  We derive the energy density associated with an electromagnetic wave passing
through a hyperbolic metamaterial (HMM). Both types of HMMs are studied. By
considering a dispersive and absorbing HMM as an effective uniaxial crystal, we
find that under the influence of an electromagnetic wave, the response of type
I (type II) HMM in the directions perpendicular to and parallel to the optical
axis are similar to those of Lorentz (Drude) and Drude (Lorentz) media,
respectively. Numerical examples are presented to reveal the general
characteristics of the direction-dependent energy storage capacity of both
types of HMMs.
