Gated Terahertz Magneto-optical Measurements of 3D Dirac Semimetals*

We report gated terahertz magneto-optical measurements of the bulk and surface states of Na$_3$Bi and Cd$_3$As$_2$. The onset of interband transitions within the bulk 3D Dirac cone is observed. A gate is used to modulate surface carriers and bulk carriers near the surface resulting in differential optical signals. Gate-modulated cyclotron resonance and Fourier transform infrared spectroscopy characterize the bulk Dirac carriers along all three crystal axes as well as the Fermi-arc carriers with Fermi Energy. The transport scattering and Fermi velocity anisotropy as well as potential fluctuations characterized near the Dirac point will be discussed.