Abstract

Tunable solid-state lasers in the spectral region between 600nm and 700nm are interesting for a variety of applications and extend the tuning range of the Ti-sapphire laser (700nm - 1000nm) to shorter wavelengths. A potential laser active transition metal ion is the trivalent manganese ion. It has a 3d⁴ electron configuration with the ⁵E level as ground state. The energy level splitting as a function of the crystal field strength is given by the Tanabe-Sugano diagram, see Fig. 1. The absorption spectrum, shown in Fig. 2 for Mn³⁺:Gd₃Ga₅O₁₂, is dominated by the spin allowed transition into the ⁵T₂ energy level around 520nm. Since the emission spectrum, also depicted in Fig. 2 for Mn³⁺:Gd₃Ga₅O₁₂, exhibits more than one emission band, a thermal coupling between different excited energy levels has to be assumed, most likely between the ⁵T₂ and the energetically lower ¹E and ¹T₂ levels. All these energy levels are strongly dependent on the crystal field strength, i.e. the intensity ratio of the different emission channels can be influenced by the host crystal. Also the metastable level lifetimes vary in the different hosts due to the different thermal population in the energy levels, e g. for Mn³⁺:Y₃Al₅O₁₂ it is 0.97ms, for Mn³⁺:Gd₃Ga₅O₁₂ 1.7μs. For the realization of tunable lasers based on Mn³⁺ doped crystals, excited state absorption processes have to be taken into account. In dependence of the crystal field strength, either the ⁵T₂ level or one of the singlet levels (¹E and ¹T₂) are energetically lower. Since no higher lying quintet levels exist, only spin-forbidden transitions from the ⁵T₂ level are present, which are expected to be at least one order of magnitude weaker than the spin-allowed fluorescence transition into the ground state In the crystals under investigation, the singlet levels are energetically lower and therefore excited state absorption transitions into higher singlet levels interfere with the emission process. Host crystals with lower crystal field strengths or anisotropic crystals with polarization dependent transition probability could be used to avoid this problem. In this paper, the influence of different crystalline hosts on the spectroscopic characteristics of the Mn³⁺ ion will be presented with respect to the realization of a tunable solid-state laser in the red spectral range.

© 1996 IEEE