Mechanisms of the blue emission of NaYF₄:Tm³⁺ nanoparticles excited by an 800 nm continuous wave laser

A thorough understanding of energy transfer and upconversion (UC) processes between trivalent lanthanide (Ln³⁺) ions is essential and important for improving UC performance. However, because of the abundant energy states of Ln³⁺ ions, UC mechanisms are very complicated, which makes it a challenge to exclusively verify and quantitatively evaluate the dominant process. In this study, the fundamental excitation processes of Tm³⁺-doped NaYF₄ nanocrystals under 800 nm continuous wave (CW) laser excitation were experimentally investigated on the basis of the quantum transition principle. An 800 nm CW laser combined with other wavelength CW lasers, including 471 nm, 657 nm, 980 nm, and 1550 nm lasers, were designed to study in-depth the excitation processes of UC luminescence via simultaneous two-wavelength laser excitation. The results indicate that the excited state absorption of ³H₆ → ³H₄ ∼∼ ³H₅ → ¹G₄ is the dominant pathway of the 481 nm and 651 nm emission bands, and two kinds of energy transfer UC pathways, uniformly expressed as ¹G₄ + ³H₄ → ¹D₂ + ³F₄, play the primary roles in the 456 nm emission band.