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Vanadium dioxide (VO2) is a material that undergoes an insulator–metal transition upon heating above 340 K. It remains debated as to whether this electronic transition is driven by a corresponding structural transition or by strong electron–electron correlations. Here, we use apertureless scattering near-field optical microscopy to compare nanoscale images of the transition in VO2 thin films acquired at both mid-infrared and terahertz frequencies, using a home-built terahertz near-field microscope. We observe a much more gradual transition when THz frequencies are utilized as a probe, in contrast to the assumptions of a classical first-order phase transition. We discuss these results in light of dynamical mean-field theory calculations of the dimer Hubbard model recently applied to VO2, which account for a continuous temperature dependence of the optical response of the VO2 in the insulating state., The insulator-to-metal transition in vanadium dioxide still has many unexplored properties. Here the authors use multi-modal THz and mid-IR nano-imaging to examine the phase transition in VO2 thin films, and discuss the unexpectedly smooth transition at THz frequencies in the context of a dimer Hubbard model.

We report the Drude oscillator strength D and the magnitude of the bulk band gap E_{g} of the epitaxially grown, topological insulator (Bi,Sb)_{2}Te_{3}. The magnitude of E_{g}, in conjunction with the model independent f-sum rule, allows us to establish an upper bound for the magnitude of D expected in a typical Dirac-like system composed of linear bands. The experimentally observed D is found to be at or below this theoretical upper bound, demonstrating the effectiveness of alloying in eliminating bulk charge carriers. Moreover, direct comparison of the measured D to magnetoresistance measurements of the same sample supports assignment of the observed low-energy conduction to topological surface states.

We have investigated the electronic structure and carrier dynamics of the topological insulator $\mathrm{Bi}{}_{2\ensuremath{-}x}\mathrm{Sb}{}_{x}\mathrm{Te}{}_{3\ensuremath{-}y}\mathrm{Se}{}_{y}$, for $x=0.5,y=1.3$ and $x=1,y=2$, using infrared spectroscopy. Our results show that both of these BSTS alloys are highly insulating in the bulk, with analysis of the infrared data indicating an upper limit to the carrier density of $4.4\ifmmode\times\else\texttimes\fi{}{10}^{17}$ ${\mathrm{cm}}^{\ensuremath{-}3}$. Furthermore, analysis of the interband transitions of $\mathrm{Bi}{}_{1.5}\mathrm{Sb}{}_{0.5}\mathrm{Te}{}_{1.7}\mathrm{Se}{}_{1.3}$ revealed distinct band-structure critical points, which suggest high crystallographic order of our crystals. Motivated by the low carrier density and crystallographic order identified in these compounds, we searched for the Landau level transitions associated with the surface states through magneto-optical measurements in the far infrared range. We failed to observe any indications of the Landau level resonances at fields up to 8 T in sharp contrast with our earlier finding for a related $\mathrm{Bi}{}_{1\ensuremath{-}x}\mathrm{Sb}{}_{x}$ alloy. We discuss factors that may be responsible for suppressed magneto-optics response of these single crystals.

© 2014 American Physical Society. A resurgence in the use of the donor-acceptor approach in synthesizing conjugated polymers has resulted in a family of high-mobility ambipolar systems with exceptionally narrow energy bandgaps below 1 eV. The ability to transport both electrons and holes is critical for device applications such as organic light-emitting diodes and transistors. Infrared spectroscopy offers direct access to the low-energy excitations associated with injected charge carriers. Here we use a diffraction-limited IR microscope to probe the spectroscopic signatures of electron and hole injection in the conduction channel of an organic field-effect transistor based on an ambipolar DA polymer polydiketopyrrolopyrrole-benzobisthiadiazole. We observe distinct polaronic absorptions for both electrons and holes and spatially map the carrier distribution from the source to drain electrodes for both unipolar and ambipolar biasing regimes. For ambipolar device configurations, we observe the spatial evolution of hole-induced to electron-induced polaron absorptions throughout the transport path. Our work provides a platform for combined transport and infrared studies of organic semiconductors on micron length scales relevant to functional devices.

We demonstrate an electrolyte-based voltage tunable vanadium dioxide (VO2) memory metasurface. Large spatial scale, low voltage, non-volatile switching of terahertz (THz) metasurface resonances is achieved through voltage application using an ionic gel to drive the insulator-to-metal transition in an underlying VO2 layer. Positive and negative voltage application can selectively tune the metasurface resonance into the “off” or “on” state by pushing the VO2 into a more conductive or insulating regime respectively. Compared to graphene based control devices, the relatively long saturation time of resonance modification in VO2 based devices suggests that this voltage-induced switching originates primarily from electrochemical effects related to oxygen migration across the electrolyte–VO2 interface.