Three Dimensional Infrared Nano-Imaging of Stripe Order in Vanadium Dioxide

We report the three dimensional landscape of the stripe state in vanadium dioxide (VO$_{2})$ films. This is achieved via direct visualization with scattering-type scanning near-field optical microscope (s-SNOM) [Phys. Rev. Lett. 111 (9), 096602 (2013)]. The VO$_{2}$ films we investigate in this study are epitaxially grown on [100]$_{\mathrm{R}}$ TiO$_{2}$ substrates and exhibit uniaxial strain induced cracking uniformly along the rutile c axis. With s-SNOM, we show that (1) monoclinic-tetragonal crystal symmetry, (2) in-plane rotational symmetry and (3) out-of-plane (z-axis) symmetry have been spontaneously broken in the vicinity of the phase transition. Our results demonstrate s-SNOM as a powerful approach for bringing new insight into mesoscopic physics in strained metal oxide thin films.