Shaking metallic grains turns them into tunable laser

PUTTING a jar full of metal beads on a subwoofer makes more than just a rattle. Add light and it can become a laser that changes frequency when the beads are shaken harder. Such a tunable laser could lead to crisper projected images and help unlock the mysterious behaviour of granular materials, seen in sand castles, dunes and avalanches.

Ordinary lasers work by bouncing photons back and forth between two semi-transparent mirrors separated by a medium such as a crystal or a gas. The medium is chosen so that as the photons travel through it, they stimulate atoms to emit new photons - all with exactly the same wavelength, phase and direction - which build the beam.

In the past decade, physicists have set about building so-called random lasers, which turn that picture inside out by embedding the mirrors - often a suite of tiny grains - inside the medium. Random lasers have been shown to remove the speckling from images created by laser projectors, but the grains used were small, making them hard to control and sensitive to temperature changes.

To address these problems, Claudio Conti at the Sapienza University of Rome in Italy and his colleagues went big. They dropped about 1500 metal spheres, each a millimetre or so in diameter, into a rectangular container with a solution of rhodamine B, which gives off light when excited, and shone light at it. Switching the subwoofer on caused the spheres to bounce inside the fluid, effectively suspending them like the smaller particles in other random lasers.

The spheres acted like mirrors and laser light was emitted in all directions. This multi-directionality is typical of random lasers. Though the light is less intense than a conventional laser, it can be marshalled for a variety of applications.

But unlike in previous random lasers, Conti's team can tune their granular laser to emit different laser light. Turning up the subwoofer produced laser light of a second wavelength, for example, while some types of shaking produced two wavelengths at once - a "laser chord" (Physical Review Letters, DOI: 10.1103/PhysRevLett.108.248002). "To our knowledge this is the first time that this kind of controlled effect has been observed," Conti says.

He believes the tunability is a result of the poorly understood properties of granular materials, in which grains are individually solid but can also flow as a group like a liquid or a gas. This split personality may be why patterns form in sand dunes, even though each sand grain moves randomly. Similarly, the beads in the laser moved randomly when they were shaken, yet exhibited patterns in their distance from each other that changed with shaking frequency. "Randomness has a richness, in this sense," says Conti.

As the laser light can act as a read-out of the underlying granular structure, Conti thinks the spheres could help reveal why granular materials behave so strangely - how avalanches travel, for example, or why sand castles don't immediately fall apart.

Cefe L?pez at the Materials Science Institute of Madrid in Spain agrees: "This study, I'm sure, is going to help in studying these complex systems."

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