Extreme-ultraviolet vortices from a free-electron laser

Extreme-ultraviolet vortices may be exploited to steer the magnetic properties of nanoparticles, increase the resolution in microscopy, and gain insight into local symmetry and chirality of a material; they might even be used to increase the bandwidth in long-distance space communications. However, in contrast to the generation of vortex beams in the infrared and visible spectral regions, production of intense, extreme-ultraviolet (XUV) and x-ray optical vortices still remains a challenge. Here, we present an in-situ and an ex-situ technique for generating intense, femtosecond, coherent optical vortices with tunable topological charge at a free-electron laser (FEL) in the XUV.

The first method takes advantage of nonlinear harmonic generation in a helical undulator and exploits the fact that such harmonics carry a topological charge of l = n-1, where n is the harmonic number. The experiment was performed at the FERMI FEL. An ultraviolet (250-nm) seed laser was used to energy modulate the electron beam (e-beam) in the first undulator (modulator), as shown in the top panel of Figure 1. The e-beam was then sent through a dispersive section (a four-dipole-magnet chicane), where the energy modulation was transformed into a current-density modulation (bunching) with Fourier components spanning many harmonics of the seed laser frequency. Such a bunched e-beam entered the helical radiator tuned to a fundamental wavelength of 31.2 nm (i.e., the 8th harmonic of the seed), producing coherent light in the XUV. The FEL was operated in the high-gain regime, close to the saturation point. Under these conditions, the interaction between the radiation at the fundamental FEL wavelength and the e-beam induced bunching at the second harmonic (15.6 nm), resulting in emission of coherent XUV vortices carrying unit topological charge (l = 1) at intensities on the order of 10−3 of the fundamental FEL emission; see bottom panel in Figure 1.

>Read more on the FERMI website

Top: The scheme to generate optical vortices at harmonics (in the present case at the 2nd harmonic) of the fundamental FEL wavelength. The optical vortex is separated from the fundamental FEL emission using a Zr filter.
Bottom: Intensity profile of the generated optical vortex with a topological charge of l =1 (left), and interference with a Gaussian beam revealing the twisted nature of the vortex (right).


Molecular dynamics on the femtosecond timescale

A photochemical reaction in its becoming has been observed with unprecedented detail at the Free Electron Laser FERMI in Trieste.

The result of the experiment published in Nature Communications paves the way for investigations that can shed new light on photochemical processes.

“Shooting the movie” of a photochemical reaction, interpreting its hidden details with the help of a computer: this is what has been done, thanks to the extraordinary capabilities of the FERMI free electron laser source in Trieste, by a research team composed of the Universities of Uppsala and Gothenburg in Sweden, the Institut Ruđer Bošković of Zagreb, the Elettra-FERMI Laboratory, the University of Trieste and the Laboratory of Physical Chemistry, Matter and Radiation in Paris. The study was published in Nature Communications.

The researchers wanted to capture the details of a chemical reaction promoted by the absorption of light (photochemical process), to understand how the excitation generated by a light beam induces changes on a target molecule. The first steps in a photochemical process involve changes in the electronic and geometric structure of a molecule over extremely short times measured in femtoseconds (1 fs = 10-15 s), which had so far hindered the accurate reconstruction of the entire sequence of the reaction.

The combination of intensity, energy resolution and very short pulse duration of the FERMI seeded free-electron laser source can now for the first time provide exceptionally detailed information on photoexcitation-deexcitation and fragmentation processes of isolated molecules in pump-probe experiments on the 50-femtosecond time scale.

Photoelectron spectroscopy with high resolution in energy and time, combined with accurate electronic structure and molecular dynamics calculations, has allowed to visualize in its entirety the temporal evolution of the prototype system chosen for the experiment: acetylacetone—a stable molecule used in environmental and medical applications.

“Besides revealing the dynamics of the reaction—explains Maria Novella Piancastelli of the University of Uppsala, principal investigator—a strong point of the experiment lies in the general applicability of the method, which leads us to consider it as the best way to investigate fundamental photochemical processes such as photosynthesis, photovoltaic energy production and vision.  The stairway that goes from simple to complex molecules, and from the understanding of phenomena to practical applications is of course a long one, and we are specifically interested in its ‘first step’.

>Read more on the FERMI website

Figure: A pictorial representation of the potential energy surfaces involved in the relaxation mechanism of acetylacetone: the ground state S0 (darker blue), two singlet S2 (ππ*) (light blue) and S1 (nπ*) (orange), and two triplet T2 (nπ*) (light green) and T1 (ππ*) (green) states. This approach based on high-resolution valence spectra backed by high-level calculations is the ultimate way to shed light on fundamental, basic photo processes such as photosynthesis, photovoltaic energy production, and vision.



Observation and Control of Laser-Enabled Auger Decay

When isolated atoms are electronically excited, they have two possible ways of releasing electronic energy: by radiation or by Auger decay. The Auger process, in which the decaying electron transfers its energy to another electron causing it to detach (ionization), has played an important part in modern physics, particularly surface science, because it is by far the strongest decay channel for core holes of light elements such as carbon, nitrogen, and oxygen. In some cases, the Auger process is energetically forbidden, because the energy being exchanged is not sufficient for ionization. In this case, new electronic mechanisms for deexcitation may be discovered that “borrow” energy from the surroundings. One of these is interatomic Coulombic decay (ICD) where the energy is “borrowed” from surrounding atoms. Another mechanism is laser enabled Auger Decay (LEAD), where the energy is “borrowed” from an ancillary laser field; up to now LEAD has been observed with low-energy photons, meaning that more than one photon must be absorbed to make the process possible.

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Time-resolved measurement of interatomic Coulombic decay

… induced by two-photon double excitation of Ne2

On the 24th of March 2017, Tsukasa Takanashi gained his doctorate from the University of Tohoku (Japan), together with the President’s Award prize (総長賞). The prize is awarded each year to the best PhD students in recognition of their outstanding academic curriculum, and particularly for the excellent results obtained during their studies. Tsukasa carried out his studies under the supervision of Professor Kiyoshi Ueda, a leading figure on the international scene of atomic and molecular physics, and until recently, a member of the FERMI Review Panel. In his thesis, Tsukasa used the light from Free Electron Lasers (FELs) to study the dynamics of highly excited molecular systems; in his home country, he utilized the Japanese FEL SACLA, and he studied the Coulomb explosion of the molecule CH2I2 (diiodomethane). This process is the fragmentation by multiple ionization of a sample, and the successive repulsion of the ions by the positive charge which is generated.

An important part of his work was carried out at FERMI, currently the only FEL source in the world able to provide Tsukasa the wavelength (75.6 nm) and temporal resolution (10-13 s) necessary to study the dynamics of his system: the Ne2 molecule, which consists of two neon atoms bound by their weak van der Waals interaction. The apparent simplicity of this system allows the detailed study of complex phenomena, such as the exchange of energy after electronic excitation, which is basic to all photochemical processes.

>Read more on the FERMI website

Image: Schematic representation of the resonant absorption of two FEL photons by a neon dimer (upper panel) and the ICD relaxation process by ionization (lower panel).