\textbf{Optical Properties of Dirac Materials}

Recently strong spin-orbit interaction and topological considerations have led to materials with 3D Weyl or Dirac energy band dispersions. Several novel phases have been predicted for these materials, including fractionalized topological insulators, chiral spin liquids, topological Weyl semimetals, axion insulators and spin liquids. These 3D materials are semimetals corresponding to zero gap semiconductors. As such their optical properties have new features not seen in conventional materials including logarithmic singularities in the dielectric function. We will present preliminary optical studies on several of these new materials. The materials studied include Dirac materials Cd$_{\mathrm{3}}$As$_{\mathrm{2}}$ and Na$_{\mathrm{3}}$Bi and the Weyl semimetal Eu$_{\mathrm{2}}$Ir$_{\mathrm{2}}$O$_{\mathrm{7}}$ which has broken time reversal symmetry in the antiferromagnetic state. The optical data give the first evidence for a Weyl state in Eu$_{\mathrm{2}}$Ir$_{\mathrm{2}}$O$_{\mathrm{7}}$. We also observe the logarithmic singularities associated with the Pauli blocking of the inter Dirac cone excitations in Cd$_{\mathrm{3}}$As$_{\mathrm{2}}$ and Na$_{\mathrm{3}}$Bi. These optical data provides important information about the 3D energy band structure of these novel materials that are not amenable to surface probes such as ARPES or STM.