Spectral behavior of nano-sized and azimuthally symmetric coated long period gratings

摘要

In this work, the experimental analysis of the sensitivity characteristics to the surrounding refractive index in nanoscale coated long period gratings (LPGs) is presented. Polymeric coatings of refractive index higher than the cladding one have been considered with thickness values ranging in hundreds of nanometers. The presence of the high refractive index coating induces the cladding to overlay mode transition depending on the overlay features (thickness and index) and the surrounding refractive index. The immediate consequence is a drastic enhancement in the sensitivity to surrounding refractive index due to a large shift of all the attenuation bands in the refractive index range where the modal transition occurs. As matter of fact the sensitivity characteristics changes from the sub-linear monotone behaviour to a resonant like shape. Here, a complete experimental analysis combined with an analytical fitting has been investigated to outline the new SRI sensitivity profile and its dependence on the overlay thickness and order mode.