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

We observed multiphoton upconversion (UC) emissions from the high-energy states (I-2(11/2), D-4(7/2), (4)G(9/2), D-2(5/2), K-2(13/2), and P-2(3/2)) of Er3+ ions in Yb3+ - Er3+ codoped beta-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+(I-4(11/2)) to Yb3+(F-2(5/2)) and 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. (C) 2010 Optical Society of America