Nature Commun. 6, 8251 (2015)

Credit: NPG

Applications for masers, the microwave predecessor to the laser, are hindered by the need to operate such devices under conditions or either low temperature or high vacuum. A solid-state maser that operates at room temperature is thus highly desirable. Although a room-temperature pulsed version was reported in 2012, scientists are still seeking a device that will support continuous-wave operation. Now, Liang Jin and co-workers from the Chinese University of Hong Kong in China and the University of Stuttgart in Germany have theoretically proposed that it could be feasible to make such a device from diamond. Their idea is to use an ensemble of nitrogen–vacancy (NV) centre spins in diamond resonantly coupled to a high-quality (Q ∼ 105) microwave sapphire resonator cavity. The Chinese and German scientists considered a 3 × 3 × 0.5 mm3 diamond crystal with an NV centre concentration of 2 ppm located inside a cylindrical sapphire dielectric resonator (radius of 15 mm) loaded in a coaxial cylindrical cavity (radius of 40 mm). The set-up was exposed to a magnetic field of 2,100 G such that the transition frequency between the ground and excitation states was resonant with the cavity mode frequency around 3 GHz. The scientists calculated the spin inversion, the microwave output power gain and the noise temperature and found that the threshold of 532 nm pump power for masing was 4.3 W. For the pump power less than 10 W, a maser output power greater than 10 nW was predicted with a coherence time of minutes.