Boron-doped a-SiOx:H films prepared by photo-CVD technique

The optoelectronic and structural properties of p-type a-SiO_x:H films have been studied. The deposition parameters e.g. chamber pressure and diborane to silane ratio are optimized to get a film with dark conductivity (σ_d) 7.9×10⁻⁶ S cm⁻¹ and photoconductivity 9.3×10⁻⁶ S cm⁻¹ for an optical gap (E₀₄) of 1.94 eV. The decrease of optical gap accompanied by the increase of conductivity is due to less oxygen incorporation in the film, which is substantiated by the decrease of the intensity of SiO absorption spectra. The properties are very much effected by the chamber pressure and diborane to silane ratio.     

Interaction of two-dimensional spatial incoherent solitons in photorefractive medium

We present a review of our recent theoretical and experimental results on the interaction of incoherent two-dimensional solitary beams in PR SBN crystals. We show that the inherent anisotropy of PR nonlinearity strongly affects the interaction between solitons. Theoretical and experimental results reveal that solitons interacting in a plane perpendicular to the direction of external biasing field always attract, whereas those colliding in a plane of the field exhibit anomalous behaviour. They may experience both attractive and repulsive forces, depending on their mutual separation. We also show that this anisotropy results in the complicated topology of soliton trajectories, featuring periodic collisions, prolonged mutual spiraling and collapse, depending on the initial conditions. Received: 16 November 1998 / Revised version: 12 February 1999 / Published online: 12 April 1999     

Diode laser spectroscopic measurement of line shape of (ν1+3ν3) band transitions of acetylene

A near-infrared diode laser spectrometer is set up to study the absorption line shape of acetylene in the 782 nm region. The second-derivative spectra recorded by source modulation technique have enhanced sensitivity. Careful choice of operating current and diode temperature leads to distortion-free line shape for six rotational components of the (ν₁+3ν₃) overtone-combination mode of acetylene. Self- and nitrogen-broadening coefficients and line-strength parameters have been extracted by fitting the observed line shape with Voigt profiles. There is no evidence of the effect of velocity changing collisions on the line shape in this near-infrared band. Received: 6 July 1998 / Revised version: 2 November 1998 / Published online: 10 March 1999     

Estimation of trap concentration in photorefractive crystals by a technique of adaptive holographic interferometry

12 GeO₂₀, Bi₁₂SiO₂₀, Bi₁₂TiO₂₀). Received: 17 June 1998/Revised version: 3 August 1998     

High quality β-SiC films obtained by low-temperature heteroepitaxy combined with a fast carbonization step

Received: 11 September 1998 / Accepted: 19 October 1998 / Published online: 24 February 1999     

Thermal tuning of a fixed Bragg grating for IR light fabricated in a LiNbO3:Ti channel waveguide

We investigate a thermally tunable integrated narrow-bandwidth interference filter for light in the infrared wavelength region around 5=1550 nm fabricated in a channel waveguide in lithium niobate. The filter is based on a holographically recorded and thermally fixed refractive-index Bragg grating. The device is connected to optical single-mode fibers and works polarization-independent. We measure a peak reflectivity of 95% and a linewidth (FWHM) of 0.09 nm. By temperature-controlling the peak wavelength of the filter is tuned. For the linear thermal expansion coefficient of lithium niobate along the c axis the value !₃₃=(4.5ǂ.5)᎒^-6 K^-1 is obtained. Permanent illumination of the filter with incoherent light allows us to adjust the peak reflectivity. Dedicated to Prof. K. Ringhofer on the occasion of his 60th birthday.     

An extended interaction oscillator based on a complex resonatorstructure

We report experimental investigation of a novel scheme for efficient interaction between a linear electron beam and an electromagnetic wave in a complex extended interaction structure. The structure of interest consists of a high R/Q, five-gap, coupled-cavity resonator that incorporates a coaxial section of a quarter plasma wavelength placed between the first and second cavities. In the coaxial section, beam and wave propagate in separate channels. The first cavity, strongly coupled to the other cavities through the wave channel of the coaxial section, serves as a buncher cavity. An inner channel running through the center conductor of the coaxial section provides a cutoff drift space for ballistic bunching of electrons, an effect that is shown to significantly enhance the interaction efficiency. Oscillation power of 2.2 kW at 16.6 GHz was demonstrated with an interaction efficiency of 30%. The total efficiency was further increased to 41% by incorporation of a two-stage depressed collector     

Seeded and spontaneous light hexagons in LiNbO3:Fe

The first observation of optical hexagons in a medium with dominating photovoltaic charge transport – in LiNbO₃:Fe – is reported. The optimum conditions for hexagon observation are revealed and discussed. The dynamics of hexagon formation are also studied. Received: 9 February 2000 / Published online: 8 March 2000     

Simulation of flow problems with moving mechanical components,fluid–structure interactions and two-fluid interfaces

The application of a stabilized space–time finite element formulation to problems involving fluid–structure interactions and two-fluid interfaces is discussed. Two sample problems are presented and the method is validated by comparison with a test problem. © 1997 John Wiley & Sons, Ltd.