The optical activity of rare-earth ions in semiconductors is discussed taking into account the large 4f-level correlation energy. In addition to crystal-field effects, three many-body excitation mechanisms of 4f-4f transitions are identified; coherent or incoherent energy transfer via electron-hole pairs and nonequilibrium electron excitation. The first two give rise to novel nonlinear optical effects, while the last promises a simple means of achieving optical gain.

• Received 20 February 1991

DOI:https://doi.org/10.1103/PhysRevLett.66.2782