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

We observed multiphoton upconversion (UC) emissions from the high-energy states ((2)I(11/2), (4)D(7/2), (4)G(9/2), (2)D(5/2), (2)K(13/2), and (2)P(3/2)) of Er(3+) ions in Yb(3+)-Er(3+) codoped beta-NaYF(4) 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 Er(3+)((4)I(11/2)) to Yb(3+)((2)F(5/2)) and subsequently the back ET from Yb(3+) to Er(3+) played important roles in populating these high-energy states of Yb(3+) and Er(3+). Several different UC processes work simultaneously to populate excited Er(3+) in this codoped sample.