  We examine the coupling of electromagnetic waves incident normal to a thin
silver film that forms an oscillatory grating embedded between two otherwise
uniform, semi-infinite half spaces. Two grating structures are considered, in
one of which the mid point of the Ag film remains fixed whereas the thickness
varies sinusoidally, while in the other the mid point oscillates sinusoidally
whereas the film thicknesses remains fixed. On reducing the light wavelength
from the long wavelength limit, we encounter signatures in the transmission, T,
and reflection, R, coefficients associated with: i) the short-range surface
plasmon mode, ii) the long-range surface plasmon mode, and iii) electromagnetic
diffraction tangent to the grating. The first two features can be regarded as
generalized (plasmon) Wood's anomalies whereas the third is the first-order
conventional (electromagnetic) Wood's anomaly. The energy density at the film
surface is enhanced for wavelengths corresponding to these three anomalies,
particularly for the long range plasmon mode in thin films. When exciting the
silver film with a pair of waves incident from opposite directions, we find
that by adjusting the grating oscillation amplitude and fixing the relative
phase of the incoming waves to be even or odd, T+R can be made to vanish for
one or the other of the plasmon modes; this corresponds to perfect coupling
(impedance matching in the language of electrical engineering) between the
incoming light and these modes.
