Ultraviolet upconversion fluorescence of Er³⁺ induced by 1560 nm laser excitation

We observed multiphoton upconversion (UC) emissions from the high-energy states (²I_11/2, ⁴ D_7/2, ⁴G_9/2, ²D_5/2, ² K _13/2, and ²P_3/2) of Er³⁺ ions in Yb³⁺ - Er³⁺ codoped β-NaYF₄ microcrystals under 1560 nm excitation. Analysis indicated that the highest-order emissions came from 11-photon UC processes. In the codoped system, the energy transfer (ET) from Er³⁺ (⁴I_11/2,) to Yb³⁺ (²F_5/2 subsequently the back ET from Yb³⁺ to Er³⁺ played important roles in populating these high-energy states of Yb³⁺ and Er³⁺. Several different UC processes work simultaneously to populate excited Er³⁺ in this codoped sample.