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arXiv2019

Ultra-High Performance of Nano-Engineered Graphene-Based Natural Jute Fiber Composites

Natural fibers composites are considered as sustainable alternative to synthetic composites due to their environmental and economic benefits. However, they suffer from poor mechanical and interfacial properties due to a random fiber orientation and weak fiber-matrix interface. Here we report nano-engineered graphene-based natural jute fiber preforms with a new fiber architecture (NFA) which significantly improves their properties and performances. Our graphene-based NFA of jute fiber perform enhances Young modulus of jute-epoxy composites by ~324% and tensile strength by ~110% more than untreated jute fiber composites, by arranging fibers in parallel direction through individualisation and nano surface engineering with graphene derivatives. This could potentially lead to manufacturing of high performance natural alternatives to synthetic composites in various stiffness driven high performance applications.

physics.app-ph↗

arXiv2019

All Inkjet-Printed Graphene-Silver Composite Inks for Highly Conductive Wearable E-Textiles Applications

Inkjet-printed wearable electronic textiles (e-textiles) are considered to be very promising due to excellent processing and environmental benefits offered by digital fabrication technique. Inkjet-printing of conductive metallic inks such as silver (Ag) nanoparticles (NPs) are well-established and that of graphene-based inks is of great interest due to multi-functional properties of graphene-based materials. However, poor ink stability at higher graphene concentration and the cost associated with the higher Ag loading in metal inks have limited their wider use. Moreover, graphene-based e-textiles reported so far are mainly based on graphene derivatives such as graphene oxide (GO) or reduced graphene oxide (rGO), which suffers from poor electrical conductivity. Here we report inkjet printing of highly conductive and cost-effective graphene/Ag composite inks for wearable e-textiles applications. The composite inks were formulated, characterised and inkjet-printed onto PEL paper first and then sintered at 150 C for 1 hr. The sheet resistance of the printed patterns is found to be in the range of ~0.08 - 4.74 ohm/sq depending on the number of print layers and the graphene/Ag ratio in the formulation. The optimised composite ink was then successfully printed onto surface pre-treated (by inkjet printing) cotton fabrics in order to produce all-inkjet-printed highly conductive and cost-effective electronic textiles.

physics.app-ph↗

arXiv2019

Effect of scanning acceleration on the leakage signal in magnetic flux leakage type of non-destructive testing

This novel work investigates the influence of the inspection system acceleration on the leakage signal in magnetic flux leakage type of non-destructive testing. The research is addressed both through designed experiments and simulations. The results showed that the leakage signal, represented by using peak to peak value, decreases between 20% and 30% under acceleration. The simulation results indicated that the main reason for the decrease is due to the difference in the distortion of the magnetic field for cases with and without acceleration, which is the result of the different eddy current distributions in the specimen. The findings will help to allow the optimisation of the MFL system to ensure the main defect features can be measured accurately during the machine acceleration. It also shows the importance of conducting measurements at constant velocity, wherever possible.

physics.app-ph↗

arXiv2019

Multi-functional integrated photonics in the mid-infrared with suspended AlGaAs on silicon

The microscale integration of mid- and longwave-infrared photonics could enable the development of fieldable, robust chemical sensors, as well as highly efficient infrared frequency converters. However, such technology would be defined by the choice of material platform, which immediately determines the strength and types of optical nonlinearities available, the optical transparency window, modal confinement, and physical robustness. In this work, we demonstrate a new platform, suspended AlGaAs waveguides integrated on silicon, providing excellent performance in all of these metrics. We demonstrate low propagation losses within a span of nearly two octaves (1.26 to 4.6 $μ$m) with exemplary performance of 0.45 dB/cm at $λ= 2.4$ $μ$m. We exploit the high nonlinearity of this platform to demonstrate 1560 nm-pumped second-harmonic generation and octave-spanning supercontinuum reaching out to 2.3 $μ$m with 3.4 pJ pump pulse energy. With mid-IR pumping, we generate supercontinuum spanning from 2.3 to 6.5 $μ$m. Finally, we demonstrate the versatility of the platform with mid-infrared passive devices such as low-loss 10 $μ$m-radius bends, compact power splitters with 96 $\pm$ 1% efficiency and edge couplers with 3.0 $\pm$ 0.1 dB loss. This platform has strong potential for multi-functional integrated photonic systems in the mid-IR.

physics.app-ph↗

arXiv2019

Unidirectional Luminescence from Quantum Well Metasurfaces

III-Nitride light emitting diodes (LEDs) are the backbone of ubiquitous lighting and display applications. Imparting directional emission is an essential requirement for many LED implementations. Although optical packaging, nano-patterning and surface roughening techniques can enhance LED extraction, directing the emitted light requires bulky optical components. Optical metasurfaces provide precise control over transmitted and reflected waveforms, suggesting a new route for directing light emission. However, it is difficult to adapt metasurface concepts for incoherent light emission, due to the lack of a phase-locking incident wave. In this Letter, we demonstrate metasurface-based design of InGaN/GaN quantum-well structures that generate narrow, unidirectional transmission and emission lobes at arbitrary engineered angles. We show that the directions and polarization of emission differ significantly from transmission, in agreement with an analytical Local Density of Optical States (LDOS) model. The results presented in this Letter open a new paradigm for exploiting metasurface functionality in light emitting devices.

physics.optics↗

arXiv2019

An electrically tunable terahertz plasmonic device based on shape memory alloys and liquid metals

An electrically tunable terahertz (THz) plasmonic device is designed and fabricated using liquid metals (eutectic gallium indium EGaIn) and shape memory alloy wires (Flexinol). The liquid metal is injected into the voids of a poly(dimethyl) siloxane (PDMS) microfluidic mold forming a periodic array of subwavelength apertures, while the wires are inserted into the elastomer below the metal plane. When a DC voltage is applied to the wires, they contract via Joule heating, reducing the aperture periodicity and blue-shifting the transmission resonances of the device. When the voltage is removed, the wires cool and elongate back to their original length, allowing the transmission spectrum to return to its original state. The magnitude of this change depends upon the applied voltage. The device is shown to thermally cycle between the relaxed state and the fully contracted state reproducibly over at least 500 thermal cycles. The asymmetric geometry of the device and the contraction process yield transmission properties that are unexpected: two closely spaced resonances, where both resonances correspond to the same scattering indices, and an increase in the transmission amplitude of the lowest order resonance upon contraction. We use numerical simulations to understand these features.

physics.app-ph↗

arXiv2019

Probabilistic quantum cloning of N quantum states

Probabilistically creating n perfect clones from m copies for one of N priori known quantum states with minimum failure probability is a long-standing problem. We provide a rigorous proof for the geometric approach to this probabilistic quantum cloning problem when N = 2. Then, we give the general geometric form of the sufficient and necessary condition of probabilistic cloning for N known quantum states. By this general geometric approach, we realize the optimal probabilistic quantum cloning of N known quantum states with priori probabilities. The results are also applicable to the identification of those N quantum states.

quant-ph↗

arXiv2019

Numerical Contribution of Micro-Pulsed Laser Effect on Copper

A numerical model is developed to study heat, fluid flow and radiation transfers during the interaction between a UV laser beam and copper. Calculations are performed for a laser of Gaussian and Lorentzian shapes of a wavelength of 400nm, a focal spot radius of 50 micrometers and duration of 80 microsec. In order to describe the transient behaviour in and above the copper target, heat and Navier-Stokes equations are linked to Lambert Beer relationship by taking into account the conduction, and the convection phenomena. The resulting equations are schemed by the finite element method. Comparison with the literature showed qualitative and quantitative agreements for Crater depths and transmission profiles for different laser pulse numbers. Then, the effects of the laser fluences, the Gaussian and Lorentzian shapes on temperature, velocities, melting and evaporation phenomena are studied.

physics.comp-ph↗

arXiv2019

Optical and mechanical properties of amorphous Mg-Si-O-N thin films deposited by reactive magnetron sputtering

In this work, amorphous thin films in Mg-Si-O-N system were prepared in order to investigate the dependence of optical and mechanical properties on Mg composition. Reactive RF magnetron co-sputtering from magnesium and silicon targets were used for the deposition of Mg-Si-O-N thin films. Films were deposited on float glass, silica wafers and sapphire substrates in an Ar, N2 and O2 gas mixture. X-ray photoelectron spectroscopy, atomic force microscopy, scanning electron microscopy, spectroscopic ellipsometry, and nanoindentation were employed to characterize the composition, surface morphology, and properties of the films.

physics.app-ph↗

arXiv2019

Dynamic meta-atoms

Interaction of electromagnetic radiation with time-variant objects is a fundamental problem whose study involves foundational principles of classical electrodynamics. Such study is a necessary preliminary step for delineating the novel research field of linear time-varying metamaterials and metasurfaces. A closer look to the literature, however, reveals that this crucial step has not been addressed and important simplifying assumptions have been made. Before proceeding to studies of linear time-varying metamaterials and metasurfaces with their effective parameters, we need to rigorously describe the electric and magnetic responses of a temporally-modulated meta-atom. Here, we introduce a theoretical model which describes a time-variant meta-atom and its interaction with incident electromagnetic waves in time domain. The developed general approach is specialized for a dipole emitter/scatterer loaded with a time-varying reactive element. We confirm the validity of the theoretical model with full-wave simulations. Our study is of major significance also in the area of nanophotonics and nano-optics because the optical properties of all-dielectric and plasmonic nanoparticles can be varied in time in order to achieve intriguing scattering phenomena.

physics.app-ph↗

arXiv2019

Origin of the resistance-area product dependence of spin transfer torque switching in perpendicular magnetic random access memory cells

We report on an experimental study of current induced switching in perpendicular magnetic random access memory (MRAM) cells with variable resistance-area products (RAs). Our results show that in addition to spin transfer torque (STT), current induced self-heating and voltage controlled magnetic anisotropy also contribute to switching and can explain the RA dependencies of switching current density and STT efficiency. Our findings suggest that thermal optimization of perpendicular MRAM cells can result in significant reduction of switching currents.

cond-mat.mes-hall↗

arXiv2019

Miniature Multi-Level Optical Memristive Switch Using Phase Change Material

The optical memristive switches are electrically activated optical switches that can memorize the current state. They can be used as optical latching switches in which the switching state is changed only by applying an electrical Write/Erase pulse and maintained without external power supply. We demonstrate an optical memristive switch based on a silicon MMI structure covered with nanoscale-size Ge2Sb2Te5 (GST) material on top. The phase change of GST is triggered by resistive heating of the silicon layer beneath GST with an electrical pulse. Experimental results reveal that the optical transmissivity can be tuned in a controllable and repeatable manner with the maximum transmission contrast exceeding 20 dB. Partial crystallization of GST is obtained by controlling the width and amplitude of the electric pulses. Crucially, we also demonstrate that both Erase and Write operations, to and from any intermediate level, are possible with accurate control of the electrical pulses. Our work marks a significant step forward towards realizing photonic memristive switches without static power consumption, which are highly demanded in high-density large-scale integrated photonics.

physics.optics↗

arXiv2019

More than 3-mm-long carrier diffusion and strong absorption over the full solar spectrum in copper oxide and selenium composite film

Usually, single-crystal material has a longer carrier diffusion length due to fewer defects. Here, we report a nano/amorphous copper oxide and selenium (COS) composite film prepared by radio frequency magnetron co-sputtering of CuO and Se and post annealing. The photo-generated carriers in the COS film can diffuse very far (3.147 mm). Additionally, the COS film can strongly absorb light over the entire solar spectrum region (250-2500 nm) (average absorptance is ~80%). These results indicate that the COS composite is an excellent material and can be used in solar cells and other photo-electric devices.

physics.app-ph↗

arXiv2019

Thermal enhanced NIR-NIR anti-Stokes emission in rare earth doped nanocrystals

Nanoparticles with anti-Stokes emissions enable many sensing applications, but their efficiencies are considerably low. The key to enable the process of anti-Stokes emissions is to create phonons and assist the excited photons to be pumped from a lower energy state onto a higher one. Increasing the temperature will generate more phonons, but it unavoidably quenches the luminescence. Here by quantifying the number of phonons being generated from the host crystal and at the surface of Yb3+/Nd3+ co-doped nanoparticles, we systematically investigated mechanisms towards the large enhancements of the phonon-assisted anti-Stokes emissions from 980 nm to 750 nm and 803 nm. Moreover, we provided direct evidence that moisture release from the nanoparticle surface at high temperature was not a main reason. We further demonstrated that the brightness of 10 nm nanoparticles were enhanced by more than two orders of magnitude, standing in stark contrast to the thermal quenching effect.

physics.optics↗

arXiv2019

Novel two-dimensional Ca-Cl crystals with metallicity, piezoelectric effect and room-temperature ferromagnetism

Recently we have reported the direct observation of two-dimensional (2D) Ca-Cl crystals on reduced graphene oxide (rGO) membranes, in which the calcium ions are only about monovalent (i.e. ~+1) and metallic rather than insulating properties are displayed by those CaCl crystals. Here, we report the experimental observation and demonstration of the formation of graphene-Ca-Cl heterojunction owing to the metallicity of 2D Ca-Cl crystals, unexpected piezoelectric effect, room-temperature ferromagnetism, as well as the distinct hydrogen storage and release capability of the Ca-Cl crystals in rGO membranes. Theoretical studies show that the formation of those abnormal crystals is attributed to the strong cation-pi interactions of the Ca2+ with the aromatic rings in the graphitic surfaces. Since strong cation-pi interactions also exist between other metal ions (such as Mg2+, Fe2+, Co2+, Cu2+, Cd2+, Cr2+ and Pb2+) and graphitic surfaces, similar 2D crystals with abnormal valence state of the metal cations and corresponding abnormal properties as well as novel applications are highly expected. Those findings further show the realistically potential applications of such abnormal CaCl material with unusual electronic properties in designing novel transistors and magnetic devices, hydrogen storage, catalyzer, high-performance conducting electrodes and sensors, with a size down to atomic scale.

physics.chem-ph↗

arXiv2019

High Thermal Boundary Conductance across Bonded Heterogeneous GaN-SiC Interfaces

GaN-based HEMTs have the potential to be widely used in high-power and high-frequency electronics while their maximum output powers are limited by high channel temperature induced by near-junction Joule-heating, which degrades device performance and reliability. Increasing the TBC between GaN and SiC will aid in the heat dissipation of GaN-on-SiC power devices, taking advantage of the high thermal conductivity of the SiC substrate. However, a good understanding of the TBC of this technically important interface is still lacking due to the complicated nature of interfacial heat transport. In this work, a lattice-mismatch-insensitive surface activated bonding method is used to bond GaN directly to SiC and thus eliminating the AlN layer altogether. This allows for the direct integration of high quality GaN layers with SiC to create a high thermal boundary conductance interface. TDTR is used to measure the thermal properties of the GaN thermal conductivity and GaN-SiC TBC. The measured GaN thermal conductivity is larger than that of GaN grown by MBE on SiC, showing the impact of reducing the dislocations in the GaN near the interface. High GaN-SiC TBC is observed for the bonded GaN-SiC interfaces, especially for the annealed interface whose TBC (230 MW/m2-K) is close to the highest values ever reported. To understand the structure-thermal property relation, STEM and EELS are used to characterize the interface structure. The results show that, for the as-bonded sample, there exists an amorphous layer near the interface for the as bonded samples. This amorphous layer is crystallized upon annealing, leading to the high TBC found in our work. Our work paves the way for thermal transport across bonded interfaces, which will impact real-world applications of semiconductor integration and packaging.

physics.app-ph↗

arXiv2019

Giant Spin Seebeck Effect through an Interface Organic Semiconductor

Interfacing an organic semiconductor C60 with a non-magnetic metallic thin film (Cu or Pt) has created a novel heterostructure that is ferromagnetic at ambient temperature, while its interface with a magnetic metal (Fe or Co) can tune the anisotropic magnetic surface property of the material. Here, we demonstrate that sandwiching C60 in between a magnetic insulator (Y3Fe5O12: YIG) and a non-magnetic, strong spin-orbit metal (Pt) promotes highly efficient spin current transport via the thermally driven spin Seebeck effect (SSE). Experiments and first principles calculations consistently show that the presence of C60 reduces significantly the conductivity mismatch between YIG and Pt and the surface perpendicular magnetic anisotropy of YIG, giving rise to enhanced spin mixing conductance across YIG/C60/Pt interfaces. As a result, a 600% increase in the SSE voltage (VLSSE) has been realized in YIG/C60/Pt relative to YIG/Pt. Temperature-dependent SSE voltage measurements on YIG/C60/Pt with varying C60 layer thicknesses also show an exponential increase in VLSSE at low temperatures below 200 K, resembling the temperature evolution of spin diffusion length of C60. Our study emphasizes the important roles of the magnetic anisotropy and the spin diffusion length of the intermediate layer in the SSE in YIG/C60/Pt structures, providing a new pathway for developing novel spin-caloric materials.

physics.app-ph↗

arXiv2019

Coverage Enhancement for NLOS mmWave Links Using Passive Reflectors

The future 5G networks are expected to use millimeter wave (mmWave) frequency bands to take advantage of large unused spectrum. However, due to the high path loss at mmWave frequencies, coverage of mmWave signals can get severely reduced, especially for non-line-of-sight (NLOS) scenarios as mmWave signals are severely attenuated when going through obstructions. In this work, we study the use of passive metallic reflectors of different shapes/sizes to improve 28 GHz mmWave signal coverage for both indoor and outdoor NLOS scenarios. We quantify the gains that can be achieved in the link quality with metallic reflectors using measurements, analytical expressions, and ray tracing simulations. In particular, we provide an analytical model for the end-to-end received power in an NLOS scenario using reflectors of different shapes and sizes. For a given size of the flat metallic sheet reflector approaching to the size of incident plane waves, we show that the reflected received power for the NLOS link is same as line-of-sight (LOS)free space received power of the same link distance. Extensive results are provided to study impact of environmental features and reflector characteristics on NLOS link quality.

eess.SP↗