Oblique frozen modes in periodic layered media

We study the classical scattering problem of a plane electromagnetic wave incident on the surface of semi-infinite periodic stratified media incorporating anisotropic dielectric layers with special oblique orientation of the anisotropy axes. We demonstrate that an obliquely incident light, upon entering the periodic slab, gets converted into an abnormal grazing mode with huge amplitude and zero normal component of the group velocity. This mode cannot be represented as a superposition of extended and evanescent contributions. Instead, it is related to a general (non-Bloch) Floquet eigenmode with the amplitude diverging linearly with the distance from the slab boundary. Remarkably, the slab reflectivity in such a situation can be very low, which means an almost 100% conversion of the incident light into the axially frozen mode with the electromagnetic energy density exceeding that of the incident wave by several orders of magnitude. The effect can be realized at any desirable frequency, including optical and UV frequency range. The only essential physical requirement is the presence of dielectric layers with proper oblique orientation of the anisotropy axes. Some practical aspects of this phenomenon are considered.Comment: text and 9 figure

Surface plasmon polaritons on deep, narrow-ridged rectangular gratings

Copyright © 2009 Optical Society of America. This paper was published in Journal of the Optical Society of America B and is made available as an electronic reprint with the permission of OSA. The paper can be found at the following URL on the OSA website: http://www.opticsinfobase.org/josab/abstract.cfm?URI=josab-26-6-1228 Systematic or multiple reproduction or distribution to multiple locations via electronic or other means is prohibited and is subject to penalties under law.The dispersion diagrams of surface plasmon polaritons have been calculated for rectangular gratings, with very narrow wires, of varying depths. For gratings with a moderate height a family of vertical-standing-wave resonances may be excited, which consist of surface plasmons, oscillating on either vertical surface, coupling together through the metal wires. These modes evolve similarly to the manner in which shallow-grating surface-plasmon dispersion curves evolve into cavity modes in the grooves of the structure. However, on further increase in grating height these vertical standing waves evolve into a second resonant feature, which is independent of yet further increases in height. This new mode is shown to be equivalent to the resonances found on infinite multilayer metal/dielectric structures illuminated at normal incidence

IMAGEtags: Quantifying mRNA Transcription in Real Time with Multiaptamer Reporters

Cell communications are essential to the organization, development, and maintenance of multicellular organisms. Much of this communication involves changes in RNA transcription and is dynamic. Most methods for studying transcription require interrupting the continuity of cellular function by sacrificing the communicating cells and capturing gene expression information by periodic sampling of individual cells or the population. The IMAGEtag technology to quantify RNA levels in living cells, demonstrated here in yeast, allows individual cells to be tracked over time as they respond to different environmental cues. IMAGEtags are short RNAs consisting of strings of a variable number of tandem aptamers that bind small-molecule ligands. The aptamer strings can vary in length and in configuration of aptamer constituents, such as to contain multiples of the same aptamer or two or more different aptamers that alternate in their occurrence. A minimum effective length is about five aptamers. The maximum length is undefined. The small-molecule ligands are enabled for imaging as fluorophore conjugates. For each IMAGEtag, two fluorophore conjugates are provided, which are FRET pairs. When a cell expresses an RNA containing an IMAGEtag sequence, the aptamers bind their ligands and bring the fluorophores into sufficiently close proximity to allow FRET. The background fluorescence of both fluorophores is minimal in the FRET channel. These features endow IMAGEtags with the sensitivity to report on mRNA expression levels in living cells