British researchers have, for the first time, built a prototype solid-state maser that works at room temperature, with no permanent applied magnetic field. Masers, which do the same thing with microwave radiation that lasers do with visible light, have not become widely used thanks to their difficult operating conditions – some require cryogenic refrigeration or vacuum chambers and sometimes strong magnetic fields. The researchers claim that their device could have a range of applications in the future – from the detection of explosives to detecting the atomic states of atoms in quantum computing.

Extreme conditions

There are two basic types of masers. Atomic and molecular masers were the first type to be invented back in 1958. They require bulky vacuum chambers and can only emit very low powers. The second and more useful type – solid state masers – exploit transitions between spin states of paramagnetic ions in a solid crystal. They are far more powerful and can produce perhaps the most sensitive, low-noise detectors of faint microwave signals yet developed. Unfortunately, to sustain the necessary population inversion in a conventional solid-state maser requires liquid-helium refrigeration, usually accompanied by a strong DC magnetic field.

The need for these extreme conditions has meant that, while NASA has been willing to invest in maintaining solid-state masers to receive the faint signals transmitted by the Voyager space probes, more everyday applications have been ruled out. "For example, you could use a maser to improve the accuracy of an airport body scanner," says lead author Mark Oxborrow of the National Physical Laboratory in Teddington, UK, "but that would increase the cost of the device considerably. So I think there are many applications that have just been rendered impracticable by the requirement of cryogenics."

New operating mechanisms

Oxborrow and colleagues at Imperial College London produced their maser by substituting a soft polymer – p-terphenyl doped with pentacene – for the usual crystalline ruby as the gain medium. In addition, instead of pumping it with a microwave source, as is traditional for a solid-state maser, they used a 585 nm medical laser designed for the treatment of vascular lesions. These changes allowed them to utilize a phenomenon known as "spin-selective intersystem crossing", which had never been used in a maser and is still not completely understood, to sustain the population inversion in the absence of cryogenic temperatures or a strong, permanent magnetic field. "It is not just that we have taken the traditional technology and just improved things in various directions to get it to work at room temperature." explains Oxborrow. "The operating mechanism of our room-temperature maser is completely different from the conventional solid-state maser."

Impressive but potentially problematic?

Aharon Blank, a chemist at the Technion-Israel Institute of Technology in Haifa, Israel, who was part of a previous, unsuccessful project 10 years ago to develop a room-temperature solid-state maser, is impressed by the research. However, he points out a number of aspects of the design that could potentially prove problematic. First, although the device can operate at zero field, a magnetic field is required to tune the magnetic field at which it operates. While inconvenient, he does not believe this would prove fatal for a commercial device based on the technology. "There are commercial devices in use today that use a static magnetic field to vary the frequency," he says, "so that is not a major problem."

One problem, however, is serious. At present, like the first lasers, the device is only capable of operating in pulsed rather than continuous mode. Masers are used mainly to detect and amplify very faint incoming microwave radiation, and the uses of a detector that cannot stay continuously on are limited. On the flip side, Oxborrow suggests that it could be used to listen for radar echoes, for example. The team are currently experimenting further with their device to ascertain whether or not it can be made to operate in continuous form and, if so, how this can be achieved.

The research is published in Nature.