Innovative Exploitation of Grating-Coupled Surface Plasmon Resonance for Sensing

Surface Plasmon Polaritons (SPPs) are confined solutions of Maxwell’s equations which propagate at the interface between a metal and a dielectric medium and have origin from the coupling of the electromagnetic field with electron-plasma density oscillations inside the metal [1]. SPPs are localized in the direction perpendicular to the interface: field intensity decays exponentially from the surface with an extension length of the same order of the wavelength inside the dielectric and almost one order shorter in the metal [2]. These features make SPPs extremely sensitive to optical and geometrical properties of the supporting interface, such as shape, roughness and refractive indices of the facing media. Since these modes have a non-radiative nature, the excitation by means of a wave illuminating the metallic surface is possible only in the configurations providing the wavevector-matching between the incident light and SPP dispersion law (Surface Plasmon Resonance – SPR, see Figure 1). Prism-Coupling SPR (PCSPR) exploits a prism in order to properly increase incident light momentum and achieve SPP excitation, however this setup suffers from cumbersome prism alignment and it is not suitable for miniaturization and integration. A more amenable and cheaper solution consists in Grating-Coupling SPR (GCSPR), where the metal surface is modulated with a periodic corrugation. The plasmonic behaviour of these modulated metallic surfaces had been discovered since the early years of the last century by R.W. Wood [3] and the connection between Wood's anomalies and surface plasmons was finally established by J.J. Cowan and E.T. Arakawa [4]. A plane-wave illuminating the patterned area is diffracted by the periodic structure and it is possible for at least one of the diffracted orders to couple with SPP modes.