Holmium doped calcium fluoride as a maser material

A requirement for a three level microwave maser is that transitions be allowed between nonadjacent energy levels. This can be accomplished by a strong hyperfine interaction which, like a noncubic crystalline field, mixes the spin states. Moreover, an energy doublet in a cubic field would have an isotropic Zeeman effect and the material could then be used in powdered form. EPR studies have shown that divalent holmium in CaF
2
has a strong hyperfine interaction and is in a field of cubic symmetry with an energy doublet lowest. This material may be used in powdered form in maser applications below 5 kMc. It is shown that Ho
²⁺
:CaF
2
is expected to have a figure-of-merit which is one-tenth that of ruby for equal concentrations of the paramagnetic ion. If further work verifies the advantage of using powder, it is possible that more favorable materials of this kind can be found.