Spectroscopic Properties and Optical Clusters in Erbium-doped CaF₂, SrF₂ and PbF₂ Crystals

As a fundamental light source and a good window for atmospheric transmission, the mid-infrared 3 mu m lasers have led many promising applications. The rare earth doped crystalline materials, such as erbium doped crystals, are some of the most important routes for generation of the lasers. However, they have an intrinsic shortcoming of self-termination because of their short lifetime of I-4(11/2) and longer lifetime of I-4(13/2). To eliminate this effect, a high concentration doping method is usually adopted to change the energy transfer process to decrease I-4(13/2) lifetime. The efficiency and output power of Er3+-doped crystals were thus limited due to their degraded thermal properties. Trivalent erbium ions are easily clustering in fluoride crystals. Distances among the ions are short and therefore energy transfer processes could be significantly improved in the crystals even doping with low concentrations. Low doping concentrations could also alleviate the thermal effect in laser operations, which enable the erbium doped fluorides to be promising candidates for high power and high efficiency mid- infrared lasers. However, connection of spectral properties and erbium clusters is unknown. Here, the first principles calculation is utilized to model the erbium ion clusters in CaF2, SrF2 and PbF2 crystals, concerning the absorption and photoluminescence properties. The results reveal that spectral properties and structures of the erbium clusters, evolve gradually with matrix crystals. Relationship between spectral properties and optical erbium clusters is determined qualitatively, which could be used to design new erbium doped mid-infrared lasers.