  It is shown that a smooth metal film (or a plasma layer) can be made
transparent for an electromagnetic wave when two identical subwavelength
diffraction gratings are placed on both sides of the film. The electromagnetic
wave transmission through the metal film is caused by excitation of evanescent
surface waves (plasmons) and their transformation into propagating waves at the
gratings. A model which is developed analytically shows that the problem of the
wave transmission is physically equivalent to the problem of excitation of two
coupled resonators of evanescent waves which are formed at the two film
surfaces.
