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 μm lasers have aroused many promising applications.The rare earth doped crystalline materials are one of the most important routes for generation of the lasers.However, the self-termination occurs because of the short lifetime of 4 I11/2 and longer lifetime of 4 I13/2 in erbium doped crystals.In order to remove this effect, the high doping concentrations were usually adopted to enhance the energy transfer process to decrease the lifetimes of 4 I13/2 level.The efficiency and output power of Er 3+ -doped crystals were thus limited due to the degraded thermal properties.Trivalent erbium ions are easily clustering in fluoride crystals.Distances among the ions are short and energy transfer processes could be significantly improved in the crystals even with low doping concentrations.Low doping concentrations could also alleviate the thermal effect in laser operations, which make the erbium doped fluorides be promising candidates for high power high efficiency mid-infrared lasers.However, connection of the spectral properties and erbium clusters is unknown.In this work, the first principles calculation is utilized to model the erbium ion clusters in CaF2, SrF2 and PbF2 crystals, which is also combined with the absorption and photoluminescence properties.The results reveal that spectral properties, as well as structures of the erbium clusters evolve gradually with matrix crystals.The relationship of spectral properties and optical erbium clusters is determined qualitatively, which could be used to design new erbium doped mid-infrared lasers.