ABSTRACT
A fundamental property of Ln3+ activator ions in insulating crystals is the availability of a large number of energy levels and hence different luminescence and absorption channels. This property provides wide-ranging possibilities for novel inquiries in the field of the interaction of radiation with activated crystalline compounds. A good example of the use of these practically inexhaustible possibilities for the realization of different versions of three- and four-level generation schemes was the discovery of the cascade principle of stimulated emission (SE) excitation and the development on this basis of a number of new operating schemes of crystalline lasers. 1,2
