Maximizing the sensitivity of thin film metal-dielectric refractometers

A technique has been developed for calculating the reflectance of refractometers with a thin-film metal-dielectric structure on the basis of a coupling prism, using multiplication of the characteristic matrices of the films. The characteristic matrices of the metallic films are obtained by numerical solution of the integral equations of the anomalous skin effect. Spectroscopic refractometers with the structure coupling-prism – gold film – dielectric film – water with a controllable refractive index were investigated. Fused quartz was chosen as the material for both the coupling prism and the dielectric film. The dielectric permittivity of the ionic lattice of gold and the constants of the electron gas in the gold films were determined from literature data on the spectral ellipsometry of gold films. It was shown that by choosing the thicknesses of the metallic and dielectric films, as well as the angle of incidence, one can combine two factors that lead to maximization of refractometer sensitivity – zero reflection from the base of the coupling prism and excitation in the dielectric film of a waveguide mode operating under conditions close to critical. When using s-polarized waves this allows the refractometer sensitivity to be increased to 9.6 $\cdot$ 10$^5$ nm/RIU. Estimates of the resolving power of thin-film metal-dielectric refractometers based on the least-squares method are presented.