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Quasicrystals are selective with terahertz light

04 Apr 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. The discovery suggests that it may be possible to use quasicrystal hole arrays to make filters that can be tuned simply by being rotated (Nature 446 517).

Previous experiments have shown that almost all light incident on an array of holes will be transmitted if the holes are smaller than the light's wavelength. This is thanks to the way photons interact with surface electrons, producing collective excitations known as "surface plasmon polaritons".

This type of enhanced transmission was thought only to occur in periodic holes in metal, but now Valy Vardeny and colleagues from the University of Utah have now shown that the effect can be even more pronounced in aperiodic, "quasicrystal" arrays.

At first glance, quasicrystals look as though their pattern ought to repeat, but on closer inspection it becomes clear 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 tunable 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.

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