<title>Polarization conversion through the excitation of electromagnetic modes on a grating</title>

Abstract

Electromagnetic modes may be excited by photons using grating coupling to provide the necessary extra momentum. Rotation of the grating so that the grooves are no longer perpendicular to the plane of incidence (conical diffraction) breaks the symmetry of the system and introduces polarization conversion. This conversion is strongly enhanced when a mode is excited. Reflectivity data are presented showing polarization enhancement by the excitation of surface plasmon polaritons (SPPs), long range surface plasmon polaritons, and resonant guided modes on surface modulated gratings. The dependence of this conversion on angle of incidence, azimuthal angle, and groove depth has been studied in detail. When the grooves are at 45 degree(s) to the plane of incidence, maximum conversion is recorded for the SPP. However, for guided modes the maximum conversion occurs at angles other than 45 degree(s). The conversion has been seen to rise monotonically with groove depth and efficiencies of over 50% are easily obtained via SPPs and guided modes. Theoretical comparisons with data are made using a rigorous differential method applicable for conical diffraction. This formulation uses a coordinate transformation between the grating surface and a planar system. The comparison between data and theory is very good and further theoretical modeling predicts that conversion efficiencies can approach 100% for appropriate circumstances.