Comparative analysis of resonant phonon THz quantum cascade lasers

We present a comparative analysis of a set of GaAs-based THz quantum cascade lasers, based on longitudinal-optical phonon scattering depopulation, by using an ensemble Monte Carlo simulation, including both carrier-carrier and carrier-phonon scattering. The simulation shows that the parasitic injection into the states below the upper laser level limits the injection efficiency and thus the device performance at the lasing threshold. Additional detrimental effects playing an important role are identified. The simulation results are in reasonable agreement with the experimental findings.Comment: 3 pages, 3 figure

Photocurrent-based detection of Terahertz radiation in graphene

Graphene is a promising candidate for the development of detectors of Terahertz (THz) radiation. A well-known detection scheme due to Dyakonov and Shur exploits the confinement of plasma waves in a field-effect transistor (FET), whereby a dc photovoltage is generated in response to a THz field. This scheme has already been experimentally studied in a graphene FET [L. Vicarelli et al., Nature Mat. 11, 865 (2012)]. In the quest for devices with a better signal-to-noise ratio, we theoretically investigate a plasma-wave photodetector in which a dc photocurrent is generated in a graphene FET. The rectified current features a peculiar change of sign when the frequency of the incoming radiation matches an even multiple of the fundamental frequency of plasma waves in the FET channel. The noise equivalent power per unit bandwidth of our device is shown to be much smaller than that of a Dyakonov-Shur detector in a wide spectral range.Comment: 5 pages, 4 figure

Se-doping dependence of the transport properties in CBE-grown InAs nanowire field effect transistors

We investigated the transport properties of lateral gate field effect transistors (FET) that have been realized by employing, as active elements, (111) B-oriented InAs nanowires grown by chemical beam epitaxy with different Se-doping concentrations. On the basis of electrical measurements, it was found that the carrier mobility increases from 103 to 104 cm2/(V × sec) by varying the ditertiarybutyl selenide (DtBSe) precursor line pressure from 0 to 0.4 Torr, leading to an increase of the carrier density in the transistor channel of more than two orders of magnitude. By keeping the DtBSe line pressure at 0.1 Torr, the carrier density in the nanowire channel measures ≈ 5 × 1017 cm-3 ensuring the best peak transconductances (> 100 mS/m) together with very low resistivity values (70 Ω × μm) and capacitances in the attofarad range. These results are particularly relevant for further optimization of the nanowire-FET terahertz detectors recently demonstrated

High performance bilayer-graphene Terahertz detectors

We report bilayer-graphene field effect transistors operating as THz broadband photodetectors based on plasma-waves excitation. By employing wide-gate geometries or buried gate configurations, we achieve a responsivity $\sim 1.2V/W (1.3 mA/W)$ and a noise equivalent power $\sim 2\times 10^{-9} W/Hz^{-1/2}$ in the 0.29-0.38 THz range, in photovoltage and photocurrent mode. The potential of this technology for scalability to higher frequencies and the development of flexible devices makes our approach competitive for a future generation of THz detection systems.Comment: 8 pages, 5 figures. Submitted to Applied Physics Letter

Terahertz near-field nanoscopy based on detectorless laser feedback interferometry under different feedback regimes

Near-field imaging techniques, at terahertz frequencies (1–10 THz), conventionally rely on bulky laser sources and detectors. Here, we employ a semiconductor heterostructure laser as a THz source and, simultaneously, as a phase-sensitive detector, exploiting optical feedback interferometry combined with scattering near-field nanoscopy. We analyze the amplitude and phase sensitivity of the proposed technique as a function of the laser driving current and of the feedback attenuation, discussing the operational conditions ideal to optimize the nano-imaging contrast and the phase sensitivity. As a targeted nanomaterial, we exploit a thin (39 nm) flake of Bi2Te2.2Se0.8, a topological insulator having infrared active optical phonon modes. The self-mixing interference fringes are analyzed within the Lang–Kobayashi formalism to rationalize the observed variations as a function of Acket's parameter C in the full range of weak feedback (C < 1)