Bandwidth Optimisation and Frequency Tuning of Plasmonic Functionalised Metasurfaces for Optical Sensing of Chemical and Biological Substances

This paper reports on a method to optimise the sensitivity of plasmonics sensors based on functionalised metasurfaces of 2D-array of Al NanoAntennas (NA) deposited on a SiO2 substrate operating in the visible region of the electromagnetic spectrum. Moreover, we analysed the characteristics of a double layer metasurface configuration where two different NA 2D-arrays are separated by a dielectric spacer. The optical properties of both the metasurface configurations have been studied analysing how their maximum transmittance and Full-Width-at-Half-Maximum (FWHM) of the transmission curve are related to the variations of the NA geometrical parameters and dielectric spacer thickness. The tailoring of the FWHM is particular important for improving the plasmonic sensors sensitivity in probing the presence of chemical/ biological substances absorbed on the NA surface when their absorption curve is superimposed with the metasurface transmission curve. In particular, better is this superposition better will be the plasmonic sensor sensitivity in probing variations of small concentrations of the adsorbed substances. The simulation results of the optical response of the designed plasmonic sensors suggest a methodology in choosing the NA parameters able to modify the bandwidth of the metasurface transmittance so fitting the absorption curve of chemical/ biological substances absorbed on them. As a case-example, we simulated the response of a plasmonic sensor on which has been deposited a 3nm-thick layer of Rhodamine-6G (R6G) proving that is possible to increase the sensor detection sensitivity of about two orders of magnitude in the measurement of the R6G absorbance. Furthermore we proved the capability of the double layer plasmonic sensors to tune the transmission curve peak wavelength without changing the main optical characteristics. (C) 2016 The Authors. Published by Elsevier Ltd.