How Light Gets Through Tiny Holes

Two new papers offer a detailed explanation for a baffling experiment published three years ago (Update 359), in which much larger-than-expected amounts of light passed through a silver-coated quartz barrier with tiny openings: namely, a periodic array of 150-nm holes up to 10 times smaller than the wavelength of the light sent through. This bodes well for scaling down optical devices to nanometer dimensions.

At the time, researchers correctly guessed that the light passed through as a result of surface plasmons (SPs), collective oscillations of electrons at the boundary between conductors and insulators.

Now, two research collaborations independently explain the results by showing that plasmons (themselves collective objects) and the photons of light form a composite object, known as a "surface plasmon polariton." A polariton is a combination of photons and another type of object; you might recall from our recent item on stopped light (Update 521) that the polariton in that case consisted of photons and atoms. Combining the plasmon polaritons with the regular arrangement of holes results in a "polaritonic crystal."

One collaboration (contact Anatoly Zayats, Queen's Univ. of Belfast, a.zayats@qub.ac.uk, 011-44-28-90273133) shows that the cooperative effect of many holes in the polaritonic crystal leads to the creation of an electromagnetic field that is most intense close to or at the location of the holes. The polaritonic crystal, says Zayats, is highly analogous to photonic crystals (structures which exclude light at certain wavelengths), and may have a similar host of applications.

Another collaboration (contact Luis Martin-Moreno, Univ. of Zaragoza, Spain, 34-97-6976-1000, lmm@posta.unizar.es) performed new experiments and calculations involving free-standing perforated metal holes, which in fact offer even more enhanced light transmission than that in the original experiments. They explain that the light gets through the holes in the form of an SP "molecule," consisting of two polaritons, one on each side of the metal film, that interact with one another with exponentially decaying electromagnetic fields, forming "molecular" levels in very much the same way that atomic electron wavefunctions interact to form molecular levels in a diatomic molecule.

Roughly speaking, the one group invokes a "molecule" picture to explain the transfer of light through the hole, while the other group focuses upon the formation of the transmitted field's spatial distribution by invoking a "crystal" picture. (Salomon et al., and Martin-Moreno et al., Physical Review Letters, 5 February.)