We demonstrate that optical transmission of a normally incident, monochromatic plane wave through a single sub-wavelength aperture in an opaque metal film can be substantially enhanced by a thin, semitransparent metal film placed parallel to the opaque metal film in front of the aperture. When the semi-transparent and the opaque metal film are separated by a proper distance, a light trapping cavity is formed and the sub-wavelength aperture exhibits a transmission maximum. An enhancement factor of ~40 is demonstrated for a cylindrical 100 nm diameter hole in a silver film.
Olkkonen, J. (2009). Light trapping cavity enhanced light transmission through a single sub-wavelength aperture in a metal film. Optics Express, 17(26), 23992-24001. https://doi.org/10.1364/OE.17.023992