Propagating transverse electric and transverse magnetic modes in liquid crystal-clad planar waveguides

ABSTRACT

In a planar dielectric waveguide, weak confinement of a propagating mode in a high index core leads to a measurable evanescent interaction with the cladding. In this work, we study the effect of a reorientable anisotropic cladding on the behaviour of Transverse Electric (TE) and Transverse Magnetic (TM) mode polarisations using a liquid crystal (LC)-clad waveguide architecture. The polarised evanescent field of a guided mode interacts with a voltage-tunable birefringent LC cladding to deflect an out-coupled beam. Experimental measurements are coupled with a theoretical framework and show good consistency with simulation results. We isolate the effect of mode confinement by changing the thickness of the high index core. Interactions between the LC index ellipsoid and the mode polarisation are probed by changing the initial alignment of the LC. Finally, we examine the difference in deflection between TE and TM modes, which incorporates both a change in mode confinement and a difference in LC index components.     

The flame photometric determination of calcium in phosphate,carbonate, and silicate rocks

A flame photometric method of determining calcium in phosphate, carbonate, and silicate locks has been developed Aluminum and phosphate interference was overcome by the addition of a large excess of magnesium. The method is rapid and suitable for routine analysis Results obtained are within ± 2% of the calcium oxide content.     

Min-max theory for the Yang-Mills-Higgs equations

In each monopole sector there exist an infinite number of finite energy solutions to the Prasad-Sommerfield limit of the SU(2) Yang-Mills-Higgs equations on ³ whose energy is greater than any finite number.National Science Foundation Postdoctoral Fellow in Mathematics     

The role of complexing ligands in the formation of non-aggregated nanoparticles of zirconia

The hydrolysis and condensation of zirconium n-propoxide in n-propanol have been chemically controlled via the complexation of the zirconium precursor with acetylacetone. The size of the zirconium oxide-based particles is mainly controlled by the complexation ratio x=[acac]/[Zr]. the mean size increases from nanometric to submicronic range when x decreases from 1 to 0.1. Amorphous colloidal particles are obtained at room temperature. They result from a competitive growth/termination mechanism of zirconium-oxo species in the presence of acac surface capping agents. However non-aggregated nanocrystalline particles of tetragonal zirconia, about 2 nm in diameter are formed upon aging at 60°C when hydrolysis is performed in the presence of paratoluene sulfonic acid (PTSA).     

Chemically-bonded stationary phases for thin-layer chromatography of metal ions

Summary A numer of bonded stationary phases were investigated for the separation of Cd²⁺, Cu²⁺, Fe³⁺, Mn²⁺, Ni²⁺, Pb²⁺, and Zn²⁺ by thin-layer chromatography. The effect of structure, surface loading, type of silica gel and mobile phase concentration was considered.     

PM3 semi-empirical study of stereoelectronic effects in the Baeyer-Villiger reaction

The Baeyer-Villiger reaction of polymethoxybenzaldehydes with peroxysuccinic acid (PSA) is studied theoretically. The semi-empirical SCF-MO/PM3 method has been used to study the reaction and to calculate the energy profiles. In addition, heat of formation computations and bond order analyses of the transition species along the reaction coordinates have been performed and are interpreted qualitatively in terms of electron flow through the system during aryl migration. Our results support the hypothesis that the second step is a concerted reaction. Our data support a definite effect of the substituent on the aryl migration. The theoretical predictions are in good agreement with experimental results.     

Studies of pyrolysis and air heating of rayon by inverse gas chromatography

Inverse gas chromatography using water probing at 110°C was carried out to characterize rayon yarns after prior heating at 200 and 250°C in both inert (nitrogen) and oxidative (dry air) atmospheres for successively longer time intervals. During the early stages of heating in either atmosphere, the affinity of the rayon for water drops rapidly. However, whereas continued heating in N₂ results in a further loss of activity, data have been obtained showing that after an initial period yarns heated in air at 250°C exhibit an increasing attraction for water even while losing more weight.     

Electron affinity of the guanine-cytosine base pair and structural perturbations upon anion formation

The adiabatic electron affinity (AEA) for the Watson-Crick guanine-cytosine (GC) DNA base pair is predicted using a range of density functional methods with double- and triple-zeta plus polarization plus diffuse (DZP++ and TZ2P++) basis sets in an effort to bracket the true electron affinity. The methods used have been calibrated against a comprehensive tabulation of experimental electron affinities (Chem.Rev. 2002, 102, 231). Optimized structures for GC and the GC anion are compared to the neutral and anionic forms of the individual bases as well as Rich's 1976 X-ray structure for sodium guanylyl-3',5'-cytidine nonahydrate, GpC.9H(2)O. Structural distortions and natural population (NPA) charge distributions of the GC anion indicate that the unpaired electron is localized primarily on the cytosine moiety. Unlike treatments using second-order perturbation theory (MP2), density functional theory consistently predicts a substantial positive adiabatic electron affinity for the GC pair (e.g., TZ2P++/B3LYP: +0.48 eV). The stabilization of C(-) via three hydrogen bonds to guanine is sufficient to facilitate adiabatic binding of an electron to GC and is also consistent with the positive experimental electron affinities obtained by photoelectron spectroscopy of cytosine anions incrementally microsolvated with water molecules. The pairing (dissociation) energy for GC(-) (35.6 kcal/mol) is determined with inclusion of electron correlation and shows the anion to have greater thermodynamic stability; the pairing energy for neutral GC (TZ2P++/B3LYP 23.9 kcal/mol) compares favorably to previous MP2/6-31G (23.4 kcal/mol) results and a debated experiment (21.0 kcal/mol).     

Cyclopentadiene annulated polycyclic aromatic hydrocarbons: investigations of electron affinities

The adiabatic electron affinities of cyclopentadiene and 10 associated benzannelated derivatives have been predicted with both density functional and Hartree-Fock theory. These systems can also be regarded as benzenoid polycyclic aromatic hydrocarbons (PAHs) augmented with five-membered rings. Like the PAHs, the electron affinities of the present systems generally increase with the number of rings. To unequivocally bind an electron, cyclopentadiene must have at least two conventionally fused benzene rings. 1H-Benz[f]indene, a naphthalene-annulated cyclopentadiene, is predicted to have a zero-point energy corrected adiabatic electron affinity of 0.13 eV. Since the experimental E(A) of naphthalene is negative (-0.19 eV), the five-membered ring appendage contributes to the stability of the naphthalene-derived 1H-benz[f]indene radical anion significantly. The key to binding the electron is a contiguous sequence of fused benzenes, since fluorene, the isomer of 1H-benz[f]indene, with separated six-membered rings, has an electron affinity of -0.07 eV. Each additional benzene ring in the sequence fused to cyclopentadiene increases the electron affinity by 0.15-0.65 eV: the most reliable predictions are cyclopentadiene (-0.63 eV), indene (-0.49 eV), fluorene (-0.07 eV), 1H-benz[f]indene (0.13 eV), 1,2-benzofluorene (0.25 eV), 2,3-benzofluorene (0.26 eV), 12H-dibenzo[b,h]fluorene (0.65 eV), 13H-indeno[1,2-b]anthracene (0.82 eV), and 1H-cyclopenta[b]naphthacene (1.10 eV). In contrast, if the six-membered ring-fusion is across the C(2)-C(3) cyclopentadiene single bond, only a single benzene is needed to bind an electron. The theoretical electron affinity of the resulting molecule, isoindene, is 0.49 eV, and this increases to 1.22 eV for 2H-benz[f]indene. The degree of aromaticity is responsible for this behavior. While the radical anions are stabilized by conjugation, which increases with the size of the system, the regular indenes, like PAHs in general, suffer from the loss of aromatic stabilization in forming their radical anions. While indene is 21 kcal mol(-1) more stable than isoindene, the corresponding radical anion isomers have almost the same energy. Nucleus-independent chemical shift calculations show that the highly aromatic molecules lose almost all aromaticity when an extra electron is present. The radical anions of cyclopentadiene and all of its annulated derivatives have remarkably low C-H bond dissociation energies (only 18-34 kcal mol(-1) for the mono-, bi-, and tricyclics considered). Hydrogen atom loss leads to the restoration of aromaticity in the highly stabilized cyclopentadienyl anion congeners.