We performed far infrared optical spectroscopy measurements on the heavy fermion compound URu 2 Si 2 as a function of temperature. The light's electric field was applied along the a axis or the c axis of the tetragonal structure. We show that in addition to a pronounced anisotropy the optical conductivity exhibits for both axis a partial suppression of spectral weight around 12 meV and below 30 K. We attribute these observations to a change in the bandstructure below 30 K. However since these changes have no noticeable impact on the entropy nor on the DC transport properties we suggest that this is a crossover phenomenon rather than a thermodynamic phase transition.
