Comparative study of the spectroscopic properties of Cr-doped LiAlO2 and LiGaO2

A detailed spectroscopic study of the optical characteristics of the tetrahedrally coordinated Cr⁴⁺ ion in LiAlO₂ and LiGaO₂ is given. From absorption and excitation measurements the crystal field parameter Dq and the Racah parameter B were determined to be Dq=1065 cm⁻¹, B=450 cm⁻¹, and Dq/B=2.4 for LiAlO₂ and Dq=1055 cm⁻¹, B=428 cm⁻¹, and Dq/B=2.5 for LiGaO₂. For the Racah parameter C only a lower limit can be given, i.e. 2417 cm⁻¹ for LiAlO₂ and 2667 cm⁻¹ for LiGaO₂. Due to the strong crystal field splitting — caused by the low site symmetry — the ³B(³T₂) crystal field component is the metastable and thus the emitting level. In the low-temperature absorption and emission spectra the expected three spin–orbit components of the ³B level are found at 8273, 8296, and 8300 cm⁻¹ for Cr⁴⁺:LiAlO₂ and 8610, 8623, and 8632 cm⁻¹ for Cr⁴⁺:LiGaO₂. The emission lifetime of Cr⁴⁺ in LiAlO₂ is 95 μs at 10 K and single exponential. In Mg-codoped LiAlO₂ and in LiGaO₂ the Cr⁴⁺ decay is double exponential. In Cr,Mg:LiAlO₂ two centers can be clearly distinguished, while in Cr:LiGaO₂ a variety of centers are observed, probably due to different charge compensation processes between Li, Ga, and Cr. The quantum efficiencies at room temperature are 42% for Cr:LiAlO₂ and 23% for Cr:LiGaO₂. Already at low temperature nonradiative decay processes occur. The temperature dependence of the lifetimes were analyzed with the model of Struck and Fonger. Excited state absorption measurements indicate that in the spectral region of the emission the excited state absorption cross-section is larger than the stimulated emission cross-section. Therefore laser oscillation is unlikely in these systems.