Analytical Approach to the Prism Coupling Problem in the Kretschmann Configuration

Prism coupling in the Kretschmann configuration is a well-known method for excitation of surface plasmon polaritons (SPP’s) in a metal film bounded from one side by a prism and from the other side by air. The analysis of the reflectance in the upper medium (prism) is based on the well-known Fresnel’s formula. Due to the fact that this formula cannot be inverted directly to give the complex dielectric permittivity and the thickness of the metal film from measured values of the reflectance at three or more different angles of incidence, an additional analysis is needed. Here, such an analysis is presented. The special case of illumination with He–Ne laser (λ = 632.8 nm) of silver film bounded by air is considered. A new asymptotic formula for the Lorentz dip is derived. Our experimental data for silver film are reported too.     

Long Range Surface Plasmons for Observation of Biomolecular Binding Events at Metallic Surfaces

A long range surface plasmon (LRSP) is an electromagnetic wave propagating along a thin metal film with an order of magnitude lower damping than conventional surface plasmon (SP) waves. Thus, the excitation of LRSP is associated with a narrower resonance and it provides larger enhancement of intensity of the electromagnetic field. In surface plasmon resonance (SPR) biosensors, these features allow a more precise observation of the binding of biomolecules in the proximity to the metal surface by using the (label-free) measurement of refractive index (RI) variations and by SP-enhanced fluorescence spectroscopy. In this contribution, we investigate LRSPs excited on a layer structure consisting of a fluoropolymer buffer layer, a thin gold film, and an aqueous sample. By implementing such structure in an SPR sensor, we achieved a 2.4- and 4.4-fold improvement of the resolution in the label-free and fluorescence-based detection, respectively, of the binding of biomolecules in the close proximity to the surface. Moreover, we demonstrate that the sensor resolution can be improved by a factor of 14 and 12 for the label-free and fluorescence-based detection, respectively, if the biomolecular binding events occur within the whole evanescent field of LRSP.