Polymer Coated Long Period Gratings: New Perspectives as High Sensitivity Chemical Sensors

Abstract

In this work, long period gratings coated with polymeric high refractive index (HRI) thin layers are proposed as high sensitive opto-chemical sensors with tailored sensitivity profile. The deposition of thin coatings with high refractive index (compared to the cladding one) induces strong changes in the field distribution related to the cladding modes. The main effect is a significant change in the sensitivity characteristic to surrounding refractive index (SRI) and to overlay refractive index. Differently from the standard LPG, where the sensitivity to SRI exhibits an exponential form, in HRI coated LPG the sensitivity shows a resonant-like profile. Since the resonant-peak can be centered around the selected SRI by acting on the overlay features (refractive index and thickness), this effect can be efficiently used to realize high performance opto-chemical sensors. In order to investigate the sensor performances the SRI sensitivity of attenuation bands related to different cladding modes have been experimentally tested. Here &form Syndiotactic Polystyrene (sPS) was selected as nanoscale HRI polymeric layer.