Coupling Efficiency of Surface Plasmon Polaritons: Far- and Near-Field Analyses

The excitation of surface plasmon polaritons (SPPs) through one-dimentional (1D) metallic (Au) grating on higher refractive index -GaP substrate is investigated. Such grating devices find potential applications in real world, only if the coupling efficiency (eta) of a free-space transverse-magnetic plane-wave into a SPPs mode is maximum. A simple and robust technique is used to estimate the eta, by simply measuring the transmission through the grating while varying slit width (a) but period (I >) and the thickness (t) remain fixed. When the wave vector (k (0) ) of the incident light is matched to that of SPP, highest eta is achieved. It is found that I >/3 < a < I >/2 yields a maximum eta where the intermediate scattering couples more incident energy to SPPs. These gratings are designed in such a way that they support only the fundamental plasmonic mode yielding higher eta. Scanning near-field optical measurements also confirm and corroborate the observations of far-field and near-field modeling (COMSOL multiphysics) results.