The first continuous-wave, solid-state maser to operate at room temperature has been created by researchers in the UK. The diamond-based device could lead to the development of ultra-sensitive microwave amplifiers that need no cryogenic cooling. Such devices could have a wide range of applications including security scanning and medical imaging.

A maser is essentially a microwave version of the laser. It preceded the laser and was also crucial to its development. While the laser has revolutionized technology from telecommunications to industrial cutting, however, the demanding operating conditions of masers has limited their practical use.

The original masers – invented in 1958 – were based on microwave transitions in atoms or molecules in a vacuum chamber. The vacuum requirement makes these devices bulky, and their power is very low. In 1960, a significant advance came with the development of the solid-state maser, which used a crystal of cryogenically cooled ruby as the cavity. Although masers have been useful in radio telescopes and atomic clocks, the need to run at very low temperatures makes them impractical for use in everyday technology such as airport body scanners.

Too hot to handle

In 2012, Mark Oxborrow of the National Physical Laboratory and Jonathan Breeze and Neil Alford of Imperial College London devised a new maser scheme in which a soft polymer – p-terphenyl doped with pentacene – was pumped with an optical laser. This could operate at room temperature, but there was a problem: their device worked only in the pulsed regime, whereas many maser applications such as microwave detectors require continuous-wave operation. Moreover, p-terphenyl is a very poor thermal conductor   which would limit its ability to dissipate the heat inevitably generated by non-radiative decay processes   and its melting point is only 230 °C. Therefore, even if an organic maser could operate continuously, such operation might rapidly destroy the device.

Now Breeze, Alford and colleagues at Imperial College have implemented a similar scheme in a maser cavity made from synthetic diamond impregnated with negatively charged nitrogen-vacancy (NV) centres. Diamond is an ideal medium because it has the highest recorded thermal conductivity of any material.

Laser pumping drives electrons into an excited state that rapidly decays to one of three spin sublevels of the electronic ground state. By applying a moderate magnetic field to the NV centres, the researchers manipulated the sub-levels' energies such that the state into which the electron most commonly decayed was above another sub-level. This allowed the laser pumping to create a population inversion between the bottom two sub-levels and therefore maser emission. The energy difference between the two sub-levels, and thus the maser frequency, could be tuned by the magnetic field. The system's stability allowed the researchers to operate their maser continuously for up to 10 hours with no degradation in its output.

Challenges overcome

The researchers are unable to speak to Physics World about its work because it has been submitted to a journal with an embargo policy. However, Pauli Kehayias of Harvard University in the US is enthusiastic about the research. "As a PhD student I was excited about the precursor work to this," he says, "I thought about whether an NV diamond maser was possible, but was discouraged after realizing the technical challenge and other disadvantages. I’m pleased to see that an NV diamond maser actually works!”

The maser is described in a preprint on the arXiv server.