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arXiv2018

Low-Power (1T1N) Skyrmionic Synapses for Spiking Neuromorphic Systems

In this work, we propose a `1-transistor 1-nanotrack' (1T1N) synapse based on movement of magnetic skyrmions using spin polarised current pulses. The proposed synaptic bit-cell has 4 terminals and fully decoupled spike transmission- and programming- paths. With careful tuning of programming parameters we ensure multi-level non-volatile conductance evolution in the proposed skyrmionic synapse. Through micromagnetic simulations, we studied in detail the impact of programming conditions (current density, pulse width) on synaptic performance parameters such as number of conductance levels and energy per transition. The programming parameters chosen used all further analysis gave rise to a synapse with 7 distinct conductance states and 1.2 fJ per conductance state transition event. Exploiting bidirectional conductance modulation the 1T1N synapse is able to undergo long-term potentiation (LTP) & depression (LTD) according to a simplified variant of biological spike timing dependent plasticity (STDP) rule. We present subthreshold CMOS spike generator circuit which when coupled with well known subthreshold integrator circuit, produces custom pre and post-neuronal spike shapes, responsible for implementing unsupervised learning with the proposed 1T1N synaptic bit-cell and consuming ~ 0.25 pJ/event. A spiking neural network (SNN) incorporating the characteristics of the 1T1N synapse was simulated for two seperate applications: pattern extraction from noisy video streams and MNIST classification.

physics.app-ph↗

arXiv2018

Optical Wireless Cochlear Implants

In the present contribution, we introduce a wireless optical communication-based system architecture which is shown to significantly improve the reliability and the spectral and power efficiency of the transcutaneous link in cochlear implants (CIs). We refer to the proposed system as optical wireless cochlear implant (OWCI).In order to provide a quantified understanding of its design parameters, we establish a theoretical framework that takes into account the channel particularities, the integration area of the internal unit, the transceivers misalignment, and the characteristics of the optical units. To this end, we derive explicit expressions for the corresponding average signal-to-noise-ratio, outage probability, ergodic spectral efficiency and capacity of the transcutaneous optical link (TOL). These expressions are subsequently used to assess the dependence of the TOL's communication quality on the transceivers design parameters and the corresponding channels characteristics. The offered analytic results are corroborated with respective results from Monte Carlo simulations. Our findings reveal that OWCI is a particularly promising architecture that drastically increases the reliability and effectiveness of the CI TOL, whilst it requires considerably lower transmit power compared to the corresponding widely-used radio frequency (RF) solution.

eess.SP↗

arXiv2018

The Effect of Thermal History on Microstructural Evolution, Cold-Work Refinement and α/\b{eta} Growth in Ti-6Al-4V Wire + Arc AM

Wire + arc additive manufacture (WAAM) is an attractive method for manufacturing large-scale aerospace components, however the microstructural changes that occur and the effect of interpass rolling are poorly understood. Therefore two fundamental studies were conducted: the first involved temperature measurement of a wrought dummy wall so that the microstructural changes in the heat affected zone (HAZ) could be related to the thermal cycle. This demonstrated that the white band in the microstructure corresponded to 825 C well below the beta-transus temperature and above this boundary the bi-modal substrate material was converted to lamellar. The second involved peening WAAM material along the side of a deposited wall before applying a typical WAAM thermal heat treatment. This showed that refinement occurred up to the first layer band in the microstructure and the smallest grains were observed just above this boundary at higher temperatures significant grain growth occurred. This study has provided the foundational understanding of microstructural changes that will facilitate future process developments.

physics.app-ph↗

arXiv2018

Wideband impedance measurement techniques in small complex cavities such as ear simulators and the human ear canal

The multimedia evolution has led to increased audio signal bandwidth in new generations of smartphones, headsets, headphones, as well as hearing aids. Full audio bandwidth performance, up to 20 kHz, is required. This calls for wider band performance of the ear simulators, and head and torso simulators used by the industry to evaluate these multimedia devices, as well as a better understanding of the high frequency behaviour of the human ear that these simulators are supposed to replicate. Acoustic input impedance measurements are an important parameter when characterizing small complex cavities such as ear simulators and the human ear. Today, this is well covered up to around 8-10 kHz. In this study a calibration procedure for measuring wideband impedance is developed, that uses multiple reference loads each defined by a detailed simulated impedance, in order to increase the applicable frequency bandwidth of the measurements. Comparing measured impedance magnitude, calibrated with this new procedure, to detailed simulations on a widely used ear simulator (IEC711 coupler) shows good agreement in the full audio bandwidth. Furthermore, the calibration procedure is streamlined to make it suited for a larger scale impedance study of the human ear, and measurements in one human ear is presented.

physics.app-ph↗

arXiv2018

Wideband impedance measurement in the human ear canal; In vivo study on 32 subjects

The human ear canal couples the external sound field to the eardrum and the solid parts of the middle ear. Therefore, knowledge of the acoustic impedance of the human ear is widely used in the industry to develop audio devices such as smartphones, headsets, and hearing aids. In this study acoustic impedance measurements in the human ear canal of 32 adult subjects is presented. Wideband measurement techniques developed specifically for this purpose enable impedance measurement to be obtained in the full audio band up to 20kHz. Full ear canal geometries of all subjects are also available from the first of its kind in vivo based magnetic resonance imaging study of the human outer ear. These ear canal geometries are used to obtain individual ear moulds of all subjects and to process the data. By utilizing a theoretical Webster's horn description, the measured impedance is propagated in each ear canal to a common theoretical reference plane across all subjects. At this plane the mean human impedance and standard deviation of the population is found. The results are further demographically divided by gender and age and compared to a widely used ear simulator (the IEC711 coupler).

physics.med-ph↗

arXiv2018

Rectifying properties in 90° rotated bilayer black phosphorus nanojunction: A first principle study

We explore the possibility of using van dar Waals bonded heterostructures of stacked together 2D bilayer black phosphorus (BP) for nanoscale device applications. The electronic property of BP in AA stacking and 90° twisted is studied with density functional theory. Further, we study the homogeneous nanojunction architecture of BP to use its anisotropic properties. Using the first principle simulations along with NEGF approach, we calculate quantum transport properties of the nanojunction setup. The interlayer directionally dependent current characteristics are explained in different setups. Our result revealed that 90° twisted nanojucntion device would be a potential rectifier despite having no p-n junction characteristic only due to the intrinsic anisotropy of the material, making tunneling between armchair- and zigzag-directional BP sheets asymmetric.

physics.app-ph↗

arXiv2018

Metalized polymer tubes for high-frequency electromagnetic waveguiding

Low loss electromagnetic energy transport over long distances motivates the development of different types of waveguiding systems. Requirements of high quality optically polished waveguide surfaces needed in high-frequency applications and low-cost manufacturing are practically incompatible in current realizations. Here we demonstrate a new paradigm solution, based on surface functionalization with subsequent electroless plating of conductive micron smooth copper layer on the inner surface of flexible non-conducting poly-carbonate tubes. The structure was shown to support moderately low loss guiding performances (~5-10 dB/m) at Ku-band. The mechanically flexible design of the system allows shaping the waveguide network almost on demand. In particular, an efficient energy guiding over a closed loop with 8 lambda radius was demonstrated. The new platform of high quality metalized flexible waveguiding systems opens new opportunities in designs of cheap and efficient networks, operating over a broad spectral range, approaching tens of GHz and even higher.

physics.app-ph↗

arXiv2018

Applications and Potentials of Reciprocal Bianisotropic Metasurfaces

We discuss different applications and potentials of reciprocal bianisotropic metasurfaces including uniform and gradient metasurfaces, in particular, with anisotropic, chiral, and omega properties. Based on an analytic analysis, we discuss the necessary conditions to observe asymmetric co- and cross- polarization reflection and/or transmission, polarization conversion and rotation, asymmetric absorption, etc. We consider two kinds of incident wave scenarios based on linear and circular polarization.

physics.optics↗

arXiv2018

Design and simulation of a low dark current metal/silicon/metal integrated plasmonic detector

Silicon-based waveguide plasmon detectors have a great research interest because of CMOS compatibility and integration capability with other plasmonic integrated circuits. In this paper, a balanced metal-semiconductor-metal (MSM) integrated plasmonic detector is proposed to isolate the output from dark current and made it suitable for low noise applications. Performance characteristics of the new device are numerically simulated. In a specific bias point (V = 3 V), the output current is estimated to be about 31.8 μA and responsivity is 0.1288 A/W for a device with 2 μm2 area. Simulation results for this balanced Plasmon detector demonstrate considerable dark current reduction compared with unbalanced plasmon detectors. Our estimated theoretical I-V characteristic, fits appropriately with experimental curve results reported before.

physics.app-ph↗

arXiv2018

Modeling and Thermal Metrology of Thermally Isolated MEMS Electrothermal Actuators for Strain Engineering of 2D Materials

We present electrothermal microelectromechanical (MEMS) actuators as a practical platform for straining 2D materials. The advantages of the electrothermal actuator is its high output force and displacement for low input voltage, but its drawback is that it is actuated by generating high amounts of heat. It is crucial to mitigate the high temperatures generated during actuation for reliable 2D material strain device implementation. Here, we implement a chevron actuator design that incorporates a thermal isolation stage in order to avoid heating the 2D material from the high temperatures generated during the actuation. By comparing experiment and simulation, we ensure our design does not compromise output displacement, while keeping the 2D material strain device stage cool. We also provide a simple analytical model useful for quickly evaluating different thermal isolation stage designs.

physics.app-ph↗

arXiv2018

Experimental 3D Coherent Diffractive Imaging from photon-sparse random projections

The routine atomic-resolution structure determination of single particles is expected to have profound implications for probing the structure-function relationship in systems ranging from energy materials to biological molecules. Extremely-bright, ultrashort-pulse X-ray sources---X-ray Free Electron Lasers (XFELs)---provide X-rays that can be used to probe ensembles of nearly identical nano-scale particles. When combined with coherent diffractive imaging, these objects can be imaged; however, as the resolution of the images approaches the atomic scale, the measured data are increasingly difficult to obtain and, during an X-ray pulse, the number of photons incident on the two-dimensional detector is much smaller than the number of pixels. This latter concern, the signal "sparsity," materially impedes the application of the method. We demonstrate an experimental analog using a synchrotron X-ray source that yields signal levels comparable to those expected from single biomolecules illuminated by focused XFEL pulses. The analog experiment provides an invaluable cross-check on the fidelity of the reconstructed data that is not available during XFEL experiments. We establish---using this experimental data---that a sparsity of order $1.3\times10^{-3}$ photons per pixel per frame can be overcome, lending vital insight to the solution of the atomic-resolution XFEL single particle imaging problem by experimentally demonstrating 3D coherent diffractive imaging from photon-sparse random projections.

physics.app-ph↗

arXiv2018

Perspectives on Astrophysics Based on Atomic, Molecular, and Optical (AMO) Techniques

About two generations ago, a large part of AMO science was dominated by experimental high energy collision studies and perturbative theoretical methods. Since then, AMO science has undergone a transition and is now dominated by quantum, ultracold, and ultrafast studies. But in the process, the field has passed over the complexity that lies between these two extremes. Most of the Universe resides in this intermediate region. We put forward that the next frontier for AMO science is to explore the AMO complexity that describes most of the Cosmos.

physics.atom-ph↗

arXiv2018

Generation of high energy laser-driven electron and proton sources with the 200 TW system VEGA 2 at the Centro de Laseres Pulsados

The Centro de Laseres Pulsados in Salamanca Spain has recently started operation phase and the first User access period on the 6 J 30 fs 200 TW system (VEGA 2) already started at the beginning of 2018. In this paper we report on two commissioning experiments recently performed on the VEGA 2 system in preparation for the user campaign. VEGA 2 system has been tested in different configurations depending on the focusing optics and targets used. One configuration (long focal length f=130 cm) is for under-dense laser-matter interaction where VEGA 2 is focused onto a low density gas-jet generating electron beams (via laser wake field acceleration mechanism) with maximum energy up to 500 MeV and an X-ray betatron source with a 10 keV critical energy. A second configuration (short focal length f=40 cm) is for over-dense laser-matter interaction where VEGA 2 is focused onto an 5 microns thick Al target generating a proton beam with a maximum energy of 10 MeV and average energy of 7-8 MeV and temperature of 2.5 MeV. In this paper we present preliminary experimental results.

physics.plasm-ph↗

arXiv2018

Concept of round non-flat thin film solar cells and their power conversion efficiency calculation

Thin-film solar cells that are considered as the second generation of solar cells are known for their low cost and acceptable efficiency. In this technology, semiconductor layers with a thickness of micrometer are deposited on thick enough substrates to maintain physical consistency. The relatively low processing temperature helps use substrates of different materials. Compared with crystalline solar cells, which are mainly made up of rigid flat plates, it is also possible to make thin-film cells on flexible or non-flat substrates. In this study, a method was first proposed to calculate the efficiency of such cells without the need for 3D simulation, and then it is investigated using non-flat conical and paraboloid substrates as a novel method to enhance the light trapping. As a result, a significant increase in the efficiency of the studied non-flat cells was observed and reported in comparison with the flat cells. In addition, the paraboloid shape shows a better performance than that of the conical, to use as the cell's substrate.

physics.app-ph↗

arXiv2018

Meta-surface-enabled ultra-sharp multimode waveguide bends on silicon

Mode-division-multiplexing (MDM) is attractive as a means to increase the link capacity of a single wavelength for optical interconnects via the use of multiple mode-channels in multimode bus waveguide.In order to route the data in MDM systems, waveguide bends are typically needed.In particular, ultra-sharp bends are desirable for reducing the chip area occupied by high-density photonic intergrated circuits (PICs). In this work, we propose and demonstrate a novel meta-surfaced waveguide structure on silicon, which enables ultral-sharp multimode waveguide bends to have low loss and low crosstalk among different mode-channels.An ultra-sharp S-bend with a bending radius of R=10μm is realized for an MDM optical interconnect with three mode-channels. The numerical simulation shows that the proposed ultal-sharp meta-surfaced multimode waveguide bend (MMWB) has low excess losses (0.2~0.5dB) and low inter-mode crosstalk (<-30dB) over a broad wavelength-band(>100nm) for all the three mode-channels. This is verified by experimental results, which show that the fabricated S-bends have excess losses of 0.5~0.8dB and inter-mode crosstalk of <-20 dB over a wavelength of >60 nm. The experimental results were partially limited by the measurement setup, particularly the on-chip mode (de)multiplexers which had inter-mode crosstalk of about -20dB. The proposed MMWB can be extended for the transmission of more than three mode-channels.Our work paves the way to use meta-surfaced silicon photonic waveguide structures for on-chip multimode optical interconnect.

physics.app-ph↗

arXiv2018

Diameter Optimization for Highest Degree of Ballisticity of Carbon Nanotube Field Effect Transistors

Carbon Nanotube (CNT) is one of the most significant materials for the development of faster and improved performance of nano-scaled transistors. This paper aims at analyzing a trade-off between device performance and device size of CNT based transistor. Acoustic and Optical Phonon scattering along with the elastic scattering lead to the non-ballistic performances of those transistors. The main focus of this work is mainly on finding an optimum diameter to obtain the highest degree of ballisticity from both single-walled and double-walled Carbon Nanotube Field Effect Transistors (CNT-FETs). At first, an n-type single-walled Carbon Nanotube Field Effect Transistor has been considered and the diameter dependence on degree of ballisticity has been simulated. The effects of drain voltage, gate voltage and channel length have been investigated for such characteristics followed by a comparison with double-walled CNT-FET structure. First of all, it has been found that degree of ballisticity along with the optimum diameter increases with the increase of (VDS VGS). The increase of channel length, however, degrades the ballistic performance demanding a higher diameter to reach the optimum point. Finally, it can be concluded that optimum diameter for DWCNT-FET reaches earlier than SWCNT-FET but at lower degree of ballisticity.

physics.app-ph↗

arXiv2018

Ultrafast Chemical Imaging by Widefield Photothermal Sensing of Infrared Absorption

Infrared (IR) imaging has become a viable tool for visualizing various chemical bonds in a specimen. The performance, however, is limited in terms of spatial resolution and imaging speed. Here, instead of measuring the loss of the IR beam, we utilize a pulsed visible light for a high-throughput, widefield sensing of the transient photothermal effect induced by absorption of single mid-IR pulses. To extract such transient signals, we built a virtual lock-in camera synchronized to the visible probe and IR light pulses with precisely-controlled delays, allowing sub-microsecond temporal resolution determined by the probe pulse width. Our widefield photothermal sensing (WPS) microscope enabled chemical imaging at a speed up to 1250 frames per second, with high spectral fidelity, and offering sub-micron spatial resolution. With the capability of imaging living cells and nanometer-scale polymer films, WPS microscopy opens a new way for high-throughput characterization of biological and material specimens.

physics.optics↗

arXiv2018

Design and Application of a Relativistic Kramers-Kronig Analysis Algorithm

Low-loss electron energy loss spectroscopy (EELS) in the scanning transmission electron microscope (STEM) probes the valence electron density and relevant optoelectronic properties such as band gap energies and other band structure transitions. The measured spectra can be formulated in a dielectric theory framework, comparable to optical spectroscopies and ab-initio simulations. Moreover, Kramers-Kronig analysis (KKA), an inverse algorithm based on the homonym relations, can be employed for the retrieval of the complex dielectric function (CDF). However, spurious contributions traditionally not considered in this framework typically impact low-loss EELS modifying the spectral shapes and precluding the correct measurement and retrieval of the dielectric information. A relativistic KKA algorithm is able to account for the bulk and surface radiative-loss contributions to low-loss EELS, revealing the correct dielectric properties. Using a synthetic low-loss EELS model, we propose some modifications on the naive implementation of this algorithm that broadens its range of application. The robustness of the algorithm is improved by regularization, appliying previous knowledge about the shape and smoothness of the correction term. Additionally, our efficient numerical integration methodology allows processing hyperspectral datasets in a reasonable amount of time. Harnessing these abilities, we show how simultaneous relativistic KKA processing of several spectra can share information to produce an improved result.

cond-mat.mtrl-sci↗