The effect of the crystal field strength on the optical spectra of Cr3+ in gallium garnet laser crystals

We have investigated the basic spectroscopic properties of Cr3+-doped gallium garnets. Weak crystal fields (Dq/B=2.39–2.55) lead to broad4 T 2→4 A 2 fluorescence in the 700–950 nm range depending on the chemical composition of the crystals. At room temperature the4 T 2→4 A 2 transition is homogeneously broadened, whereas the 4 K spectra of the2 E→4 A 2 transition show inequivalent Cr3+ sites and inhomogeneous linewidths of 8 cm−1 in these laser crystals. From 300 to 4 K, the oscillator strength of the4 T 2→4 A 2 transition is reduced by a factor of about 0.6 due to the temperature dependent electron phonon coupling. The transition probability of the2 E→4 A 2 transition correlates with the energetic position of the4 T 2 level, which is admixed into the2 E level via spin-orbit coupling. The comparison of the2 E→4 A 2 phonon sideband structure in garnets of different chemical compositions allows the assignment of some peaks to GaO4-tetrahedron modes of the garnet structure.