DC-powered Fe3+: Sapphire maser and its sensitivity to ultraviolet light

Alas, all negative! The 385 nm LED inside the copper can did not function (no light) when cold. The only noticeable effect upon injecting 385 nm radiation via the plastic light pipe was a slow transient shift in the maser frequency (taking several seconds to complete) consistent with heating. The optical power at 255 nm shone onto the sapphire crystal was estimated to be 1.63 mW (see Table 4 below). This could be turned on and off within a few tens of ms with a mechanical shutter. No substantial difference in the maser signal power could be detected when toggling between an open and closed shutter. The results of these optical-illumination experiments are, needless to say, preliminary. Using lasers or installing an optical build-up cavity would drive the excitation to t_2g(π*) harder; synchronous detection would help to detect a weak effect. It is both salutary and intriguing to note that the ESR strengths of Cr³⁺:sapph. (i.e. ruby) were not altered upon irradiating it with sufficient ⁶⁰Co γ rays to turn it orange in colour [40].