  General relations for the stored reactive field energy density in passive
linear artificial microwave materials are established. These relations account
for dispersion and absorption effects in these materials, and they are valid
also in the regions where the real parts of the material parameters are
negative. These relations always give physically sound positive values for the
energy density in passive metamaterials. The energy density and field solutions
in active metamaterials with non-dispersive negative parameters are also
considered. Basic physical limitations on the frequency dispersion of material
parameters of artificial passive materials with negative real parts of the
effective parameters are discussed. It is shown that field solutions in
hypothetical materials with negative and non-dispersive parameters are
unstable.
