Topological insulators are a class of materials which have raised a great interest over the last decade, thanks to their intriguing conduction properties. Indeed, they are insulating in the bulk and metallic at the surface. Moreover, these metallic surface states have linear Dirac dispersion as in the case of graphene [1,2]. Bi2Se3 is among the most promising topological insulators, since its band structure provides only one Dirac cone, while the bulk gap is pretty large (about 300 meV) [3]. It has been demonstrated that by terahertz-infrared spectroscopy it is possible to detect the Dirac surface state by patterning thin films of Bi2Se3 with ribbons of width from 2 to 20 ?m. In this way, a Dirac plasmon is excited and its dispersion recovers very well the theoretical dispersion, calculated by using the parameters of the Dirac carriers [4]. In this scenario, we present here our investigation on the nonlinear regime of patterned films of Bi2Se3 with ribbons of width of 4 and 20 ?m. By exploiting the intense THz electric field of the TeraFERMI beamline at the FEL (Free Electron Laser) Fermi in Trieste [5], we were able to induce a nonlinear behaviour of the Dirac plasmon. Indeed, we observed a redshift of the plasmonic peak as the incoming THz electric field increases (up to MV/cm).

We have measured the electric signal downconverted from a terahertz frequency by an unbiased high mobility two-dimensional electron-gas (2DEG) device. The 2DEG was confined in an asymmetric plasmonic microcavity, and the radiation frequency was continuously tuned in the 0.2-0.4 THz range. The presence of resonant peaks at three frequencies corresponding to three plasma oscillation modes of the ungated 2DEG clearly points to the intrinsic nature of the hydrodynamic nonlinearity responsible for the downconversion as opposed to previously proposed plasmonic cavity configurations where the 2DEG oscillates under the metal gate that also acts as the source of the nonlinearity.

We target the nanofabrication of free-standing nanostructures made of epitaxial semiconductor material layers of high crystallinity quality and high heterostructure complexity for optical applications at the nanoscale. Here we demonstrate the fabrication method in the case of epitaxial germanium grown on a silicon substrate but the method can be applied to any heterostructure material. The nanostructures are fabricated out of planar epitaxial wafers in the form of pillars with arbitrary section and high aspect ratio by electron-beam lithography and deep reactive-ion etching. The patterned SiGe structures are then released by focused ion-beam milling of the pillar base. In this way, they become free-standing and can be relocated on a suitable substrate by using a nanomanipulator. Microscopic characterizations are ongoing to verify that the high crystal quality typical of epitaxial layers grown on a large-area substrate is preserved throughout the different fabrication steps.

In this work, we investigated the magnetotransport properties of a two dimensional electron gas hosted in an AlGaN/AlN/GaN heterostructure and one-dimensional devices fabricated on it. At cryogenic temperature, high mobility and long mean free path is achieved, allowing ballistic transport experiments. Longitudinal resistivity measured in Hall bar geometry shows well-developed Shubnikov-de Haas oscillations with amplitude modulation. Amongst possible mechanisms, the zero-field spin splitting may be the origin of the observed effects. Split gate quantum point contacts were fabricated by electron beam lithography. Linear conductance measurements at zero magnetic field show clear quantized conductance plateaus at 2e (2)/h and 4e (2)/h. Non-perfectly quantized conductance values are found for higher plateaus, suggesting the presence of impurity scattering.

Imaging arrays of direct detectors in the 0.5-5 THz range are being experimentally developed. Terahertz active imaging with amplitude-modulated quantum cascade lasers emitting at 2.5 and 4.4 THz performed by using an antenna-coupled superconducting microbolometer. We then present two room-temperature terahertz detector technologies compatible with monolithic arrays: i) GaAs Schottky diodes with air-bridge sub-micron anodes; ii) high electron mobility transistors with sub-micron Schottky gate. Performances, requirements and fabrication costs of the different detector technologies are compared.

Active spectroscopic imaging is based on arrays of broadband, short-response-time detectors observing a scene illuminated by a number of THz sources. Here we present the system design, the fabrication process and the single-pixel test for three detector technologies: GaAs Schottky diodes, GaN transistors, Nb bolometers.

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.

We explore the possibility of using an AlGaN/GaN high electron mobility transistor (HEMT) as a free-space coupled homodyne mixer. We used 150 GHz radiation from a Free Electron Laser, hence 5 times higher than the HEMT nominal band center at 30 GHz. The homodyne mixing provides a quasidc output signal, which makes the HEMT a detector of the radiation at 150 GHz.

In this work we present the electrical characterization of non self-aligned p-channel thin film transistors fabricated by using laser doping technique for source/drain contact formation and gate oxide deposited at room temperature by Electron Cyclotron Resonance Plasma Enhanced Chemical Vapour Deposition. These techniques are suitable for a very low temperature process for TFT fabrication. The output characteristics show a current increase at high drain voltage, (bkinkQ effect) rather moderate, if compared to self aligned polysilicon TFTs, probably due to the gradual doping profile induced by laser doping process. After bias stress at low gate voltage and high drain voltage condition a strong reduction of kink current has been observed in the output characteristics at high drain voltage, whereas minor changes has been observed in the transfer characteristics. This behaviour is similar to what observed in n-channel Gate Overlapped Thin Film Transistors. In the high gate voltage and high drain voltage bias-stress condition we observed only a rigid shift of the transfer characteristics probably related to charge injection in the gate oxide, while subthreshold slope remains constant.