Light, because of its wave-corpulse duality, shows many surprising behaviors. A recent PRL paper [1] adds one more. Imagine you fabricate a gold film on a glass substrate and punch a regular array of sub-wavelength holes in this film. Now you shine light upon it. It is so thin--about 20 nm--that it becomes semi-transparent. Now you look at the transmission, which unexpectedly turns out being smaller than without holes. That is, the brutalized film, contrary to expectations, makes an obacure view. Nevertheless, if the film is much thicker, say, 100nm thick, the scenario will be the opposite: the transmission is greatly increased [2].
The authors think that, Fano analysis may lend an explanation. According to them, there are two interfering wavelets contributing to the transmitted waves, which are the resonantly scattered and the nonresonantly scattered, respectively. The former involves resonant excitations of surface plasmon, whereas the latter enters directly through holes. It turns out that, Fano resonance hinges on a single parameter, which is the quotient of a ratio to the line width (which measures how coherent a light is). The ratio concerns the resonant wave amplitude and the directly transmited amplitude. They argue that, this parameter is large for thick films but rather small for extremely thin films, which may then give rise to the observations.
It is worth seeing that, the surface plasma may play a central role. The interaction between light and plasma is obviously an interesting subject. This interaction can carry light to pass through very small holes. The light is at first coupled to the plasma and then the plasma carries it to the destination [2].
[1]Phys. Rev. Lett. 103, 203901 (2009)
[2]Nature (London) 391, 667 (1998)
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