Growth, structure, spectroscopic, and laser properties of Ho-doped yttrium gallium garnet crystal

We report on the growth of a 2.86 at.% Ho:YGG crystal using the optical floating zone technique in an oxygen-rich environment, followed by the study of its structure, optical spectroscopy and first demonstration of continuous-wave laser operation at 2.1 µm. The crystal structure refined through the Rietveld method reaffirms the cubic symmetry (sp. gr. Ia3¯d, a = 12.2796(9) Å). The absorption, stimulated-emission (SE), and gain cross-sections for the ⁵I₈ ↔ ⁵I₇ Ho³⁺ transition were determined from the spectroscopic studies. The SE cross-section, σ _SE, amounts to 9.5 × 10^-21 cm² at 2084 nm, and the luminescence lifetime of the ⁵I₇ state is 8.94 ms. Among the Ho-doped garnets, Ho:YGG is characterized by smoother gain cross sections and a long luminescence lifetime. Laser operation of the Ho:YGG crystal is showcased using a 1908 nm Tm-fiber laser as a pump source. The in-band pumped Ho:YGG laser delivers 976 mW at 2085 and 2109 nm with a slope efficiency of 50.1%.