Determining the Interfacial Refractive Index via Ultrasensitive\nPlasmonic Sensors

Abstract

Plasmonic\nsensors are promising for ultrasensitive chemical and\nbiological analysis. However, there are increasing experimental findings\nthat cannot be well addressed by theoretical calculations, including\nthe nonlinear dependence of the plasmonic peak wavelength on the refractive\nindex (RI) and the ultrahigh sensitivity beyond the theoretical limit.\nThe gap between experiments and theoretical calculations is that the\nbulk RI (BRI) used for calculation could be different from the interfacial\nRI (IRI) determining the electromagnetic response as a result of the\ninteraction of molecules with the surface. But there is still no method\nto determine the IRI. Herein, we quantitatively determine the IRI\nby disentangling the surface RI (SRI) from the BRI. The obtained IRI\ncan be directly applied in theoretical calculations to reliably reflect\nthe experimental response and rigorously guide the design of plasmonic\nsensors. Moreover, it can be a fundamental dimensionless number to\ndescribe the light–matter interaction at the interface.