In the new study, Eugene Tkalya from the Institute of Nuclear Physics at Moscow State University has theoretically proven how the stimulated gamma emission of thorium nuclei can emit coherent visible light. Although the nuclear gamma-ray laser emits light based on stimulated emission, it operates a bit differently than a normal laser.
well, no, not Gamma-ray. 162 nm UV; Thorium 229 has a nuclear excited state at just 7 kev. There are a few other atoms with a first excited state this low, too.
The trick is exciting a nuclei with that LITTLE energy. In this case, you can use a more random photon source.
It seems to me, though, that is you can find nuclei with excited states below their neutron or proton separation energy that you can turn a rather random energy source into a laser of the appropriate wavelength. Of course the absorption of photons by nuclei is a lot less common than that of absorption by electrons around the atoms, so your efficiency will be extremely low, unless your energy source is something screwy like low energy excited neutral particles. Say, 100 keV pi-0 mesons or something.