Conductivity anisotropy in strained VO$_{2}$ thin films, probed by THz Time Domain Spectroscopy*

We used THz time domain spectroscopy to measure the temperature and polarization dependent far-infrared conductivity of high quality strained VO$_{2}$ thin films epitaxially grown on (100) TiO$_{2}$ substrates. A large conductivity anisotropy is observed in the metallic phase of our VO$_{2}$ films with the conductivity along the rutile axis $\sim $30 times larger than the orthogonal direction. The MIT temperature also exhibits anisotropy with a value of 360K along the rutile c axis and 340K along the perpendicular direction. Our results are consistent with substrate induced strain modulation of the energy and bandwidth associated with the vanadium 3d orbitals.