Optical chemo-sensor based on long period gratings and ultrathin sensitive polymer films for water monitoring

Abstract

In this work, the numerical and experimental analysis of coated Long Period Gratings (LPGs) as high sensitivity optochemical sensor is presented. The proposed structure relies on LPGs coated with nanoscale high refractive index (HRI) overlays. When azimuthally symmetric nano-scale HRI coatings are deposited along LPGs devices, a significant modification of the cladding modes distribution occurs, depending on the layer features (refractive index and thickness) and on the external refractive index. If these parameters are properly chosen, a strong field enhancement within the overlay occurs, leading to an excellent sensitivity to the coating properties. Here, the effects of the overlay thickness and the cladding mode order on sensor sensitivity and response time have been numerically and experimentally investigated. In order to provide a high sensitivity and species specific opto-chemical sensor, the LPGs were coated with nanoscale overlays of Syndiotactic Polystyrene (sPS) in the nanoporous crystalline δ form. The sensitive material was chosen in light of its selectivity and high sorption properties towards chlorinated and aromatic compounds. Sensor probes were prepared by using dip coating technique and a proprietary procedure to obtain the form sPS. Experimental demonstration of the sensor capability to perform sub ppm detection of chloroform in water at room temperature is also reported.