Cascade Stimulated Emission in Crystals with Several Metastable States of Ln³ Ions

Abstract

Compounds of Pr³⁺, Ho³⁺, Er³⁺ and Tm³⁺ have become of great interest in studies of oxide and fluoride laser crystals. Such activators possess several metastable states allowing stimulated emission (SE) to be excited at great number of intermultiplet transitions. For example, using Ho³⁺ ions, 13 generation channels were observed in the range from 0.55 to 3.94 µm [1, 2]. The other interesting peculiarity of systems with several metastable states, which was first noticed [3, 4], is that SE can also be excited in a series of cascade transitions. As shown by many experiments this new principle of laser action can very effectively shift the IR boundary of lasing in insulating crystals, the energy characteristics of lasing being improved. Nowdays, more than 15 casacde schemes were achieved using oxide and fluoride crystals activated by Ln³⁺ ions [1–16]. In particular such media were first shown to exhibit lasing in the mid-IR-range from 3.4 to about 5.15 µm [9, 12, 14, 16].