Mechanism of coupling and interference in nano-slit - art. no. 68320E

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.