Nonlinearity of InP-Doped Fibers

By combining nano-technology with fiber technology, an optical fiber doped with semiconductor nano-particles as InP is fabricated by using the modified chemical vapor deposition （MCVD） method. Proved by experiment, the fiber has excellent waveguide characteristics, and the concentration of InP is approximately 0.1~o. By using a scanning electron microscope, a stereo-scan photograph of the fiber is obtained, and based on the graph, presentations of the fiber under both magnetic and electronic fields are simulated, the effective core area Aeff≈ 10μm^2 is calculated, and so is the nonlinear index γ = 10.53 W^-1/Km of the fiber. This research leads a new method of high nonlinearity fiber fabrication.     

Core-level study of WSi2 (110)

Angle resolved core level studies of the Si 2p and W 4f levels have been carried out on the (110) surface of a WSi₂ single crystal using synchrotron radiation. Surface shifted components have been revealed both in the Si 2p and W 4f spectra. Investigations were carried out at two different annealing temperatures. The results indicate Si enrichment at the surface, and a larger enrichment after the higher temperature anneals. The reactivity upon initial oxygen exposure was investigated. Strong Si oxidation was observed but chemically shifted W 4f components could also be detected.     

Local effective polarization and absorption potentials from cross-sections for e-atom scattering

A possibility of obtaining local, effective, energy-dependent polarization and absorption potentials from experimental cross-sections for electron scattering is investigated. Potentials have been fitted with the help of the genetic algorithm on large sets of experimental data for e-He, e-Ne and e-Ar elastic scattering at impact energies 20-3000 eV. The obtained potentials reproduce the cross-sections within experimental errors.     

XRD and XPS characterization of mixed valence Mn3O4 hausmannite thin films prepared by chemical spray pyrolysis technique

Spray pyrolysis technique has been employed successfully for the synthesis of single phase mixed valence spinel hausmannite (Mn₃O₄) thin films using alcoholic start solution of manganese acetate (Mn(CH₃COO)₂·4H₂O) on pyrex glass substrates at atmospheric pressure using air as a carrier gas. Thermal decomposition of the precursor in the temperature range 320-490 °C led to the formation of Mn₃O₄ phase as revealed from the thermogravimetry analysis. Prepared samples are characterized by X-ray diffraction that shows spinel structure with space group I4₁/amd. Pure and well crystallized specimen is subjected to X-ray photoelectron spectroscopy for the surface chemistry investigation of these systems at a molecular level. Surface Mn/O ratio is compared to the bulk composition of the sample. Atomic force micrographs revealed that the morphology and the surface grains of the films largely influenced by the substrate temperature.     

Experimental and theoretical studies on X-ray induced secondary electron yields in Ti and TiO2

Generation of X-ray induced secondary electrons in Ti and TiO₂ was studied from both experimental and theoretical approaches, using X-ray photoelectron spectroscopy (XPS) attached to a synchrotron radiation facility and Monte Carlo simulation, respectively.The experiment revealed that the yields of secondary electrons induced by X-rays (electrons/photon) at photon energies to 4950 and 5000 eV for Ti and TiO₂ are δ_Ti(4950 eV) = 0.002 and δ_Ti(5000 eV) = 0.014 while those for TiO₂ are δTiO₂(4950 eV)=0.003 and δTiO₂(5000 eV)=0.018.A novel approach to obtain the escape depth of secondary electrons has been proposed and applied to Ti and TiO₂. The approach agreed very well with the experimental data reported so far. The Monte Carlo simulation predicted; and while and .An experimental examination on the contribution of X-ray induced secondary electrons to photocatalysis in TiO₂ has also been proposed.     

Germania nanocrystals in Modified Chemical Vapour Deposition

The Modified Chemical Vapour Deposition process for fabrication of silica-based optical fibres is used to generate nanoscale soot particles which are deposited on a substrate by thermophoresis and then sintered and collapsed into a preform from which optical fibre can be drawn. To increase the refractive index of the fibre core silica and germania soots are often generated together and co-deposited. The silica soot is invariably amorphous in character but for certain configurations of the preform lathe it has been found that germania soot particles may be crystalline. A systematic study has identified parameters controlling germania crystallinity in soot nanoparticles and transition from crystalline germania nanoparticles to glassy germania has been observed. The implications of possible retention of germania nanocrystals in preform and fibre are discussed with regard to fibre quality and optical performance.     

Electronic excitations during grazing scattering of hydrogen atoms on KI(001) and LiF(001) surfaces

The energy loss of hydrogen atoms with energies of 400 eV and 1 keV is studied in coincidence with the number of emitted electrons during grazing scattering from atomically clean and flat KI(001) and LiF(001) surfaces. The energy loss spectra for specific numbers of emitted electrons are analyzed in terms of a binary interaction model based on the formation of transient negative ions via local capture of valence band electrons from anion sites. Based on computer simulations we derive for this interaction scenario probabilities for the production of surface excitons, for electron loss to the conduction band of KI, for emission of electrons, and for formation of negative hydrogen ions. The pronounced differences of data obtained for the two surfaces are attributed to the different electronic structures of KI and LiF.     

Fibre Bragg Gratings Inscribed in Homemade Microstructured Fibres

Fibre Bragg gratings （FBGs） are inscribed in homemade microstructured fibres by the standard phase mask method. Enhanced couplings between the forward fundamental mode and backward cladding modes are obtained. The mode coupling and spectral characteristics are investigated experimentally. The cladding mode resonances can be affected by filling active materials into the air holes, which will be useful for the implementation of tunable photonic devices in optical fibre communication and sensing systems.     

Near field induced defects and influence of the liquid layer thickness in Steam Laser Cleaning of silicon wafers

The removal of particles from commercial silicon wafers by Steam Laser Cleaning was examined. Polystyrene colloids were used as model contaminants due to their well defined size and shape. In contrast to previous studies, where the experimental conditions on the surface were only roughly determined, special care was taken to control the amount of liquid applied to the surface. We report measurements of the cleaning threshold for different particle sizes. The comparability of the results was ensured by the reproducible conditions on the surface. Moreover, we studied the influence of different liquid film thicknesses on the cleaning process. Investigations of laser induced liquid evaporation showed that the cleaning threshold coincides with the fluence necessary for the onset of explosive vaporization. After particle removal, the surface was examined with an atomic force microscope. These investigations demonstrated that near field enhancement may cause defects on the nm-scale, but also showed that Steam Laser Cleaning possesses the capability of achieving damage-free removal for a large range of different particle sizes. Received: 14 January 2003 / Accepted: 16 January 2003 / Published online: 28 March 2003 RID="*" ID="*"Corresponding author. Fax: +49-7531/88-3127, E-mail: florian.lang@uni-konstanz.de     

Use of wavelets in potential scattering problems

Application of the Daubechies compact support wavelets to problems of nonrelativistic potential scattering described by integral Lippmann-Schwinger equation is discussed. Structure of wavelet representation of various physical operators is investigated. It is shown that for a special class of potentials wavelets enable sparse approximation of the kernel of the Lippmann-Schwinger equation. Constraints for such potentials are derived. This paper is dedicated to Prof. J. Bičák on the occasion of his 60th birthday.