Alessandra Di Gaspare

Terahertz current oscillations in a gated two-dimensional electron gas with antenna integrated at the channel ends

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 0.18–0.72u2009THz 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 300u2009V/W and noise equivalent power of 1u2009nW/Hz0.5

Heterodyne and subharmonic mixing at 0.6 THz in an AlGaAs/InGaAs/AlGaAs heterostructure field effect transistor

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 1u2009μm. We investigated frequency mixing in the transistors channel by measuring, with a quasi-optical setup, the heterodyne, second- and third-order subharmonic mixing signal at 0.592u2009THz. 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.

Topologically protected Dirac plasmons and their evolution across the quantum phase transition in a (Bi1−xInx)2Se3 topological insulator

A 3D Topological Insulator (TI) is an intrinsically stratified electronic material characterized by an insulating bulk and Dirac free electrons at the interface with vacuum or another dielectric. In this paper, we investigate, through terahertz (THz) spectroscopy, the plasmon excitation of Dirac electrons on thin films of (Bi1-xInx)2Se3 TI patterned in the form of a micro-ribbon array, across a Quantum Phase Transition (QPT) from the topological to a trivial insulating phase. The latter is achieved by In doping onto the Bi-site and is characterized by massive electrons at the surface. While the plasmon frequency is nearly independent of In content, the plasmon width undergoes a sudden broadening across the QPT, perfectly mirroring the single particle (free electron) behavior as measured on the same films. This strongly suggests that the topological protection from backscattering characterizing Dirac electrons in the topological phase extends also to their plasmon excitations.

Spectroscopic study of plasma wave resonances of a two-dimensional electron gas in a microcavity at low temperatures

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 0.15–0.4xa0THz 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 10xa0K is found at specific radiation frequencies, indicating the formation of standing plasma waves in the microcavity formed by the channel.

Differential Fano interference spectroscopy of subwavelength hole arrays for mid-infrared mass sensors

We studied mid-infrared sensors based on the wavelength shift of Surface Plasmon Polariton resonances upon solid substance deposition on subwavelength hole arrays in a thin metal film (metal meshes). We present an experimental and numerical investigation of the mid-infrared transmission of metal meshes with and without a dielectric substrate, and we develop an analytical model which describes the Fano interference between the Bethe continuum and the SPP resonances. Fitting the differential transmission signal, measured before and after deposition of a target solid film, we demonstrate sensitivity down to few molecular monolayers, at least one order of magnitude better than mid-infrared vibrational spectroscopy. Sensor calibration was performed on thin polymer films and an example of real application is then provided by measuring the optical density of phospholipid membrane complexes with thickness in the range 2-10 nm.

Progress in producing terahertz detector arrays

A ‘terahertz camera,’ capable of detecting electromagnetic radiation at frequencies between 300GHz and 3THz, would enable production of real-time, noncontact images of objects concealed behind dielectric materials, including clothing, packages, envelopes, paper, and plastic wraps. With a sufficiently high resolution for direct object identification (see Figure 1), such a device would trigger development of innovative security systems with a double advantage of increased effectiveness and higher throughput over visual/manual inspection. But so far, no practical, mainstream technology has emerged to generate, handle, and detect terahertz radiation. The first route toward the terahertz camera was paved by radio astronomers, who employed cryogenically cooled superconducting electronics to image the cosmic-background radiation. Such systems have excellent performance, but they are low-volume applications manufactured at very high cost. Micrometric-textured materials, such as paper, plastics, and clothing, are opaque to visible and IR light because of light scattering rather than absorption. The transparency of these materials to terahertz radiation derives from its wavelength (0.1–1.0mm). The terahertz wavefront passes through small obstacles without significant deviations, just like sound waves can freely travel through a cornfield or water waves can propagate through a cane thicket. For the same reason, terahertz radiation can travel much further than IR light through aerosols made of micrometric particles like dust, smoke, or fog. With these capabilities in mind, a second route to terahertz imaging adapts existing thermal imagers—long-wave IR cameras—by changing their pixel size and optics.1 However, blackbody emission from room-temperature objects rapidly drops for wavelengths longer than 10 m, making passive terahertz imaging difficult. In addition, active thermal sensors discard both phase and spectroscopic information about the radiation they detect. Figure 1. A transmission image at 2.5THz, generated with a cryogenically cooled quantum-cascade laser and a superconducting detector, shows a miniature box cutter (black, zero transmission) inside an opaque polyethylene bag (orange, 60% transmission). This highperformance cryogenic imaging system demonstrates the capability of terahertz imaging for security applications. (Courtesy: S. Cibella.2)

Subharmonic mixing at 0.6 THz in an AlGaAs/InGaAS/AlGaAs field effect transistor

We measured the nonlinear response of field effect transistors fabricated with GaAs-based heterostructures by performing direct detection, heterodyne and subharmonic mixing measurements. The study of the spectral responsivity as a function of different antenna coupling is presented in the 0.18-0.4 GHz range. We also verified the subharmonic and heterodyne mixing at 0.6 THz in a HFET detector with a broadband antenna.

Antenna-coupled heterostructure field effect transistors for integrated terahertz heterodyne mixers

We present the realization of high electron mobility transistors on GaN-heterostructures usable for mixing and rectification in the THz range. Device fabrication is fully compatible with industrial processes employed for millimetre wave integrated circuits. On-chip, integrated, polarization-sensitive, planar antennas were designed to allow selective coupling of THz radiation to the three terminals of field effect transistors in order to explore different mixing schemes for frequencies well above the cutoff frequency for amplification. The polarization dependence of the spectral response in the 0.18-0.40 THz range clearly demonstrated the possible use as integrated heterodyne mixers.

Monolithic focal plane arrays for terahertz active spectroscopic imaging: an experimental study

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.