Simplified description of an active medium in quantum plasmonics
  In order to simplify the theoretical description of spasers, a gain medium is commonly represented by a two-level system. A realistic model, however, should have four levels. By using the Lindblad equations we develop a description of such a system and show that depending on ratios of the Rabi frequency and the rate of relaxation of the polarization, a four-level system may be reduced to one of two effective two-level systems that reproduce the key properties of a four-level system. 
Evgeny D. Chubchev, Eugeny S. Andrianov, Alexey P. Vinogradov, and   Alexander A. Lisyansky
cond-mat.other