Divalent (Cr²⁺), trivalent (Cr³⁺), and tetravalent (Cr⁴⁺) chromium ion-doped tunable solid-state lasers operating in the near and mid-infrared spectral regions

The chromium ion is among the most widely explored and used laser-active ions. Depending on the charge state of the laser-active chromium ion in the host, tunable chromium-doped lasers can be categorized under three groups as Cr²⁺, Cr³⁺, and Cr⁴⁺ lasers. In this paper, we provide a comprehensive overview of recent research conducted with chromium ion-doped tunable solid-state lasers, covering primarily the last 2 decades. After an introduction of the key physical parameters of tunable solid-state lasers, recent experimental results obtained with Cr³⁺, Cr⁴⁺, and Cr²⁺ ion-doped lasers are presented with an emphasis on alexandrite (Cr³⁺:BeAl₂O₄), Cr:LiCAF (Cr³⁺:LiCaAlF₆), Cr:LiSAF (Cr³⁺:LiSrAlF₆), Cr:LiSGaF (Cr³⁺:LiSrGaF₆), Cr⁴⁺:forsterite (Cr⁴⁺:Mg₂SiO₄), Cr⁴⁺:YAG (Cr⁴⁺:Y₃Al₅O₁₂), Cr²⁺:ZnSe, and Cr²⁺:ZnS lasers. For each laser system, recent developments involving power scaling, different excitation schemes, continuous-wave power performance, Q-switched/gain-switched operation, mode-locked operation, and emerging scientific/technological/biomedical applications are discussed. The paper is concluded with an overall assessment of these laser media and discussion about possible directions of research in future.