Particle swarm optimizer for the surface plasmon resonance effect on metal gratings

In this work, the spectral dependence of optimum parameters of the surface plasmon resonance (SPR) effect on metallized sinusoidal diffraction gratings, under normal incidence, was determined using the particle swarm optimization method. The method was chosen due to its simplicity and effectiveness in providing reliable results, relative to direct search or gradient methods. The Rayleigh's hypothesis, which restricts the analysis to the case of shallow gratings, is used to model the diffracted fields across the interface between the sensing medium and metal. A penalty function was applied to avoid the occurrence of singularities and violation of the validity of the Rayleigh hypothesis. Using this procedure, the optimum values of grating periodicity and amplitude that maximized the sensitivity function for gold, silver, copper, and aluminum-metals that yield high quality factor SPR effects-were determined in a wavelength range between 500 and 1600 nm, for both gaseous and aqueous sensing media.