Opt. Lett. 38, 4320–4323 (2013)

Researchers in the USA and the UK have produced a whispering-gallery-mode resonator made from extremely high purity diamond. By minimizing leakage of impurities into the synthesis chamber and using high-purity gases, they produced a diamond containing only about 20 ppb of nitrogen impurities. The resonator made from this diamond had a Q-factor of 2.4 × 107, which is two orders of magnitude higher than the previous highest Q-factor for a monolithic diamond resonator. The measured Q-factor corresponds to an absorption coefficient of 4 × 10−3 cm−1. Previous measurements of the absorption coefficient of diamond have been dominated by scattering losses from surfaces, and thus do not reflect the true absorption of bulk diamond. In contrast, the researchers estimate that surface scattering contributes only about 10% to their measured absorption coefficient. Further modest improvements to the diamond purity should enable such resonators to be used in devices such as ultrastable optical etalons, cavity-stabilized lasers and optoelectronic radio-frequency oscillators.