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 (Bi 2 Se 3). 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. © 2013 Macmillan Publishers Limited. All rights reserved.

The channel of a high electron mobility transistor can work as a resonant microcavity for plasma waves, provided that the plasmon decay length is much larger than the cavity length. We have performed a spectroscopic study in the 0.15-0.4 THz range of the power absorbed by the micrometric channel of a two-dimensional electron gas (2DEG) transistor, where the active layer is formed by a remotely doped AlGaAs/InGaAs/AlGaAs quantum well where the electron mobility increases with decreasing temperature. The radiation emitted by a tunable frequency-multiplied THz oscillator was coupled to the cavity by an integrated lens-antenna optical system. The rectified signal is measured as a function of frequency and a strong increase upon cooling to 10 K is found at specific radiation frequencies, indicating the formation of standing plasma waves in the microcavity formed by the channel.

We fabricated a two-dimensional-electron-gas field effect transistor with an asymmetric terahertz antenna connected to the channel terminals and a gate length of 1 lm. We investigated frequency mixing in the transistor's channel by measuring, with a quasi-optical setup, the heterodyne, secondand third-order subharmonic mixing signal at 0.592 THz. The dependence on the gate voltage and on the radiation power of both the local-oscillator and the radio-frequency signals was studied for all mixing orders. The conditions for full-plasmonic-mixing are fulfilled in our transistor at room temperature.

We report on the fabrication and electrical characterization of Schottky diodes on epitaxial relaxed germanium for THz detection, which implement both a sub-micron junction area and the air-bridge technology. The small footprint necessary for low capacitances and T-shaped contacts are obtained by using a triple layer of electronic resists with different sensitivity. The cross-sectional analyses revealed a clearly suspended air-bridge and the T-shape of the metal contact with footprint width below 400 nm. The electrical characterization at room temperature shows well defined Schottky diode behavior with ideality factor between 1 and 2 and low series resistance suitable for THz detection.

We are developing microelectronic terahertz rectifiers by scaling down the dimensions of GaAs Schottky diodes and field-effect transistors to the sub-micron range, and by investigating the effect of on-chip parasitic capacitances on the square-law power detection signal. For broadband operation at THz frequencies the terahertz oscillating signal is fed to the device by integrated lithographic planar antennas, suitably coupled to a silicon substrate lens. Such room-temperature THz detectors can be fabricated in arrays and naturally provide picosecond response time, suitable for detection of coherent THz radiation produced by single electron bunches in accelerators.

We studied terahertz current oscillations induced by a frequency-tunable radiation source in a AlGaAs/InGaAs/AlGaAs heterostructure field effect transistor channel. A planar antenna was integrated on-chip, and a substrate lens was used for broadband coupling of free-space radiation at 0.18-0.72 THz to the channel ends. Through spectral analysis of the detection signal, we identified two different mixing mechanisms: one related to channel current oscillations and the other to modulation of the gate-to-channel potential. Depending on gate bias and radiation frequency, the two mechanisms either compete or cooperate, leading to responsivity up to 300 V/W and noise equivalent power of 1 nW/Hz0.5

Multiferroic LuFe2O4 (LFO) exhibits three-dimensional (3D) charge order below TCO ~ 350 K and strong electroresistance (ER) above a static threshold field Eth. By measuring simultaneously, in LFO single crystals, the dc current and the far-infrared reflectivity along different axes, we do not detect any insulator-to-metal transition above Eth. Combined current-temperature measurements confirm that the ER is due to self-heating of LFO, as recently reported. The data can be fit by the standard activation law for an intrinsic semiconductor, with a gap value = 0.57 eV. This value is consistent with that of the optical gap reported for LFO in the literature.

We demonstrate a rectification mechanism based on quantum tunneling through the narrow Schottky barrier of a sub-micrometric Au/Ti/n-GaAs junction, which is capable of efficient power detection of free-space terahertz radiation beams even without an applied dc bias. Three-dimensional shaping of the junction geometry provides an enhanced zero-bias tunneling probability due to increased electric fields at the junction, resulting in cutoff frequencies up to 0.55 THz, responsivity up to 200 V/W, and noise equivalent power better than 10^-9 W/sqrt(Hz) without applied dc bias.

A midinfrared mass sensor based on high quality factor surface plasmon modes was designed,fabricated, and tested by infrared spectroscopy for the detection of nanometric layers of dielectric materials. Substrate removal below a metal mesh with period of 2 m results in the coupling between degenerate surface plasmon modes on the two surfaces, resulting in a quality factor up to33 for the antisymmetric mode. The presented substrateless metal mesh integrates mass sensing capability together with midinfrared spectroscopy, and is therefore of potential interest for substance-selective environmental and biomedical sensing applications

Absorption due to conduction intersubband transitions is studied in n-type s-Si/SiGe multiquantum wells MQW of different well widths and barrier composition grown by UHV-chemical vapor deposition CVD. The measured intersubband transition energies are compared with the theoretical results of a tight-binding model which provides the electronic band structure of the complete MQW system throughout the whole Brillouin zone. Our findings demonstrate both the high quality of the CVD grown MQWs and the effectiveness of the adopted tight-binding model in describing band profiles and electronic structures of SiGe multilayer systems. In particular we have evaluated the conduction band offsets in the investigated structures.

By studying the optical conductivity of Bi2Sr2-xLaxCuO6 and Y0.97Ca0.03Ba2Cu3O6, we show that the metal-to-insulator transition in these hole-doped cuprates is driven by the opening of a small gap at low T in the far infrared. Its width is consistent with the observations of angle-resolved photoemission spectroscopy in other cuprates, along the nodal line of the k space. The gap forms as the Drude term turns into a far-infrared absorption, whose peak frequency can be approximately predicted on the basis of a Mott-like transition. Another band in the midinfrared softens with doping but is less sensitive to the metal-to-insulator transition.

High electron mobility transistors can work as room-temperature direct detectors of radiation at frequency much higher than their cutoff frequency. Here, we present a tool based on a Free Electron Laser source to study the detection mechanism and the coupling of the high frequency signal into the transistor channel. We performed a mapping over a wide area of the coupling of 0.15 THz radiation to an AlGaN/GaN transistors with cut-off frequency of 30 GHz. Local, polarization-dependent irradiation allowed us to selectively couple the signal to the channel either directly or through individual transistor bias lines, in order to study the nonlinear properties of the transistor channel. Our results indicate that HEMT technology can be used to design a millimeter-wave focal plane array with integrated planar antennas and readout electronics.