  Here the multipole approach [13], in combination with the density matrix
formalism is used for establishing of the model for MMs with gain. This
approach allows us to investigate analytically or semi-analytically the
interplay between gain and magnetic properties of the MMs, the influence of
internally unstable operation mode for spasers (MAs coupled with emitters,
which the MMs consist of) on the propagation characteristics, and finally to
optimize MM design. Moreover, the presented model is in line with the
previously presented approach [16] (actually is its natural extension on the
problem of plane wave propagation) and from the other side pretty clear and
observable. The results presented in this paper resolve the discussion excited
by [12] followed by [35-38].
