Tunable single-frequency operation of a diode-pumped vertical external-cavity laser at the cesium D<Subscript>2</Subscript> line

We report on a diode-pumped vertical external-cavity surface-emitting laser emitting around 852 nm for Cesium atomic clocks experiments. We have designed a 7-quantum-well semiconductor structure optimized for low laser threshold. An output power of 330 mW was achieved for 1.1 W of incident pump power. Furthermore, a compact setup was built for low-power single-frequency emission. We obtained an output power of 17 mW in a single longitudinal mode, exhibiting both broad (9 nm) and continuous (14 GHz) tunability around the Cesium D₂ line. The laser frequency has been stabilized on an atomic transition with residual frequency fluctuations ∼300 kHz. Through a beatnote experiment the −3 dB laser linewidth has been measured to <500 kHz over 10 ms.     

Spatial photorefractive solitons with picosecond laser pulses

We report on the formation of spatial optical solitons in photorefractive media using picosecond laser pulses. Solitons are generated with laser pulses in the visible and infrared wavelength region using photorefractive strontium barium niobate. The dynamics of the soliton formation and the region of existence is studied in detail.     

Ultrafast X-ray experiments on structural changes in single crystals of polar molecules

Intramolecular charge transfer in aminobenzonitrile (DIABN) single crystals is studied by a combination of ultrafast techniques. The intramolecular dynamics is probed by time-resolved spectroscopy in the UV/Vis and mid-IR. The intermolecular structural response is investigated by transient X-ray scattering and transmission experiments. A theoretical analysis shows that diffuse scattering and dipole solvation determine the X-ray data. The observed dynamics results from charge transfer reaction leading to strong local changes of molecular dipole moments and related structural rearrangements in the crystal.     

混流叶轮S2流面的展向掺混模型与应用

本文在S2流面流动方程中加入了"展向掺混"模型,模拟叶栅端部二次流动、湍流扩散等对局部低动量流体的展向输运效应。方法用于混流叶轮内流动分析,结果表明:由于混流叶轮子午流道的大曲率,叶轮端部的低动量流体在展向的扩散远大于轴流叶栅,叶轮出口的展向均匀化趋势更加明显。合理的选择掺混扩散系数,可以获得与三维N-S计算的周向平均值类似的子午面分布,从而提高了S2流面分析和设计的可靠性。     

Structural modifications of diamond like carbon films induced by MeV nitrogen ion irradiation

Diamond-like carbon (DLC) films were deposited on Si(1 0 0) substrates using plasma deposition technique. The deposited films were irradiated using 2 MeV N⁺ ions at fluences of 1×10¹⁴, 1×10¹⁵ and 5×10¹⁵ ions/cm². Samples have been characterized by using Raman spectroscopy, X-ray photoelectron spectroscopy (XPS) and high-resolution transmission electron microscopy (HRTEM). Analysis of Raman spectra shows a gradual shift of both D and G band peaks towards higher frequencies along with an increase of the intensity ratio, I(D)/I(G), with increasing ion fluence in irradiation. These results are consistent with an increase of sp² bonding. XPS results also show a monotonic increase of sp²/sp³ hybridization ratio with increasing ion fluence. Plan view TEM images show the formation of clusters in the irradiated DLC films. HRTEM micrographs from the samples irradiated at a fluence of 5×10¹⁵ ions/cm² show the lattice image with an average interplanar spacing of 0.34 nm, revealing that the clusters are graphite clusters. The crystallographic planes in these clusters are somewhat distorted compared to the perfect graphite structure.     

Electronic structure of dysprosium silicide films grown on a Si(1 1 1) surface

The thickness-dependent electronic structures of Dy silicide films grown on a Si(1 1 1) surface have been investigated by angle-resolved photoelectron spectroscopy. Two (1×1) periodic bands, both of them cross the Fermi level, have been observed in the silicide films formed by Dy coverages of 1.0 monolayer and below, and more than five () periodic bands have been observed in thicker films. Taking the () periodic structure of Dy atoms in the submonolayer silicide film into account, the periodicity of the two metallic bands indicate that they mainly originate from the orbitals of Si atoms, which form a (1×1) structure. Of the () periodic bands observed in thick films, four of them are well explained by the folding of the (1×1) bands into a () periodicity. Regarding the other band, the three () periodic bands would originate from the electronic states related to the inner Si layers that form a () structure, and the one observed in the 3.0 ML film only might originate from the electron located at the interface between bulk Si and the Dy silicide film.     

In situ identification of organic components of ink used in books from the 1900s by atmospheric pressure matrix assisted laser desorption ionization mass spectrometry

Transport of N, O, and Ti during dc magnetron sputtering deposition of nanoscopic TiN/Ti and TiN structures on plasma nitrided M2 tool steel, as well as transport of metallic species composing the plasma nitrided steel substrates were investigated. N and O depth distributions were determined with subnanometric resolution using narrow resonant nuclear reaction profiling, whereas Ti was profiled, also with subnanometric depth resolution, by medium energy ion scattering. The surface elementary compositions of the TiN/Ti/nitrided steel and TiN/nitrided steel structures were determined by low energy ion scattering. The chemical compounds formed during deposition were accessed by X-ray photoelectron spectroscopy, indicating the presence of TiN, TiO₂, Ti oxynitrides, as well as other metallic nitrides and oxynitrides, but no metallic Ti was observed. Owing to the observed intensive atom mobility, the compositions of the deposited films on plasma nitrided steel structures varied continuously on a nanoscopic scale, from the core of the steel substrate to the bulk of the stoichiometric TiN films. The Ti interlayer assists interdiffusion of all species, in contrast to the TiN film layer, which is known to be a diffusion barrier. The improved adhesion of TiN hard coatings to plasma nitrided steel under working conditions is discussed in terms of the gradual compositional change around the interfaces and the atomic mobility during their formation.     

Computational study of plasma-surface interaction in plasma-assisted technologies

The influence of the unevenness of substrates immersed into plasma important for plasma-based treatment of materials were studied by computer experiment. The role of both substrate properties and plasma parameters was investigated. For this analysis the combination of multidimensional fluid modelling and particle simulation was used. The fluid part of our model consisted of continuity equations for all charged species, energy balance equation for electrons and Poisson equation. The basic scattering processes were also included. The particle simulation technique was used both for the calculation of electron energy distribution function and for the derivation of quantities characterising plasma-surface interaction. This approach enabled us to study in detail the structure of the sheath and presheath near metal substrates with realistic geometries and finite dimensions. The main attention was devoted to the influence of substrate geometry in both macroscopic and microscopic spatial scales on the local electric fields in plasma.     

The double M-effect induced by noble gases activated with negative ions

The M-effect (monochromatization-effect) is a powerful tool which can give high intensity monochromatic spectra with a certain wavelength depending on the type of used gas mixtures to generate plasma state. The effect consists in the emission of a single spectral line of plasmas ignited in certain gas mixtures. The main condition to obtain the M effect is the presence of an electropositive and an electronegative gas mixture. For example, in the case of Ne+H₂ monochrome radiation was obtained, the wavelength of Ne being 585.3 nm (1s²–2p⁵). In this paper we prove the general character of this effect, i.e. if the optical emission spectra reduced to nearly one line can be observed also in other gas mixture discharges, for example in the case of one electronegative gas and two electropositive gases. Different other mixtures, as Xe+Ne+H₂ and Xe+Ar+H₂ have been studied. In all these cases, the M-effect appeared without doubt.     

Temperature and density spectroscopic measurements in different laser-generated plasmas

A pulsed Nd:Yag laser, at intensities of the order of 10¹⁰ W/cm², is employed to irradiate different thick metallic targets (Ti, Fe, Ag, and Ni) placed in vacuum. The obtained non-equilibrium plasmas are investigated with various analytical techniques. An electrostatic ion energy analyzer and different ion collectors are employed to monitor in situ the ions ejected from the plasma and to determine the core plasma temperature, the ion energy distributions and the ion angular emission. An optical spectrometer is employed to analyze the plasma corona emitted light vs. wavelength and to identify the emitted characteristic lines. The optical spectroscopy permitted to evaluate the electron temperatures and densities. Results show strong temperature and density gradients occurring in the laser-generated plasma plume.