Laser-related spectroscopy of ion-doped crystals for tunable solid-state lasers

In this work an overview of transition metal (TM) ion- and rare earth (RE) ion-doped crystals for application as tunable solid-state lasers will be given. Spectroscopic and laser results will be presented including recent research and advances in this field. Within this work tunability is defined as the possibility to achieve laser oscillation in the vibronic sideband of a transition. Tunable solid-state lasers are of interest for a wide field of applications, e.g. in scientific research, in medicine, for measurement and testing techniques, ultra short pulse generation, and communication. They can also be used as coherent light sources for second-harmonic generation, for optical parametric oscillators, and for sum- and difference-frequency generation. Tunable laser media based on 3d ↔ 3d transitions of transition-metal ions and 4f ↔ 5d transitions of rare-earth ions cover nowadays almost the whole spectral range between 270 nm and 4500 nm, see Fig. 1 [1-15]. In comparison to laser systems based on the 4f ↔ 4f transitions of trivalent rare-earth ions, tunable lasers based on 3d ↔ 3d and 4f ↔ 5d transitions are in general affected by a higher probability of excited-state absorption (ESA), a higher probability of non-radiative decay, and a higher saturation intensity leading to higher laser thresholds. Often laser oscillation cannot be obtained at all. These general topics will be considered in Sect. 1, where the basic aspects of tunable solid-state lasers are discussed: these are the preparational, the spectroscopic, and the laser aspect. In Sect. 2, the investigation of transition metal ion-doped crystals with respect to the realization of tunable laser oscillation is presented. The work is focused on transition-metal ions of the 3d row (Fe row) and divided into two subsections according to the octahedral and tetrahedral coordinations of the ion investigated. Each subsection is structured according to the electron configurations: 3d1, 3d3, 3d4, and 3d8 for the octahedrally coordinated ions and 3d1, 3d2, and 3d4 for the tetrahedrally coordinated ions. Section 3 deals with interconfigurational transitions of divalent and trivalent rare-earth ions. Finally, in Sect. 4 the work is summarized.