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arXiv2018

Wave propagation in infinite nonlinear acoustic metamaterial beam by considering the third harmonic generation

Nonlinear acoustic metamaterial (NAM) initiates new fields for controlling elastic waves. In this work, the flexural wave propagation in the half-infinite NAM beam consisting of periodic Duffing resonators is reported by considering the third harmonic generation (THG). Different analytical methods are proposed to describe the wave propagation in the equivalent homogenous medium. Then their effectiveness and accuracy are demonstrated in comparison with the finite element methods. We unveil analytically and numerically extensive physical properties of the strongly nonlinear AM, including the nonlinear resonance in a cell, the effective density, nonlinear locally resonant (NLR) bandgap, propagations and couplings of the fundamental and the third harmonics. These characteristics are highly interrelated, which facilitates the prediction of functionalities. In the near field, the identical bifurcation frequency of these features acts as the start frequency of the NLR bandgap for fundamental waves, whose width is narrower for a stronger nonlinearity. While in the far field, the NLR bandgap characterizes a distance-amplitude-dependent behavior leading to a self-adaptive bandwidth. Moreover, the transmission in the passband of the infinite NAM is different from the chaotic band effect of finite NAMs, and it is influenced by the shifted NLR gap. Our work will promote future studies and constructions of NAMs with novel properties.

physics.app-ph↗

arXiv2018

On Mechanical Behavior of Elastomeric Networks: Effects of Random Porous Microstructure

An assumption in micromechanical analysis of polymers is that the constitutive polymeric media is non-porous. Non-porosity of media, however, is merely a simplifying assumption. In this paper, we neglect this assumption and studied polymer networks with a different porosity volume fraction. A random morphology description function is used to model the porosity of the network and nonlinear finite element analyses are conducted to perform structural analysis of porous polymer networks. The results show that the porosity effect is significant in mechanical behavior of polymer networks and may increase the maximum Von-Mises stress drastically.

cond-mat.soft↗

arXiv2018

Microscopic Origin of Regeneration Noise in Relaxation Oscillator and its Macroscopic Circuit Manifestation

This paper augments the existing macroscopic circuit noise model for phase noise in relaxation oscillators by showing the microscopic origins of the noise and explains temperature dependency. The noise arises from fluctuation of the energy accompanying the excess carriers on device (transistors) capacitors in the oscillator. Such fluctuation has its physical origin from the noise of such carriers, which, microscopically, are distributed across the energy levels (Fermi-Dirac). Furthermore this energy can be interpreted, circuit-wise, such that its gradient, with respect to circuit state variables, correspond to time evolution of current and voltage i.e. the oscillator dynamics. Three methods: potential energy based (macroscopic), free energy based (microscopic), Langevin equation based, are used to develop the noise model. The model temperature variation over range of 77K-300K was compared to measured results on oscillators fabricated in 0.13 μm CMOS technology. The trend agree reasonably well, where above a crossover temperature, the phase noise is a monotonic increasing function of temperature, while below the crossover temperature, the phase noise stays relatively constant and an explanation based on Langevin equation, extended to quantum regime, is offered.

cond-mat.stat-mech↗

arXiv2018

Digital Biosensing By Foundry-Fabricated Graphene Sensors

Biomedical and environmental testing currently require access to highly specialized facilities or extensive training. Current biochemical tests require complex reagents to achieve a simple human readable result such as a color change. This results in low-information tests, with high reagent costs. To maximize information from testing while minimizing costs, biotechnology should leverage complex analysis enabled by advanced portable computing power and use simplified reagents. Nanotechnologists have created proof-of-concept devices to move diagnosis and monitoring from large core labs to hand-held, easy-to-use personal devices, but none have achieved large-scale manufacturability. We demonstrate the first economical nanoelectronics produced in a commercial foundry. The mass-manufacture of these graphene-based digital biosensors is achieved by successfully integrating delicate graphene material into standard electronic high-volume production processes. We describe critical manufacturing parameters and the results of biosensing measurements that establish graphene biosensors as a portable, low-cost method to gain precise data without labeled, complex reagents. The low power and resource usage of these biosensors enable individuals to gain personal, immediate control over precise biological and environmental data.

physics.app-ph↗

arXiv2018

Electrical Signatures of Corrosion and Solder Bond Failure in c-Si Solar Cells and Modules

Moisture- and temperature-activated corrosion of metal fingers, mechanical stress induced delamination, and failure of solder bonds rank among the leading failure mechanisms of solar modules. The physics of moisture ingress, diffusion and reaction have been explored in detail, but the electrical implications of corrosion and delamination on specific front-surface grid geometry is not fully understood. In this paper, we show that the module efficiency loss due to corrosion, delamination, and solder bond failure (CDS) involves a complex interplay of voltage/current redistribution, reflected as a loss in photocurrent as well as decrease/increase in shunt/series resistances. Our work will redefine the interpretation of experimental J-V characteristics features due to degradation mechanisms, integrate a variety of scattered and counter-intuitive experimental results within a common theoretical framework, and inform CDS-resistant grid design for solar modules.

physics.app-ph↗

arXiv2018

High responsivity, low dark current ultraviolet photodetector based on AlGaN/GaN interdigitated transducer

An ultraviolet (UV) photodetector employing the two-dimensional electron gas (2DEG) formed at the AlGaN/GaN interface as an interdigitated transducer (IDT) is characterized under optical stimulus. The 2DEG-IDT photodetector exhibits a record high normalized photocurrent-to-dark current ratio (NPDR, $6\times10^{14}$). In addition, we observe a high responsivity ($7,800$ A/W) and ultraviolet-visible rejection-ratio ($10^{6}$), among the highest reported values for any GaN photodetector architecture. We propose a gain mechanism to explain the high responsivity of this device architecture, which corresponds to an internal gain of $26,000$. We argue that the valence band offset in the AlGaN/GaN heterostructure is essential in achieving this high responsivity, allowing for large gains without necessitating the presence of trap states, in contrast to common metal-semiconductor-metal (MSM) photodetector architectures. Our proposed gain mechanism is consistent with measurements of the scaling of gain with device channel width and incident power. In addition to high performance, this photodetector architecture has a simple two-step fabrication flow that is monolithically compatible with AlGaN/GaN high electron mobility transistor (HEMT) processing. This unique combination of low dark current, high responsivity and compatibility with HEMT processing is attractive for a variety of UV sensing applications.

physics.app-ph↗

arXiv2018

Defect Engineering for Modulating the Trap States in Two-dimensional Photoconductor

Defect induced trap states are essential in determining the performance of semiconductor photodetectors. The de-trap time of carriers from a deep trap could be prolonged by several orders of magnitude as compared to shallow trap, resulting in additional decay/response time of the device. Here, we demonstrate that the trap states in two-dimensional ReS2 could be efficiently modulated by defect engineering through molecule decoration. The deep traps that greatly prolong the response time could be mostly filled by Protoporphyrin (H2PP) molecules. At the same time, carrier recombination and shallow traps would in-turn play dominant roles in determining the decay time of the device, which can be several orders of magnitude faster than the as-prepared device. Moreover, the specific detectivity of the device is enhanced (as high as ~1.89 x 10^13 Jones) due to the significant reduction of dark current through charge transfer between ReS2 and molecules. Defect engineering of trap states therefore provides a solution to achieve photodetectors with both high responsivity and fast response.

cond-mat.mtrl-sci↗

arXiv2018

Indoor Coverage Enhancement for mmWave Systems with Passive Reflectors: Measurements and Ray Tracing Simulations

The future 5G networks are expected to use millimeter wave (mmWave) frequency bands, mainly due to the availability of large unused spectrum. However, due to high path loss at mmWave frequencies, coverage of mmWave signals can get severely reduced, especially for non-line-of-sight (NLOS) scenarios. In this work, we study the use of passive metallic reflectors of different shapes/sizes to improve mmWave signal coverage for indoor NLOS scenarios. Software defined radio based mmWave transceiver platforms operating at 28 GHz are used for indoor measurements. Subsequently, ray tracing (RT) simulations are carried out in a similar environment using Remcom Wireless InSite software. The cumulative distribution functions of the received signal strength for the RT simulations in the area of interest are observed to be reasonably close with those obtained from the measurements. Our measurements and RT simulations both show that there is significant (on the order of 20 dB) power gain obtained with square metallic reflectors, when compared to no reflector scenario for an indoor corridor. We also observe that overall mmWave signal coverage can be improved utilizing reflectors of different shapes and orientations.

eess.SP↗

arXiv2018

Tunable colloidal photonic crystals

Spherical colloids arranged in a crystalline order are known to produce structural colors. The intensity and brilliance of such photonic crystals require high size-monodispersity of the colloids, a low number of lattice defects and disorder, as well as a relatively large refractive index contrast between the scattering colloids and the continuous background. Here we present the unexpected photonic properties of aqueous suspensions of charge-stabilized, 186 nm large, fluorinated colloids with a refractive index of 1.37. Employing reflectivity, optical observation, small angle x-ray scattering measurements and reflectivity modeling, we demonstrate that these suspensions become partially transparent while showing strong, almost angle-independent color in reflection despite the very small refractive index difference. Under certain conditions, additional sharp Bragg reflections are observed. We were able to tune the observed structural colors continuously across the entire visible range by simply changing the volume fraction of these colloidal suspensions, which show a white appearance when dilute, structural color and Bragg peaks when concentrated enough to form Wiegner crystal, and angle-independent color when very concentrated and in a glassy phase.

physics.optics↗

arXiv2018

A disclosure of the role of frequency on the plasma electrolytic oxidation of AZ31 magnesium alloy in aluminate-tungstate electrolyte

Plasma electrolytic oxidation (PEO) was carried out to form black ceramic coatings on AZ31 magnesium alloy in aluminate-tungstate electrolyte. Influences of current frequency (such as 100, 1000 and 2000 Hz) on properties of the PEO coatings have been explored. It was found that increment of frequency led to coatings with higher thickness, higher roughness and darker appearance. In order to explore the coating formation mechanism at different frequencies, the micro-discharges at different frequencies were investigated by real-time imaging method. Microstructure of coatings was characterized by scanning electron microscopy (SEM) assisted with energy dispersive X-ray spectroscopy (EDS). Coatings formed at 100 Hz show the most compact microstructure while porosity of the coatings, both in the pore number and pore size, is greatly increased at higher frequencies. The coatings formed at higher frequencies show an increased atomic ratio of W/Al. The cancel of the negative pulse from the pulsed waveform can also lead to a higher W/Al ratio, and hence a darker coating. The higher porosity of the coatings was attributed to the stronger plasma discharges and gas evolution at higher frequencies. The stronger plasma discharge is induced by the fact that there is not sufficient time for the cooling down of discharge channels at higher frequencies.

physics.app-ph↗

arXiv2018

Modelling a conductive-capacitive medium using the boundary element method

In this paper a generalized fundamental solution using the boundary element method to solve the Helmholtz equation is proposed. It is observed that the commonly used fundamental solution is only valid for good conductors since the capacitive effect of the considered medium is always neglected. By the use of the well-known Lorentz gauge condition a fundamental solution which incorporates the phase as well as the attenuation transmission coefficients is derived by the authors. Next, a model is developed using this proposed fundamental solution for modelling a coating layer of a buried pipeline. Subsequently, a model of a buried coated pipeline in close proximity to a high voltage power line is developed and numerically implemented. Finally, the model is used to simulate two configurations in order the verify the proposed general fundamental solution of the boundary element method for electromagnetic field problems. Hereby, its validity is proven and it is shown that the generalized solution and related model can be used for industrial applications.

physics.app-ph↗

arXiv2018

Growth and Characterization of Multi-Walled Carbon Nanotubes using Chemical Vapor Deposition

In this study, the synthesis of multi-walled carbon nanotubes (MWCNTs) was carried out by chemical vapor deposition (CVD) using propane as the carbon source and Si as the catalyst support. The effect of CVD process variables such as temperature, choice of catalyst, etc on the growth behavior of nanotubes has been examined to understand the catalytic growth of CNTs. The transition metal catalysts, Fe and Ni, were used in both elemental metal form and in a metal complex form. In the case of elemental metal catalysts, the respective metals were deposited over the Si substrate using thermal evaporation following which nanotubes were synthesized by means of CVD. Subsequent studies of the synthesized carbon nanostructures employing elemental metal catalysts revealed a significant influence of the temperature and the catalyst material on the structure of CNTs. The CNTs synthesized using Ni catalyst were bamboo-like whereas the CNTs developed employing Fe catalyst were straight tubes with partial metal filling. Consequently, growth models for the different growth mechanisms have been proposed. Certain limitations of the above process have been overcome by employing spin-coating of a metal complex catalyst material on the Si substrate. The CVD synthesis of nanotubes using metal complex catalysts always resulted in partially catalyst filled CNTs. More importantly, the metal complex catalyst could be easily patterned on the Si substrate using spin-coating and photolithography, which resulted in site selective growth of partially catalyst filled MWCNTs. Since the entire process of site selective growth is suitable for conventional device fabrication, this method is a promising and practical pathway for large-scale fabrication of several magnetic material filled CNT-based devices.

physics.app-ph↗

arXiv2018

Compact 60GHz On-Chip Antenna in 65nm CMOS Technology With Circular and Linear Polarizations for Millimeter-Wave Applications

In this paper, design and simulation of a compact on-chip antenna for 60 GHz band applications is presented. Antenna is designed in 65nm TSMC technology. This technology consists of 9 metal layers. A 0.7 mm * 0.7 mm patch with slots is located on 9th layer and first layer is used as ground plane. Total chip size is 0.8 mm * 0.8 mm. In order to enhance radiation efficiency and control axial ratio, four slots are adjusted on ground plane. Circular and linear polarizations are achievable by proper port excitation. Maximum efficiency of the antenna is about 60% at 59.6 GHz. Simulated antenna bandwidth ranges from 58.7 GHz as band start and 62.3 GHz as band stop. Peak gain values for circularly and linearly polarized modes are -0.6 dB and -0.9 dB at 0 respectively.

physics.app-ph↗

arXiv2018

Intrinsic nonlinearity of a PN-junction diode and higher order harmonic generation

Voltage current characteristics of a PN-junction diode are intrinsically nonlinear in nature. It is shown in this paper that a mathematical form of nonlinearity of a PN-junction diode resembles the nonlinear response of electric polarization of a dielectric medium to the electric field. Nonlinearity of a PN-junction can be expressed in a series of successively increasing orders of the nonlinearity. For a PN-junction diode, higher order nonlinear terms become significant as a voltage across the diode is increased. In this paper, a gradual emergence of a nonlinear regime with the amplitude of a sinusoidal voltage is presented. Higher order harmonics are produced by utilizing the nonlinearity of a single PN-junction diode. An experimental realization of a frequency comb with the highest frequency up to the twentieth harmonics is also presented. In addition, in the same circuit by making the nonlinearity significant up to the second order, an experiment on generation of the sum and difference of frequencies is realized.

physics.app-ph↗

arXiv2018

Investigation of the performance of photovoltaics installed close to river

The effect of temperature and relative humidity on photovoltaics close to river in calabar was studied. A KT-908 precision digital hygrometer and thermometer, and a M890C+ digital multimeter were used in the process. Results obtained shows that the effect of relative humidity on current, power and efficiency are the same. The results also show that voltage remains fairly stable between 65% to 75% relative humidity. While efficiency increases with temperature up to the maximum operating temperature of the photovoltaic module before starting to decrease. Peak in efficiency was observed at a relative humidity value of 65% and module temperature of about 470C at noon.

physics.app-ph↗

arXiv2018

Brightness of nonequilibrium photoemission in metallic photocathodes near threshold

The operation of photoemission electron sources with wavelengths near the photoemission threshold has been shown to dramatically decrease the minimum achievable photocathode emittance, but at the cost of significantly reduced quantum efficiency (QE). In this work, we show that for femtosecond laser and electron pulses, the increase in required laser intensities due to the low QE drives the photocathode electronic distribution far from static equilibrium. We adapt an existing dynamic model of the electron occupation under high intensity laser illumination to predict the time-dependent effects of the nonequilibrium electron distribution on the QE, mean transverse energy (MTE), and emission brightness of metal phtocathodes. We find that multiphoton photoemission dramatically alters the MTE as compared to thermal equilibrium models, causing the MTE to no longer be a monotonic function of photon excess energy.

physics.acc-ph↗

arXiv2018

Design of a Plasmonic Absorption Electro-optical Modulator Based on n-doped Silicon and Barium Titanate

In this paper, a numerical solution for a plasmonic absorption modulator is presented with a six-layer structure consisting of an air superstrate, a gold layer, a barium titanate layer, a n type silicon layer, a gold layer and a thin-shell nanolattice aluminium oxide substrate. Regarding the suggested structure, the parameters related to the absorption modulation are investigated at different thicknesses. Here, the Pockels effect and the free carrier dispersion effect are considered simultaneously. The dispersion relation of this structure is analytically obtained and numerically solved by the Nelder-Mead method. The maximum calculated figure of merit is 12.79. Furthermore, according to our results, it is understood that this modulator has a high ability to be utilized in optical communication systems. Also, it could be integrated to the microelectronic systems and it is compatible with CMOS technology.

physics.app-ph↗

arXiv2018

An axis symmetric 2D description of the dynamics of a single crystal micro fiber growth from the melt by micro- pulling- down method. Part.1

In this paper the first part of an axis symmetric description of a single crystal micro fiber growth process by micro-pulling-down method is presented.the description concerns the following aspects:the free surface equation and the pressure difference across the free surface (section 2);limits of the pressure difference p across the free surface (section3;the fiber radius change due to the change of the pressure difference p,and the fiber radius size control(section 4);the static stability of the free surface (section 5). The above aspects are numerically illustrated in case of the growth of a silicon fiber of radius equal to 0.0001 m by using the Maple 17 software. This description can be helpful in the better understanding of the growth process and in the automation of manufacturing

physics.app-ph↗