Optical Excitation and Relaxation of Solids with Defects

Abstract

As color centers in alkali halides are the simplest of all the point defects in solids, they have been studied as a model-case since the beginning of this century. Color centers display an optical cycle that very often produces luminescence having a very high efficiency after excitation and relaxation. This optical characteristic has been utilized for the last 20 years to produce color center lasers, with a high spectral purity emission tunable from the visible to the near infrared. Among the laser active color centers, the FA family is the most popular, with its very peculiar linear and nonlinear optical properties. Moreover, it is still supplying new experimental data that imply energy-trans fer phenomena not contemplated by previous accepted models. An accurate description of the FA centers in alkali halides is given together with a historical account of successes and failures, the latter due to chance human events, to limited technical choices, and to a lack of scientific curiosity.