Ursula J. Gibson

Multiple determination of the optical constants of thin-film coating materials

The seven participating laboratories received films of two different thicknesses of Sc2O3 and Rh. All samples of each material were prepared in a single deposition run. Brief descriptions are given of the various methods used for determination of the optical constants of these coating materials. The measurement data are presented, and the results are compared. The mean of the variances of the Sc2O3 refractive-index determinations in the 0.40–0.75-nm spectral region was 0.03. The corresponding variances for the refractive index and absorption coefficient of Rh were 0.35 and 0.26, respectively.

Electrodeposition and room temperature ferromagnetic anisotropy of Co and Ni-doped ZnO nanowire arrays

Cobalt and nickel doped ZnO nanowire arrays were synthesized by an electrochemical process at a temperature of 90°C. Energy dispersive x-ray spectroscopy and x-ray diffraction show that the dopants are incorporated into the wurtzite-structure ZnO. Anisotropic ferromagnetism with an easy direction of magnetization either perpendicular or parallel to the wire axis, depending on the wire geometry and density, was observed in 1.7% Co and 2.2% Ni-doped ZnO nanowires at room temperature. The anisotropic magnetism was explained in terms of a competition between self-demagnetization and magnetostatic coupling among the nanowires.

Low-temperature growth and field emission of ZnO nanowire arrays

Structural, optical, and field-emission properties of ZnO nanowire arrays grown at 90°C are investigated. Single-crystalline ZnO nanowires with low level of oxygen vacancies are obtained at low temperatures. The nanowire growth is strongly dependent on the seeding method used but independent of the substrate materials, which enable large scale growth of ZnO arrays on all kinds of substrates including polymers. We have demonstrated stable electron emission at low-field strengths for nanowires grown on polystyrene and polyethylene foils, making them promising candidates for fabrication of flexible cold cathodes. Deposition of a few nanometers of gold on ZnO nanowires significantly lowers the field required for electron emission, which is explained in terms of additional field enhancement from Au islands on top of the ZnO nanowires.

Simple versatile method for fabricating guided-wave gratings

A simplifed method which requires only a single mirror for making guided-wave gratings (periodic, chirped, and curved) is described, and the properties of gratings fabricated by this method are reported.

Prism coupling into ZnS waveguides: a classic example of a nonlinear coupler.

Prism coupling of an argon laser into a nonlinear ZnS waveguide was investigated for power slew rates much less than the nonlinearity relaxation time. The angular variation in coupling efficiency becomes progressively more asymmetric with increasing power until switching occurs on one side of the curve. For large detunings on the switching side, increasing absorption bistability was observed.

Synthesis and magnetic properties of Co-doped ZnO nanowires

An electrochemical approach to the synthesis of Co-doped ZnO nanowire arrays on conducting substrates at 90 °C is reported. The structure and properties of as-grown and annealed nanowires were studied by x-ray diffraction, scanning electron microscope, energy dispersive x-ray spectroscopy, photoluminescence, and vibrating-sample magnetometry. The Co-doped ZnO nanowires exhibit a broadened band-edge emission at 386nm with a strong emission band around 550nm. Room-temperature ferromagnetism was observed in the doped ZnO nanowires, which makes them potentially useful as building components for spintronics. Effects of Co concentration and annealing on the magnetic properties were investigated.

Effects of low substrate temperature and ion assisted deposition on composition, optical properties, and stress of ZnS thin films.

ZnS thin films were deposited with and without ion assisted deposition onto substrates held at temperatures ranging from -120 to 50 degrees C. Effects on chemical and crystalline composition, film microstructure, refractive index, stress, and waveguide losses were investigated. We observed that minimum stress and minimum losses occurred at the same deposition temperature (-50 degrees C) in ZnS films, which also corresponded to a quenching of crystallinity.

Orientation of optically trapped nonspherical birefringent particles

While the alignment and rotation of microparticles in optical traps have received increased attention recently, one of the earliest examples has been almost totally neglected--the alignment of particles relative to the beam axis, as opposed to about the beam axis. However, since the alignment torques determine how particles align in a trap, they are directly relevant to practical applications. Lysozyme crystals are an ideal model system to study factors determining the orientation of nonspherical birefringent particles in a trap. Both their size and their aspect ratio can be controlled by the growth parameters, and their regular shape makes computational modeling feasible. We show that both external (shape) and internal (birefringence) anisotropy contribute to the alignment torque. Three-dimensionally trapped elongated objects either align with their long axis parallel or perpendicular to the beam axis depending on their size. The shape-dependent torque can exceed the torque due to birefringence, and can align negative uniaxial particles with their optic axis parallel to the electric field, allowing an application of optical torque about the beam axis.

Ion beam processing for coating MgF2 onto ambient temperature substrates.

In this paper, we report on the use of inert gas ion bombardment during the deposition of MgF2 films on ambient temperature substrates. The low energy (<250-eV/ion) bombardment is shown to increase significantly the abrasion resistance and adherence of the films without significantly degrading their optical performance. The success of this technique has wide-ranging implications for the application of durable coatings to temperature sensitive substrates.

Measurement of planar waveguide losses using a coherent fiber bundle

We report on a nondestructive and accurate method for measuring the mode propagation losses of planar waveguides. A coherent optical fiber bundle images the scattered light streak at a remote detector plane. Measurements have been made over a wide range from low loss (≤1 dB/cm) to high loss (≥20 dB/cm) with a spatial resolution of 0.2 mm and a repeatability of 2%. Waveguides prepared by thermal evaporation, ion-exchange, and sputtering were characterized.