Signature of Hidden Order in URu$_2$Si$_2$ in the c-axis Optical Conductivity

We present high quality c-axis far infrared optical data for the heavy fermion compound URu$_2$Si$_2$. In particular, we compare the signature of the as yet poorly understood 'hidden order' state along the a- and c-axes of the tetragonal structure. The results presented here demonstrate the presence of the hidden order in the ac plane along the c- direction, although there is a very pronounced difference from the absorption at 5 meV seen along the a-axis. We present an assessment of the nature and significance of the a-c anisotropy of the hidden order signature.