Optical conductivity of the Hidden order phase in URu$_2$Si$_2$

We measured the optical conductivity of an URu$_2$Si$_2$ single crystal as a function of temperature. URu$_2$Si$_2$ shows a coherent transport transformation around 70 K and a phase transition to a still unknown (hidden) order below 17 K. Below the coherence temperature we observe a consistent development of a very low frequency Drude-like peak. At the hidden order transition our results show a large spectral weight redistribution below 15 meV with an optical conductivity typical of a density wave gap. This spectral weight redistribution goes together with the appearance of a sharp peak at 5 meV. We will discuss effects of the hidden order transition on the carriers scattering rate and effective mass and compare our data with the earlier work of Bonn {\it et al.}, PRL {\bf 61}, 1305 (1988).