(Top) Schematic of the subwavelength hole array. (Bottom) Scattering electron microscope image of the subwavelength hole array in gold film.
If you punch holes into a semitransparent gold film, will the thin sheet transmit more or less light? Surprisingly, it will let less light through, thanks to surface plasmons.
Bruno Gompf and colleagues at the University of Stuttgart (Germany) found the intriguing result (Phys. Rev. Lett. 103, 203901) while researching the optical properties of nanostructures in thin films.
In 1998, a team headed by T.W. Ebbesen of NEC Research Institute (U.S.A.) learned that a metal film that is thick enough to be completely opaque when solid—roughly 200 nm—can pass extraordinary amounts of light when pierced by subwavelength-sized holes in an orderly array, thanks to surface plasmon polaritons. However, no one had investigated the properties of metal films much thinner than the size of their piercings.
The Stuttgart researchers placed a 20-nm-thick layer of gold atop a glass substrate. At this depth, gold transmits about 50 percent of the incident light, as they confirmed with measurements. The team then used standard lithographic processes to poke 200-nm-wide holes with a period of 300 nm in the thin gold.
The scientists measured how polarized light, ranging from the ultraviolet to the near-infrared (230 nm to 1.8 µm), passes through the gold film at a variety of angles of incidence. The resulting transmission spectra for the pierced film showed strong absorption around 1.96 eV or 633 nm, shifting to lower energies and longer wavelengths when the plane of the light’s incidence was parallel or perpendicular to the rectangular array of holes.
A theoretical explanation for the phenomenon lies in an analysis of the surface plasmon polariton modes. The plasmons on each surface of the ultrathin gold film are coupled to each other much more strongly than those on thick films, and the non-resonantly scattered light greatly exceeds the light resonantly scattered by surface plasmons. The short-range surface plasmons can be excited when they absorb light, but they do not re-emit it.
According to Gompf, gold has a low plasmon frequency. The same effect would happen with a silver film, but the strong absorption would occur at a different frequency. The pierced thin films could be used to create tiny polarizing filters and other components of photonic devices. Next, Gompf and his colleagues are studying the effects of varying periodicities, hexagonal lattices and unordered hole arrays on the thin metal films.
