For a recent review on coherent population trapping see E. Arimondo in Progress in Optics, Vol. XXXV, ed. E. Wolf (North-Holland, Amsterdam, 1996) p. 257 and references therein.
I.V. Jyotsna and G.S. Agarwal, Phys. Rev. A 53, 1690 (1996).
C.M. Bowden, A.S. Manka, J.P. Dowling and M. Fleischhauer, in Coherence and Quantum Optics, eds. J.H. Eberly, L. Mandel and E. Wolf (Plenum, NewYork, 1996) p. 271.
G.S. Agarwal, Quantum Optics (Springer-Verlag, Berlin, 1974) Sec. 6.
M. Born and E. Wolf, Principles of Optics, 6th ed. (Pergamon, Oxford, 1980) Chap. 2.
R. Friedberg, S.R. Hartmann, and Jamal T. Manassah, Phys. Rev. A 40, 2446 (1989); Phys. Rev. A 42, 494 (1990).
J.J. Maki, M.S. Malcuit, J.E. Sipe, and R.W. Boyd, Phys. Rev. Lett. 67, 972 (1991).
Nonlinear density matrix equations have been used earlier in quantum optics, see e.g. [2,3] and G.S. Agarwal, Phys. Rev. A 4, 1791 (1971); K. Molmer and Y. Castin, Ref. 3, p. 193.