Crystal growth, spectroscopic and 2 μm mid-infrared laser performance of Tm: Gd3Ga3Al2O12 crystals

Tm³⁺ doped Gd₃Ga₃Al₂O₁₂ (GAGG) crystals with varying Tm³⁺ ion concentrations were grown by the Czochralski method. We have made a detailed analysis of the effect of Tm³⁺ concentration on the thermal and optical properties of the crystals. As the concentration of Tm³⁺ increases, the thermal conductivity of the crystals decreases from 6.29 W m⁻¹ K⁻¹ (4.1 at% Tm: GAGG) to 5.72 W m⁻¹ K⁻¹ (6.3 at% Tm: GAGG). Combined with Judd-Ofelt theory, we analyzed the absorption spectral parameters of GAGG with varying Tm³⁺ ion concentrations. The calculated spectral parameters: Ω₂ = 0.48 × 10⁻²⁰cm², Ω₄ = 1.18 × 10⁻²⁰cm², Ω₆ = 0.60 × 10⁻²⁰cm² (4.1 at% Tm: GAGG), Ω₂ = 0.51 × 10⁻²⁰cm², Ω₄ = 1.32 × 10⁻²⁰cm², Ω₆ = 0.84 × 10⁻²⁰cm² (6.3 at% Tm: GAGG) were used to obtain fluorescence branching ratios, radiative transitions and radiative lifetimes. The effect of Tm³⁺ concentration on fluorescence spectra was investigated, with the fluorescence lifetime decreasing from 8.82 ms to 8.45 ms. And the luminescent quantum efficiency also decreases from 90 % to 80.5 %, which is due to the enhancement of non-radiative phenomena. The 4.1 at% Tm: GAGG crystal possesses the largest fluorescence intensity near 1800 nm and the smooth spectral is also more favourable for obtaining tunable and pulsed laser output. We have demonstrated that 4.1 at% is the optimal Tm³⁺ concentration by continuous laser experiments, achieving a maximum output power of 2.79 W and a slope efficiency of 41.08 %.