Upconversion luminescence mechanisms of Er³⁺ ions under excitation of an 800 nm laser

The fundamental processes of upconversion (UC) emissions from trivalent lanthanide ions are essential for designing and improving the performance of UC materials. However, UC mechanisms involve multiple processes in one system because of the abundant energy states, which make it a challenge to exclusively verify and quantitatively evaluate the dominant process. In this paper, the mechanisms of green and red emissions in Er³⁺ doped NaYF₄-glass ceramics under 800 nm continuous wave laser excitation are studied via excitation power-dependence, polarization-dependence, and the transient evolution. These UC emission properties are compared with those under single-/two-wavelength excitation in a pump-probe system of 800 nm and 1500 nm femtosecond lasers. These results indicate that the excited state absorption pathway of ⁴I_15/2 → ⁴I_9/2 → ⁴I_13/2 → ²H_11/2/⁴S_3/2 plays the dominant role in the emission of the green band, and the cross relaxation pathway of ⁴I_11/2 + ⁴I_13/2 → ⁴F_9/2 + ⁴I_15/2 plays the main role in the emission of the red band.