ABSTRACT
This chapter discusses the resonant energy transfer process and related phenomena, such as the sensitization and quenching of luminescence. Energy transfer processes are often used in practical phosphors in order to enhance the emission efficiency. The mechanisms of resonant energy transfer can be classified into several types based on the character of the transitions. Dexter's theory of resonant energy transfer has elucidated that two optical centers within a certain distance may be in resonance and transfer the excitation energy from one (donor) to the other (acceptor). The close proximity of the centers enables them to be connected by the electrostatic interaction. If the concentration of an activator is higher than an appropriate value, the emission of the phosphor is usually lowered, this effect is concentration quenching. In some rare-earth activated phosphors, the effect of concentration quenching is so small that even stoichiometric phosphors, in which all of the host constituent cations are substituted by the activator ions, have been developed.
