Flying upconversion fluorescent particles and direct observation of energy transfer and depopulation processes

Flying particles with visible fluorescence were observed when a target, ZnF₂:Tm³⁺/Yb³⁺, was irradiated by using a 953.6 nm pulsed infrared laser. Narrow blue beams showed the ejecting particles with upconversion emissions. The changing brightness along fluorescence traces exhibited the energy transfer from Yb³⁺ ions to Tm³⁺ ions and the depopulation of excited Tm³⁺ ions. Parameters of microexplosions induced by the pulsed laser were estimated by analyzing these flying particles. Compared with the target, these particles have strong infrared-to-ultraviolet upconversion ability. These results not only suggest a way to explore microexplosions induced by pulsed lasers but also provide a feasible scheme for developing ultraviolet upconversion materials.