Electromagnetic diffraction of optical waves at a subwavelength aperture becomes a focus of recent research interest in
order to explain theoretically and experimentally the transmission enhancement of arrays of nano-size holes in metallic
screens. Some theories propose that the surface waves exited at a subwavelength aperture by diffraction is propagated to
the neighboring aperture and interfered with the incident optical beam at that aperture, resulting in enhancing or
suppressing of the transmitted energy through the aperture. In those theories one considers implicitly the interference of
the two waves in the exit side of the aperture. However, in the entrance side of the aperture we note that the surface
waves and the incident beam have perpendicular wavevectors and orthogonal polarizations before being coupled into a
slit. It is then important to investigate how this interference occurs with mediation of mutual conversion between the
surface waves and bulk waves at the nano-slit.
We find that a part of the surface wave energy scattered by the slit edge leaks into the slit and induces diagonal
electrical charge dipole, which radiates new bulk waves and surface waves on the slit's side-walls. Multiple reflection
Fabry-Perot resonators are formed in both horizontal and vertical directions over the slit, depending on the slit's width
and depth. We demonstrate the hypothetic interference between the fields induced by the surface waves coupled into the
slit and the normal incident wave, which induces also electric dipoles and new surface waves at the slit. Our calculation
fits well with the experimental results. The work is significant for development of the optical diffraction theory on the
metallic nano-size apertures.