Upconversion Emission Mechanism In Holmium Doped Oxide And Fluoride Materials

The study of rare-earth doping upconversion (UC) materials has attracted a great deal of attentions, owing to the important applications in solid-state lasers, bio-probes, undersea-probes and display devices so on. The popular of high power infralaser diode promotes the investigation of short wavelength all-solid lasers with rare-earth doping upconversion materials. However, Nd³⁺, Er³⁺, Tm³⁺ ions were mainly used as activator in upconversion lasers nowadays. The Ho³⁺ ion emission whose intensity corresponds to that of Er³⁺ ion was neglect investigated. Furthermore, the Ho³⁺ ion has outstanding fluorescence character. Based on Ho³⁺/Yb³⁺ ions doped fluoride and oxide materials, we performed the investigations on disputable nearinfared UC emission mechanisms of Ho³⁺ ion, the ultraviolet UC emissions of Ho³⁺ ion, the energy transfer between Ho³⁺ and Gd³⁺ ions, UC emission enhancement of Ho³⁺ ion.The controversial nearinfrared UC fluorescence emission (754 nm) of Ho³⁺ were studied. Ho³⁺ and Yb³⁺ ions with different concentrations co-doping Y₂O₃,Gd₂O₃ bulk ceramics were synthesized by sol-gel method. The analysis of UC fluorescence emissions, luminescence lifetimes, the pumping power dependence of the UC emissions demonstrated that the NIR emission was associated with the 5F4/5S2→5I7 transition of Ho³⁺ ions for Ho³⁺/Yb³⁺ codoped Y₂O₃ and Gd₂O₃ bulk ceramics. Moreover, the steady-state rate equations provide theory explaination for the mechanism of NIR emission.Ultraviolet UC fluorescence emissions of Ho³⁺ ion in Y₂O₃ bulk ceramics were studied. The UC emission spectra of Ho³⁺ ion were obtained with the changing of Yb³⁺ concentration. The intensity of the ultraviolet UC luminescence centered at 360 nm always increased with Yb³⁺ concentration from 2 to 15 mol%. The max intensity of UC emissions centered at (554, 754) and (392, 428, 667) nm could reach when the Yb³⁺ ion concentrations are 5 and 11 mol%, respectively. This means that we can further increase the Yb³⁺ ion concentration to increase the intensity of the UV UC emission at 360 nm, and at the same time, decrease the intensity of other UC emissions.The investigation of energy transfer between Ho³⁺ and Gd³⁺ ions in Y₂O₃ bulk ceramics were carried out. The ultraviolet UC emissions of Gd³⁺ ion were studied by the analysis of the pumping power dependence of the UC emissions and the theoretical data, which indicated that Ho³⁺ ion could transfer energy directly to Gd³⁺ ions by a four-photon UC process.The enhancement of UC fluorescence emission intensities of Ho³⁺ ion were invesitigated in Y₂O₃ bulk ceramics. The enhanced UC flurescence emission intensities of Ho³⁺ ion were obtained with Gd³⁺ ion concentrations under a 976 nm diode laser excitation. The analyses of XRD, luminescence lifetimes and theoretical were used to propose the mechanisms for the enhancement. The significant increase of the UC emissions of Ho³⁺ ion was induced by lengthening the lifetimes of Ho³⁺ ion's energy levels involved.Enhanced ultraviolet UC fluorescence emissions of Ho³⁺ ion in fluoride synthesized by hydrothermal methord were studied. The energy transfer processes (ET) from Gd³⁺ to Ho³⁺ ions were confirmed by investigating the UC fluorescence spectra and the pumping power dependence of the UC emissions of Ho³⁺ and Gd³⁺ ions. The ET process makes the ultraviolet UC emission centered at 340 and 360 nm intensity enhance.