Up- and down-conversion photoluminescence properties of Ho2O3 doped YbAG single crystals prepared by optical floating zone method

Abstract

Single crystals of garnets with composition Yb_2.96-xY_xHo_0.04Al₅O₁₂ (x = 0, 2.92, 2.96) were grown by optical floating zone method, and shown by XRD measurements to be in the cubic phase. EDS spectra show that Ho³⁺ and Yb³⁺ were homogenously distributed in the structure. As shown by the transmission and absorption spectra, the Yb_2.96-xY_xHo_0.04Al₅O₁₂ crystals are highly transparent and can function as excellent optical crystals. Compared to the Y_2.96Ho_0.04Al₅O₁₂ crystal, the Yb_2.96Ho_0.04Al₅O₁₂ crystal has an additional strong broad absorption in the 836–1070 nm range, which increases its absorption of 980 nm photons. The optical band gaps of the Yb_2.96-xY_xHo_0.04Al₅O₁₂ crystals decrease with increasing Yb₂O₃ concentration. Furthermore, differences with Yb₂O₃ concentrations in the Yb_2.96-xY_xHo_0.04Al₅O₁₂ crystals were observed for the first time in the up- and down-conversion photoluminescence. Under excitation with a 980 nm laser, the up-conversion PL spectra of the Yb_2.96-xY_xHo_0.04Al₅O₁₂ crystals show several emission peaks at 548 nm, 665 nm and 775 nm, and their intensities increase with Yb₂O₃ concentrations. In the Yb_2.96Ho_0.04Al₅O₁₂ crystal, Yb³⁺ not only is a component of the garnet structure, but it also acts as a sensitizer. In regards of down-conversion, under excitation at 454 nm, the down-conversion PL spectra of the Yb_2.96-xY_xHo_0.04Al₅O₁₂ crystals show strong emission peaks at 550 nm, and their intensities decrease with increasing Yb₂O₃ concentrations, which is considered to be the consequence of increasing matrix density.