Influence of Host Lattice Ions on the Dynamics of Transient Multiband Upconversion in Yb–Er Codoped NaLnF₄ and LiLnF₄ Microcrystals (Ln: Y, Lu, Gd)

Inorganic host matrices provide a tunable luminescence environment for lanthanide ions, allowing for the modulation of upconversion luminescence (UCL) properties. AREF₄ (A = alkali metal, RE = rare earth) have a low phonon energy and a high optical damage threshold, making them widely used as the host matrix for UCL materials. However, the impact mechanism of alkali metal ions and lanthanide lattice ions on transient UCL dynamics in AREF₄ remains unclear. This study utilized a high-power nanosecond-pulsed laser at 976 nm to excite Yb-Er codoped NaLnF₄ and LiLnF₄ (Ln: Y, Lu, and Gd) microcrystals (MCs). All samples exhibit multiband emission, and the transient UC dynamics are discussed in detail. Compared with LiLnF₄, NaLnF₄ has higher UC efficiency and red to green (R/G) ratio. Lanthanide ions (Y, Lu, and Gd) affect the energy transfer (ET) distance in Yb-Er codoped systems, thereby altering UC efficiency and the R/G ratio. The energy level coupling between Gd³⁺ and Er³⁺ prolongs the duration of the UC emission. Specifically, the red emission lifetime of NaGdF₄ is five times longer than that of NaYF₄. Our research contributes to exploring excellent alternative host matrices for NaYF₄ in the fields of rapid-response optoelectronic devices, micro-nano lasers, and stimulated emission depletion (STED) microscopy.