Ultraviolet and violet upconversion luminescence in Ho³⁺-doped Y₂O₃ ceramic induced by 532-nm CW laser

Ultraviolet and violet upconversion luminescence spectra of holmium-doped Y₂O₃ were produced under the excitation of a compact continues-wave 532 nm solid-state laser. Emissions around 306, 362, 412, 390 and 428 nm can be assigned to the transitions of ³D₃ → ⁵I_J (J = 8, 7, 6), ⁵G₄ → ⁵I₈ and ⁵G₅ → ⁵I₈, respectively. Power dependence and upconversion dynamics analysis demonstrated that both the energy transfer upconversion (ETU) and the excited state absorption (ESA) processes were involved in the population of ³D₃ state via the coupled intermediate states ⁵S₂/⁵F₄. Fluorescence spectra in the visible and infrared ranges showed that ⁵G₄ and ⁵G₅ states were populated by the ESA process from ⁵I₆ and ⁵I₇ states, respectively, while the ⁵I₆ and ⁵I₇ states were radiatively populated from the excited ⁵S₂/ ⁵F₄ states. Upconversion mechanisms have been evaluated based on a rate equation model.