Power switched multiphoton upconversion emissions of Er^3+ in Yb^3+/Er^3+ codoped β-NaYF_4 microcrystals induced by 980 nm excitation

Multiphoton upconversion (UC) emissions from the high-energy states ((4)G(9/2), (4)G(7/2), (2)K(13/2), and (2)P(3/2)) of Er(3+) ions were observed under 980 nm excitation. These high-energy excited states were populated by a five-photon 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 (4)S(3/2) and (4)F(9/2) of Er(3+) under different NIR pump power. A mechanism of the power density-dependent multiphoton UC processes was proposed based on experimental data and analysis.