  When Th nuclei are doped in CaF$_2$ crystals, a set of electronic defect
states appears in the crystal bandgap which would otherwise provide complete
transparency to vacuum-ultraviolet radiation. The coupling of these defect
states to the 8 eV $^{229m}$Th nuclear isomer in the CaF$_2$ crystal is
investigated theoretically. We show that although previously viewed as a
nuisance, the defect states provide a starting point for nuclear excitation via
electronic bridge mechanisms involving stimulated emission or absorption using
an optical laser. The rates of these processes are at least two orders of
magnitude larger than direct photoexcitation of the isomeric state using
available light sources. The nuclear isomer population can also undergo
quenching when triggered by the reverse mechanism, leading to a fast and
controlled decay via the electronic shell. These findings are relevant for a
possible solid-state nuclear clock based on the $^{229m}$Th isomeric
transition.
