Liquid crystal based continuous phase retarder: From optically neutral to a quarter waveplate in 200 microseconds

Liquid crystal variable phase retarders have been incorporated into prototype devices for optical communications system applications, both as endless polarization controllers 1,2,3, and as holographic beam steerers 4. Nematic liquid crystals allow continuous control of the degree of retardation induced at relatively slow switching speeds, while ferroelectric liquid crystal based devices allow fast (sub millisecond) switching, but only between two bistable states. The flexoelectro-optic effect 5,6 in short-pitch chiral nematic liquid crystals allows both fast switching of the optic axis and continuous, electric field dependent control of the degree of rotation of the optic axis. A novel geometry for the flexoelectro-optic effect is presented here, in which the helical axis of the chiral nematic is perpendicular to the cell walls (grandjean texture) and the electric field is applied in the plane of the cell. This facilitates deflection of the optic axis of the uniaxial negatively birefringent material from lying along the direction of propagation to having some component in the polarization plane of the light. The device is therefore optically neutral at zero field for telecommunications wavelengths (1550nm), and allows a continuously variable degree of phase excursion to be induced, up to 2π/3 radians achieved so far in a 40μm thick cell. The retardation has been shown both to appear, on application of the field, and disappear on removal, at speeds of 100-500 μs. The direction of deflection of the optic axis is also dependent on the direction of the field, allowing the possibility, in a converging electrode "cartwheel cell", of endless rotation of the liquid crystal waveplate at a higher rate than achievable through dielectric coupling to plain nematic materials.

Microbead dynamics on quartz crystal microbalance at elevated amplitudes

The application of the Quartz Crystal Microbalance (QCM) for biochemical sensing is well known. However, utilizing the nonlinear response of the QCM at elevated amplitudes has received sporadic attention. This study presents results for QCM-analyte interaction that provide insight into the nonlinear dynamics of the QCM with attached analyte. In particular, interactions of the QCM with polystyrene microbeads physisorbed via self-assembled monolayer (SAM) were studied through experiments and modelling. It was found that the response of the QCM coupled to these surface adsorbents is anharmonic even at low oscillation amplitudes and that the nonlinear signals from such interactions are much higher than those for bare quartz. Therefore, these signals can potentially be used as sensitive signatures of adsorbents and their kinetics on the surface. ©2009 IEEE.

Novel wideband active waveguide photonic crystal polarisation selector

We have designed and fabricated a broadband and compact polarisation selector using a photonic crystal at the junction of two intersecting active waveguides. The crystal shows >8dB polarisation selectivity over a 70nm range. © 2003 Optical Society of America.

Fabrication and properties of TBCCO films via air-atomising spray deposition of thallium-free precursors

The high-field properties of polycrystalline superconducting TlBaCaCuO films fabricated by the incorporation of thallium vapour into air-atomised BaCaCuO precursors are described. Thick films with Tc values in the range 106-111 K have been prepared on polycrystalline yttria-stabilised zirconia substrates. The surface morphology, crystal structure and composition of the films are related to their high-field transport and magnetisation properties. Typical 10 mm × 9 mm films show Jc values > 1×104 A/cm2 at 77 K (0 T). The best film has a Jc=1.3×104 A/cm2 (Ic=3.6 A) at 77 K (0 T). Films prepared on 26 mm×9 mm substrates show typical large-area Jc values > 0.5×104 A/cm2 (77 K, 0 T). A square planar specimen of dimensions 4.3 mm ×4.3 mm exhibited magnetisation Jc values=1.2×105 A/cm2 at 4.2 K (0.1 T), 9.3×104 A/cm2 at 10 K (0.1 T), 3.3×104 A/ cm2 at 4 K (8 T), and 1.6×104 A/cm2 at 10 K (8 T). © 1994.

Assembly and inspection of liquid crystal on silicon devices

Liquid crystal on silicon (LCOS) is one of the most exciting technologies, combining the optical modulation characteristics of liquid crystals with the power and compactness of a silicon backplane. The objective of our work is to improve cell assembly and inspection methods by introducing new equipment for automated assembly and by using an optical inspection microscope. A Suss-Micro'Tec Universal device bonder is used for precision assembly and device packaging and an Olympus BX51 high resolution microscope is employed for device inspection. ©2009 Optical Society of America.

Low-temperature (≤600 °C) semi-insulating oxygen-doped silicon films by the PECVD technique for large-area power applications

This work describes the deposition, annealing and characterisation of semi-insulating oxygen-doped silicon films at temperatures compatible with polysilicon circuitry on glass. The semi-insulating layers are deposited by the plasma enhanced chemical vapour deposition technique from silane (SiH4), nitrous oxide (N2O) and helium (He) gas mixtures at a temperature of 350 °C. The as-deposited films are then furnace annealed at 600 °C which is the maximum process temperature. Raman analysis shows the as-deposited and annealed films to be completely amorphous. The most important deposition variable is the N2O SiH4 gas ratio. By varying the N2O SiH4 ratio the conductivity of the annealed films can be accurately controlled, for the first time, down to a minimum of ≈10-7Ω-1cm-1 where they exhibit a T -1 4 temperature dependence indicative of a hopping conduction mechanism. Helium dilution of the reactant gases is shown to improve both film uniformity and reproducibility. A model for the microstructure of these semi-insulating amorphous oxygen-doped silicon films is proposed to explain the observed physical and electrical properties. © 1995.

Low temperature (≤ 600°C) semi-insulating oxygen-doped silicon films by the PECVD technique for large area power applications

This work describes the annealing and characterisation of semi-insulating oxygen-doped silicon films deposited by the Plasma Enhanced Chemical Vapour Deposition (PECVD) technique from silane (SiH4), nitrous oxide (N2O) and helium (He) gas mixtures. The maximum process temperature is chosen to be compatible with large area polycrystalline silicon (poly-Si) circuitry on glass. The most important deposition variable is shown to be the N2O SiH4 gas ratio. Helium dilution results in improved film uniformity and reproducibility. Raman analysis shows the 'as-deposited' and annealed films to be completely amorphous. A model for the microstructure of these Semi-Insulating Amorphous Oxygen-doped Silicon (SIAOS) films is proposed to explain the observed physical and electrical properties. © 1995.

ROLE OF CHLORINE AND ALKALI PROMOTERS IN ETHYLENE EPOXIDATION OVER SINGLE CRYSTAL SILVER SURFACES.

Experiments with N//2O were carried out with a view to obtaining additional information about the reactivity of oxygen surface species. On clean Ag, N//2O decomposition was found to be an activated process which led exclusively to the deposition of O(a) species. The presence of preadsorbed oxygen or subsurface oxygen served to enhance the deposition rate of O(a). Subsequent dosing with ethylene at 300 K of such an oxygen-populated surface followed by TPR examination showed it to be active for ethylene oxide formation. Control experiments established that adventitious decomposition of N//2O at the reactor walls or specimen supports followed by possible re-absorption of O//2(a) was an entirely negligible process. ) The oxidation activity of N//2O was also investigated at elevated pressures in the batch reactor.