The non-equilibrium control of emergent phenomena in solids is an important research frontier, encompassing effects like the optical enhancement of superconductivity 1 . Recently, nonlinear excitation 2 , 3 of certain phonons in bilayer cuprates was shown to induce superconducting-like optical properties at temperatures far above T c 4,5,6 . This effect was accompanied by the disruption of competing charge-density-wave correlations 7,8 , which explained some but not all of the experimental results. Here, we report a similar phenomenon in a very different compound. By exciting metallic K 3 C 60 with mid-infrared optical pulses, we induce a large increase in carrier mobility, accompanied by the opening of a gap in the optical conductivity. Strikingly, these sameReprints and permissions information is available online at www.nature.com/reprints.Users may view, print, copy, and download text and data-mine the content in such documents, for the purposes of academic research, subject always to the full Conditions of use:http://www.nature.com/authors/editorial_policies/license.html#termsCorrespondence and request for materials should be addressed to An.C. (andrea.cavalleri@mpsd.mpg.de). Author Contributions

Plasmons are quantized collective oscillations of electrons and have been observed in metals and doped semiconductors. The plasmons of ordinary, massive electrons have been the basic ingredients of research in plasmonics and in optical metamaterials for a long time. However, plasmons of massless Dirac electrons have only recently been observed in graphene, a purely two-dimensional electron system. Their properties are promising for novel tunable plasmonic metamaterials in the terahertz and mid-infrared frequency range. Dirac fermions also occur in the two-dimensional electron gas that forms at the surface of topological insulators as a result of the strong spin-orbit interaction existing in the insulating bulk phase. One may therefore look for their collective excitations using infrared spectroscopy. Here we report the first experimental evidence of plasmonic excitations in a topological insulator (Bi2Se3). The material was prepared in thin micro-ribbon arrays of different widths W and periods 2W to select suitable values of the plasmon wavevector k. The linewidth of the plasmon was found to remain nearly constant at temperatures between 6 K and 300 K, as expected when exciting topological carriers. Moreover, by changing W and measuring the plasmon frequency in the terahertz range versus k we show, without using any fitting parameter, that the dispersion curve agrees quantitatively with that predicted for Dirac plasmons.

Raman and combined trasmission and reflectivity mid infrared measurements have been carried out on monoclinic VO2 at room temperature over the 0-19 GPa and 0-14 GPa pressure ranges, respectively. The pressure dependence obtained for both lattice dynamics and optical gap shows a remarkable stability of the system up to P*∼10 GPa. Evidence of subtle modifications of V ion arrangements within the monoclinic lattice together with the onset of a metallization process via band gap filling are observed for P>P*. Differently from ambient pressure, where the VO2 metal phase is found only in conjunction with the rutile structure above 340 K, a new room temperature metallic phase coupled to a monoclinic structure appears accessible in the high pressure regime, thus opening to new important queries on the physics of VO2. PACS numbers:Since the first observation of the metal to insulator transition (MIT) in several vanadium oxides, these materials attracted considerable interest because of the huge and abrupt change of the electrical properties at the MIT. As usual in transition metal oxides, electronic correlation strongly affects the conduction regime of vanadium oxides, although, in some compounds, lattice degrees of freedom seem to play an important role. This is the case of VO 2 , which undergoes a first order transition from a high temperature metallic rutile (R) phase to a low temperature insulating monoclinic (M1) one. At the MIT temperature, T c =340 K, the opening of an optical gap in the mid-infrared (MIR) conductivity and a jump of several order of magnitude in the resistivity are observed [1]. The interest on this compound is thus mainly focused on understanding the role and the relative importance of the electron-electron and the electron-lattice interaction in driving the MIT. Despite the great experimental and theoretical efforts [2], the understanding of this transition is still far from being complete [3,4,5,6,7]. In the R phase the V atoms, each surrounded by an oxygen octahedron, are equally spaced along linear chains in the c-axis direction and form a body-centered tetragonal lattice. On entering the M1 insulating phase the dimerization of the vanadium atoms and the tilting of the pairs with respect to the c axis lead to a doubling of the unit cell, with space group changing from C 5 2h (R) to D 14 4h (M1) [8,9]. As first proposed by Goodenough [10], the V-V pairing and the off-axis zig-zag displacement of the dimers lead to a band splitting with the formation of a Peierls-like gap at the Fermi level. First principle electronic structure calculations based on local density approximation (LDA) showed the band splitting on entering the monoclinic phase, but failed to yield the opening of the band gap [11,12]. In fact, as early pointed out [13], the electron-electron correlation has to be taken into account to obtain the insulating phase. A recent theoretical paper where the electronic Coulomb repulsion U is properly accounted for, shows that calculations carried out joining dynamical mean field theory with...