J. F. Suyver, A. Aebischer, D. Biner, P. Gerner, J. Grimm, S. Heer, K. W. Krämer, C. Reinhard and H. U. Güdel
Novel materials doped with trivalent lanthanides and transition metal ions showing near-infrared to visible photon upconversion
Opt. Mater. 27, 1111-1130 (2005)                 

Abstract: This paper presents an overview of the recent results on upconversion spectroscopy obtained in our group. After a brief introduction into the upconversion field, three different topics will be addressed. First, the near-infrared (NIR) to red/green or blue upconversion efficiencies are discussed for the very efficient upconversion lattice NaYF₄ codoped with Er³⁺, Yb³⁺/Er³⁺ or Yb³⁺/Tm³⁺, respectively. It will be demonstrated that as much as 50% of the NIR excitation photons contribute to the upconversion emission. Possible application of such a phosphor for enhancing the energy conversion efficiency of solar cells will be discussed. Next, the upconversion spectroscopy of nanocrystalline solutions will be discussed. Most importantly, optically transparent solutions showing intense visible emission under near-infrared excitation, an essential first step for application in new luminescent nanolabels, will be presented. Finally, upconversion spectroscopy of mixed transition metal/rare earth systems will be discussed. Both systems where the sensitizer is the rare earth ion and upconversion occurs on the transition metal ion, and their counterparts (transition metal sensitized upconversion) are presented and their underlying mechanisms will be described. The possibilities for chemical tuning of upconversion properties in such systems are presented.