ABSTRACT
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9.1 Infrared up-conversion phosphors 9.1.1 Introduction In this section, characteristic features of infrared-to-visible up-conversion phosphors doped with trivalent rare-earth ions are described. Devices made with these materials and important applications of such materials and devices are also mentioned. It is widely known that certain kinds of trivalent rare-earth-activated phosphors emit intense visible luminescence under near-infrared light excitation. This up-conversion process takes place in the following way: higher lying excited states of a rare-earth ion are populated by two-or three-step successive excitations with infrared quanta or by energy transfers. A downward transition to the ground level or to an intermediate excited level produces the visible luminescence. The up-conversion efficiency is known to be strongly dependent upon choice of host materials, activator and sensitizer concentrations, etc. However, since elementary processes governing the up-conversion processes are physically simple in nature and well explored, it seems now possible to review the entire up-conversion phenomenon in a consistent manner.
