Power switched multiphoton upconversion emissions of Er³⁺ in Yb³⁺/Er³⁺ codoped β-NaYF₄ microcrystals induced by 980 nm excitation

Multiphoton upconversion (UC) emissions from the high-energy states ( ⁴G_9/2, ⁴G_7/2, ²K _13/2, and ²P_3/2) of Er³⁺ ions were observed under 980 nm excitation. These high-energy excited states were populated by a fivephoton or a four-photon UC process conditionally, which depended on the near-infrared (NIR) pump density. Experiments exhibited that the power dependence originated from the varied populating routes of intermediated ⁴S_3/2 and ⁴F_9/2 of Er³⁺ under different NIR pump power. A mechanism of the power density-dependent multiphoton UC processes was proposed based on experimental data and analysis.