In a recently published paper ͓U. Schröter and D. Heitmann, Phys. Rev. B 60, 4992 ͑1999͔͒ an unexpected result occurred when light was incident upon a periodically corrugated thin metal film when the corrugations on the two interfaces were identical and in phase with each other. It was observed that it was not possible to excite the surface plasmon polariton on the metal surface facing away from the incoming light, and they ascribed this to the lack of a thickness variation within the metal. In this paper a somewhat different interpretation of their results is presented, which shows that the surface plasmon polariton ͑SSP͒ is in fact very weakly excited on the transmission side of such structures. It is explained why this coupling is so weak in terms of the cancellation of the evanescent diffracted orders from the two diffractive surfaces and how, by changing the phase between the grating on either surface, this coupling becomes much stronger. An explanation for the observation that SPP excitation on such structures may lead to either transmission maxima or minima is also presented. DOI: 10.1103/PhysRevB.67.235404 PACS number͑s͒: 73.20.Mf, 42.70.Qs, 78.66.Bz, 41.20.Jb A surface plasmon polariton ͑SPP͒ is an electromagnetic excitation at the interface between a material with a negative permittivity ͑often a metal͒ and a dielectric. 1 It consists of a surface charge density oscillation coupled to electromagnetic fields, which are bound to the surface and which decay exponentially into both media. With an appropriate coupling geometry ͑in-plane momentum matching͒ SPP's may be excited by incident radiation. This results in resonant coupling and enhanced optical fields, which is one of the reasons why SPP's may be of use in such fields as surface-enhanced Raman spectroscopy or in nonlinear optics. Interest in SPP's has also been heightened in the last few years after it was discovered that they may mediate enhanced transmission of light through arrays of subwavelength apertures in classically opaque metal films. 2,3 When light is incident upon a planar metal surface SPP's cannot be excited since the wave vector parallel to the surface of the SPP is greater than that available to the incident radiation. However, by periodically corrugating the metal surface the wave vector of the incident light may effectively gain integer values of the grating vector, and therefore coupling may occur when the wave vector matching condition is satisfied. Since the increase in the wave vector of the incident light occurs due to diffraction, it is the evanescent diffracted orders of the system which excite the SPP's.If an optically thin metal film bounded by dielectrics is investigated, then it is possible that SPP's may be excited at both metal and dielectric interfaces, and in each case it is the evanescent-diffracted orders ͑corresponding to diffraction in each bounding dielectric medium͒ which excite the SPP. It is this case which Schröter and Heitmann recently investigated. 4 There is also the possibility of exciting coupled...