Crystal Growth and Characterization of Terbium-Based Borate Crystals of Sr$_{3}$Tb(BO$_{3}$)$_{3}$, Li$_{6}$Tb(BO$_{3}$)$_{3}$, and TbCa$_{4}$O(BO$_{3}$)$_{3}$: Color Centers, Spectroscopic Properties, and Optical Gain

International audience; This work aims to evaluate the prospect for visible laser applications of three terbium-based borate single crystals, viz. Sr$_{3}$Tb(BO$_{3}$)$_{3}$ (STB), Li$_{6}$Tb(BO$_{3}$)$_{3}$ (LTB), and TbCa$_{4}$O(BO$_{3}$)$_{3}$ (TbCOB). Crystals with good optical quality were grown by the Czochralski technique. Formation of color centers was observed in the crystals grown in air atmosphere and was then circumvented by applying a reducing atmosphere during the growth process. The color centers were further studied by absorption and electron paramagnetic resonance spectroscopic characterization. Thermal properties that can impact laser operation, including thermal conductivity, specific heat capacity, and thermal expansion, were characterized using the as-grown crystals. The principal refractive indices of STB and LTB were measured experimentally. Cross sections of the 4f$^{8}$ → 4f$^{8}$ transitions of Tb$^{3+}$ in these crystals were derived from the recorded absorption and emission spectra as well as the Judd–Ofelt calculations. The fluorescence decay curves from the $^{5}$D$_{4}$ excited state were recorded to yield the fluorescence lifetimes. Experimental results and theoretical calculations indicate that these terbium-based crystals have minor nonradiative processes from the $^{5}$D$_{4}$ manifold, and the transition cross sections in the visible spectra range are comparable to those of the previously reported gain media. Lastly, optical gain at 543 nm was measured in these crystals in a pump–probe scheme.