Transition Metal Ion Lasers Other Than Cr3+

In this chapter, the background and status of transition-metal-based solid-state lasers are discussed [1,2]. The transition metal ions that have been demonstrated to lase include Ti³⁺, V²⁺, Cr²⁺, Cr³⁺, Cr⁴⁺, Mn⁵⁺, Fe²⁺, Co²⁺ and Ni²⁺. All of these ions will be discussed in this chapter with the exception of Cr³⁺. These transition metal ions have been demonstrated to laser oscillate in a cavity, by serving as the activators in various host media including oxide, fluoride, sulphide and selenide crystals. Laser oscillation has never been reported for any transition-metal-doped glass: all known laser systems are based on crystalline host media. They are incorporated onto sites of either fourfold or sixfold coordination, referred to as tetrahedral and octahedral sites, respectively. In terms of their practical use, Ti³⁺ in sapphire (Ti:Al₂O₃) is, by a wide margin, the most common transition metal laser. The next most common transition metal lasers include the Cr³⁺ lasers, alexandrite and Cr:LiSAF, forsterite (Cr⁴⁺ in Mg₂SiO₄) and Co:MgF₂. Several other materials are still being explored. Lasers based on V²⁺ and Ni²⁺ do not seem to have taken hold significantly, in spite of the many years that have elapsed since the initial demonstration of their lasing capability. Some ions have been reported to exhibit gain but an ability to oscillate has not yet been demonstrated, for example Rh²⁺ [3] and Cu⁺ [4], as discussed later.