Frontiers Physics World  December 2017

Cryo-free maser runs continuously at room temperature

Candid camera Masers could someday be used in security scanners. (CC BY-SA 3.0 / Brijot)

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 a laser, in which coherent electromagnetic radiation is emitted when stimulated electrons relax to a lower energy level and a resonant cavity then amplifies the light. But while the laser has revolutionized the world, 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 made these early devices bulky and low in power, but a big step forward occurred two years later with the development of the solid-state maser, which used a crystal of cryogenically cooled ruby as the cavity.

Although masers have proved useful in radio telescopes and atomic clocks, they need to run at very low temperatures, which has made them impractical for everyday technology such as airport body scanners. In 2012, however, Mark Oxborrow of the UK’s National Physical Laboratory, along with Jonathan Breeze and Neil Alford of Imperial College London, devised a new maser scheme in which an optical laser “pumped” electrons from a lower energy level to a higher one in a soft polymer – p‑terphenyl – doped with pentacene.

Their device could operate at room temperature, but worked only in the pulsed regime, whereas many maser applications, such as microwave detectors, require continuous-wave operation. Moreover, p‑terphenyl is a poor thermal conductor, limiting its ability to dissipate the heat generated by non-radiative decay processes. The material also melts at just 230 °C, meaning that even if an organic maser could operate continuously, it would be rapidly destroyed.

Now, however, Breeze, Alford and colleagues 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 sub-levels of the ground state. By applying a moderate magnetic field to the NV centres, the researchers manipulated the sub-levels’ energies so that the electrons most commonly decayed into a state lying above another sub-level.

The resulting population inversion between the bottom two sub-levels leads to maser emission, with the researchers able to tune the energy gap – and thus the maser frequency – by tweaking the magnetic field. Being so stable, the maser was operated continuously for up to 10 hours with no drop in output (arXiv:1710.07726).

Tim Wogan