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In the development of materials with valuable luminescence properties, we have used the optical floating zone method to prepare single crystals in the cubic phase of yttria-stabilized zirconia (YSZ) doped with 0.75 mol% Ho2O3 and various concentrations of Yb2O3. The influence of Yb3+ contents was determined for absorption and photoluminescence spectra, and fluorescence decay curves at room temperature. The absorption spectra consisted of several peaks that could be assigned to various transitions involving Yb3+ and Ho3+ ions, and oscillator strength parameters Ωt (t = 2, 4, 6) were determined using the Judd-Ofelt theory, along with the transition probability, lifetime, and branching ratio from the absorption and emission spectra of the crystal of YSZ co-doped with 0.75 mol% Ho2O3 and 1.5 mol% Yb2O3. The Ω2 value of ~14 × 10–20 cm2 indicates that the bonding around Ho3+ involves lower symmetry and higher covalency than in several other hosts, and the value of the ratio Ω4/Ω6 of 1.93 shows that this sample is a good candidate material for high quality laser output. Up-conversion emission spectra showed that Yb3+ doping significantly increased the emission intensity of 0.75 mol% Ho2O3 doped YSZ at green (557 nm), red (670 nm) and NIR (758 nm) wavelengths when excited by a 980 nm laser, and reached a maximum for 1.5 mol% Yb2O3. The emission at 557 nm was much sharper than that reported for nanocrystals of similar composition, where the maximum intensity was observed at 540 nm. Down-conversion PL spectra showed two prominent emission peaks at 1040 nm and 1213 nm, and the fluorescence decay curves indicate that energy transfer between Yb3+ and Ho3+ influenced the lifetime of the excited state of Ho3+ which varied with Yb2O3 concentration. Overall these results indicate that these Yb/Ho co-doped YSZ crystals have considerable potential for laser and luminescence applications.

Rubies (Cr: Al2O3 single crystals) with 0.025–0.4 mol% Cr2O3 were prepared by the optical floating zone method, and characterized by X-ray diffraction (XRD), electron paramagnetic resonance (EPR) spectroscopy, optical transmission and photoluminescence (PL). Crystals were free of inclusions and cracks, and had high transmittance in the visible region. In addition to the well-known signals from Cr3+ in the α-Al2O3 structure, the EPR spectra indicated the presence of very weak signals, which may correspond to paramagnetic centres associated with an oxygen vacancy, and an electron-loss oxygen centre, O−. The concentration of these paramagnetic centres was at a minimum for Cr2O3 concentration of 0.15 mol%, and additional Cr3+-derived peaks were also observed with Cr2O3 concentrations >0.15 mol%. The PL spectra also varied with Cr2O3 concentrations; the emission intensities of the PL spectra increased with Cr2O3 concentration for concentrations ≤0.15 mol%, then decreased with the further increases in Cr2O3. Thus the maximum luminescence emission intensity and the minimum concentration of paramagnetic structural defects were observed with 0.15 mol % Cr2O3, and the presence of additional interactions involving Cr3+ ions may contribute to the partial quenching of the luminescence at higher Cr3+ concentrations. Overall, the optical floating zone method represents an effective and economic approach to the preparation of high quality rubies, and their optimum optical properties were observed with Cr2O3 concentrations of 0.15 mol%.

High-quality yttria-stabilized zirconia (YSZ) single crystals doped with different concentrations of Er2O3 were grown by the optical floating zone method. The cubic phase of ZrO2 was retained when Er3+ partially replaced Y3+ in the YSZ matrix. Up-conversion luminescence was observed when the crystals were excited by a 980 nm laser diode. The optical power curve was obtained by plotting the up-conversion luminescence intensity against the laser power, and both the green and red light up-conversion were shown to involve two photons. Seven characteristic visible and three infrared absorption peaks were observed in the absorption spectrum of the 1.50 mol% Er2O3: YSZ. The up-conversion spectra produced three prominent peaks at 547(green), 561 (green) and 679 (red) nm. Green and red light maxima were observed with Er2O3 concentrations of 1.5 mol% and 0.5 mol%, respectively. Oscillator strength parameters Ωt (t = 2, 4, 6) were determined using the Judd-Ofelt theory and Ω2 = 9.90 × 10−20 cm2, Ω4 = 2.92 × 10−20 cm2, Ω6 = 3.32 × 10−20 cm2. The Ω4/Ω6 ratio (0.88), which is the key parameter for laser performance of reactive matrix materials, indicates that Er3+-doped YSZ has potential laser applications.

Single crystals of yttria-stabilized zirconia (YSZ) doped with different contents of holmium were prepared by the optical floating zone method, and their structures and phase composition were characterized by XRD and Raman spectroscopy. These showed that the metastable tetragonal phase was stabilized by the substitutions. Transmittance of each of the Ho3+-doped YSZ single crystals was larger than 85% in the visible region. Measurements of fluorescence spectra indicated a strong emission at a wavelength of 553.5 nm, and two weak emission peaks at 671 nm and 759 nm when the crystals were excited by light with a wavelength of 448 nm. The YSZ doped with 0.75 mol% of Ho2O3 had a relatively higher fluorescence intensity and possessed high color purity (99.7%) in the green region, indicating that it can be used to produce efficient green emission.