Optical spectroscopic investigation of Ba3Tb(PO4)3 single crystals for visible laser applications

Abstract

Optical spectroscopic characterization of Ba3Tb(PO4)3 single crystals grown by the Czochralski technique was performed in order to evaluate the potential for visible laser application. Powder X-ray diffraction patterns at different temperatures were recorded to calculate the thermal expansion for temperatures between 298 and 1073 K. Refractive indices in the visible spectral region were determined and fitted into a Sellmeier equation. Absorption spectra were measured in the UV and visible range to obtain the ground-state absorption cross-sections, which were used for a Judd-Ofelt analysis. Electrostatic and spin-orbit interaction parameters were fitted, allowing for more precise Judd-Ofelt calculations. The Judd-Ofelt intensity parameters were calculated and employed to study the radiative transition probabilities, branching ratios and radiative lifetime associated to the potential upper laser level 5D4. The fluorescence spectrum shows several lines in the visible range, which can be assigned to 5D4 → 7FJ (J=6,5,4,3,2,1,0) transitions of Tb3+. These data enabled the determination of the branching ratio of the transitions from the 5D4 excited state. The stimulated emission cross-section of the 5D4 → 7F5 transition was found to be 5.9 × 10−22 cm2 at a wavelength of 549.6 nm by using the Füchtbauer-Ladenburg equation. Fluorescence decay curves were recorded to obtain the fluorescence lifetime. The resulting value of 3.17 ms is in reasonable agreement with the theoretical value of 3.54 ms pointing towards a high radiative quantum efficiency of 90% despite a high doping ion density of 3.5 × 1021 cm−3 and revealing the low effect of detrimental inter-ionic processes in Tb3+.