D. M. Bird

Bend loss in all-solid bandgap fibres

Experimental measurements of all-solid photonic bandgap fibres with an array of high-index rods in a low-index background revealed an unexpected variation of bend loss across different bandgaps. This behaviour was confirmed by calculations of photonic band structure, and explained with reference to the differing field distributions of the modes of the cladding rods. Our understanding was confirmed by further experiments, leading to proposals for the improvement of these fibres.

Robust photonic band gaps for hollow core guidance in PCF made from high index glass

We report a new type of photonic bandgap that becomes substantial at remarkably low air-filling fractions (~60%) in triangularlattice photonic crystal fibres (PCF) made from high index glass (n / 2.0). The ratio of inter-hole spacing to wavelength makes these new structures ideal for the experimental realisation of hollow-core PCF in the mid/farinfrared, where suitable glasses (e.g., tellurites and chalcogenides) tend to have high refractive indices.

Approximate band structure calculation for photonic bandgap fibres

An approximate method for finding the band structure of simple photonic bandgap fibres is presented. Our simple model is an isolated high-index rod in a circular unit cell with two alternative boundary conditions. Band plots calculated this way are found to correspond closely to calculations using an accurate numerical method.

Metallic mode confinement in microstructured fibres

We report the first long, uniform, optical fibers in which visible light is guided in a single mode by metallic reflection. We describe the fabrication, experiment and characterization of these metallic optical fibers and compare them with theoretical calculations.

Scaling laws and vector effects in bandgap-guiding fibres

Scaling laws for photonic bandgaps in photonic crystal fibres are described. Although only strictly valid for small refractive index contrast, they successfully identify corresponding features in structures with large index contrast. Furthermore, deviations from the scaling laws distinguish features that are vector phenomena unique to electromagnetic waves from those that would be expected for generic scalar waves.

Experimental demonstration of the frequency shift of bandgaps in photonic crystal fibers due to refractive index scaling

Experimental demonstration of the frequency shift of photonic bandgaps due to refractive index scaling using D2O-filled hollow-core photonic crystal fibers is presented. The results confirm a simple scaling law for bandgaps in fibers in which the low-index medium is varied.

Electron-hole pair creation by atoms incident on a metal surface

Electron–hole pair creation by an adsorbate incident on a metal surface is described using ab initio methods. The approach starts with standard first principles electronic structure theory, and proceeds to combine classical, quantum oscillator, and time dependent density functional methods to provide a consistent description of the nonadiabatic energy transfer from adsorbate to substrate. Of particular interest is the conservation of the total energy at each level of approximation, and the importance of a spin transition as a function of the adsorbate/surface separation. Results are presented and discussed for H and D atoms incident on the Cu(111) surface.

Errors and correction term for HOLZ line simulations

Abstract Some of the errors associated with the use of the kinematic or plane-wave approximation to simulate higher-order Laue zone (HOLZ) lines observed in convergent-beam electron diffraction patterns are discussed. The applicability of a proposed formula for estimating the required correction is tested on a variety of materials (with atomic number ranging from 14 to 69) and zone axes. The formula is straightforward to compute and involves only the structure factors of the material; it does not require a many-beam dynamical calculation. It is shown that the formula works best for weak axes. The effects of composition on HOLZ line positions can thus be readily estimated.

An improved photonic bandgap fiber based on an array of rings.

We describe the modeling, fabrication and characterization of a silica-core photonic bandgap fiber based on a 2-d array of raised-index cladding rings. The use of rings to form the cladding is shown to re-order the cladding modes in such a way as to broaden the photonic band gaps and reduce bend sensitivity. We compare the performance of the ring fiber with that of a similar fiber made using solid rods.

Electronic nonadiabatic effects in the adsorption of hydrogen atoms on metals

The time-dependent, mean-field Newns-Anderson model for a spin-polarized adsorbate approaching a metallic surface is solved in the wide-band limit. Equations for the time evolution of the occupation of the spin dependent adsorbate states and for the nonadiabatic and nearly adiabatic adsorbate-surface energy transfer rates are derived. Numerical solutions are obtained using characteristic parameters derived from density functional theory calculations for the H/Cu(111) system. The time evolution of the model system is shown to be strongly nonadiabatic in the vicinity of the transition point between spin-polarized and nonpolarized ground states. Away from the spin transition the nonadiabatic energy transfer is in close agreement with the nearly adiabatic limit. Near the transition, nonadiabatic effects are large and the nearly adiabatic approximation fails.