Multibeam Borrmann effect in crystalline gamma-ray lasers

The effects of crystallinity are of crucial importance in considerations regarding gamma-ray lasers, bringing in new elements of laser theory which could well prove decisive in determining whether lasing can or cannot be achieved. Certain multibeam Borrmann eigenmodes of the radiation field within a perfect crystal containing resonant Mossbauer nuclei exhibit much less photoabsorption and much stronger coupling to the nuclei (provided the nuclear transition is of multipolarity M1 or higher) than do the single plane wave channels of an amorphous system (or equivalently, off-Bragg in the crystal). These modes offer the possibility of achieving lasing with orders of magnitude less population inversion density than would otherwise be the case, provided a nondestructive means is found to achieve a population inversion. The radiation in these modes would emerge from the crystal in a number of well-defined beams, each collimated by Bragg diffraction to an angular divergence of from 10-5to 10-4 rad. The properties of these multibeam modes as they relate to gamma-ray lasers, as well as the effects of crystallinity on the optics of systems which decay by superradiant rather than stimulate demission, are discussed.