Comparative study of the spectroscopic properties of Cr4+-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.