Sensitivity enhancement in polymer coated long period gratings: towards high performance opto-chemical sensors

In this work, long period gratings coated with polymeric high refractive index (HRI) thin layers are proposed as high sensitive opto-chemical sensors. The deposition of thin coatings with high refractive index (compared to the cladding one) induces strong changes in the filed distribution related to the cladding modes. The main consequence is a significant enhancement in the device sensitivity to the refractive index of the eternal medium and of the overlay, respectively. This effect was here efficiently used to realize high performance opto-chemical sensors. In order to investigate the sensor performances different film thicknesses have been tested and the attenuation bands related to different cladding modes have been monitored. The attention was focused on the measurements of the attenuation bands central wavelengths and peak losses changes induced by a controlled modification of the external and overlay refractive indexes. Here, as nanoscale HRI polymeric layer, the nanoporous crystalline 5 form syndiotactic polystyrene (sPS) was selected since it can adsorb reversibly certain analytes, whose size and shape well fit the nanocavities