Alberto Simoncig

Toward an integrated device for spatiotemporal superposition of free-electron lasers and laser pulses

Free-electron lasers (FELs) currently represent a step forward on time-resolved investigations on any phase of matter through pump-probe methods involving FELs and laser beams. That class of experiments requires an accurate spatial and temporal superposition of pump and probe beams on the sample, which at present is still a critical procedure. More efficient approaches are demanded to quickly achieve the superposition and synchronization of the beams. Here, we present what we believe is a novel technique based on an integrated device allowing the simultaneous characterization and the fast spatial and temporal overlapping of the beams, reducing the alignment procedure from hours to minutes.

Four-wave-mixing experiments and beyond: the TIMER/mini-TIMER setups at FERMI

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 chemical specificity.

Timing methodologies and studies at the FERMI free-electron laser

Time-resolved investigations have begun a new era of chemistry and physics, enabling the monitoring in real time of the dynamics of chemical reactions and matter. Induced transient optical absorption is a basic ultrafast electronic effect, originated by a partial depletion of the valence band, that can be triggered by exposing insulators and semiconductors to sub-picosecond extreme-ultraviolet pulses. Besides its scientific and fundamental implications, this process is very important as it is routinely applied in free-electron laser (FEL) facilities to achieve the temporal superposition between FEL and optical laser pulses with tens of femtoseconds accuracy. Here, a set of methodologies developed at the FERMI facility based on ultrafast effects in condensed materials and employed to effectively determine the FEL/laser cross correlation are presented.

Characterization of ultrafast free-electron laser pulses using extreme-ultraviolet transient gratings

The characterization of the time structure of ultrafast photon pulses in the extreme-ultraviolet (EUV) and soft X-ray spectral ranges is of high relevance for a number of scientific applications and photon diagnostics. Such measurements can be performed following different strategies and often require large setups and rather high pulse energies. Here, high-quality measurements carried out by exploiting the transient grating process, i.e. a third-order non-linear process sensitive to the time-overlap between two crossed EUV pulses, is reported. From such measurements it is possible to obtain information on both the second-order intensity autocorrelation function and on the coherence length of the pulses. It was found that the pulse energy density needed to carry out such measurements on solid state samples can be as low as a few mJ cm-2. Furthermore, the possibility to control the arrival time of the crossed pulses independently might permit the development of a number of coherent spectroscopies in the EUV and soft X-ray regime, such as, for example, photon echo and two-dimensional spectroscopy.

EUV stimulated emission from MgO pumped by FEL pulses

Stimulated emission is a fundamental process in nature that deserves to be investigated and understood in the extreme ultra-violet (EUV) and x-ray regimes. Today, this is definitely possible through high energy density free electron laser (FEL) beams. In this context, 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. We develop a novel 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 EIS beamline at the seeded free-electron laser FERMI

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.

Nonlinear optics with coherent free electron lasers

We interpreted the recent construction of free electron laser (FELs) facilities worldwide as an unprecedented opportunity to bring concepts and methods from the scientific community working with optical lasers into the domain of x-ray science. This motivated our efforts towards the realization of FEL-based wave-mixing applications. In this article we present new extreme ultraviolet transient grating (X-TG) data from vitreous SiO2, collected using two crossed FEL pulses (photon frequency 38 eV) to generate the X-TG and a phase matched optical probing pulse (photon frequency 3.1 eV). This experiment extends our previous investigation, which was carried out on a nominally identical sample using a different FEL photon frequency (45 eV) to excite the X-TG. The present data are featured by a peak intensity of the X-TG signal substantially larger than that previously reported and by slower modulations of the X-TG signal at positive delays. These differences could be ascribed to the different FEL photon energy used in the two experiments or to differences in the sample properties. A systematic X-TG study on the same sample as a function of the FEL wavelength is needed to draw a consistent conclusion. We also discuss how the advances in the performance of the FELs, in terms of generation of fully coherent photon pulses and multi-color FEL emission, may push the development of original experimental strategies to study matter at the femtosecond–nanometer time–length scales, with the unique option of element and chemical state specificity. This would allow the development of advanced experimental tools based on wave-mixing processes, which may have a tremendous impact in the study of a large array of phenomena, ranging from nano-dynamics in complex materials to charge and energy transfer processes.

Generation of acoustic waves by an extreme ultra violet free electron laser in a transient grating experiment

The use of lasers to generate acoustic waves revolutionized the field of ultrasound and enabled numerous key developments in both fundamental research and applications. In the past decade, remarkable progress has been achieved in developing coherent sources of radiation operating in the extreme ultraviolet (EUV) and x-ray ranges, such as free electron lasers, which already yielded many breakthroughs in different fields of science. We expect that the field of ultrasonics will also greatly benefit from the availability of coherent EUV and x-ray sources. So far, a number of studies explored the use of coherent EUV and x-ray radiation for detection of acoustic waves. In this report, we describe the first experiment on the generation of surface and bulk acoustic waves in the tens of GHz range by EUV light.

Amplified spontaneous and stimulated Mg L emissions from MgO pumped by FEL pulses

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.

Damping of the tunneling mechanism in high-order harmonic generation processes induced by femtosecond visible laser pulses

The high-order harmonic generation (HHG) of coherent light pulses in gases is strictly dependent on the ionization dynamics that delocalizes the electron wave function in the laser field continuum. Here we report on the investigation of such dynamics by varying the energy of a 400 nm, 60 fs driving laser pulse. The experimental data provide the unambiguous evidence of a hybrid dynamical region, not described by the full-tunneling approximation models, where the HHG photon number exponentially decays when the Keldysh parameter is varied from 1 to 3. Finally, a phenomenological model, suitable to account for this hybrid regime, is reported.