Highly efficient photon upconversion in solution is demonstrated for the first time. Transparent colloidal solutions of Yb/Er and Yb/Tm doped NaYF₄ nanocrystals show bright upconversion emission in the red, green, and blue spectral region by excitation around 1 μm with a laser of moderate power. These new systems show a substantial improvement on the order of 10⁸ compared to previously reported systems.

This work was carried out in the group of Prof. Hans Ulrich Güdel.

References:

• S. Heer, K. Kömpe, H.-U. Güdel, M. Haase;
  "Highly Efficient Multicolour Upconversion Emission in Transparent Colloids of Lanthanide-Doped NaYF₄ Nanocrystals"
  Adv. Mater., 16, 2102-2105, (2004); doi:10.1002/adma.200400772.