Mixed valency of the rare earth and actinium ions in solid states

There has been a number of papers [2, 3, 4 and 8] published which deal with the prediction of intrinsic optical bistability of coupled ion pairs in a coherent radiation field. This coupling can be different, either electromagnetic near-field coupling of the transition dipoles or other more general interactions. In the last year Malyshev et al. published a paper in which the authors deny the existence of this bistability. In this paper the reason is discussed for this discrepancy, which seems to be due to different approaches of introducing the external phase destroying perturbations. The bistability is based on an inversion-dependent resonance frequency of coupled ions which can generally be important for phase-sensitive effects.