Nonlinearity and optical bistability caused by local field effects in   the interaction between a two level atomic system and a metamaterial
  We present a theoretical analysis of the nonlinear response of a metamaterial consisting of an array of split ring resonators coupled to a two level atomic system (2LS) without inversion. We calculated the total complex susceptibility and the linear, third and fifth-order nonlinear solution of the Maxwell Bloch equations that describe the coupled system. We analyzed the impact of the nonlinear effects on the dynamics of an active metamaterial/2LS compound in dependence of the magnitude of the external electric field when the 2LS is pumped by an external pump source and population inversion is achieved. We observed that nonlinear damping in the active metamaterial/2LS compound significantly reduces the effect of spiking. Finally, we investigated the conditions under which the system can exhibit the phenomenon of optical bistability. We observed that optical bistability can only be obtained when the atomic volume density of the 2LS attains unphysically high values and thus is not expected in the system under investigation. 
J. L. Garcia-Pomar, R. Beigang, M. Rahm
physics.optics cond-mat.mtrl-sci