Grating couplers for surface plasmons excited on thin metal films in the Kretschmann-Raether configuration

We investigate the optical properties of periodically modulated thin metal films in the Kretschmann-Raether configuration. We find that excitation of surface plasmons via the grating coupler effect at the metal surface not facing the incoming light is not possible in the case of a conformal modulation of the two metal surfaces, but only if the thickness of the metal film is varied over the grating period. In this case there is strong interaction between the surface plasmons on both surfaces and photonic band gaps open up in the dispersion. Whereas in reflection spectra the surface plasmons are seen as minima, they can appear as maxima, minima, or Fano-type resonances in transmission spectra. We show how thin film grating couplers can be tailored to achieve a field strength enhancement at a particular interface or pronounced photonic band gaps. [S0163-1829(99)00931-5].