Photon management with lanthanides

Efficient conversion of photons from high energy radiation (e.g. ultraviolet or X-rays) to lower energies (visible) has been optimized by using luminescent materials based on the optical properties of lanthanide ions. Presently, luminescent materials with efficiencies close to the theoretical maximum are applied in e.g. fluorescent tubes, X-ray imaging and color television. Contrary to the mature status of luminescent materials in these fields, areas requiring new luminescent materials are emerging. There is great challenge in research on up- and downconversion materials and lanthanide ions are the prime candidates to achieve efficient materials. Here downconversion processes will be discussed for VUV phosphors with Pr3+. The efficiency of resonant energy transfer of the 1S0–1I6 energy from Pr3+ to Eu3+and Mn2+ is investigated. The aim is to convert the 405 nm photon of the first step of the well-known cascade emission of Pr3+ into a more useful visible photon. For co-doping with Eu3+ it is observed that the Pr3+ emission is quenched, most probably through a metal-to-metal charge-transfer state. The energy transfer from Pr3+ to Mn2+ is found to be inefficient.An alternative for downconversion through resonant energy transfer is the non-resonant process of cooperative sensitization. For the Tb–Yb couple efficient cooperative energy transfer from the 5D4 level of Tb3+ to two Yb3+ neighbors is observed with a transfer rate of 0.26 ms−1. This corresponds to an upper limit of 188% for the conversion efficiency of visible (490 nm) photons to infrared (∼1000 nm) photons.