SURFACE-PROFILE DEPENDENCE OF PHOTON PLASMON-POLARITON COUPLING AT A CORRUGATED SILVER SURFACE

The influence of the surface-profile Fourier decomposition on photon-surface-plasmon-polariton (SPP) coupling at a corrugated silver surface, as probed by changes in diffraction-order intensities, line shapes, and peak positions, is explored. The effect of higher spatial harmonics on diffraction orders is considered for coupling to the SPP in the vicinity of a grating-induced minigap as well as away from the minigap region. Numerical results are obtained with the reduced Rayleigh equations, which are accurate in the range of corrugation strengths that optimize photon-SPP coupling for silver. Under first-order-coupling conditions away from the minigap region the presence of a particular spatial harmonic in the surface profile enhances a specific diffraction order in an easily predictable way. The selectivity, sensitivity, and associated Line-shape changes for enhancement are examined as a function of harmonic amplitude for several higher harmonics. In connection with recent experiments [J. Opt. Soc. Am. B 8, 1348 (1991)] we investigate the influence of the second and the third spatial harmonics on the propagating orders in the vicinity of the (+1, -2) minigap region of an 870-nm grating at 633 nm. The interaction between first- and second-order couplings to the opposite branches of the SPP dispersion relation is tuned by variation of the corrugation depth. It is found that changes in the profile Fourier decomposition with corrugation depth, which arise in the holographic fabrication of diffraction gratings, can explain the observed momentum gap in the -1 diffraction order in this case.