On the resonant optical bistability condition
  We address a two level system in an environment interacting with the electromagnetic field in the dipole approximation. The resonant optical bistability induced by local-field effects is studied by considering the relationship between the population difference and the excitation field. The diversity of various systems is included by accounting for the system self-action via the surface part of the Green's dyadic in the general form. The bistability condition and the exact solution of the steady state optical Bloch equations at the absolute bistability threshold are derived analytically. 
A. V. Malyshev
physics.optics cond-mat.mtrl-sci