Angular and Intensity Modes Self-Referenced Refractive Index Sensor Based on Thin Dielectric Grating Combined With Thin Metal Film

The combination of a thin dielectric grating with a thin metal film is shown to reveal the existence of multiple plasmons, some are short range and others long range both represented by resonant dips in the reflectivity. Usually the plasmons come in pairs, where one is excited at the substrate metal interface while the other at the grating metal interface. One of these dips is sensitive to the refractive index (RI) of the medium adjacent to the metal surface while the other to that near the grating interfaces. Using an optimum design it is possible to obtain high sensitivity to the RI variations of one of the adjacent media but not to the other, hence a self-referenced biosensor can be built using this design. Two configurations are shown to reveal unique features in the angular mode: 1) the possibility of using both angular and intensity sensitivity to detect variations in the RI of the analyte, 2) the possibility of using the excited multiple sharp plasmons that cause multiple resonances (dips) in the reflectivity, where part of these resonances are red-shifted due to variations in the RI of the analyte, while the others are blue shifted. Hence, by measuring the shift of one with respect to the other the angular sensitivity is improved, 3) multiple dips can be used for reference, and 4) high figure of merit is obtained. The thin dielectric grating is shown to have two roles, one to provide the momentum matching whereas the other is to act effectively as a dielectric layer underneath the metal film to enable the excitation of both the long and short range surface plasmons.