Probing and modelling the localized self-mixing in a GaN/AlGaN field-effect terahertz detector

In a GaN/AlGaN field-effect terahertz detector, the directional photocurrent is mapped in the two-dimensional space of the gate voltage and the drain/source bias. It is found that not only the magnitude, but also the polarity, of the photocurrent can be tuned. A quasistatic self-mixing model taking into account the localized terahertz field provides a quantitative description of the detector characteristics. Strongly localized self-mixing is confirmed. It is therefore important to engineer the spatial distribution of the terahertz field and its coupling to the field-effect channel on the sub-micron scale.Comment: 12 pages, 4 figures, submitted to AP

Vibration Damping of Carbon Nanotube Assembly Materials

Vibration reduction is of great importance in various engineering applications, and a material that exhibits good vibration damping along with high strength and modulus has become more and more vital. Owing to the superior mechanical property of carbon nanotube (CNT), new types of vibration damping material can be developed. This paper presents recent advancements, including our progresses, in the development of high-damping macroscopic CNT assembly materials, such as forests, gels, films, and fibers. In these assemblies, structural deformation of CNTs, zipping and unzipping at CNT connection nodes, strengthening and welding of the nodes, and sliding between CNTs or CNT bundles are playing important roles in determining the viscoelasticity, and elasticity as well. Towards the damping enhancement, strategies for micro-structure and interface design are also discussed

Bio-Inspired Aggregation Control of Carbon Nanotubes for Ultra-Strong Composites

High performance nanocomposites require well dispersion and high alignment of the nanometer-sized components, at a high mass or volume fraction as well. However, the road towards such composite structure is severely hindered due to the easy aggregation of these nanometer-sized components. Here we demonstrate a big step to approach the ideal composite structure for carbon nanotube (CNT) where all the CNTs were highly packed, aligned, and unaggregated, with the impregnated polymers acting as interfacial adhesions and mortars to build up the composite structure. The strategy was based on a bio-inspired aggregation control to limit the CNT aggregation to be sub 20--50 nm, a dimension determined by the CNT growth. After being stretched with full structural relaxation in a multi-step way, the CNT/polymer (bismaleimide) composite yielded super-high tensile strengths up to 6.27--6.94 GPa, more than 100% higher than those of carbon fiber/epoxy composites, and toughnesses up to 117--192 MPa. We anticipate that the present study can be generalized for developing multifunctional and smart nanocomposites where all the surfaces of nanometer-sized components can take part in shear transfer of mechanical, thermal, and electrical signals

Spin transfer nano-oscillators

The use of spin transfer nano-oscillators (STNOs) to generate microwave signal in nanoscale devices have aroused tremendous and continuous research interest in recent years. Their key features are frequency tunability, nanoscale size, broad working temperature, and easy integration with standard silicon technology. In this feature article, we give an overview of recent developments and breakthroughs in the materials, geometry design and properties of STNOs. We focus in more depth on our latest advances in STNOs with perpendicular anisotropy showing a way to improve the output power of STNO towards the {\mu}W range. Challenges and perspectives of the STNOs that might be productive topics for future research were also briefly discussed.Comment: 11 pages, 10 figures, nanoscale 201

Detection of incoherent broadband terahertz light using antenna-coupled high-electron-mobility field-effect transistors

The sensitivity of direct terahertz detectors based on self-mixing of terahertz electromagnetic wave in field-effect transistors is being improved with noise-equivalent power close to that of Schottky-barrier-diode detectors. Here we report such detectors based on AlGaN/GaN two-dimensional electron gas at 77~K are able to sense broadband and incoherent terahertz radiation. The measured photocurrent as a function of the gate voltage agrees well with the self-mixing model and the spectral response is mainly determined by the antenna. A Fourier-transform spectrometer equipped with detectors designed for 340, 650 and 900~GHz bands allows for terahertz spectroscopy in a frequency range from 0.1 to 2.0~THz. The 900~GHz detector at 77~K offers an optical sensitivity about $1~\mathrm{pW/\sqrt{Hz}}$ being comparable to a commercial silicon bolometer at 4.2~K. By further improving the sensitivity, room-temperature detectors would find applications in active/passive terahertz imaging and terahertz spectroscopy.Comment: 4.5 pages, 5 figure

Enhanced photoelectric and photothermal responses on silicon platform by plasmonic absorber and omni-schottky junction

Recent progresses in plasmon-induced hot electrons open up the possibility to achieve photon harvesting beyond the fundamental limit imposed by band-to-band transitions in semiconductors. To obtain high efficiency, both the optical absorption and electron emission/collection are crucial factors that need to be addressed in the design of hot electron devices. Here, we demonstrate a photoresponse as high as 3.3mA/W at 1500nm on a silicon platform by plasmonic absorber (PA) and omni-Schottky junction integrated photodetector, reverse biased at 5V and illuminated with 10mW. The PA fabricated on silicon consists of a monolayer of random Au nanoparticles (NPs), a wide-band gap semiconductor (TiO2) and an optically thick Au electrode, resulting in broadband near-infrared (NIR) absorption and efficient hot-electron transfer via an all-around Schottky emission path. Meanwhile, time and spectral-resolved photoresponse measurements reveal that embedded NPs with superior absorption resembling plasmonic local heating sources can transfer their energy to electricity via the photothermal mechanism, which until now has not been adequately assessed or rigorously differentiated from the photoelectric process in plasmon-mediated photon harvesting nano-systems

Enhancement of Friction between Carbon Nanotubes: An Efficient Strategy to Strengthen Fibers

Interfacial friction plays a crucial role in the mechanical properties of carbon nanotube based fibers, composites, and devices. Here we use molecular dynamics simulation to investigate the pressure effect on the friction within carbon nanotube bundles. It reveals that the intertube frictional force can be increased by a factor of 1.5 ~ 4, depending on tube chirality and radius, when all tubes collapse above a critical pressure and when the bundle remains collapsed with unloading down to atmospheric pressure. Furthermore, the overall cross-sectional area also decreases significantly for the collapsed structure, making the bundle stronger. Our study suggests a new and efficient way to reinforce nanotube fibers, possibly stronger than carbon fibers, for usage at ambient conditions.Comment: revtex, 5 pages, accepted by ACS Nano 10 Dec 200

Metamaterial absorber integrated microfluidic terahertz sensors

Spatial overlap between the electromagnetic fields and the analytes is a key factor for strong light-matter interaction leading to high sensitivity for label-free refractive index sensing. Usually, the overlap and therefore the sensitivity are limited by either the localized near field of plasmonic antennas or the decayed resonant mode outside the cavity applied to monitor the refractive index variation. In this paper, by constructing a metal microstructure array-dielectric-metal (MDM) structure, a novel metamaterial absorber integrated microfluidic (MAIM) sensor is proposed and demonstrated in terahertz (THz) range, where the dielectric layer of the MDM structure is hollow and acts as the microfluidic channel. Tuning the electromagnetic parameters of metamaterial absorber, greatly confined electromagnetic fields can be obtained in the channel resulting in significantly enhanced interaction between the analytes and the THz wave. A high sensitivity of 3.5 THz/RIU is predicted. The experimental results of devices working around 1 THz agree with the simulation ones well. The proposed idea to integrate metamaterial and microfluid with a large light-matter interaction can be extended to other frequency regions and has promising applications in matter detection and biosensing

Wide-Range Tunable Dynamic Property of Carbon Nanotube-Based Fibers

Carbon nanotube (CNT) fiber is formed by assembling millions of individual tubes. The assembly feature provides the fiber with rich interface structures and thus various ways of energy dissipation, as reflected by the non-zero loss tangent (>0.028--0.045) at low vibration frequencies. A fiber containing entangled CNTs possesses higher loss tangents than a fiber spun from aligned CNTs. Liquid densification and polymer infiltration, the two common ways to increase the interfacial friction and thus the fiber's tensile strength and modulus, are found to efficiently reduce the damping coefficient. This is because the sliding tendency between CNT bundles can also be well suppressed by the high packing density and the formation of covalent polymer cross-links within the fiber. The CNT/bismaleimide composite fiber exhibited the smallest loss tangent, nearly as the same as that of carbon fibers. At a higher level of the assembly structure, namely a multi-ply CNT yarn, the inter-fiber friction and sliding tendency obviously influence the yarn's damping performance, and the loss tangent can be tuned within a wide range, as similar to carbon fibers, nylon yarns, or cotton yarns. The wide-range tunable dynamic properties allow new applications ranging from high quality factor materials to dissipative systems

The effect of Ga-doped nanocrystalline ZnO electrode on deep-ultraviolet enhanced GaN photodetector

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