Upconversion channels in Er3+:ZBLALiP fluoride glass microspheres D. G. O'Shea, J. M. Ward, B. J. Shortt, M. Mortier, P. Féron and S. Nic Chormaic Eur. Phys. J. Appl. Phys., 40 2 (2007) 181-188 Published online: 31 October 2007 DOI: 10.1051/epjap:2007148