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The conical diffraction mounting in which the direction of incident light belongs to a plane parallel to the direction of the grooves has the unique property of maintaining high diffraction efficiency, even in the extreme-ultraviolet (EUV) region. This property is useful for designing high-throughput time-delay-compensated monochromators for the spectral selection of ultrashort EUV pulses as the high-order harmonics generated by the interaction between an ultrashort laser pulse and a gas jet. The time compensation allows one to exploit the femtosecond scale duration of the harmonics both to have high intensity and to reach an unprecedented temporal resolution for pump and probe experiments. Because two gratings have to be used for time compensation, the high diffraction efficiency becomes an essential requirement, which can be fulfilled by the conical diffraction mounting. Measurements recently accomplished at the Bending Magnet for Emission Absorption and Reflectivity (BEAR) beam line (ELETTRA Synchrotron, Trieste, Italy) for three gratings in the 10--90 nm region are reported here that show a peak efficiency of as much as 0.7 in the first order. A model computing the electromagnetic propagation and the grating efficiency, implemented and tested with the experimental data, permits the study and design of rather complex systems operating in the conical mounting. Basic physical principles and mathematical aspects of the model are discussed here. OCIS codes: 050.1950, 050.1960, 300.6540, 320.0320.

A Mo-Si multilayer mirror has been used for determination of the ellipticity and higher-order content of a synchrotron beam. The method is based on the angular measure of multilayer reflectivity in the region of Bragg first- and second-order reflections. Beam parameters were derived by a fitting procedure. [C] 2006 Optical Society of America OCIS codes: 120.4140, 340.6720, 310.6860.

Resonant soft X-ray reflectivity at the carbon K edge, with linearly polarized light, was used to derive quantitative information of film morphology, molecular arrangement, and electronic orbital anisotropies of an ultrathin 3,4,9,10-perylene tetracarboxylic dianhydride (PTCDA) film on Au(111). The experimental spectra were simulated by computing the propagation of the electromagnetic field in a trilayer system (vacuum/PTCDA/Au), where the organic film was treated as an anisotropic medium. Optical constants were derived from the calculated (through density functional theory) absorption cross sections of the single molecule along the three principal molecular axes. These were used to construct the dielectric tensor of the film, assuming the molecules to be lying flat with respect to the substrate and with a herringbone arrangement parallel to the substrate plane. Resonant soft X-ray reflectivity proved to be extremely sensitive to film thickness, down to the single molecular layer. The best agreement between simulation and experiment was found for a film of 1.6 nm, with flat laying configuration of the molecules. The high sensitivity to experimental geometries in terms of beam incidence and light polarization was also clarified through simulations. The optical anisotropies of the organic film were experimentally determined and through the comparison with calculations, it was possible to relate them to the orbital symmetry of the empty electronic states. [ABSTRACT FROM AUTHOR]

Interface formation between Ru and Si(001) has been studied by x-ray and ultraviolet photoemissions. The film properties were investigated, for metal deposition at room temperature, as a function of the Ru film thickness and as a function of the annealing temperature of a thick grown film. From the evolution of the Ru and Si core levels, we find that alloying takes place at the interface during growth and estimate the thickness of the intermixed region to be of the order of 15–20 ML. Annealing at increasingly high temperatures causes the formation of different silicide phases, which are discussed in relation to theoretical and experimental data on related transition metal silicides. [ABSTRACT FROM AUTHOR]

Time-resolved transient grating (TG) spectroscopy facilitates detailed studies of electron dynamics and transport phenomena by means of a periodic excitation of matter with coherent ultrashort light pulses. Several current and next generation free-electron laser (FEL) facilities provide fully coherent pulses with few femtosecond pulse durations and extreme ultraviolet (XUV) photon energies. Thus, they allow for transient grating experiments with periodicities as small as tens of nanometers and with element specific photon energies. Here, we demonstrate the element specificity of XUV TG (X-TG) experiments by tuning the photon energy across the Si L2,3-edge of Si3N4. We observe a shortening of the signal decay when increasing the XUV photon energy above the absorption edge. The analysis of the wavelength dependent signal shows that the faster decay is driven by the increase in the charge carrier density. From the decay constants the interband Auger coefficient at elevated temperatures and high electron densities has been determined. [ABSTRACT FROM AUTHOR]

The development of free electron laser (FEL) sources has provided an unprecedented bridge between the scientific communities working with ultrafast lasers and extreme ultraviolet (XUV) and X-ray radiation. Indeed, in recent years an increasing number of FEL-based applications have exploited methods and concepts typical of advanced optical approaches. In this context, we recently used a seeded FEL to demonstrate a four-wave-mixing (FWM) process stimulated by coherent XUV radiation, namely the XUV transient grating (X-TG). We hereby report on X-TG measurements carried out on a sample of silicon nitride (Si3N4). The recorded data bears evidence for two distinct signal decay mechanisms: one occurring on a sub-ps timescale and one following slower dynamics extending throughout and beyond the probed timescale range (100 ps). The latter is compatible with a slower relaxation (time decay ＞ ns), that may be interpreted as the signature of thermal diffusion modes. From the peak intensity of the X-TG signal we could estimate a value of the effective third-order susceptibility which is substantially larger than that found in SiO2, so far the only sample with available X-TG data. Furthermore, the intensity of the time-coincidence peak shows a linear dependence on the intensity of the three input beams, indicating that the measurements were performed in the weak field regime. However, the timescale of the ultrafast relaxation exhibits a dependence on the intensity of the XUV radiation. We interpreted the observed behaviour as the generation of a population grating of free-electrons and holes that, on the sub-ps timescale, relaxes to generate lattice excitations. The background free detection inherent to the X-TG approach allowed the determination of FEL-induced electron dynamics with a sensitivity largely exceeding that of transient reflectivity and transmissivity measurements, usually employed for this purpose. [ABSTRACT FROM AUTHOR]

FERMI@Elettra, the first vacuum ultraviolet and soft X-ray free-electron laser (FEL) using by default a 'seeded' scheme, became operational in 2011 and has been opened to users since December 2012. The parameters of the seeded FERMI FEL pulses and, in particular, the superior control of emitted radiation in terms of spectral purity and stability meet the stringent requirements for single-shot and resonant coherent diffraction imaging (CDI) experiments. The advantages of the intense seeded FERMI pulses with variable polarization have been demonstrated with the first experiments performed using the multipurpose experimental station operated at the diffraction and projection imaging (DiProI) beamline. The results reported here were obtained with fixed non-periodic targets during the commissioning period in 2012 using 20-32 nm wavelength range. They demonstrate that the performance of the FERMI FEL source and the experimental station meets the requirements of CDI, holography, and resonant magnetic scattering in both multi- and single-shot modes. Moreover, we present the first magnetic scattering experiments employing the fully circularly polarized FERMI pulses. The ongoing developments aim at pushing the lateral resolution by using shorter wavelengths provided by double-stage cascaded FERMI FEL-2 and probing ultrafast dynamic processes using different pump-probe schemes, including jitter-free seed laser pump or FEL-pump/FEL-probe with two color FEL pulses generated by the same electron bunch. [ABSTRACT FROM AUTHOR]

We present the results of the characterization of Mg/Co periodic multilayers and their thermal stability for the EUV range. The annealing study is performed up to a temperature of 400C. Images obtained by scanning transmission electron microscopy and electron energy loss spectroscopy clearly show a good quality of the multilayer structure. The measurements of the EUV reflectivity around 25 nm (∼49 eV) indicate that the reflectivity decreases when the annealing temperature increases above 300C. X-ray emission spectroscopy is performed to determine the chemical state of the Mg atoms within the Mg/Co multilayer. Nuclear magnetic resonance used to determine the chemical state of the Co atoms and scanning electron microscopy images of cross sections of the Mg/Co multilayers reveal changes in the morphology of the stack from an annealing temperature of 305C. This explains the observed reflectivity loss. [ABSTRACT FROM AUTHOR]

We report on the study of multilayer mirrors that were designed to compress extreme-ultraviolet pulses below 50 as. The mirrors were optimized in the time domain to obtain the desired pulse shape and duration when combined with a broadband hybrid filter. Moreover, they appeared to be very robust to environmental parameters, ensuring a pulse compression in various conditions. We manufactured the mirrors and characterized them on a synchrotron facility both in amplitude and in phase. A measurement of the latter was carried out using the photocurrent technique, which is the only method that allows one to access the mirrors' absolute spectral phase. This led to a measurement of the offset induced by the mirrors on the carrier-envelope phase of attosecond pulses. [ABSTRACT FROM AUTHOR]

We report boron K-edge x-ray absorption near-edge structure measurements on borophosphosilicate glasses and density functional theory calculations aimed at determining the most stable local geometries for boron and phosphorous. We demonstrate that phosphorous induces a modification in the local structure of boron, from a trigonal to a tetrahedral geometry. The microscopic mechanisms involved are discussed. © 2003 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Abstract: FERMI@Elettra, the first seeded EUV-SXR free electron laser (FEL) facility located at Elettra Sincrotrone Trieste has been conceived to provide very short (10–100fs) pulses with ultrahigh peak brightness and wavelengths from 100nm to 4nm. A section fully dedicated to the photon transport and analysis diagnostics, named PADReS, has already been installed and commissioned. Three of the beamlines, EIS-TIMEX, DiProI and LDM, installed after the PADReS section, are in advanced commissioning state and will accept the first users in December 2012. These beam lines employ active X-ray optics in order to focus the FEL beam as well as to perform a controlled beam-shaping at focus. Starting from mirror surface metrology characterization, it is difficult to predict the focal spot shape applying only methods based on geometrical optics such as the ray tracing. Within the geometrical optics approach one cannot take into account the diffraction effect from the optics edges, i.e. the aperture diffraction, and the impact of different surface spatial wavelengths to the spot size degradation. Both these effects are strongly dependent on the photon beam energy and mirror incident angles. We employed a method based on physical optics, which applies the Huygens–Fresnel principle to reflection (on which the WISE code is based). In this work we report the results of the first measurements of the focal spot in the DiProI beamline end-station and compare them to the predictions computed with Shadow code and WISE code, starting from the mirror surface profile characterization. [Copyright &y& Elsevier]

Ultrashort, coherent x-ray pulses of a free-electron laser are used to holographically image the magnetization dynamics within a magnetic domain pattern after creation of a localized excitation via an optical standing wave. We observe a spatially confined reduction of the magnetization within a couple of hundred femtoseconds followed by its slower recovery. Additionally, the experimental results show evidence of a spatial evolution of magnetization, which we attribute to ultrafast transport of nonequilibrium spin-polarized electrons for early times and to a fluence-dependent remagnetization rate for later times. [ABSTRACT FROM AUTHOR]

Abstract: Electronic properties of graphene nano-structures are highly anisotropic and correlated to their basal plane orientation. In this work, we tried to study and explain how deposition temperature can draw a definite plane growth direction. Carbon films have been deposited by Pulsed Laser Ablation (Nd:YAG, 2nd harmonic: λ=532 nm, hν=2.33 eV, τ=7 ns, ν=10 Hz, Φ ≈7 J/cm2), from a pyrolytic graphite target on Si <100> substrates. PLD depositions were performed in vacuum, at increasing substrate temperature, ranging from room temperature (RT) to 900 °C. Near-edge X-ray absorption fine structure (NEXAFS), SEM and Raman spectroscopy have been used to characterise the films. Using linearly polarized Synchrotron radiation and HOPG (Highly Oriented Pyrolytic Graphite) as a reference system, we investigated, ex situ, the occurrence of an average orientation distribution of the nano-sized graphene clusters, grown at high temperature. The results of the angle-dependent study by NEXAFS of film grown at 900 °C provide evidence that at high temperature the graphene sheets show a tendency to grow with basal planes oriented perpendicularly to the substrate surface. At low temperature, the film is less structured. The features ascribable to graphitic character are less pronounced and broadened; the absence of an angle dependence of NEXAFS measurements suggest that C-structures are randomly oriented. The corresponding Raman spectrum substantiates the prevailing amorphous nature of the film. [Copyright &y& Elsevier]

The extension of nonlinear optical techniques to the extreme-ultraviolet (EUV), soft and hard x-ray regime represents one of the open challenges of modern science since it would combine chemical specificity with background-free detection and ultrafast time resolution. We report on the first observation of a four-wave-mixing (FWM) response from solid-state samples stimulated exclusively by EUV pulses. The all-EUV FWM signal was generated by the diffraction of high-order harmonics of the FERMI free-electron laser (FEL) from the standing wave resulting from the interference of two crossed FEL pulses at the fundamental wavelength. From the intensity of the FWM signal, we are able to extract the first-ever estimate of an effective value of ~6×10-24m²V-2 for the third-order nonlinear susceptibility in the EUV regime. This proof of principle experiment represents a significant advance in the field of nonlinear optics and sets the starting point for a manifold of techniques, including frequency and phase-resolved FWM methods, that are unprecedented in this photon-energy regime. [ABSTRACT FROM AUTHOR]