Anomalies in the Magnetic Susceptibility in the Second-Order Phase Transitions beyond the Curie Point

Taking into account the inexhaustible interest in studying the peculiarities of physical properties in the neighborhood of phase transitions and the growth of experimental investigations of cobalt fluoride, we have studied the peculiarities of magnetic susceptibility in the vicinity of the critical field H_C at which cobalt fluoride performs the second-order phase transition from the antiferromagnetic phase to the angular phase. It is discovered that in the magnetic field H ‖ C₄, the magnetic susceptibility becomes infinite at H → H_C. It is shown that as the magnetic field direction deviates from the C₄ axis, the magnetic susceptibility in the critical field H_C proves to be finite. It is also shown that the change in the magnetic susceptibility with the change in the magnetic field considerably decreases at extremely insignificant deviations of the field H from the C₄ axis. Since the calculations are performed in terms of the Landau theory of phase transitions, we pay attention to the similarity and difference between the obtained results and those in the vicinity of the Curie point obtained by using the Landau theory of phase transitions.     

A spectroscopic prism coupler for measuring the refractive indices and thicknesses of dielectric films

A spectroscopic prism coupler is created for measuring refractive indices n_f and thicknesses H_f of dielectric films. The operating principle of the device is based on the simultaneous resonance excitation of several waveguide modes in a film by a focused TE or TM polarized light beam in the geometry of frustrated total internal reflection. Calculations of n_f and H_f are performed using measured angular positions θ_m of dark m-lines in the cross section of the specularly reflected beam. Using obtained angles θ_m, we can calculate effective refractive indices β_m of modes. By solving a set of nonlinear dispersion equations for the modes of a planar waveguide, we can calculate refractive index n_f and thickness H_f of a film. The proposed prism coupler has no moving parts and allows us to measure the optical parameters of films 0.5–10 μm thick in the 400–1100 nm range of wavelengths. The device can also be used as a spectroscopic refractometer for measuring the refractive indices of bulk media. The device is used to measure refractive index and thickness of a SiO film and the refractive index of TF4 glass.     

An experimental and kinetic modeling study of a premixed nitromethane flame at low pressure

A premixed nitromethane/oxygen/argon flame at low pressure (4.67 kPa) has been investigated using tunable vacuum ultraviolet (VUV) photoionization and molecular-beam mass spectrometry. About 30 flame species including hydrocarbons, oxygenated and nitrogenous intermediates have been identified by measurements of photoionization efficiency spectra. Mole fraction profiles of the flame species have been determined by scanning burner position at some selected photon energies. The results indicate that N₂ and NO are the major nitrogenous products in the nitromethane flame. Compared with previous studies on nitromethane combustion, a number of unreported intermediates, including C₃H₄, C₄H₆, C₄H₈, C₂H₂O, C₂H₄O, CH₃CN, H₂CNHO, C₃H₃N and C₃H₇N, are observed in this work. Based on our experimental results and previous modeling studies, a detailed oxidation mechanism including 69 species and 314 reactions has been developed to simulate the flame structure. Despite some small discrepancies, the predictions by the modeling study are in reasonable agreement with the experimental results.     

Adsorption of ethylene on clean and oxygen precovered Cu(110) surfaces

UV photoemission spectroscopy (UPS) with He I and He II radiation is used to study the interaction of C₂H₄ with clean and oxygen precovered Cu(110) surfaces at 90 K. On the clean surface only-bonding of the C₂H₄ molecules is observed whereas preadsorbed oxygen causes a second molecular orbital to be involved in the chemisorption. This result is consistent with the differing behaviour of the work function change during thermal desorption of C₂H₄.     

Laser beam defocusing at 1.06 μm by carrier excitation in silicon

Laser beams from a 15 ns pulsed Nd: YAG laser are defocused after passing silicon crystals with 400 m thickness. The beam profile changes into a ring structure at incident laser energies up to 30 mJ and energy densities of 440 mJ/cm². The experimental deflection angles agree with calculations assuming refractive index changes due to electronhole pairs produced by interband absorption.     

Detection of acetylene by electronic resonance-enhanced coherent anti-Stokes Raman scattering

We report the detection of acetylene (C₂H₂) at low concentrations by electronic resonance-enhanced coherent anti-Stokes Raman scattering (ERE-CARS). Visible pump and Stokes beams are tuned into resonance with Q-branch transitions in the v₂ Raman band of acetylene. An ultraviolet probe beam is tuned into resonance with the – electronic transition of C₂H₂, resulting in significant electronic resonance enhancement of the CARS signal. The signal is found to increase significantly with rising pressure for the pressure range 0.1–8 bar at 300 K. Collisional narrowing of the spectra appears to be important at 2 bar and above. A detection limit of approximately 25 ppm at 300 K and 1 bar is achieved for our experimental conditions. The signal magnitudes and the shape of the C₂H₂ spectrum are essentially constant for UV probe wavelengths from 233.0 to 238.5 nm, thus indicating that significant resonant enhancement is achieved even without tuning the probe beam into resonance with a specific electronic resonance transition. PACS 42.65.Dr; 42.62.Fi; 42.65.-k     

Effect of boron concentration on the UV photosensitivity of silica glass film for planar lightwave circuit

The photosensitivity dynamics in SiO₂ glass with a composition similar to that of silica planar lightwave circuit (PLC) devices was investigated as a fundamental study prior to device fabrication. Silica bulk glasses with similar composition to the core layer of PLC devices were prepared with various concentrations of B₂O₃. The photosensitivity in boron and germanium co-doped amorphous SiO₂ yields a refractive index change Δn as high as 10⁻³ after irradiation with a KrF UV laser beam. The index modulation disappeared after thermal annealing. The result of annealing experiment and UV absorption/Raman spectra revealed that the molar volume change by UV irradiation is responsible for the index variation in the material.     

The temperature dependence of the third-order nonlinear susceptibility χ(3) of CS2 and dodecylbenzene

The dependence of the third-order nonlinear susceptibility ⁽³⁾ of CS₂ and C₁₈H₃₀ (dodecylbenzene) on temperature was measured by the degenerate four-wave mixing method. The empirical formula of this dependence was obtained. An explanation of this relationship was also given through the application of the model of a polar molecular moving in a viscous medium under the action of a laser beam.     

Formation of pseudoisocyanine J-aggregates during thin-film formation

Thin-film formation of J-aggregated pseudoisocyanine iodide with long alkyl substituents C₁₈H₃₇ (PIC 2-18) and C₁₀H₂₁ (PIC 2-10) and ethyl substituents (PIC 2-2) with added anion was studied by spectrophotometry directly during spin-coating of its solutions. It is found that a bathochromic shift of the monomer dye absorption band maximum and extensive growth of a J-peak that is not compensated by the monomer decrease take place as the dye film forms. The refractive index and absorption coefficient of a solid dye thin film in monomeric (n _max = 2.1) and J-aggregated (n _max = 3.05) forms were measured as functions of the dispersion by spectral ellipsometry. The influence of a change in the local field factor on the spectral properties of the pseudoisocyanine dye solution in the course of both spin-coating and solid J-aggregated thin-film formation is considered. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 76, No. 1, pp. 76–83, January–February, 2009.     

Thermal conductivity of the Nb3Ge layer prepared by the continuous deposition method

The paper describes the measurement of thermal conductivity of the stainless steel tape on which the superconductive Nb₃Ge layer was vapour-deposited on both sides by the continuous method. The stainless steel 50 m substrate covered by the 2 m Nb layer was deposited with the layer of Nb₃Ge of the thickness of 10 m. Thermal conductivity in the temperature range within 5 up to 80 K was measured in lengthwise direction using the thermopotentiometric method in the bath cryostat. In the same experimental arrangement the measurement of thermal conductivity of the substrate and of the tape with the deposited layer of Nb₃Ge was performed. Specific thermal conductivity of the Nb₃Ge layer was calculated on the basis of measured values.     

Numerical approach to laser beam propagation through turbulent atmosphere and evaluation of beam quality factor

In this paper, propagation of a Gaussian laser beam through turbulent atmosphere is evaluated numerically. The beam quality factor for the propagated beam has been estimated for different turbulent conditions that are characterized by parameter C_n. The calculations show that the beam quality can be affected dramatically by atmospheric turbulence and the laser beam size and wavelength have major role in the results. Furthermore, the propagation of laser beam in longer distance results in more spatial perturbation of the beam. The results of these calculations and evaluation of beam quality factor M² can be used for estimating the refractive index structure parameter (or atmosphere turbulent parameter) C_n.     

Pulse duration dependence of laser damage mechanisms

A number of mechanisms for internal laser damage in transparent dielectrics are examined for pulse duration dependence and the results are compared with experimental measurements when possible. The differences in dependence on pulse duration and other variables from one mechanism to another suggest that different mechanisms may initiate damage in different parameter ranges. Experimental identification of these mechanisms will be aided by measuring the pulse duration dependence of the damage threshold.Symbols bulk absorption coefficient - thermal volume expansion coefficient - C heat capacity per unit volume - c velocity of light in vacuum - D thermal diffusivity - E instantaneous electric field strength - E ₀ electric field amplitude - e charge on an electron - e spectral emissivity at the laser wavelength - _c band gap energy - _d energy density (energy per unit cross-sectional area) - _e electron energy - k volume compressibility - k wave vector - L sample length - L _f focusing length - wavelength in vacuum - M multiplication factor to correct particle cross-section for Mie effect - m ^* electron effective mass - N conduction electron number density - N _b value ofN required for breakdown - n number of photons absorbed inn-photon ionisation - n refractive index - n ₀ original refractive index - n ₂ coefficient for quadratic term in expression for refractive index - n change in refractive index - P _d power density (power per unit cross-sectional area) - R inclusion radius - R _s sample radius - r distance from centre of inclusion - r _b beam radius - S breaking strength, damaging stress - T temperature - t time - t time required to accelerate an electron to the energy of the conduction band - t pulse duration - time required for acoustic wave to propagate across the beam - _e conduction electron lifetime - U velocity - angular velocity (frequency times 2)     

Influence of surroundings on photorefractive effect in lithium niobate crystals

In the paper results of the investigation of the influence of electric properties of the environment surrounding LiNbO₃ crystals on photorefractive effect induced in these crystals by Gaussian Ar⁺ laser beam with various intensities are presented. We show spatial and temporal dependences of changes of the refractive index obtained experimentally in LiNbO₃: Fe and LiNbO₃: Fe:Mn samples surrounded by media with different electric conductivities and different permittivities (water, air, water solution of CaCl₂). The space and time dependences of the refractive index changes induced by the Ar⁺ laser beam are observed by means of the Mach–Zehnder interferometer using light from HeNe laser. The experimentally obtained results are in a good agreement with those following from numerical calculations using the manifold mirroring method. The agreement between calculated and experimental results indicates that the polarization charge at the photorefractive crystal/surrounding medium boundary significantly influences the photorefractive process in the crystal. The experimentally observed slow spontaneous decrease of the refractive index change in a sample placed into a slightly conducting medium (air) after switching off the beam also indicates that the polarization charge in the sample's surroundings affects the photorefraction.     

Effect of chemical etching on the Cu/Ni metallization of poly (ether ether ketone)/carbon fiber composites

Poly(ether ether ketone)/carbon fiber composites (PEEK/Cf) were chemical etched by Cr₂O₃/H₂SO₄ solution, electroless plated with copper and then electroplated with nickel. The effects of chemical etching time and temperature on the adhesive strength between PEEK/Cf and Cu/Ni layers were studied by thermal shock method. The electrical resistance of some samples was measured. X-ray photoelectron spectroscopy (XPS) was used to analyze the surface composition and functional groups. Scanning electron microscopy (SEM) was performed to observe the surface morphology of the composite, the chemical etched sample, the plated sample and the peeled metal layer. The results indicated that CO bond increased after chemical etching. With the increasing of etching temperature and time, more and more cracks and partially exposed carbon fibers appeared at the surface of PEEK/Cf composites, and the adhesive strength increased consequently. When the composites were etched at 60 °C for 25 min and at 70-80 °C for more than 15 min, the Cu/Ni metallization layer could withstand four thermal shock cycles without bubbling, and the electrical resistivity of the metal layer of these samples increased with the increasing of etching temperature and time.     

Effect of the morphology on the mechanisms of the magnetization reversal of multilayered thin Co/Pd films

A comparative study of the magnetization curves of continuous and porous multilayered Pd_10nm/[Co_0.3nm]/Pd_0.55nm]15/Pd_2nm films deposited on an anodized TiO₂ template was performed by magnetometry. Based on the comparison of the dependences of coercive field H_C on angle θ between the easy-magnetization axis and the direction of external magnetic field H with theoretical dependences H_C(θ) for the magnetization reversal by domain walls motion (according to the Kondorski model) and the rotation of magnetic moments (by the Stoner–Wohlfarth model), the differences in the mechanisms of magnetization reversal for two mentioned types of the films were revealed. The correlation between the difference in the morphologies of the continuous and porous films and revealed change in the mechanisms of the magnetization reversal, as well as the changes in values of H_C and calculated constants of the magnetic anisotropy, is discussed.     

The effect of silver on the optical,spectral-luminescent,and crystallization properties of bromide photo-thermo-refractive glasses

The effect of silver on the optical, spectral-luminescent, and crystallization properties of bromide photo-thermo-refractive glasses is studied. Multicomponent photosensitive glasses of the Na₂O–ZnO–Al₂O₃–SiO₂ system with photosensitizing agents (cerium, antimony, silver) and halogenides (fluorine and bromine) are synthesized. Ultraviolet irradiation and thermal treatment below the glass-transition temperature of the glasses cause the formation of silver molecular clusters, which exhibit luminescence in the visible and infrared regions. UV irradiation and thermal treatment of glasses above the glass-transition temperature lead to the growth of silver nanoparticles with plasmon resonance peak in the region of 420 nm. Further thermal treatment of glasses above the glass-transition temperature shifts the plasmon-resonance maximum by 70 nm to longer wavelengths, which is related to the growth of a crystalline shell consisting of mixed silver and sodium bromides on nanoparticles. This formation of a crystalline phase on colloidal centers results in a local increase in the refractive index of the irradiated region by +Δn ~ 900 ppm compared to the nonirradiated region. Photo-thermo-refractive glasses with increased silver concentration are promising photosensitive materials for creating holographic optical elements and devices for line narrowing and stabilizing filters, spectral beam combiners, and filters for increasing the spectral brightness of laser diodes. A positive change in the refractive index of Photo-thermo-refractive glasses provides the possibility of recording in them 3D waveguide and integrated-optical structures.     

Growth and basic crystallographical parameters of cytosine hydrochloride single crystals

Single crystals of cytosine hydrochloride (C₄H₅N₃O.HCl, mol. weight 147·56) weighing about 4 g were grown using the technique of cooling the solution. The composition of the crystals and some of their properties are described and basic crystallographical parameters were determined by means of a reflection two-circle goniometer and X-ray diffraction methods (monoclinic system, 2/m, P2₁/c).The authors thank Mr. R.Wágner for refractive index measurements and Ing. A.Slivkaniová for performing the analyses.     

On the formation of nanocomposite TiC/a-C:H coatings by the method of the magnetron sputtering of Ti in an electron-beam activated Ar/C2H2 mixture

Composite TiC/a-C:H coatings with a thickness of 5 μm are produced by the reactive magnetron sputtering of titanium in an Ar/C₂H₂ gas mixture, which was additionally irradiated by a wide electron beam with an energy of 100 eV and an electric current of up to 1 A. The composition of the coatings is controlled by changing the C₂H₂ flow rate in the range from 1 to 16 mL/min under a constant Ar flow rate (40 mL/min) and magnetron current (2 A, 10 μs, 50 kHz). X-ray diffraction (XRD) investigations and high resolution transmission electron microscopy (HRTEM) methods confirm the formation of nanostructured coatings with a thickness of 4 to 9 nm. The hardness of the coatings nonmonotonically depends on the acetylene-flow rate. A maximum microhardness of 30 GPa is obtained when the atomic concentration of titanium in the coating determined by the Auger spectroscopy method is close to 38%. It is established that the electron-beam irradiation of a gas mixture during the deposition of coatings promotes the accelerated decomposition of acetylene and leads to a multiple (from 10 to 2 mL/min) decrease in the C₂H₂ flow rate, at which the maximum microhardness of coatings is reached.     

Optically induced index change in GaAs by laser diode: Measurement method and analysis to realize an all-optical switch

In this paper the possibility to realize an all-optical switch on GaAs, using a semiconductor laser as a pump beam, is investigated. An estimate for the mean value of the refractive index change (n) of the material is calculated as a function of the pump power. A method for the measurement of n is proposed, based on the mathematical relationships between the propagation characteristics of the guided mode and the refractive index change.     

玻璃基板中热光效应折射率分布的研究和测定

几乎所有的光学材料都具有热致折射率变化的特性,当光束经过局域热源作用下折射率呈梯度分布的介质时会发生偏向,利用这种特性可做成各种热光器件.本文在玻璃基板表面上制备了条形Ti薄膜热源,通过测定出射光束的偏向角,利用解折方法,得出基极截面的折射率分布.根据能量守恒,得到该材料的折射率温度系数,与公布值比较,验证了本方法的正确性.本文对研究和分析热光材料和器件提供了一种有益的方法.