Bimodal behavior and isobestic transition pathway in surface plasmon resonance sensing

In traditional interpretation of surface plasmon resonance (SPR) sensing and imaging data, total surface coverage of adsorbed or deposited chemical and biological molecules is generally assumed. This homogenous assumption leads to the modeling of monomodal propagation of plasmons on the surface of the metallic film corresponding to a certain relative permittivity and thickness of the medium-such as molecular thin film-next to the metal. In actual SPR Imaging (SPRI) and SPR sensing situations, the plasmonics-active platforms (e.g., biochips) employed may capture the biomolecular targets as aggregates of different domain sizes on the surface of the thin metallic films. Indeed, such binding of target material always has a finite thickness and is characterized by aggregate lateral sizes possibly varying from tens of nanometers to hundreds of micrometers. This paper studies the propagation of surface plasmons in metallic films, with dielectric domain sizes varying within such ranges. Through rigorous coupled wave analysis (RCWA) calculations, it is indicated that when the domain size is small, only a single mode of propagation-i.e. 'monomodal' propagation behavior-occurs as indicated by only one dip in the angular reflectance curves associated with metallic film having a periodically structured array of molecules on its surface. On the other hand, as the domain size is increased, there is a transition from the 'monomodal propagation behavior' to the existence of a 'mixture of monomodal and bimodal propagation behavior', which changes to a purely 'bimodal behavior' after the size of the domain periodicity is increased beyond about ten micron. Such a transition pathway clearly exhibits isobestic points. The calculations presented in this paper can enable correct interpretation of experimental angular or spectral reflectance data.