Abstract

The absorption of single photons by coupled pairs of ions in rare-earth solids was first observed by Varsanyi and Dieke.¹ They noticed that weak absorption of light occurred in PrCl₃ at frequencies close to the sum of transition frequencies of individual Pr ions, giving rise to rare-earth fluorescence. This effect was explained by the perturbation theory of Dexter.² He showed that weak multipole or exchange interactions between rare-earth ions permitted the simultaneous promotion of two ions to excited states by the action of one absorbed photon in an electric dipole allowed transition. Similarly, two ions initially in excited states could emit one sum frequency photon in a cooperative manner.

© 1987 Optical Society of America