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as being analogous to that between transistors and valves, and the way the development of the transistor effectively superseded valve technology in the 1950s.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.”