Wireless communications in the terahertz band (0.1-10 THz) is a promising and key wireless technology enabling ultra-high data rate communication over multi-gigahertz-wide bandwidths, thus fulfilling the demand for denser networks. The complex propagation environment at such high frequencies introduces several challenges, such as high spreading and molecular absorption losses. As such, intelligent reflecting surfaces have been proposed as a promising solution to enable communication in the presence of blockage or to aid a resource-limited quasi-omnidirectional transmitter direct its radiated power. In this paper, we present a metallic reflectarray design achieving controlled non-specular reflection at true terahertz frequencies (i.e., 1-1.05 THz). We conduct extensive experiments to further characterize and validate its working principle using terahertz time-domain spectroscopy and demonstrate its effectiveness with information-carrying signals using a continuous-wave terahertz testbed. Our results show that the reflectarray can help facilitate robust communication links over non-specular paths and improve the reliability of terahertz communications, thereby unleashing the true potential of the terahertz band.

Terahertz (THz) plasma oscillations represent a potential path to implement ultrafast electronic devices and circuits. Here, we present an approach to generate on-chip THz signals that relies on plasma-wave stabilization in nanoscale transistors with specific structural asymmetry. A hydrodynamic treatment shows how the transistor asymmetry supports plasma-wave amplification, giving rise to pronounced negative differential conductance (NDC). A demonstration of these behaviors is provided in InGaAs high-mobility transistors, which exhibit NDC in accordance with their designed asymmetry. The NDC onsets once the drift velocity in the channel reaches a threshold value, triggering the initial plasma instability. We also show how this feature can be made to persist beyond room temperature (to at least 75 °C), when the gating is configured to facilitate a transition between the hydrodynamic and ballistic regimes (of electron-electron transport). Our findings represent a significant step forward for efforts to develop active components for THz electronics.

Terahertz time domain spectroscopy (THz TDS) is used to measure the melting kinetics of fructose molecular crystals. Combining single-crystal anisotropy measurements with density functional calculations, we assign the phonon frequencies and interrogate how specific phonons behave with melting. While nearly all the low-frequency phonons continuously red-shift with heating and melting, the lowest-energy phonon polarized along the c-axis blue-shifts at the melting temperature, suggesting an initial structural change immediately before melting. We find that the kinetics follow a 3D growth model with large activation energies, consistent with previous differential scanning calorimetry (DSC) measurements. The large activation energies indicate that multiple H-bonds must break collectively for the transition. The results suggest the generality of the kinetics for molecular crystals and that THz TDS with picosecond resolution could be used to measure ultrafast kinetics.

We demonstrate a computational study used to evaluate drop-on-demand printability of liquid metals via a contactless magnetohydrodynamic (MHD) pumping method. We show that the ejection regimes of pure liquid metal droplets can be categorized using two dimensionless quantities: We and a new dimensionless quantity S=Ha2Ca. By plotting We vs S, a linear relationship emerges which relates the velocity through the ejection orifice to the applied magnetic flux density. Additionally, satellite-free droplet generation is shown to be bounded by the ranges 1000≲S≲2000 and 10≲We≲20. These ranges, coupled with the linear We vs S relationship, allow one to predict the critical magnetic flux necessary to eject a satellite-free liquid metal droplet for any liquid metal with a very low viscosity to surface tension ratio (Oh<0.005). We discuss the physics underlying the MHD ejection process and relate the pump action to the dimensionless quantities. We use an MHD finite element model to parametrically sweep through applied magnetic fields and explore two-phase ejection of Al, Cu, Fe, Li, Sn, Ti, Zn, and Zr droplets from a 200 μm orifice. The model is validated using experimental high speed video ejection of Zn and Al, and the reported relationship between We and S can be used to connect the input flux density to the resulting ejection regime.

In addition to student assessment, curriculum assessment is a critical element to any pedagogy. It helps the educator assess the teaching of concepts, determine what may be lacking, and make changes for continual improvement. Meaningful assessment can be complicated when disciplines converge or when new approaches are implemented. To facilitate this, we present a network-based visualization schema to represent a materials informatics curriculum that combines materials science and data science concepts. We analyze the curriculum using network representations and relevant concepts from graph theory. This reveals established connections, linkages between materials science and data science, and the extent to which different concepts are connected. We also describe how some materials science topics are introduced from a data perspective, and present an illustrative case study from the curriculum.

Terahertz (THz)-band (0.1 THz to 10 THz) communication is envisioned as a key technology to meet the demand for faster, more ubiquitous wireless communication networks. For many years, the lack of compact, fast and efficient ways to generate, modulate, detect and demodulate THz-band signals has limited the feasibility of such communication systems. Recently, major progress within different device technologies is finally closing the so-called THz gap. For the time being, communication testbeds have been developed at sub-THz frequencies, i.e., at or near the boundary with millimeter-wave communication systems. Nonetheless, higher carrier frequencies and their associated bandwidth are needed to meet the demand for much higher data rates. In this paper, the TeraNova platform, i.e., the first integrated testbed for ultra-broadband wireless communications at true THz-band frequencies, is presented. The system consists of a transmitter and a receiver based on Schottky-diode frequency multiplying and mixing chains able to up & down-convert an information-bearing intermediate frequency (IF) signal up to 40 GHz-wide between 1 and 1.05 THz, i.e., the first absorption-defined transmission window above 1 THz. Guided by the experimental characterization of the THz channel in terms of path-loss and noise, tailored framing, time synchronization, channel estimation and single- and multi-carrier modulation techniques are implemented in software and realized by a state-of-the-art arbitrary waveform generator and a digital storage oscilloscope at the transmitter and the receiver, respectively. Experimental results are presented herein to highlight the opportunities and challenges to unleash the potential of the THz band.

Terahertz time domain spectroscopy is used to measure the melting kinetics of fructose molecular crystals. Combining single crystal anisotropy measurements with density functional calculations we assign the phonon frequencies and interrogate how specific phonons behave with melting. While nearly all the low frequency phonons continuously red shift with heating and melting, the lowest energy phonon polarized along the c-axis blue shifts at the melting temperature, suggesting an initial structural change immediately before melting. We find that the kinetics follow a 3D growth model with large activation energies consistent with previous DSC measurements. The large activation energies indicate multiple H-bonds must break collectively for the transition. The results suggest THz TDS with sub picosecond resolution could be used to measure ultra-fast kinetics.

Terahertz (THz)-band (0.1-10 THz) communication is envisioned as a key wireless technology to fulfill the demand for increasing data rates and to accommodate denser networks. The THz-band, however, suffers from very high propagation losses further aggravated by the presence of obstacles in common scenarios, that behave as opaque barriers at THz frequencies. Engineering non-line-of-sight (NLoS) communication links with-smart reflectarrays is one possible method of overcoming the complex THz communication model. However, existing reflectarray designs at lower frequencies cannot be simply repurposed due to the operating failure of the traditional control elements at THz frequencies. In this direction, the use of 2D nanomaterials, such as graphene, to design tuning elements and integrate these into THz reflectarrays is being explored. This paper presents a novel graphene-based tuning element for continuous phase control of the reflecting element, in situ. The fundamental radiating element is designed to have high reflection efficiency and tunability by leveraging the properties of metals and graphene, respectively. First, the working principle and design of the proposed tuning element, comprised of a graphene-based plasmonic waveguide, is described and explained. Second, the trade-offs in the design of the hybrid tunable reflecting element, resulting from the integration of the tuning element with a metallic patch, are exhaustively studied. After discussing the integration of multiple reflecting elements in a reflectarray, the ability to perform complete continuous dynamic beamforming is presented.

Terahertz (THz)-band (0.1 - 10 THz) communication is envisioned as a key wireless technology to fulfill the demand for dense networks and higher data rates. To overcome the complex THz communication channel, smart reflecting surfaces can be utilized to facilitate Non-Line of Sight (NLOS) communication and increase efficiency. In this light, the use of new 2D nanomaterials, such as graphene, to design reprogrammable reflectarrays is being explored. This paper presents a novel graphene–metal hybrid reflectarray for THz communications. The radiating element of the reflectarray is designed to have strong reflection efficiency and high tunability. The trade-offs in the design of the hybrid element are exhaustively studied. The ability to perform continuous dynamic beamforming is presented. Extensive numerical results are provided to demonstrate the functionality of the reflectarray to engineer NLOS paths for communication.

We report the false appearance of dispersion in non-dispersive materials when measured by terahertz time-domain spectroscopy. This occurs when the material is measured in reflection geometry and has a bulk metal interface opposite to the incident interface, for example, when a substrate is supported by a metal stage with the THz light incident on it from above. We explain this effect in terms of the frequency-dependent response of the material when it is represented by a shorted transmission line model.

In this work, we realized the requisite asymmetric boundary conditions across a plasmonic cavity, which is formed inside the gated region of a high electron mobility transistor (HEMT), to trigger the Dyakonov-Shur plasma wave instability.

Terahertz time-domain spectroscopy (THz-TDS) relies heavily on knowing precisely the thickness or refractive index of a material. In practice, one of these values is assumed to be known, or their product is numerically optimized to converge on suitable values. Both approaches are prone to errors and may mask some real features or properties of the material being studied. To eliminate these errors, we use THz-TDS in reflection geometry to accurately and independently determine both thickness and refractive index by illuminating the step-edge of a substrate atop a metal stage. This method relies solely on the relative time delay among three reflected pulses, and therefore forgoes the need for optimization or assumption of substrate parameters.

We fabricated terahertz-frequency plasmonic antenna arrays using patterned metallic structures atop a layer of graphene. The antenna design was developed through numerical simulations that were informed by experimentally obtained graphene parameters. Experimental characterization reveals a clear reflection enhancement at 3λ/2, the intensity of which was modified by underlying graphene.

We present that the electrical conduction type in carbon nanotube field-effect transistors (CNT-FETs) can be converted by induced charges in a polyvinyl alcohol (PVA) insulator. When the CNT channels are covered with pure PVA, the FET characteristics clearly change from unipolar p-type to ambipolar. The addition of ammonium ions (NH4+) in the PVA leads to further conversion to unipolar n-type conduction. The capacitance − voltage characteristics indicate that a high density of positive charges is induced at the PVA/SiO2 interface and within the bulk PVA. Electrons are electrostatically accumulated in the CNT channels due to the presence of the positive charges, and thus, stable n-type conduction of PVA-coated CNT-FETs is observed, even under ambient conditions. The mechanism for conversion of the conduction type is considered to be electrostatic doping due to the large amount of positive charges in the PVA. A blue-shift of the Raman G-band peak was observed for CNTs coated with NH4+-doped PVA, which corresponds to unipolar n-type CNT-FET behavior. These results confirm that carrier polarity engineering in CNT-FETs can be achieved with a charged PVA passivation layer.

Polymer and nanomaterial composites, known as nanocomposites, are advanced materials that have many potential applications. One type of thin-film nanocomposite is a polymer-carbon nanotube (CNT) nanocomposite, whose properties are strongly dependent on the uniformity and alignment of CNTs in the nanocomposite. However, the control of CNT alignment in these nanocomposites is still difficult to achieve. Here, we propose a facile single-step method, a capillary flow intrusion method, to fabricate uniform polymer nanocomposite thin films of vertically aligned (VA) single-/multi-walled CNTs that range from 15 to 300 mu m in thickness. Raman scattering spectroscopy and polarized Raman spectroscopy measurements, and cross-sectional scanning electron microscopy (SEM) observations confirmed that the polymer was uniformly infiltrated into VACNTs, such that the alignment of CNTs in the nanocomposite was preserved and there were no excess portions of the polymer. Experimental and numerical calculation results indicated that capillary flow rate, wettability, and polymer shrinkage are important factors in the capillary flow intrusion method. (C) 2018 The Japan Society of Applied Physics

Graphene's ability to support surface plasmon polaritons (SPPs) is of particular interest in the design of nanoscale plasmonic antennas. Since a dielectric-conductor interface is required to excite and sustain SPPs, a negative dielectric function becomes a defining property for graphene. We use terahertz time-domain spectroscopy (THz-TDS) to determine the complex dielectric function of graphene based on the extracted complex conductivity. These optical properties help us ascertain if a graphene sample is capable of supporting plasmons, and the appropriate dimensions to define a resonant cavity that would act as an antenna in the THz range.