Statistical Time-Resolved Spectroscopic Study on Upconversion Luminescence

Lanthanide phosphors have attracted great interest because of their highly efficient photonic upconversion processes. The upconversion spectra and time-resolved relaxation are influenced not only by the chemical composition of phosphors but also by the excitation characteristics of incident radiation. Despite numerous reports that upconversion spectra strongly depend on the experimental conditions, little research has been reported on the relationship between the temporal profile of excitation and the resulting luminescence spectra. In this study, we introduced time- and frequency-domain pulsed modulation spectroscopy to investigate statistical energy accumulation and relaxation in a NaYF₄:Yb³⁺,Er³⁺ upconversion process. The modulation of a laser pulse width and interval showed that complete depopulation of hot phosphors required more than 5 ms. Furthermore, we demonstrated that the intermediate state (⁴I₁₃/₂) is populated in hot phosphors and affects upconversion luminescence.