The effects of Er ^{3 +} ion concentration on 2.0-μ m emission performance in Ho ^{3 +} /Tm ^{3 +} co-doped Na ₅Y ₉F₃₂ single crystal under 800-nm excitation

Na₅Y₉F₃₂ single crystals doped with ∼ 0.8-mol% Ho^{3 +} , ∼ 1-mol% Tm^{3 +} , and various Er^{3 +} ion concentrations were prepared by a modified Bridgman method. The effects of Er^{3 +} ion concentration on 2.0-μ m emission excited by an 800-nm laser diode were investigated with the help of their spectroscopic properties. The intensity of 2.0-μ m emission reached to maximum when the Er^{3 +} ion concentration was ∼ 1 mol%. The energy transfer mechanisms between Er^{3 +} , Ho^{3 +} , and Tm^{3 +} ions were identified from the change of the absorption spectra, the emission spectra, and the measured decay curves. The maximum 2.0-μ m emission cross section of the Er^{3 +} /Ho^{3 +} /Tm^{3 +} tri-doped Na₅Y₉F₃₂ single crystal reached 5.26 × 10 ^-21 cm². The gain cross section spectra were calculated according to the absorption and emission cross section spectra. The cross section for ∼ 2.0-μ m emission became a positive gain once the inversion level of population was reached 30%. The energy transfer efficiency was further increased by 11.81% through the incorporation of Er^{3 +} ion into Ho^{3 +} /Tm^{3 +} system estimated from the measured lifetimes of Ho^{3 +} /Tm^{3 +} -and Er^{3 +} /Ho^{3 +} /Tm^{3 +} -doped Na₅Y₉F₃₂ single crystals. The present results illustrated that the Er^{3 +} /Ho^{3 +} /Tm^{3 +} tri-doped Na₅Y₉F₃₂ single crystals can be used as promising candidate for 2.0-μ m laser.