Far-infrared properties of resonant modes and tunnelling states in rare-earth-doped calcium fluoride

The low-temperature observation of a broad distribution of two-level systems in chemically disordered fluorite mixed crystals has heightened interest in the spectroscopic properties of the corresponding lightly doped crystals. The low-temperature far-air absorption of calcium fluoride crystals doped with Y³⁺, La³⁺ or Lu³⁺ in the range less than 1 mol% shows three completely different types of defect-induced spectra and the temperature and concentration dependence are found to vary widely. In addition, the far-ir properties of CaF₂ doped with Y³⁺ or Lu³⁺ can be dramatically altered by heat treatment, indicating that clustering plays an important role. By comparing the spectroscopic signatures with those previously observed for other defect systems, the lowest-lying excitations for these doped CaF₂ crystals have been assigned as follows: tunnelling transitions in two different elastic configurations for Y³⁺, a variety of ground state tunnelling transitions for the La³⁺ doping and an anharmonic resonant mode for the Lu³⁺ doped system. The surprising result is that a broad distribution of two-level systems is clearly evident in two of the three systems and the optical activity of this distribution appears to be controlled by the optical activity of the dominant defect dynamics.