Erbium as a probe of everything?
Erbium is a lanthanide ion with unique electronic and optical properties. In its trivalent state it is composed of an incompletely filled 4f inner shell and two closed outer shells. By employing these properties in specific material systems, Er can be used to probe point defects, oxygen, OH, Er, radiation defects, network structure, excitons, optical density of states, optical modes, and photonic bandstructure.
Polman, A. (2001). Erbium as a probe of everything?. Physica B, 300, 78–90.