  The optical conductivity of heavy fermions can reveal fundamental properties
of the charge carrier dynamics in these strongly correlated electron systems.
Here we extend the conventional techniques of infrared optics on heavy fermions
by measuring the transmission and phase shift of THz radiation that passes
through a thin film of UNi2Al3, a material with hexagonal crystal structure. We
deduce the optical conductivity in a previously not accessible frequency range,
and furthermore we resolve the anisotropy of the optical response (parallel and
perpendicular to the hexagonal planes). At frequencies around 7cm^-1, we find a
strongly temperature-dependent and anisotropic optical conductivity that -
surprisingly - roughly follows the dc behavior.
