Low Bias Current Research Articles

Subcarrier Pairwise Coding for Short-Haul L/E-ACO-OFDM

Dispersion-induced power fading in intensity modulated direct-detection systems degrades the higher frequency subcarriers’ signal qualities. In layered/enhanced asymmetrically clipped optical OFDM (ACO-OFDM), which is a spectrally efficient version of ACO-OFDM, the error propagation in the iterative decoding can further degrade the performance, especially when a low bias current is used for the directly modulated laser. In this letter, we propose using pairwise coding within each layer to improve the bit error rate. We transmitted ~5-GBb 16-QAM OFDM signals over a 19.8-km single mode fiber, and found a 3.6-dB improvement in the $Q^{2}$ -factor.

Robust low-bias negative differential resistance in graphene superlattices

In this work, we present a detailed theoretical study on the low bias current–voltage (I–V) characteristic of biased planar graphene superlattice (PGSL), provided by a heterostructured substrate and a series of grounded metallic planes placed over a graphene sheet, which induce a periodically modulated Dirac gap and Fermi velocity barrier, respectively. We investigate the effect of PGSL parameters on the I–V characteristic and the appearance of multipeak negative differential resistance (NDR) in the proposed device within the Landauer–Buttiker formalism and adopted transfer matrix method. Moreover‚ we propose a novel venue to control the NDR in PGSL with Fermi velocity barrier. Different regimes of NDR have been recognized, based on the PGSL parameters and external bias. From this viewpoint‚ we obtain multipeak NDR through miniband aligning in PGSL. The maximum pick to valley ratio (PVR) up to 167 obtained for , the Fermi velocity correlation (ratio of Fermi velocity in barrier and well region), is 1.9 at bias voltages between 70–130 mV. Our findings have good agreement with experiments and can be considered in designing multi-valued memory‚ functional circuit, low power and high-speed nanoelectronic device applications.

Development of a Sensitive Multiplexed Open Circuit Potential System for the Detection of Prostate Cancer Biomarkers

We report the development of a sensitive label-free, cost-effective detection system with simultaneous multi-channel measurement of open circuit potential (OCP) variations for the detection of prostate specific antigen (PSA). We demonstrate a significant increase of 600 times in the sensitivity as compared to the reported literature. To accurately measure OCP variations, a complete monolithic field-effect transistor (FET)-input ultra-low input bias current instrumentation amplifier is used to form the electronic circuit to measure the variation between a working electrode and a reference electrode. This amplifier electronic system setup provides a differential voltage measurement with high input impedance and low input bias current. Since no current is applied to the electrochemical system, a true and accurate measurement of the variation can be performed. This is the first report on the use of DNA aptamers with an OCP system where we employed a DNA aptamer against PSA. An optimised ratio of anti-PSA DNA aptamer with 6-mercapto-1-hexanol (MCH) was used to fabricate the aptasensor using gold electrodes. The electrodes are hosted in a cell with an automated flow system. A wide range of concentrations of PSA (0.1 to 100 ng/mL) were injected through the system. The sensor could potentially differentiate 0.1 ng/mL PSA from blank measurement, which is well below the required clinical range (>1 ng/mL). The sensor was also challenged with 4% human serum albumin and human kallikrein2 as control proteins where the sensor demonstrated excellent selectivity. The developed system can be further generalised to various other targets using specific probes.

Superconducting single X-ray photon detector based on W0.8Si0.2

We fabricated a superconducting single X-ray photon detector based on W0.8Si0.2, and we characterized its basic detection performance for keV-photons at different temperatures. The detector has a critical temperature of 4.97 K, and it is able to be operated up to 4.8 K, just below the critical temperature. The detector starts to react to X-ray photons at relatively low bias currents, less than 1% of Ic at T = 1.8 K, and it shows a saturated count rate dependence on bias current at all temperatures, indicating that the optimum internal quantum efficiency can always be reached. Dark counts are negligible up to the highest investigated bias currents (99% of Ic) and operating temperature (4.8 K). The latching effect affects the detector performance at all temperatures due to the fast recovery of the bias current; however, further modifications of the device geometry are expected to reduce the tendency for latching.

A Modelling and Nonlinear Equalization Technique for a 20 Gb/s 0.77 pJ/b VCSEL Transmitter in 32 nm SOI CMOS

This paper describes an ultralow-power VCSEL transmitter in 32 nm SOI CMOS. To increase its power efficiency, the VCSEL is driven at a low bias current. Driving the VCSEL in this condition increases its inherent nonlinearity. Conventional pre-emphasis techniques cannot compensate for this effect because they have a linear response. To overcome this limitation, a nonlinear equalization scheme is proposed. A dynamic VCSEL modelling technique is used to generate the time-domain optical responses for “one” and “zero” bits. Based on the asymmetry of the two responses, the rising and falling edges are equalized separately. Additionally, instead of using fixed bit delays, the equalization delay is selected based on the bias current of the VCSEL. The efficiency of the proposed modelling and equalization technique is evaluated through simulations and measurements. The transmitter achieves energy efficiency of 0.77 pJ/b at 20 Gb/s and occupies $100 \upmu\text{m} \times 60 \upmu\text{m}$ active silicon area.

Effect of electrical properties of glass electrodes on the performance of RPC detectors for the INO-ICAL experiment

The India-based Neutrino Observatory (INO) collaboration has chosen glass Resistive Plate Chambers (RPCs) as the active detector elements for the Iron Calorimeter (ICAL) experiment. In the present work, we study the electrical properties such as bulk resistivity and relative permittivity of the glasses from two different manufacturers and compared the performances of RPCs built using these glasses. We conclude that the glass electrodes with larger bulk resistivity and permittivity are better suited for manufacturing RPCs for the ICAL experiment, as these detectors could be operated at lower bias currents and voltages, and produce better time resolution compared to those built with glass electrodes of smaller bulk resistivity and permittivity.

P-doping and efficient carrier injection induced by graphene oxide for high performing WSe2 rectification devices

In this work, we fabricated multi-layer WSe2 rectifying diodes using graphene oxide (GO) as p-doping material on one side of the contacting electrodes. This GO layer can reduce the contact resistance by forming a tunneling barrier for efficient hole injection, while it increases the contact resistance for the injection of electrons. Results of Raman shift spectra and the opto-electric response of the device confirmed the p-doping effect caused by the GO layer and the formation of a barrier, respectively. We observed a gate tunable rectification effect with a forward/reverse current ratio of 104 and low reverse bias current of 10−10 A. Applying a GO layer in the fabrication of two-dimensional transition metal dichalcogenides based devices is a very useful method in the applications in future nanotechnologies.

Mechanisms of NH3 and NO2 detection in carbon-nanotube-based sensors: An ab initio investigation

The mechanisms of NH3 and NO2 detection by single-walled carbon nanotube-based devices are investigated by ab initio calculations and the non-equilibrium Greens function (NEGF) methodology. While both NH3 and NO2 can physisorb to a pristine carbon nanotube, we show that their adsorption only results in small current changes through the device. For a carbon nanotube (CNT) attached to gold nanowire leads, the most sensitive detection site is at the CNT near the CNT-Au contact, where chemisorption occurs. The resulting change in electron transmission and low-bias current can lead to over 30% sensitivity. While both NH3 and NO2 can also chemisorb at the Au electrodes, their adsorption results in only a small change in the plurality of the conducting levels of the gold layers, and thus a small effect on current. In order to enhance the detection sensitivity, it is thus beneficial to mask the electrodes to prevent chemisorption. Furthermore, the length of the pure CNT segment does not strongly affect the relative sensitivity. Our results suggest that a short-CNT device with exposed contact regions and masked electrodes would have the greatest sensitivity.

Adaptive and Non-adaptive Variable Structure Controls with Sliding Mode for Active Magnetic Bearings (AMBs) and Magnetic Levitation (MAGLEV) Systems: A Comparative Study

Abstract: In this paper we will present on two methods for high performance active magnetic bearings (AMBs). These methods based on robust Variable Structure Control (VSC) with sliding mode in non-adaptive and adaptive cases. Our approach combines two simple architectures often referred to as Dynamic Feedback Linearization (DFL) and recent advances in robust control design techniques to form a new approach for AMB for two cases. The procedure enables the designer to explicitly define the desired closed loop dynamics and enables the design of a robust stabilizing controller that forces the system dynamics to the specified desired dynamics, despite gravitational and centrifugal disturbances, modeling uncertainties, and variations in mass, moments of inertia, radii of eccentricities and center of mass of the rotor. Our design hangs the magnetic bearing on gravitational parabolic sliding surface by switching electronics and sliding mode control components. Thus, electromagnets do not constitute cross forces on rotor shaft that affects against each other. This operation guarantees the lowest power losses, by considering low bias currents method that the currents affects against. The methods also guarante both stability and performance robustness and enable the design for other unstable magnetic levitation systems. When the methods are compared with each other, the results in adaptive case are more efficient about high speed and wide range of parameter uncertainties.

1060-nm 28-Gb/s <inline-formula> <tex-math notation="LaTeX">$\times 4$ </tex-math></inline-formula>-Channel Modules Operated Over the Case Temperature Range

To reduce the power consumption of 4-channel 28-Gb/s miniature optical modules, a 4-channel double intra-cavity 1060-nm InGaAs/GaAs quantum-well vertical-cavity surface-emitting laser (VCSEL) array with 5-μm-wide apertures, a high differential gain, and a low temperature-insensitive differential resistance in conjunction with a specially optimized thermo-electric cooler heat sink for a reduced thermal resistance are used. Most important is the excellent resistance compatibility between the VCSEL and a low-power SiGe BiCMOS driver IC. Consequently, a 4-channel 28-Gb/s NRZ PRBS 2 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">31</sup> - 1 link operation at a bit error rate of 10-12 for an operational case temperature ranging from 25 °C to 70 °C was achieved at an unprecedentedly low VCSEL bias current of only 4 mA.

External Optical Feedback Effects in Semiconductor Nanolasers

The response of nanolasers subject to external optical feedback has been analyzed. Calculations have been performed using rate equations, which include the Purcell cavity-enhanced spontaneous emission factor F and the spontaneous emission coupling factor β. In the analysis, the influence of F and β is evaluated for varying distance from external mirror, current, and feedback rate. It is observed that, in general, increased F and β at low bias currents increases the critical feedback for which chaos occurs as compared to conventional lasers, whereas at higher bias currents, chaos occurs at lower feedback rates. It is also found that for larger F , when increasing the distance from external mirror, the feedback rate at which chaos occurs increases.

Quasiparticle recombination in hotspots in superconducting current-carrying nanowires

We describe a kinetic model of recombination of nonequilibrium quasiparticles generated by single photon absorption in superconducting current-carrying nanowires. The model is developed to interpret two-photon detection experiments in which a single photon does not possess sufficient energy for breaking superconductivity at a fixed low bias current. We show that quasiparticle self-recombination in relaxing hotspots dominates diffusion expansion effects and explains the observed strong bias current, wavelength, and temperature dependencies of hotspot relaxation in tungsten silicide superconducting nanowire single-photon detectors.

High-Speed VCSELs With Strong Confinement of Optical Fields and Carriers

We present the design, fabrication, and performance of our latest generation high-speed oxide-confined 850-nm vertical-cavity surface-emitting lasers. Excellent high-speed properties are obtained by strong confinement of optical fields and carriers. High-speed modulation is facilitated by using the shortest possible cavity length of one half wavelength and placing oxide apertures close to the active region to efficiently confine charge carriers. The resulting strong current confinement boosts internal quantum efficiency, leading to low threshold currents, high wall-plug efficiency, and state-of-the-art high-speed properties at low bias currents. The temperature dependent static and dynamic performance is analyzed by current-power-voltage and small-signal modulation measurements.

Preparation and characterization of PTFE coating in new polymer quartz piezoelectric crystal sensor for testing liquor products**Project supported by the National High Technology Research and Development Program of China (Grant No. 2013AA030901).

A new method was developed based on the electron beam vacuum dispersion (EBVD) technology to prepare the PTFE polymer coating of the new polymer quartz piezoelectric crystal sensor for testing liquor products. The new method was applied in the new EBVD equipment which we designed. A real-time system monitoring the polymer coating’s thickness was designed for the new EBVD equipment according to the quartz crystal microbalance (QCM) principle, playing an important role in preparing stable and uniform PTFE polymer coatings of the same thickness. 30 pieces of PTFE polymer coatings on the surface of the quartz crystal basis were prepared with the PTFE polymer ultrafine powder (purity ≥ 99.99%) as the starting material. We obtained 30 pieces of new PTFE polymer sensors. By using scanning electron microscopy (SEM), the structure of the PTFE polymer coating’s column clusters was studied. One sample from the 30 pieces of new PTFE polymer sensors was analysed by SEM in four scales, i.e., 400×, 1000×, 10000×, and 25000×. It was shown that under the condition of high bias voltage and low bias current, uniformly PTFE polymer coating could be achieved, which indicates that the new EBVD equipment is suitable for mass production of stable and uniform polymer coating.

Electron-vibron coupling effects on electron transport via a single-molecule magnet

We investigate how the electron-vibron coupling influences electron transport via an anisotropic magnetic molecule, such as a single-molecule magnet (SMM) Fe$_4$, by using a model Hamiltonian with parameter values obtained from density-functional theory (DFT). Magnetic anisotropy parameters, vibrational energies, and electron-vibron coupling strengths of the Fe$_4$ are computed using DFT. A giant spin model is applied to the Fe$_4$ with only two charge states, specifically a neutral state with the total spin $S=5$ and a singly charged state with $S=9/2$, which is consistent with our DFT result and experiments on Fe$_4$ single-molecule transistors. In sequential electron tunneling, we find that the magnetic anisotropy gives rise to new features in conductance peaks arising from vibrational excitations. In particular, the peak height shows a strong, unusual dependence on the direction as well as magnitude of applied B field. The magnetic anisotropy also introduces vibrational satellite peaks whose position and height are modified with the direction and magnitude of applied B field. Furthermore, when multiple vibrational modes with considerable electron-vibron coupling have energies close to one another, a low-bias current is suppressed, independently of gate voltage and applied B field, although that is not the case for a single mode with the similar electron-vibron coupling. In the former case, the conductance peaks reveal a stronger B-field dependence than in the latter case. The new features appear because the magnetic anisotropy barrier is of the same order of magnitude as the energies of vibrational modes with significant electron-vibron coupling. Our findings clearly show the interesting interplay between magnetic anisotropy and electron-vibron coupling in electron transport via the Fe$_4$. The similar behavior can be observed in transport via other anisotropic magnetic molecules.

Multisource and Battery-Free Energy Harvesting Architecture for Aeronautics Applications

We suggest an innovative architecture for an efficient energy generator devoted to the powering of a wireless sensor network deployed for aircraft health monitoring. This battery-free generator captures energy from its environment (transient thermal gradients as a main source, and vibrations as a secondary source allowing early biasing of the generator) and stores this energy in ultracapacitors. In this way, this multisource architecture benefits from the synergy between energy scavenging and harvesting: vibrations bring low but early and permanent energy. They also contribute to energy harvesting during cruise while thermal gradients have vanished. The use of active diodes and of a very low bias current of 10 nA/branch allow achieving ultralow power consumption, experimentally demonstrated on two different CMOS technologies. It is also proven that enough energy could be delivered to power the functions of a wireless sensor node.

Dual-gated MoS2/WSe2 van der Waals tunnel diodes and transistors.

Two-dimensional layered semiconductors present a promising material platform for band-to-band-tunneling devices given their homogeneous band edge steepness due to their atomically flat thickness. Here, we experimentally demonstrate interlayer band-to-band tunneling in vertical MoS2/WSe2 van der Waals (vdW) heterostructures using a dual-gate device architecture. The electric potential and carrier concentration of MoS2 and WSe2 layers are independently controlled by the two symmetric gates. The same device can be gate modulated to behave as either an Esaki diode with negative differential resistance, a backward diode with large reverse bias tunneling current, or a forward rectifying diode with low reverse bias current. Notably, a high gate coupling efficiency of ∼80% is obtained for tuning the interlayer band alignments, arising from weak electrostatic screening by the atomically thin layers. This work presents an advance in the fundamental understanding of the interlayer coupling and electron tunneling in semiconductor vdW heterostructures with important implications toward the design of atomically thin tunnel transistors.

1.55-μm distributed feedback laser monolithically integrated with amplifier array.

We present a laterally coupled 1.55-μm distributed feedback laser monolithically integrated with multistage multimode interferences and semiconductor optical amplifiers, using low-bias currents and providing an output power of ∼100 mW with a quasi-single spatial-mode far-field pattern and low divergence angle of 3.5° in the horizontal direction. The fabrication techniques are based on side-wall gratings and quantum-well intermixing and offer a simple, flexible, and low cost alternative to conventional methods.

The effect of Duschinsky rotation on charge transport properties of molecular junctions in the sequential tunneling regime.

We present here a systematic theoretical study on the effect of Duschinsky rotation on charge transport properties of molecular junctions in the sequential tunneling regime. In the simulations we assume that only two electronic charging states each coupled to a two dimensional vibrational potential energy surface (PES) are involved in the transport process. The Duschinsky rotation effect is accounted by varying the rotational angle between the two sets of displaced PESs. Both harmonic potential and anharmonic Morse potential have been considered for the cases of the intermediate to strong electron-vibration couplings. Our calculations show that the inclusion of the Duschinsky rotation effect can significantly change the charge transport properties of a molecular junction. Such an effect makes the otherwise symmetric Coulomb diamond become asymmetric in harmonic potentials. Depending on the angle of the rotation, the low bias current could be significantly suppressed or enhanced. This effect is particularly prominent in the Franck-Condon (FC) blockade regime where the electron-vibration coupling is strong. These changes are caused by the variation of the FC factors which are closely related to the rotational angle between the two sets of PESs involved in the charge transport process. For a molecular junction with Morse potentials, the changes caused by Duschinsky rotation are much more complicated. Both the amplitude and shape of the Coulomb diamond are closely dependent on the rotational angle in the whole range from 0 to 2π. One interesting result is that with a rotation angle of π (and also π/2 for certain cases) symmetric Coulomb diamonds can even be formed from the intrinsically asymmetric Morse potential. These results could be important for the interpretation of experimental observations.

Significant modulation of electrical spin accumulation by efficient thermal spin injection

We have investigated the dc bias current dependence of the nonlocal spin valve signal in CoFeAl/Cu lateral spin valves. The spin signal is found to increase monotonically with the bias current. Surprisingly, the modulation amplitude from $\ensuremath{-}1$ mA to 1 mA exceeds 30 percent of the spin signal at low bias current. From the analysis based on the one-dimensional spin diffusion model and considering the bias-current heating effect, we find that the contribution of the thermal spin injection is much larger than the influence of the reduction of the spin diffusion length due to the Joule heating. We also show that the second harmonic lock-in signal precisely extracts the contribution of the thermal spin injection from the mixed spin signal.