  Anisotropic homogeneous metamaterials that are neither wholly dissipative nor
wholly active at a specific frequency are permitted by classical
electromagnetic theory. Well-established homogenization formalisms indicate
that such a metamaterial may be realized quite simply as a random mixture of
electrically small (possibly nanoscale) spheroidal particles of at least two
different isotropic dielectric materials, one of which must be dissipative but
the other active. The dielectric properties of this metamaterial are influenced
by the volume fraction, spatial distribution, particle shape and size, and the
relative permittivities of the component materials. Similar metamaterials with
more complicated linear as well as nonlinear constitutive properties are
possible. Dynamic control of the active component material, for example via
stimulated Raman scattering, affords dynamic control of the metamaterial.
