Quasicrystals are selective with terahertz light
Mar 29, 2007
Physicists in the US have found that they can transmit terahertz light at selective wavelengths by shining it onto a metal film perforated in an aperiodic, "quasicrystal" pattern. Until now, this sort of enhanced light transmission had only been achieved using holes in periodic patterns, and had not been wavelength selective. This discovery demonstrates that it may be possible to use quasicrystal hole arrays to make filters that can be tuned simply by being rotated (Nature 446 517).
Shine light through the large holes of, say, a kitchen colander and you get just a portion of it coming out the other side. Shine it through arrays of holes smaller than the light's wavelength, however, and almost all of it can be transmitted. This is thanks to the way photons interact with surface electrons, producing collective excitations known as "surface plasmon polaritons". Previously, enhanced transmission was thought only to occur in periodic holes in metal. But now Valy Vardeny and colleagues from the University of Utah have shown that the effect can be even more pronounced in aperiodic, "quasicrystal" arrays.
At a glance, quasicrystals look as though their pattern ought to repeat, but at closer inspection one finds that there are always subtle irregularities that preclude any of the translational symmetry that is found in normal crystals. Quasicrystals do, however, have rotational symmetry, meaning that at a certain number of intermediate points in a complete revolution their pattern will be the same.
Vardeny's team made different arrays of holes in 75-µm-thick stainless steel foil varying from quasicrystal to totally random patterns. They then shone light through the foils and measured the spectra of light emitted from the other side.
They found that the foils with random holes attenuated the light output fairly evenly over the spectra. The quasicrystal arrays of holes, on the other hand, let sharp peaks of the light's frequency pass through, which were directly related to the spacing between the holes in the structure. In addition, the precise transmission could be tuned by simply rotating the foil. For patterns that were neither well-defined enough to be termed quasicrystals, nor totally random – what Vardeny calls "quasicrystal approximates" – the transmission peaks were less prominent.
Vardeny told Physics Web that the foils could be developed as tuneable filters for use in communications. Terahertz radiation, which lies sandwiched between the microwave and infrared regions of the electromagnetic spectrum, is currently fairly underexploited, but could enable large amounts of data to be transmitted at high speeds. Vardeny's team is now looking at other aperiodic structures for use in the terahertz region.
About the author
Jon Cartwright is a reporter for Physics Web