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Ever-smaller and ever-faster microelectronics devices with increased storage space, more communications and other functions, and much-reduced battery usage, are part of the incentive behind research into photonic crystals. These materials are bringing us closer to a technologically viable optical transistor that will form the building blocks of future optoelectronics that use photons instead of electrons to process information.

The optical properties of photonic crystals vary in a regular pattern on a scale of hundreds of nanometres. This physical structure means that light entering a photonic crystal can be controlled. For instance, a photonic crystal can transmit light of one particular wavelength, and block all others.

Rana Biswas

The simplest material of this kind has a layered structure, like a film of oil on water. Such one-dimensional structures are used as mirrors, non-reflective coatings, and paints whose colours change with the viewing angle. While nature does not exactly abound with photonic crystals, the natural gemstone, opal, is a photonic crystal, which is what gives it its unique shifting and shimmering colours. Synthetic photonic crystals have been on the science agenda since the nineteenth century, but it is only with the advent of modern fabrication techniques that designer 3D photonic crystals have become attainable.

Optical computers aside, there is a second thread woven into the fabric of photonic crystal research – telecommunications.

Now, researchers at the US Department of Energy’s Ames Laboratory have come up with what might be the perfect way to sort and distribute vast quantities of data through optical fibres. The new technology is based on a filter constructed from a three-dimensional photonic crystal and could allow multiple wavelength channels to be carried along the same stretch of optical fibre without loss and without error. The so-called add-drop filter could ultimately give us the all-optical transmission links that require no electronic components along the route.

There are up to 160 wavelength channels travelling through an optical fibre at the same time, explains Ames physicist Rana Biswas, That means a lot of dialogue is going on simultaneously. He adds that as data is carried along the fibre it is necessary to drop off individual wavelength channels at different points.

When the data being transported in multiple frequency channels over an optical fibre comes to a receiving station, you want to be able to pick off just one of those frequencies and send it to an individual end user, explains Biswas, That’s where these 3D photonic crystals come into play. The same filter technology will also allow optimal use of the fibre’s bandwidth.

The idea of add-drop filters was first conceived in the 1990s, but work focused on 2D photonic crystals until now. The Ames team created a 3D photonic crystal device that contains an entrance waveguide and an exit waveguide for channelling light, which means there is none of the light intensity loss seen with 2D photonics.

There is still at least one hurdle to jump before the 3D add-drop filter can be used in fibre optic communications and that is to scale down the device to the wavelengths of light used in Internet communications – 1.5 micrometres. That remains a big challenge confesses Biswas.

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