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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. [ABSTRACT FROM AUTHOR]

We studied surface plasmon sensors based on micrometric metal meshes by optical transmission spectroscopy as a function of the angle of incidence. The mesh period was set to 2 μm for operation at mid-infrared wavelengths. Metal meshes on dielectric substrates were compared to suspended meshes obtained with a lift-off-free fabrication process, which reduces plasmon damping and increases the quality factor up to 25. We have numerically calculated the electric field distribution of 'dark' quadrupole-like modes and found that the suspended mesh provides an enhanced interaction volume extending up to hundreds of nanometers in free space. Our sensors have been experimentally tested and they exhibited a sensitivity up to 1.4 · 10 nm, at least 1 order of magnitude better than standard mid-infrared absorption spectroscopy. [ABSTRACT FROM AUTHOR]

We make use of a niobium film to produce a micrometric vacuum-bridge superconducting bolometer responding to THz frequency. The bolometer works anywhere in the temperature range 2-7 K, which can be easily reached in helium bath cryostats or closed-cycle cryocoolers. In this work the bolometer is mounted on a pulse tube refrigerator and operated to measure the equivalent noise power (NEP) and the response to fast (μs) terahertz pulses. The NEP above 100 Hz equals that measured in a liquid helium cryostat showing that potential drawbacks related to the use of a pulse tube refrigerator (like mechanical and thermal oscillations, electromagnetic interference, noise) are irrelevant. At low frequency, instead, the pulse tube expansion-compression cycles originate lines at 1 Hz and harmonics in the noise spectrum. The bolometer was illuminated with THz single pulses coming either from a Quantum Cascade Laser operating at liquid nitrogen temperature or from a frequency-multiplied electronic oscillator. The response of the bolometer to the single pulses show that the device can track signals with a rise time as fast as about 450 ns. [ABSTRACT FROM AUTHOR]

We present a fast spectrometer working in the 0.7-4.8 THz range. Broadband radiation from a blackbody source is focused first on a rotating silicon wafer, whose surface was patterned with 18 metal band-pass filters, then on the sample under test and finally is detected by a superconducting microbolometer with microsecond time constant. The bolometer sensor is coupled to a spiral antenna whose frequency band matches the spectral range of the filters. The spectral resolution is set by the filters quality factor of about 3. A dynamic range of 100 and a S/N ratio of 20 are achieved by integrating for less than 10 second. The detector can operate up to 6 K in a closed-cycle cooler, hence making the present apparatus suitable for building up a simple terahertz video-rate spectrometer. [ABSTRACT FROM AUTHOR]

We report on the fabrication and optical properties of thin metal films periodically patterned with square hole arrays of 2 micron pitch, which behave as substrateless plasmonic devices at mid-infrared frequencies. Large (3×3 mm) meshes were fabricated by metallizing a patterned silicon nitride membrane. The mid-infrared spectra display resonant absorption lines with a Q-factor up to 22 in both transmission and reflection, due to the interaction of the radiation with surface plasmon modes on both faces of the film, allowed by substrate removal. The devices can be used to fabricate surface plasmon-based chemical sensors employing mid-infrared radiation. [ABSTRACT FROM AUTHOR]

The optical conductivity of La–Ca, Nd–Sr, and Bi–Sr manganites with commensurate charge order (CO) has been first observed in the sub-THz region. Below, the ordering temperature T CO well-defined peaks are observed, associated with side bands. They are assigned to pinned phasons, and to combinations of phasons and amplitudons, respectively. In La1− n/8Ca n/8MnO3 with n=5, 6, one can thus determine the electron-phonon coupling, the effective mass of the CO system, and its contribution to the dielectric constant. Their values support a description of the CO in La–Ca manganites in terms of a CDW approach. In Bi0.5Sr0.5MnO3, where T CO> 500 K and the coupling is stronger, only the combination bands are clearly detected. [ABSTRACT FROM AUTHOR]

The in-plane optical conductivity of La2−xSrxCuO4 with 0.07 ≤ x ≤ 0.19 exhibits strong peaks in the far infrared. These features, which depend both on doping and temperature, can be associated with excitations of charge stripes, basing on results of different techniques on the same system and on recent theoretical calculations. [ABSTRACT FROM AUTHOR]

The in-plane optical conductivity of three metallic La2− xSr xCuO4 single crystals with 0.10 ≤ x ≤ 0.15 has been studied between 30 and 295 K. Strong peaks in the far-infrared are observed, which cannot be explained by Drude-like models. Their similarity with peaks reported in Cu–O “ladders” with one-dimensional charge-ordering, their extremely low frequency, and their behavior with temperature allows us to conclude that those anomalous features are excitations of charge stripes in the Cu–O planes. [ABSTRACT FROM AUTHOR]

The in-plane optical conductivity and dc resistivity of La1.875Ba0.125−YSr YCuO4 with y = 0.085 was measured from 30 to 20,000 cm−1 and from 295 to 8 K. A strong extra-Drude absorption at finite frequency is detected both in the normal and superconducting state, and remains unchanged when the free carriers condensate. The present results confirm and extend recent infrared data on La2−xSrxCuO4. [ABSTRACT FROM AUTHOR]

We demonstrate third harmonic generation in plasmonic antennas consisting of highly doped germanium grown on silicon substrates and designed to be resonant in the mid-infrared frequency range that is inaccessible with conventional nonlinear plasmonic materials. Owing to the near-field enhancement, the result is an ultrafast, subdiffraction, coherent light source with a wavelength tunable between 3 and 5 µm, and ideally overlapping with the fingerprint region of molecular vibrations. To observe the nonlinearity in this challenging spectral window, a high-power femtosecond laser system equipped with parametric frequency conversion in combination with an all-reflective confocal microscope setup is employed. We demonstrate spatially resolved maps of the linear scattering cross section and the nonlinear emission of single isolated antenna structures. A clear third-order power dependence as well as mid-infrared emission spectra prove the nonlinear nature of the light emission. Simulations support the observed resonance length of the double-rod antenna and demonstrate that the field enhancement inside the antenna material is responsible for the nonlinear frequency mixing., Competing Interests: The authors declare that they have no conflict of interest.