An analysis of optical waveguide tapers

The characteristics of slab waveguide tapers with finite-thickness cladding are investigated by approximating the tapered section as a series of uniform five-layer waveguides with successive changes in the core and cladding thickness. The behaviour of multimode coupling and transmission characteristics for typical tapers are studied in detail by computer simulation, employing parameter values appropriate for practical systems. Strong coupling and efficient conversion between core and cladding modes are demonstrated. In addition, it is shown that for tapers of moderate slopes, the nearest-neighbour coupling approximation is quite adequate for the calculation of the individual mode amplitudes. For steeper tapers, however, additional neighbouring modes have to be taken into account, and the modal energy tends to spread further towards the higher-order modes. It is also demonstrated that relatively large step-size can be used in the iterative numerical calculations. This, in conjunction with the choice of an appropriate coupling scheme (e.g., the nearest-neighbour approximation for mild tapers), makes it possible to reduce the cost of computer simulation. The relevance of the present analysis to fiber tapers is also discussed. Supported in part by the Department of Communications of Canada     

The binary alloy system Au/Pd; a SIMS study

Sputtering yields, positive secondary ion yields and relative degrees of ionization of sputtered atoms under bombardement of 10keV O₂⁺-ions have been determined for the single-phase binary alloy system Au/Pd. A weak minimum of the total sputtering yields was observed at a Pd-concentration around 75% atomic.Strong surface topography was found after high-dose ion bombardment. Sputtering yields, together with surface enrichment factors determined by Auger Electron Spectroscopy have been used to calculate surface binding energies for both alloy components.The apparent degree of ionization for positive ions shows a different behaviour for the two alloy components; whereas an almost constant value was found for Au, a distinct maximum at a Pd-concentration of about 40% was observed for Pd. The measurements indicate that, in the concentration range investigated, the concept of constant relative sensitivity factors would yield large analytical errors when applied to the quantization of secondary ion mass spectra of Au/Pd alloys.     

The positive-divergence and blowing-up properties

A property of ergodic finite-alphabet processes, called the blowing-up property, is shown to imply exponential rates of convergence for frequencies and entropy, which in turn imply a positive-divergence property. Furthermore, processes with the blowing-up property-divergence property. Furthermore, processes with the blowing-up property are finitely determined and the finitely determined property plus exponential rates of convergence for frequencies and for entropy implies blowing-up. It is also shown that finitary codings of i.i.d. processes have the blowing-up property. Partially supported by Hungarian National Foundation for Scientific Research Grant OTKA 1906. Partially supported by NSF grant DMS-9024240.     

New Experimental Limit on the Pauli Exclusion Principle Violation by Electrons��The VIP Experiment

We present an experimental test of the validity of the Pauli Exclusion Principle for electrons based on the concept put forward a few years ago by Ramberg and Snow (Ramberg and Snow in Phys. Lett. B 238:438, 1990). In this experiment we perform a very accurate search of X-rays from the Pauli-forbidden atomic transitions of electrons in the already filled 1S shells of copper atoms. Although the experiment has a simple structure, it poses deep conceptual and interpretational problems. Here we describe the experimental method and recent experimental results, which we interpret as an upper limit for the probability to violate the Pauli Exclusion Principle. We present also future plans to upgrade the experimental apparatus using Silicon Drift Detectors.     

Metadynamics simulations of the high-pressure phases of silicon employing a high-dimensional neural network potential

We study in a systematic way the complex sequence of the high-pressure phases of silicon obtained upon compression by combining an accurate high-dimensional neural network representation of the density-functional theory potential-energy surface with the metadynamics scheme. Starting from the thermodynamically stable diamond structure at ambient conditions we are able to identify all structural phase transitions up to the highest-pressure fcc phase at about 100 GPa. The results are in excellent agreement with experiment. The method developed promises to be of great value in the study of inorganic solids, including those having metallic phases.     

An Artificial Intelligence Approach for Tackling Conformational Energy Uncertainties in Chiroptical Spectroscopies

Determination of the absolute configuration of chiral molecules is a prerequisite for obtaining a fundamental understanding in any chirality-related field. The interaction with polarised light has proven to be a powerful means to determine this absolute configuration, but its application rests on the comparison between experimental and computed spectra for which the inherent uncertainty in conformational Boltzmann factors has proven to be extremely hard to tackle. Here we present a novel approach that overcomes this issue by combining a genetic algorithm that identifies the relevant conformers by accounting for the uncertainties in DFT relative energies, and a hierarchical clustering algorithm that analyses the trends in the spectra of the considered conformers and identifies on-the-fly when a given chiroptical technique is not able to make reliable predictions. The effectiveness of this approach is demonstrated by considering the challenging cases of papuamine and haliclonadiamine, two bis-indane natural products with eight chiral centres and considerable conformational heterogeneity that could not be assigned unambiguously with current approaches.