Published online 26 February 2010 | Nature | doi:10.1038/news.2010.92

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Sound-based lasers could improve imaging and electronics.

One of the two new devices creates standing waves between tiny drums. ANDREW LAMBERT PHOTOGRAPHY / SCIENCE PHOTO LIBRARY

Using tiny drum heads and vibrating towers, two research groups have made impressive advances in developing a new type of laser that emits sound rather than light.

Such sound-based lasers — dubbed sasers by some researchers — are still in their infancy, but they could one day lead to everything from more detailed ultrasound imaging to faster electronics, say the research teams. Their papers appear in the latest issue of the journal Physical Review Letters1,2.

The idea of the saser has been around for as long as that of the laser. Lasers work by amplifying light of a particular colour, or frequency. Two highly reflective mirrors are set apart at a distance that resonates with the selected frequency, and the cavity in between is filled with a gas that amplifies the light. As the light bounces back and forth between the mirrors, the gas increases its power. When the light passes through one of the ends of the device, the result is a beam that is 'coherent' — all light particles are the same colour and beat in unison.

The maths for sound and light waves are nearly identical, and under the right circumstances it should be possible to create a coherent beam of sound waves inside a solid block of material. Because sound waves travel more slowly, the wavelength of sound at a particular frequency is much shorter than that of light. Sound waves of short wavelength could have some extremely useful practical applications, according to Kerry Vahala, a physicist at the California Institute of Technology in Pasadena. For example, beams of sound waves could provide extremely high-resolution ultrasound machines capable of resolving objects that even the most powerful microscope could miss.

But the same short wavelengths that make sasers so attractive also make them enormously difficult to construct. In normal lasers, it is easy to amplify just one frequency because similar frequencies won't resonate inside the same cavity. But the short wavelengths of sound waves mean that many more frequencies compete inside a cavity. Those extra frequencies sap sasers of their energy. Previously, Vahala worked with another team to build a saser-like device at extremely low temperatures3, but to date, no one has built a saser that could work under everyday conditions.

Drums and towers

To isolate just one frequency, Vahala and his group used a laser to drive two silica drums just a few tens of micrometres across1. Light from the laser ran around the rim of the drum heads like a car around a racetrack. Its energy made the two heads vibrate at specific frequencies. By looking at how the laser flickered as it exited the drums, Vahala and his team were able to verify that a single frequency was being amplified in one of the drum heads. When the system crossed a threshold, that head beat with a pure tone. Adjusting the gap between the two drums changed the feedback between them and allowed the team to tune the frequency of the sound.

A second group of researchers used an entirely different setup to make a saser sing2. Tony Kent of the University of Nottingham, UK, and his team began with a tower made of alternating layers of semiconducting gallium arsenide and aluminium arsenide. When a laser struck the top of the tower, it created a sound that caused electrons in the gallium arsenide to quantum-mechanically tunnel through the aluminium arsenide layer. The tunnelling amplified the sound at a specific frequency, which in turn caused more electrons to tunnel. The amplification effect was only briefly coherent, Kent says, but it nevertheless demonstrates the saser concept.

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Both sets of results are interesting and will probably have advantages and drawbacks, says Jérôme Faist, a researcher at the Swiss Federal Institute of Technology in Zurich (ETH Zurich), Switzerland. Vahala's group has created a true sound laser, Faist says: "It is a very clever piece of work." But, he adds, the device operates at megahertz frequencies — lower than the frequencies that would be useful for many applications. In addition, it will be difficult to tap the sound waves from the drum head and transmit them through a solid.

Kent's tower, by contrast, uses existing semiconductor technology, making it easier to join up with semiconducting circuits. On top of that, it operates at hundreds of gigahertz: a frequency more in tune with modern electronic devices. But it is still losing too much of its energy to competing frequencies to be useful, Faist says.

Ultimately, it's still rather unclear what sasers will be useful for. "They will find applications, but honestly I don't know where or for what," Faist says. Vahala agrees, but adds that for the moment, just building one is reward enough: "It's a cool thing to do," he says.