Non-perturbative approach to high-index-contrast variations in electromagnetic systems

We present a method that formally calculates exact frequency shifts of an electromagnetic field for arbitrary changes in the refractive index. The possible refractive index changes include both anisotropic changes and boundary shifts. Degenerate eigenmode frequencies pose no problems in the presented method. The approach relies on operator algebra to derive an equation for the frequency shifts, which eventually turn out in a simple and physically sound form. Numerically the equations are well-behaved, easy implementable, and can be solved very fast. Like in perturbation theory a reference system is first considered, which then subsequently is used to solve another related, but different system. For our method precision is only limited by the reference system basis functions and the error induced in frequency is of second order for first-order basis set error. As an example we apply our method to the problem of variations in the air-hole diameter in a photonic crystal fiber.     

Slow-light enhanced optical detection in liquid-infiltrated photonic crystals

Slow-light enhanced optical detection in liquid-infiltrated photonic crystals is theoretically studied. Using a scattering-matrix approach and the Wigner–Smith delay time concept, we show that optical absorbance benefits both from slow-light phenomena as well as a high filling factor of the energy residing in the liquid. Utilizing strongly dispersive photonic crystal structures, we numerically demonstrate how liquid-infiltrated photonic crystals facilitate enhanced light–matter interactions, by potentially up to an order of magnitude. The proposed concept provides strong opportunities for improving existing miniaturized absorbance cells for optical detection in lab-on-a-chip systems.     

Geometrical and fluidic tuning of periodically modulated thin metal films

We numerically demonstrate near-zero transmission of light through two-dimensional arrays of isolated gold rings. The analysis of the device as an optofluidic sensor is presented to demonstrate the tuning of the device in relation to variations of volume and refractive index of an isotropic fluid positioned over the structure. We also evaluate the performance of the device with respect to geometrical parameters of the rings.     

Modeling of realistic cladding structures for air-core photonic bandgap fibers

Cladding structures of photonic bandgap fibers often have airholes of noncircular shape, and, typically, close-to-hexagonal airholes with curved corners are observed. We study photonic bandgaps in such structures by aid of a two-parameter representation of the size and curvature. For the fundamental bandgap we find that the bandgap edges (the intersections with the air line) shift toward shorter wavelengths when the air-filling fraction f is increased. The bandgap also broadens, and the relative bandwidth increases exponentially with f2. Compared with recent experiments [Nature 424, 657 (2003)] we find very good agreement.     

A continuous parametric shape model

In this paper we propose a flexible continuous parametric shape model for star-shaped planar objects. The model is based on a polar Fourier expansion of the normalized radius-vector function. The expected phase amplitudes are modelled by a simple regression with parameters having nice geometric interpretations. The suggestedgeneralized p-order model is an extension of first- and second-order Gaussian shape models, and in particular the Gaussian assumption is relaxed. The statistical analysis is straightforward, as demonstrated by an application concerning shape discrimination of two cell nuclei populations.     

Real-time monitoring of the progress of polymerization reactions directly on surfaces at open atmosphere by ambient mass spectrometry

The progress of an on-surface polymerization process involving alkyl and perfluoroalkyl silanes and siloxanes was monitored in real-time via easy ambient sonic spray ionization mass spectrometry (EASI-MS). When sprayed on surfaces, the organosilicon compounds present in commercially available nanofilm products (NFPs) react by condensation to form a polymeric coating. A NFP for coating of floor materials (NFP-1) and a second NFP for coating tiles and ceramics (NFP-2) were applied to glass, filter paper or cotton surfaces and the progress of the polymerization was monitored by slowly scanning the surface. Via EASI(+)-MS monitoring, significant changes in the composition of hydrolysates and condensates of 1H,1H,2H,2H-perfluorooctyl triisopropoxysilane (NFP-1) and hexadecyl triethoxysilane (NFP-2) were observed over time. The abundances of the hydrolyzed species decreased compared with those of the non-hydrolysed species for both NFP-1 and NFP-2 and the heavier oligomers became relatively more abundant over a period of 15-20 min. A similar tendency favouring the heavier oligomers was observed via EASI(-)-MS. This work illustrates the potential of ambient mass spectrometry for the direct monitoring of polymerization reactions on surfaces.     

Surface-plasmon-polariton-induced suppressed transmission through ultrathin metal disk arrays

We report surface-plasmon-polariton-induced suppressed transmission through two-dimensional arrays of isolated metal disks with a thickness comparable to optical skin depth of the metal. A transmittance dip of -17.5 dB is achieved at the resonant wavelength of 1524 nm, compared to -12 dB for closed film. Coupling the light into the surface-plasmon polariton results in enhanced absorption, which is potentially interesting in solar cell applications.     

Fulleropyrrolidine end-capped molecular wires for molecular electronics--synthesis, spectroscopic, electrochemical, and theoretical characterization

In continuation of previous studies showing promising metal-molecule contact properties a variety of C(60) end-capped "molecular wires" for molecular electronics were prepared by variants of the Prato 1,3-dipolar cycloaddition reaction. Either benzene or fluorene was chosen as the central wire, and synthetic protocols for derivatives terminated with one or two fullero[c]pyrrolidine "electrode anchoring" groups were developed. An aryl-substituted aziridine could in some cases be employed directly as the azomethine ylide precursor for the Prato reaction without the need of having an electron-withdrawing ester group present. The effect of extending the π-system of the central wire from 1,4-phenylenediamine to 2,7-fluorenediamine was investigated by absorption, fluorescence, and electrochemical methods. The central wire and the C(60) end-groups were found not to electronically communicate in the ground state. However, the fluorescence of C(60) was quenched by charge transfer from the wire to C(60). Quantum chemical calculations predict and explain the collapse of coherent electronic transmission through one of the fulleropyrrolidine-terminated molecular wires.     

Contact resistance of quantum tubes

We consider the conductance of a quantum tube connected to a metallic contact. The number of angular momentum states that the tube can support depends on the strength of the radial confinement. We calculate the transmission coefficients which yield the conductance via the Landauer formula, and discuss the relation of our results to armchair carbon nanotubes embedded in a metal. For Al and Au contacts and tubes with a realistic radial confinement we find that the transmission can be close to unity corresponding to a contact resistance close to h / 2 e²per band at the Fermi level in the carbon nanotube.