  We report on infrared (IR) optical experiments on Bi$_2$Te$_3$ and Mn-doped
Bi$_2$Te$_3$ epitaxial thin films. In the latter film, dilute Mn doping (4.5\%)
of the topologically nontrivial semiconductor host results in
time-reversal-symmetry-breaking ferromagnetic order below $T_C$=15 K. Our
spectroscopic study shows both materials share the Bi$_2$Te$_3$ crystal
structure, as well as classification as bulk degenerate semiconductors. Hence
the Fermi energy is located in the Bi$_2$Te$_3$ conduction band in both
materials, and furthermore, there is no need to invoke topological surface
states to describe the conductivity spectra. We also demonstrate that the Drude
oscillator strength gives a simple metric with which to distinguish the
possibility of topological surface state origins of the low frequency
conductance, and conclude that in both the pristine and Mn-doped Bi$_2$Te$_3$
samples the electromagnetic response is indeed dominated by the bulk material
properties, rather than those of the surface. An encouraging aspect for taking
advantage of the interplay between nontrivial topology and magnetism, however,
is that the temperature dependence of the Mn-doped Bi$_2$Te$_3$ film suggests
bulk charge carriers do not play a significant role in mediating
ferromagnetism. Thus, a truly insulating bulk may still be suitable for the
formation of a ferromagnetic ground state in this dilute magnetic topological
semiconductor.
