An interferometric determination of the refractive part of optical constants for carbon and silver across soft X-ray absorption edges

International audienceInterferometric, direct determinations of the f 1 scattering factor near absorption edges in the soft x-ray range is demonstrated. The interferometric system, which is based on wave front division (no beam splitter) with plane mirrors only, produces a linear fringe pattern. The principle consists in direct measuring of the fringe shift occurring upon insertion of a sample into one interferometer arm, by means of a dedicated detection system. This provides the optical thickness, which in turn gives the f 1 factor, knowing either the sample mass per unit surface, or the sample thickness and density. With the sample being probed in transmission under near normal incidence, the determination of f 1 is not perturbed by the absorption part of the complex scattering factor. Therefore, f 1 data obtained here can be said new and independent with respect to those obtained previously, in the sense that they are obtained from a new, purely experimental technique, and are neither deduced from nor perturbed by absorption. The interferometer design used can be implemented in a very large spectral range. For demonstrating the ability of this interferometric system to provide such new f 1 data, a thin free-standing carbon foil near the K edge (from 4.1 to 4.65 nm, 302-267 eV), and a membrane-supported silver layer near an M edge (from 2.6 to 3.8 nm, 477-326 eV) were used as test objects

Scaling Theory and Numerical Simulations of Aerogel Sintering

A simple scaling theory for the sintering of fractal aerogels is presented. The densification at small scales is described by an increase of the lower cut-off length $a$ accompanied by a decrease of the upper cut-off length $\xi$, in order to conserve the total mass of the system. Scaling laws are derived which predict how $a$, $\xi$ and the specific pore surface area $\Sigma$ should depend on the density $\rho$. Following the general ideas of the theory, numerical simulations of sintering are proposed starting from computer simulations of aerogel structure based on a diffusion-limited cluster-cluster aggregation gelling process. The numerical results for $a$, $\xi$ and $\Sigma$ as a function of $\rho$ are discussed according to the initial aerogel density. The scaling theory is only fully recovered in the limit of very low density where the original values of $a$ and $\xi$ are well separated. These numerical results are compared with experiments on partially densified aerogels.Comment: RevTex, 17 pages + 6 postscript figures appended using "uufiles". To appear in J. of Non-Cryst. Solid

Internal frequency conversion extreme ultraviolet interferometer using mutual coherence properties of two high-order-harmonic sources

International audienceWe report on an innovative two-dimensional imaging extreme ultraviolet (XUV) interferometer operating at 32 nm based on the mutual coherence of two laser high order harmonics (HOH) sources, separately generated in gas. We give the first evidence that the two mutually coherent HOH sources can be produced in two independent spatially separated gas jets, allowing for probing centimeter-sized objects. A magnification factor of 10 leads to a micron resolution associated with a subpicosecond temporal resolution. Single shot interferograms with a fringe visibility better than 30% are routinely produced. As a test of the XUV interferometer, we measure a maximum electronic density of 3×10^20 cm^−3 1.1 ns after the creation of a plasma on aluminum target

Momentum meets value investing in a small European market

In this paper, we investigate two prominent market anomalies documented in the finance literature – the momentum effect and value-growth effect. We conduct an out- of-sample test to the link between these two anomalies recurring to a sample of Portuguese stocks during the period 1988–2015. We find that the momentum of value and growth stocks is significantly different: growth stocks exhibit a much larger momentum than value stocks. A combined value and momentum strategy can generate statistically significant excess annual returns of 10.8%. These findings persist across several holding periods up to a year. Moreover, we show that macroeconomic variables fail to explain value and momentum of individual and combined returns. Collectively, our results contradict market efficiency at the weak form and pose a challenge to existing asset pricing theories.info:eu-repo/semantics/publishedVersio

High-resolution broad-bandwidth Fourier-transform absorption spectroscopy in the VUV range down to 40 nm

International audienceVacuum-ultraviolet (VUV) high-resolution absorption spectroscopy is a unique tool for the study of gas-phase atomic and molecular electronic structure. To date, it has been performed by using lasers or synchrotron radiation-based grating spectrometers, but none of these techniques can offer simultaneous high resolution, wavelength accuracy and broad tunability. The only technique combining these three important features is Fourier-transform spectroscopy, but this is limited to the mid-UV range (down to 140 nm; ref. 1) because of a lack of beamsplitters. Here, we present a new instrument based on a wavefront-division scanning interferometer, applied for the first time to the VUV range. This instrument, coupled to the DESIRS beamline at synchrotron SOLEIL, covers a broad range of wavelengths (typically 7%, adjustable in the 250–40 nm range), a resolving power of ~1 × 106, an extrinsic absolute wavelength accuracy of 1 × 10−7 and a high signal-to-noise ratio

Application of XUV interferometry to the determination of the temporal coherence of X-ray lasers

International audienc

Nanometric deformations of thin Nb layers under a strong electric field using soft x-ray laser interferometry

International audienceWe present measurements of in situ nanometric-resolution topographical modifications of thin niobium layers subjected to strong electric fields. The Nb layers, deposited on a fused silica substrate, are interferometrically flash probed using soft x-ray laser (XRL) at the wavelength of 21.2 nm. Its pulses are reflected by the probed sample under grazing incidence angle, and the information about surface deformation is obtained by a Fresnel wave-front-division interferometer. It was experimentally established that the probing pulses at the soft x-ray wavelength do neither produce any measurable photoelectric-field emission, nor alter the topographical features of the probed surface. The examined Nb electrodes were periodically probed while the electric field was increased up to 80 MV/m, and alterations of their topographical characteristics with a resolution of ∼2 nm in the relief elevation were obtained. It was found that behavior of the Nb layer strongly depends on the polarity of the applied voltage. Only small modifications are observed with the Nb surface at the positive potential, whereas the negative potential induces significant transient surface perturbations, with peak-to-valley elevation differences ∼50 nm. The qualitative nature of these modifications was observed to be unrelated to intermittent parasitic breakdowns