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Observation of Dirac plasmons in a topological insulator


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 time1. However, plasmons of massless Dirac electrons have only recently been observed in graphene, a purely two-dimensional electron system2. Their properties are promising for novel tunable plasmonic metamaterials in the terahertz and mid-infrared frequency range3. 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 phase4. 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.

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Figure 1: Extinction coefficients of the microribbon arrays of Bi2Se3 topological insulators in the terahertz range.
Figure 2: Extraction of the bare plasmon and phonon contributions from the extinction data through a Fano fit.
Figure 3: Experimental and theoretical dispersion of plasmons in Bi2Se3.

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The authors thank M. Polini for fruitful discussions about Dirac and massive plasmonic dispersions. The work at Rutgers was supported by IAMDN of Rutgers University, NSF DMR-0845464 and ONR N000141210456. The work at ‘La Sapienza’, was supported by the INFN, TERASPARC experiment.

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M.B., N.B., N.K. and S.O. fabricated and characterized the Bi2Se3 films. M.O., A.D.G. and V.G. performed electron-beam lithography and etching. P.D.P., F.G., O.L. and M.O. carried out the terahertz experiments and data analysis. P.C., M.O. and S.L. planned and managed the project, with inputs from all authors. All authors discussed the results. P.C., M.O. and S.L. wrote the manuscript.

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Correspondence to S. Lupi.

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Di Pietro, P., Ortolani, M., Limaj, O. et al. Observation of Dirac plasmons in a topological insulator. Nature Nanotech 8, 556–560 (2013).

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