Self-cooling crystal makes room-temperature maser

Trading rubies for artificial look-a-likes might seem like a bad idea. But that could be what finally turns masers into valuable technology, ranging in application from medical imaging to chemical sensors.

First dreamed up in the 1950s, the maser is a precursor to the now ubiquitous laser that emits concentrated microwave radiation instead of visible light.

Although masers have been shown to amplify signals with little background noise, conventional solid-state masers work only under extreme conditions, such as in a vacuum or at temperatures near absolute zero. Masers also need strong magnetic fields to function.

Cryogenically cooled ruby masers have been used in antennas to boost the microwave signals from deep space missions such as NASA's Voyager probes. But for the most part, lasers have outpaced their predecessors here on Earth.

Now, using a pink crystal grown from hydrocarbons ? p-terphenyl doped with pentacene ? Mark Oxborrow of the UK's National Physical Laboratory and colleagues have created the first maser that works in air at room temperature and with no added magnetic field.

Inner beauty

Oxborrow and his team exploited a phenomenon called inter-system crossing, found to occur in pentacene, which produces a very cold system even while at room temperature. This allows the maser to work as though it's operating within thousandths of a degree of absolute zero.

The truly remarkable thing about this "internal refrigerator" is that with it, the maser's microwave emission is even greater at room temperatures than one under cryogenic conditions, says Oxborrow.

Aharon Blank, an associate professor at the Israel Institute of Technology, came close to making room-temperature masers 10 years ago. Although enthusiastic about Oxborrow's work, he points out that the new maser can only operate for pulses of about 300 microseconds.

"One needs to have continuous operation for most applications, so this very important aspect is still missing," says Blank.

Oxborrow concedes the limitations of his research, but he's confident that renewed interest in masers could lead to significant advances.

"Through modest investment there is scope for substantial improvements," he says.

Journal reference: Nature, DOI: 10.1038/nature11339

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