Status of research in gamma-ray lasers

This article reviews conceptual devices designed to generate coherent high-quantum-energy radiation by stimulating nuclear transitions in solids; and delineates some of the promising areas of research. Solid state γ-ray lasers can benefit from (1) Moessbauer effect or recoilless emission, which suppresses first-order Doppler broadening, and brings the emitting nuclei close to a common resonant frequency; and (2) Borrmann effect, which suppresses non-resonant scattering and absorption, and may allow a crystal lattice to act as a distributed resonator. Some of the basic physics that governs the performance of conceptual γ-ray lasers, and some of the classic approaches to design are discussed. Many of these early schemes were shown to be unworkable decades ago, but re-emerge in different forms generally without attention to the fallacies already revealed. Other schemes have languished for lack of a champion. Some exciting new approaches are also described particularly those utilizing techniques for suppressing resonant absorption, which can realize gain without a true inversion. (author)