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We have investigated the surface-supported 2D periodic array of gold nanorings and found that the sensing figure of merit can be significantly improved by coherent interactions. The experiments on the periodic nanostructures fabricated on glass substrate have shown that the sensing characteristics depend on the lattice constant and the character of the substrate grating order at a grazing angle. In the evanescent grating order range, as the lattice constant increases, the plasmon peak red-shifts, line shape strongly narrows and sensitivity decreases, but the figure of merit increases. The reason for the decrease in sensitivity is found to be the decreased field confinement and enhanced substrate effects caused by the coherent interactions. In the radiative grating order range, as the lattice constant increases, the plasmon peak blue-shifts, the line shape significantly broadens and the figure of merit decreases. The experimental results were confirmed by numerical calculations using 3D finite difference time domain method. The simulation results predict that, compared to the single nanoparticle, the bulk sensing figure of merit of the periodic array can be improved by more than three times and the surface sensing figure of merit can be improved by around 2.5 times. The grating-induced modes related to the substrate grating order found only in simulations are also discussed and show strongly suppressed sensitivity and low figure of merit.

SYNOPSISAn investigation was made of the optical and waveguiding properties of thin films fabricated from solutions of chitosan-acetic acid (chitosan/HAc) and chitosan/HAc doped with rareearth metal ions (Er+++ or Nd+++). For all three films, the refractive indices were approximately 1.5 and there was nearly no absorption in the range of 300 to 2700 nm. The optical loss in a waveguides was less than 0.5 dB/cm. Morphological observations disclosed that all the films possessed a dense and homogeneous amorphous structure with smooth surfaces. Extrinsic scattering, especially the scattering caused by surface impurities, was the dominating factor affecting the optical loss value. It is also interesting to note that for all the films, doped with rare-earth metal ions or not, the morphological characteristics were alike and the optical properties were similar. Doping rare-earth metal ions into chitosan thin films did not seriously influence optical waveguiding. This paper reports, we believe, the first study of chitosan films for optical applications. The experimental results demonstrate that chitosan and its derivatives are potential candidates for optical materials. 0

A double nanohole in a metal film can optically trap nanoparticles such as polystyrene/silica spheres, encapsulated quantum dots and up-converting nanoparticles. Here we study the dynamics of trapped particles, showing a skewed distribution and low roll-off frequency that are indicative of Kramers-hopping at the nanoscale. Numerical simulations of trapped particles show a double-well potential normally found in Kramers-hopping systems, as well as providing quantitative agreement with the overall trapping potential. In addition, we demonstrate co-trapping of bovine serum albumin (BSA) with anti-BSA by sequential delivery in a microfluidic channel. This co-trapping opens up exciting possibilities for the study of protein interactions at the single particle level.