Published online 18 October 2009 | Nature | doi:10.1038/news.2009.1013

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Crystals that can confine both light and vibrations could create better biosensors.

The crystal traps vibrations and light in the same place. Eichenfield M., et al.

Light and mechanical vibrations have been imprisoned together for the first time ever in the same place. An artificial crystal that traps both could lead to ultra-sensitive biosensors, create an interface for devices on a chip and provide an elegant cooling mechanism to help test quantum limits.

The communications industry has long used specially patterned materials, called photonic crystals, to guide light through optical fibres. Similar structures, known as phononic crystals, manipulate mechanical vibrations and are used, for instance, in mobile phones to filter out unwanted frequencies. Now Oskar Painter at the California Institute of Technology in Pasadena and his colleagues have created an 'optomechanical' crystal that combines these qualities to control, guide and trap light and mechanical vibrations together.

Although photonic and phononic crystals have different uses, their physical make-up is almost identical, says Painter. "If you make one type of crystal, you almost automatically make the other type," he says. This has not been exploited earlier, he says, because the vibrations set up in crystals of the size commonly used in the optics industry are extremely small and difficult to detect.

The team fashioned their crystal from a 10-micrometre long strip of silicon, etching out rectangular sections of the strip at regular intervals to create what looks like a mini railway track. Computer simulations showed that light waves passing along the track should be partially reflected at each track boundary. Although most wavelengths would still eventually pass across, interference between forward and reflected light with wavelengths that are multiples of the distance between the rails should 'lock' that light at the centre of the track, says Painter. At the same time, mechanical vibrations with the same wavelength should also become trapped, making the central rails of the track oscillate back and forth.

Bounce back

Painter's team tested that this was the case by firing laser light of various frequencies across their crystal. In most cases, all the laser light was transmitted to the other side. But at certain resonant frequencies, they saw a dip in transmission, indicating that some of the light had become trapped. To find out where it had collected, they poked their silicon-railway track with a glass-tipped probe and saw light scatter out from the track's centre.

To check that the centre of the track was also vibrating, they examined how the transmission of light varied for a small range of laser frequencies below the resonance. "The transmission intensity oscillated up and down because the vibrations of the crystal were affecting the transmission of light," says Painter.

Optical physicist Marko Lončar at Harvard School of Engineering and Applied Sciences in Cambridge, Massachusetts, says that the results are "surprising and impressive". "It's not an easy experiment," he says. "Intuitively, I wouldn't have expected those [crystals] to shake so strongly that the vibrations could be picked up."

The fact that the team has built its optomechanical crystal from silicon has implications for the computing industry. "For the first time we can start talking about optomechanical circuits," says Florian Marquardt an expert on optomechanics at the Ludwig-Maximilians University of Munich, Germany. "I can envisage chips that integrate these crystals."

Quantum bridge

Painter thinks that one of the crystal's main future uses will be to bridge together different types of quantum-computing processors on chips. "Right now, quantum-computing systems have many niches — some are based on atoms, some on photons, some on superconductors — but these experiments can't be put together because they need to be connected by light of very different frequencies," he says. Optomechanical crystals provide a way to dump quantum information carried by light from one type of quantum processor into vibrations, which can then be translated back into light of a different frequency to be sent to another processor, explains Painter. "I think these crystals will be ideal interfaces in quantum hybrid systems," says Painter.

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The fact that light at frequencies just off resonance is so sensitive to the crystal's vibrations could also be used in highly sensitive biosensors that can detect DNA sequences or dangerous pathogens. Adrian Ionescu, an expert on mass sensors at the Swiss Federal Institute in Lausanne (EPFL), Switzerland, thinks that the crystals could be used to help develop arrays to detect single gas molecules in real time, beyond the limits of any existing gas sensor devices. "This is an exciting field of research with high potential for innovative applications in sensing," he says.

Painter also hopes to use the technology for a slightly more esoteric purpose — exploring the limits of the quantum world. There has been a lot of interest in whether quantum effects can survive in relatively large mechanical systems, made of a few billion atoms. But these delicate quantum features are usually obscured by jitters caused by heat noise. The light-matter interactions within optomechanical crystals could help get round the problem, by providing an elegant cooling system.

"Light can suck the thermal energy out of a mechanical system, taking that energy into the light field," says Painter. "It would really be great to use this to see if we can then see quantum effects in a real physical mass with a size that we can relate to."