longitudinal coherences, high peak brilliance, and full control of the polarization. Such state of the art performances recently opened the doors to a new class of time-resolved spectroscopies, difficult or even impossible to be performed using self-amplified spontaneous sources (SASE) light sources. FERMI is currently equipped with three operating beamlines opened to external users (DiProI, LDM and EIS), while two more are under commissioning (MagneDYN and TeraFERMI). Here, we present the recent highlights of the EIS (Elastic and Inelastic Scattering) beamline, which has been purposely designed to take full advantage from the coherence, the intensity, the harmonics content, and the temporal duration of the pulses. EIS is a flexible experimental facility for time-resolved EUV scattering experiments on condensed matter systems, consisting of two independent end-stations. The first one (EIS-TIMEX) aims to study materials in metastable and warm dense matter (WDM) conditions, while the second end-station (EIS-TIMER) is fully oriented to the extension of four-wave mixing (FWM) spectroscopies towards the EUV spectral regions, trying to reveal the behavior of matter in portions of the mesoscopic regime of exchanged momentum impossible to be probed using conventional light sources.
Stimulated emission is a fundamental process in nature that deserves to be investigated and understood in the EUV and X-ray regimes. Today this is definitely possible through high energy density FEL beams. In this context, we show evidence for soft x-ray stimulated emission from a MgO solid target pumped by extreme ultraviolet 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. We have developed a theoretical framework, based on coupled rate and transport equations taking into account the solid density plasma state of the target. 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.
The development of free electron laser (FEL) sources, which provide extreme ultraviolet (XUV) and soft x-ray radiation
of unprecedented coherence and almost transform-limited pulse structure, has opened up the realm of XUV/x-ray
non-linear optics. In particular, XUV four-wave-mixing (XFWM) experiments may allow, e.g., to probe correlations
among low-energy excitations and core states, and to access the “mesoscopic” wavevector range (0.1-1 nm-1), inaccessible
so far and fundamental to investigate nanostructures and disordered systems. In this manuscript we report on the latest
advances and future developments of the TIMER setup at FERMI (Elettra, Italy), specifically conceived for XFWM
experiments. In particular, we discuss the improvements on the XUV-probe and on the pump transport. Moreover, TIMER
and mini-TIMER (a test setup available at the DiProI end station) are also suitable for time-resolved second order nonlinear
experiments, which are intrinsically surface sensitive due to symmetry restrictions. We hereby discuss the foreseen
extension to the XUV of interface specific probing of electronic processes, for example charge and energy transfer, with
Extreme Ultraviolet (EUV) multilayer (ML) technology has been intensively applied in many scientific and technological fields such as solar physics and photolithography. More recently, the advent of free electron lasers (FEL) emitting bright sub-ps pulses with very high quality in term of intensity stability, coherence and temporal shape has encouraged the usage of multilayer coatings also in the transport and manipulation of FEL radiation. In fact, conventional single layers coated mirrors provide negligible reflectance in the EUV spectral range whereas ML mirrors can reach high efficiency at normal incidence without affecting the pulses characteristics. Such optical elements have been also exploited at FERMI@ELETTRA FEL where novel multilayer coatings specifically conceived for pump and probe experiment and ultrafast absorption spectroscopy have been designed. The main results are reported.
In this manuscript we report on a compact experimental set-up (“mini-TIMER”) conceived for transient grating (TG) experiments based on free electron laser (FEL) radiation. This set-up has been tested at the seeded FEL facility FERMI (Elettra, Trieste, Italy) and allowed us to observe the first FEL-stimulated TG signal. This experimental result is of the greatest relevance in the context of developing coherent non-linear optical methods into the extreme ultraviolet (EUV) and soft X-ray (SXR) range. Such a challenging task will be addressed in the next future at FERMI by using the present set-up and the forthcoming EIS-TIMER beamline, which is being installed at FERMI and will start the commissioning phase in the second semester 2015. The possibility to use TGs generated by FEL radiation at sub-optical wavelengths would allow developing EUV/SXR four-wave-mixing (FWM) applications, so far considered only theoretically and widely believed to be potentially able to provide major breakthroughs in several fields of science.
FERMI@Elettra is the first seeded VUV/soft X-ray FEL source. It is composed of two undulatory chains: the low energy branch (FELl) covering the wavelength range from 20 nm up to 100 nm, and the high energy branch (FEL2, employing a double stage cascade), covering the wavelength range from 4 nm up to 20 nm. At the end of 2012 FELl has been opened to external users while FEL2 has been turned on for the first time having demonstrated that a double cascade scheme is suitable for generating high intensity coherent FEL radiation. In this paper we will share our experience and will show our most recent results for both FERMI FELl and FEL2 sources. We will also present a brand new machine scheme that allows to perform two-colour pump and probe experiments as well as the first experimental results.
The development of Free Electron Laser sources is opening up the possibility to probe dynamics at the femtosecondnanometer time-length scales. A remarkable step forward towards this goal would be achieved by extending the Four Wave Mixing (FWM) approach at VUV/soft x-ray wavelengths. FWM-based techniques allow a coherent control in both the stimulating and probing processes of photon-induced excitations. We propose to exploit the FERMI@Elettra seeded Free Electron Laser (FEL) source to put on practice the VUV/soft x-ray FWM approach, yet theoretically conceived one decade ago. Moreover, the exploitation of VUV/soft x-ray wavelengths allows adding site-sensitivity to FWM methods by exploiting core resonances of selected elements in the sample.
FERMI@Elettra is a VUV/Soft X-ray Free Electron Laser (FEL) user facility under commissioning in Trieste, Italy. It provides a spatially coherent transform-limited photon beam in the sub-ps regime with high fluence and tunable wavelength. One of the FERMI beamlines, TIMEX, will be dedicated to the study of matter under extreme and metastable conditions, created and probed by the FEL radiation. Moreover, an active optics dedicated to perform the beam shaping at focus is needed in order to provide the necessary flat-top intensity distribution for heating the sample uniformly. In this work the principles of the beam shaping applied to the TIMEX beamline will be discussed as well as the adopted solution. Ray tracing simulations will be shown for theoretical mirror profiles as well as the metrological measurements with an interferometer and the Long Trace Profiler (LTP).
FERMI@Elettra is a new free-electron-laser (FEL) facility, presently under commissioning, able to generate
subpicosecond photon pulses of high intensity in the far ultraviolet and soft X-ray range (λ=100-20 nm for
the present FEL1 source, extended in future to 4 nm with the FEL2 source). Here we briefly describe the
present status of the TIMEX end-station, devoted to perform experiments on condensed matter under extreme
conditions. The layout of the end-station, presently in the final stages of construction, is reported showing the
details of the optics and sample environment. The potential for transmission, reflection, scattering, as well as
pump-and-probe experiments is discussed taking into account that FEL pulses can heat thin samples up to
the warm dense matter (WDM) regime. The calculated deposited energy in selected elemental films, including
saturation effects, shows that homogeneous heating up to very high temperatures (1-10 eV for the electrons) can
be easily reached with a suitable tuning of the energy and focus of the soft x-ray pulses of FERMI@Elettra. The
results of the first test of the TIMEX end-station are also reported.