Infrared-to-green upconversion luminescence and mechanism of Ho³⁺, Nd³⁺ and Yb³⁺ ions in oxyfluoride glass ceramics

New oxyfluoride glasses and glass ceramics co-doped with Nd³⁺, Yb³⁺ and Ho³⁺ were prepared. The upconversion of infrared radiation into green fluorescence has been studied for Nd³⁺, Yb³⁺ and Ho³⁺ in the transparent oxyfluoride glass ceramics. At room temperature very strong green upconversion luminescence due to the Ho³⁺: (⁵F₄, ⁵S₂)→⁵I₈ transition under 800 nm excitation was observed in the glass ceramics. The intensity of the green upconversion luminescence in a 1mol% YbF₃-containing glass ceramic was found to be about 120 times stronger than that in the precursor oxyfluoride glass. The reason for the highly efficient Ho³⁺ upconversion luminescence in the oxyfluoride glass ceramics is discussed. The upconversion mechanism is also investigated.