Few laypeople, perhaps, know the term “light amplification by stimulated emission of radiation”, although everyone knows the technology by its familiar name, the laser. And “microwave amplification of stimulated emission of radiation”, or maser, is much more obscure – despite having been invented more than 50 years ago before the laser.
Could we one day be talking of masers as familiarly as we do of lasers? There are reasons why the technology has not developed to attain the ubiquity of the latter. Getting a maser to work has required a lot of equipment: either extremely low pressures supplied by special vacuum chambers and pumps, or freezing conditions at temperatures close to absolute zero (-273.15°C). With these constraints, the maser, which delivers a concentrated beam of microwaves as opposed to light, has never been able to develop in the manner of its laser cousin.
But scientists at the National Physical Laboratory and Imperial College London believe this could change. In the summer they reported a milestone in research into masers, which could herald a new phase of industrial development for the technology. The breakthrough does indeed seem significant, because the scientists were able to demonstrate a solid-state maser that operated at room temperature, working in air and with no applied magnetic field.
This means the technology could be miniaturised and made portable, without the need for bulky refrigeration equipment. Future applications could include more sensitive body scanners that could detect small tumours or hidden explosives, more sensitive radar, and sensitive radio telescopes.
Previously, getting results from the maser was a tough job, says Dr Mark Oxborrow, co-author of the NPL study on the solid-state maser. “To get the maser to work it had to sit in the bottom of a big refrigerator just to get it down to close to absolute zero. So the paraphernalia that you needed just to sustain this amplifier was expensive, and big and bulky. It wasn’t possible to make a mobile amplifier because it consumed too much electricity, or needed to be supplied with cryogenic fluids from time to time.”
Most of the applications of masers are as amplifiers, says Oxborrow. They work by amplifying electromagnetic signals, with the advantage that they are very low noise. “Only in extreme situations – where one was prepared to pay anything to get a slightly lower noise amplifier – were masers viable.”
This does not mean that they have not been used. For example, Nasa still uses masers to detect signals coming from deep space. “A maser is good at amplifying weak signals,” he says. “When you have a small signal coming, an amplified copy of that signal comes out – and if you didn’t have a low noise amplifier, the output would contain noise and fuzz.”
Oxborrow adds: “When we get data back from the Curiosity rover [which is exploring Mars], the signal back on Earth is tiny, really puny. The satellite picking up the signal from the surface of Mars only has solar cells, and not much to power its transponder. When the signal reaches home, it’s the order of -210dBm. You need to amplify it to a useful level without introducing a huge amount of noise. That’s why masers are still in use by Nasa today to boost signals from distant space probes – despite the cryogenic problem.”
Conventional maser technology involves hard inorganic crystals – ruby is sometimes used – which work at a very low temperature. The NPL/Imperial team’s key discovery was that a different type of crystal, p-terphenyl doped with pentacene, could replace ruby and replicate the same “masing” process at room temperature.
In the long term, the challenge will be to identify different materials that can mase at room temperature while consuming less power than pentacene-doped p-terphenyl. New designs of maser that would make it smaller and more portable will also be investigated.
More immediate problems to be tackled include getting the technology to work continuously, rather than in pulses. The team is also looking at getting the maser to operate over a range of microwave frequencies, instead of a narrow bandwidth, which would make the technology more useful. It could potentially be substantially improved from just minimal investment.
The maser is defined as solid-state because of its solid-bodied composition, as opposed to an atomic maser, in which atoms fly through empty space in a vacuum; it is this type of maser that requires vacuum equipment to function. Oxborrow says one can think of the difference as being analogous to that between transistors and valves, and the way the development of the transistor effectively superseded valve technology in the 1950s. “Solid state means robustness and durability – that’s why the transistor radio became more popular than the valve one,” he says.
But the maser could also go on to take on the latest transistor technology, the researchers believe, such as amplifiers employing high-electron mobility transistors. For instance, after decades of investment, the lowest noise “temperatures” of these amplifiers are a few tens of kelvins. By contrast, the noise temperature of the researchers’ solid-state maser is already a few hundred mK. Oxborrow believes that there is a lot of room for improvement, and that the relative merits of masers versus semiconductor amplifiers depend on the application, with both likely to find niches.
How did Oxborrow and his fellow researchers make their maser breakthrough? It began, as with any academic project worth its salt, with a lot of time spent in the library. Oxborrow says he personally read more than 200 scientific papers. “We stitched things together. We took one magic parameter from one paper, and another from a different paper, and by looking at all the different possibilities were able to work out the properties of p-terphenyl doped with pentacene. When I put them into what I call the ‘threshold equation’ or ‘viability equation’ that determined whether they would work or not, it looked like they would. It wasn’t a shot in the dark: we didn’t just try every chemical on the shelf.”
When the experiment worked, the scientists were ecstatic. “We were jubilant,” says Oxborrow. “It was still a long shot. It seemed too good to be true. We thought we were competent, we thought we had done the maths properly, but still we couldn’t really believe our luck – and we couldn’t believe that someone else hadn’t done it before.”
That preparation helped, but there is more work to do, he says. “Sometimes you’ve got to do your homework before you can start exploiting things. I think the maser we have reported has to be improved, and there are various ways in which we believe we
can improve it.”
