Highly efficient ultraviolet and visible light emissions from upconversion transitions in Ho/Tm/Yb co-doped lutecia-stabilized zirconia single crystals excited by a 980 nm laser

High-quality transparent cubic lutecia-stabilized zirconia (LSZ) and LSZ:Yb_0.25Tm_0.5Ho_0.03 single crystals were fabricated by the OFZ (optical floating zone) method for the first time. For comparison, YSZ and YSZ:Yb_0.25Tm_0.5Ho_0.03 single crystals were also fabricated. The transmittance of the LSZ single crystal (80.7%) is slightly lower than that of the YSZ single crystal (86.3%) in the visible light region, which is attributed to the LSZ single crystal with a higher density (6.604 g cm⁻³) in comparison with that of the YSZ single crystal (5.973 g cm⁻³). No absorption peaks were observed in the transmission and absorption spectra of LSZ and YSZ within 200–1100 nm, indicating that both crystals are excellent host crystals. The density of LSZ:Yb_0.25Tm_0.5Ho_0.03 single crystals (6.596 g cm⁻³) is also higher than that of YSZ:Yb_0.25Tm_0.5Ho_0.03 single crystals (6.005 g cm⁻³). The absorption spectra of LSZ:Yb_0.25Tm_0.5Ho_0.03 and YSZ:Yb_0.25Tm_0.5Ho_0.03 single crystals exhibited several absorption peaks at 363, 453, 678, and 785 nm and a strong broad absorption band within 850–1000 nm. The upconversion photoluminescence (UCPL) spectrum of the LSZ:Yb_0.25Tm_0.5Ho_0.03 single crystal excited at 980 nm exhibited several emission peaks at 308, 373, 488, 540, 550, 655 and 794 nm; similar emission peaks were observed in the UCPL spectrum of the YSZ:Yb_0.25Tm_0.5Ho_0.03 single crystal. The intensities of the emission peaks of the former are higher than those of the corresponding peaks of the latter, which is due to the former having a higher density and lower phonon energy in comparison with the latter. Power curves indicated that the 308 and 373 nm emissions are 4-photon upconversion processes, the 488 nm emission is a 3-photon process, and the 540 nm and 655 nm emissions are 2-photon processes. These results indicate that under the excitation at 980 nm, the LSZ:Yb_0.25Tm_0.5Ho_0.03 single crystal has high efficiency over the YSZ:Yb_0.25Tm_0.5Ho_0.03 single crystal in ultraviolet and visible light emissions.