ABSTRACT
Since the discovery of sensitized luminescence caused by energy transfer (ET) from a luminescent center acting as a sensitizer to another acting as an activator, numerous phosphors have been developed based on ET, which enables efficient and color tunable luminescence as a function of types and concentrations of sensitizers and/or activators, showing attractive potential for design of new phosphors used in solid-state lighting, bio-imaging, various light sources, and so on. This chapter presents elementary analytical method of ET based on spectroscopy and time evolution data in some phosphors covering visible-to-infrared spectral region. Moreover, a new type of ET in contrast to the general one, namely, round trip ET, is demonstrated. In the round trip process, center A transfers energy to center B with subsequent energy transferred back to the same A not another A, but bringing A to an energy state lower than before. As a result, the round trip transfer can facilitate the population relaxation to a lower state and, thus, change luminescence color.
