Temperature-dependent six-photon upconversion fluorescence of Er3+

Yb3+/Er3+ codoped β-NaYF4 microcrystals were synthesized through a facile EDTA-assisted hydrothermal method. Under 980 nm excitation, 244, 256, and 276 nm upconversion (UC) emissions were observed in NaYF4:Yb3+/Er3+ microcrystals, which were assigned to the 2I11/2 → 4I15/2, 4D7/2 → 4I15/2, and 4G9/2 → 4I15/2 transitions of Er3+ ions, respectively. Successive energy transfers (ETs) from Yb3+ to Er3+ played crucial roles in populating the high-energy states of Er3+ ions. Power dependence analysis exhibited that 244 and 256 nm UC emissions came from six-photon processes. Temperature-dependent UC emissions of 4D7/2 → 4I15/2 and 2I11/2 → 4I15/2 transitions of Er3+ were discussed and the nonradiative relaxation (NR) process of 2I11/2 → 4D7/2 was confirmed.