Comparison of Kretschmann-Raether angular regimes for measuring changes in bulk refractive index.

We compare two angular regimes for the measurement of changes in the real refractive index of bulk fluid analytes. The measurements are based on the use of the Kretschmann-Raether configuration to sense a change in reflectivity with index. Specifically, we numerically simulate the relative sensitivities of the total internal reflection (TIR) and the surface-plasmon resonance (SPR) regimes. For a fixed-angle apparatus, the method that gives the greatest change in reflectivity varies with metal film thickness. For films thicker than the skin depth, the SPR regime is the most sensitive to index changes. For thinner films, however, the TIR angle is then dominant, with increases in sensitivity on the order of 75% for 10-nm gold or silver media.