Fig. 1.

General processes of dopant interactions responsible for depopulation of an excited state. a Cross-relaxation and phonon-assisted energy transfer that depopulates an excited state locally. To counteract the depopulation associated with the localized energy exchange interaction, a high excitation power is needed to enhance the excitation process. b Long-distance energy migration through the dopant sublattice that takes the energy to lattice or surface defects. The energy migration-induced depopulation can be alleviated by spatially confining the excitation energy