Hands up: who has heard of extraordinary optical transmission? That's what I thought. It's quite simple: take a metal layer, thick enough that it is just barely opaque. Now, drill a bunch of holes in it. Make sure that the holes are regularly spaced and less than a wavelength in diameter. Shine a light on the sample and behold, the light doth shine through. What got people excited was that if you drilled a single hole that had the same total area as all the little holes, less light would get through. Such is the extraordinary optical transmission.
That experiment was carried out a while ago, but arguments over the hows, whys and wherefores are ongoing; experimentally, there didn't seem to be a lot to do. Until a group of physicists from University of Stuttgart decided to make the metal film thinner. In their experiment, the metallic film was thin enough to transmit up to 50 percent of the light shone on it. When they drilled a regular array of small holes in this material, they found that it transmitted less light than before. Cue further arguments.
So, what might cause such a thing? The explanation for both extraordinary transmission and extraordinary absorption are thought to arise from the behavior of surface plasmon resonances and polaritons. Briefly, surface plasmons are a strong coupling between the incident light field and the motion of free electrons along the surface of the metal. The light field excites coherent motion of the electrons—meaning many of the electrons move together.
In doing so, the electrons generate quite large and concentrated electric fields that propagate both the light field and the surface plasmon. Depending on the arrangement, surface plasmons can either move (this would be the polariton variety), or stay in one place (the resonance species).
In the case of extraordinary optical transmission, the surface plasmons are thought to aid transmission because they can fit through the tiny holes, while the free-standing light field cannot. So, the light excites the surface plasmon, which runs along the surface, through the hole, and out the other side. Once there, the plasmon radiates, to the delight of astonished physicists.
The surface plasmons that make it through the holes have to be coherent with each other—something that is dictated by the light field, the spacing of the holes, and the properties of the metal. Any incoherently generated surface plasmons are thought to run along the surface until they hit some disruption—like a hole, for instance—whereupon they radiate their energy as light on the same side as the incident light. Hence, lower transmission, even when a large hole is present.
Now, you might notice that there are a few... ahem... holes in that description. That is because the details of the matter are still under debate. However, this general description is thought to be largely correct.
It turns out that a similarly broad description exists for extraordinary optical absorption as well. The thinner films are not really transparent. Instead, if the metal is sufficiently thin, the fields generated by a surface plasmon on one surface can excite a surface plasmon on the other side. In the solid film, the light hits one side of the metal and generates some surface plasmons, which generate surface plasmons on the other side of the metal, allowing energy to be transferred from one surface to the other. Eventually, these plasmons radiate, transmitting light through the surface.
However, by drilling holes in the metal, fewer coherent surface plasmons are created, and many more incoherent surface plasmons result. The incoherent plasmons don't live for very long because they run into a hole and radiate their energy instead of transferring their energy to the other surface of the metal. In fact, there seems to be no combination of hole spacing and width that increases the transmission for any color up to that of the plain metal film.
It's a set of very strange results, which will, no doubt, set off more arguments in the community of physicists who play in this area.
Physical Review Letters, 2009, DOI: 10.1103/PhysRevLett.103.203901