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arXiv2019

Near-field radiative heat transfer between rough surfaces modeled using effective media with gradient distribution of dielectric function

Near-field radiative heat transfer (NFRHT) between rough surfaces, due to its widespread presence in engineering practice of near-field energy utilization, requires indepth studies, especially from the perspective of physical mechanism. In this paper, an effective multilayer model is built to approach the NFRHT between random rough surfaces of silicon carbide (SiC). Using the effective medium theory (EMT), the effective dielectric function of each layer is obtained, which forms a gradient distribution of dielectric function along the depth of the medium. The influence of the effective dielectric function on surface phonon polaritons (SPhPs) is analyzed, showing that the effective layers with small filling fraction of SiC feature lower SPhP resonance frequencies than SiC bulk. The coupling of SPhPs from the gradient distribution of dielectric function produces new surface modes that dominates the NFRHT. Investigation on the effect of root mean square height (RMS height, σ) reveals that the peaks of local density of states (LDOS) and spectral heat flux are red-shifted as σ increases, while the spectral heat flux below the peak frequency gets larger. This can be attributed to the coupling of SPhPs inside the rough layer. We also found the total net heat flux between rough surfaces separated by an average distance exceeds that between smooth plates and increases with increasing σ, which offer a new way to enhance NFRHT. This work provides a reference for the simulation and understanding of the NFRHT between rough surfaces.

physics.app-ph↗

arXiv2019

Visualizing the melting processes in ultrashort intense laser triggered gold mesh with high energy electron radiography

High energy electron radiography (HEER) is a promising tool for high energy density physics diagnostics, apart from other tools like X/γ ray shadowgraphy and high energy proton radiography. Impressive progresses have been made in development and application of HEER in past few years, and proved its potentials for high-resolution imaging of static opaque objects. By taking advantages of short pulse duration and tunable time structure of high energy electron probes, time-resolved imaging measurement of high energy density gold irradiated by ultrashort intense lasers has been performed. Phenomena of different time periods from picosecond to microsecond have been observed, thus proving feasibilities of this technique for imaging of static and dynamic objects.

physics.app-ph↗

arXiv2019

Deep learning-based quality filtering of mechanically exfoliated 2D crystals

Two-dimensional (2D) crystals are attracting growing interest in various research fields such as engineering, physics, chemistry, pharmacy and biology owing to their low dimensionality and dramatic change of properties compared to the bulk counterparts. Among the various techniques used to manufacture 2D crystals, mechanical exfoliation has been essential to practical applications and fundamental research. However, mechanically exfoliated crystals on substrates contain relatively thick flakes that must be found and removed manually, limiting high-throughput manufacturing of atomic 2D crystals and van der Waals heterostructures. Here we present a deep learning-based method to segment and identify the thickness of atomic layer flakes from optical microscopy images. Through carefully designing a neural network based on U-Net, we found that our neural network based on U-net trained only with the data based on 24 images successfully distinguish monolayer and bilayer MoS2 with a success rate of 70%, which is a practical value in the first screening process for choosing monolayer and bilayer flakes of MoS2 of all flakes on substrates without human eye. The remarkable results highlight the possibility that a large fraction of manual laboratory work can be replaced by AI-based systems, boosting productivity.

physics.app-ph↗

arXiv2019

Leveraging viscous peeling in soft actuators and reconfigurable microchannel networks

The research fields of microfluidics and soft robotics both involve complex small-scale internal channel networks, embedded within a solid structure. This work examines leveraging viscous peeling as a mechanism to create and activate soft actuators and microchannel networks, including complex elements such as valves, without the need for fabrication of structures with micron-scale internal cavities. We consider configurations composed of an internal slender structure embedded within another elastic solid. Pressurized viscous fluid is introduced into the interface between the two solids, thus peeling the two elastic structures and creating internal cavities. Since the gap between the solids is determined by the externally applied pressure, the characteristic size of the fluidic network may vary in time and be much smaller than the resolution of the fabrication method. This work presents a model for the highly nonlinear elastic-viscous dynamics governing the flow and deformation of such configurations. Fabrication and experimental demonstrations of micron-scale valves and channel-networks created from millimeter scale structures are presented, as well as the transient dynamics of viscous peeling based soft actuators. The experimental data is compared with the suggested model, showing very good agreement.

physics.flu-dyn↗

arXiv2019

Work function tuning of graphene oxide by using cesium applied to low work function tethers

Low Work Function Tethers (LWT) is long conductors with a segment coated with a material that has loosely-bounded electrons. Unlike a standard bare tether equipped with an active electron emitter, LWTs close the electrical circuit (cathodic contact) with the ambient plasma in a passive manner thanks to the thermionic and photoelectric effects. This work presents experimental results on a novel procedure for manufacturing LWT samples. Conductive substrates (aluminum and copper) have been coated with graphene oxide doped with Cesium by using a scalable, simple and cost-effective coating method with an airbrush. Both GO and Cs-doped GO (GO/Cs) samples were characterized utilizing X-ray spectroscopy, field emission scanning electron microscopy, four-point probe, and ultraviolet photoelectron spectroscopy (UPS). They showed that GO sheets were successfully doped with Cs atoms, which do not agglomerate on the graphene sheets and do not form particles. The UPS results indicated that by doping GO sheets with Cs, the work function of GO decreases from 4.6 eV to 3.09 eV, thus increasing the applicability of GO as an electron emitter material.

physics.space-ph↗

arXiv2019

Earthquake Response Analysis of Yielding Structures Coupled with Rocking Walls

This paper investigates the inelastic response of a yielding structure coupled with a rocking wall which can be vertically restrained. The paper first derives the nonlinear equations of motion of a yielding oscillator coupled with a vertically restrained rocking wall and the dependability of the one-degree of freedom idealization is validated against the nonlinear time-history response analysis of a well-known 9-story moment-resisting steel frame that is coupled with a stepping rocking wall. While, the coupling of weak building frames with rocking walls is an efficient strategy that controls inelastic deformations by enforcing a uniform interstory-drift distribution, therefore, avoiding mid-story failures, the paper shows that even for medium-rise buildings the effect of vertical tendons on the inelastic structural response is marginal, with the exception of increasing the vertical reactions at the pivoting points of the rocking wall. Accordingly, the paper, concludes that for medium- to high-rise buildings vertical tendons in rocking walls are not beneficial.

physics.app-ph↗

arXiv2019

Temperature Dependence of a Width of $Δ$H = $Δ$B Region in 5 wt.\% (Fe, Ti) Paticle-Doped MgB$_2$ Superconductor

A temperature dependence of a width of $Δ$H = $Δ$B region has been studied for 5 wt.\% (Fe, Ti) particle-doped MgB$_2$ superconductor. The result revealed that widths of the region are linear along temperature. Here we show the meaning of the result and details of the calculation. In previous report, we represented a theory that a width of $Δ$H = $Δ$B region is related with upper critical field of the superconductor, which is that pinned fluxes at volume defect are picked out and move in $Δ$H = $Δ$B region when a distance between them is the same as that of upper critical field. Thus, we inspected the relationship between a width of the region and upper critical field along temperature. The theory would gain another justification if temperature dependence of a width of the region is proportional to that of upper critical field. We discussed several topics for $Δ$H = $Δ$B region of 5 wt.\% (Fe, Ti) particle-doped MgB$_2$ superconductor, which are Fe of (Fe, Ti) particle, Bean model, volume dependence of the region, etc..

physics.app-ph↗

arXiv2019

Integrated on chip platform with quantum emitters in layered materials

Integrated quantum photonic circuitry is an emerging topic that requires efficient coupling of quantum light sources to waveguides and optical resonators. So far, great effort has been devoted to engineering on-chip systems from three-dimensional crystals such as diamond or gallium arsenide. In this study, we demonstrate room temperature coupling of quantum emitters embedded within a layered hexagonal boron nitride to an on-chip aluminium nitride waveguide. We achieved 1.2% light coupling efficiency of the device and realise transmission of single photons through the waveguide. Our results serve as a foundation for the integration of layered materials with on-chip components and for the realisation of integrated quantum photonic circuitry.

physics.app-ph↗

arXiv2019

Cascaded Förster Resonance Energy Transfer and Role of the Relay Dye

The effect of introducing a relay dye on the energy transfer efficiency in a new donor-acceptor system has been studied. The values of the critical transfer distance, the reduced concentration, and the energy transfer efficiency of the cascaded system (consisting of three dyes- donor-relay dye-acceptor) are compared with that of the two dye system. Experimentally, it has been observed that the presence of a relay dye increases the transfer efficiency from the donor to the acceptor.

physics.chem-ph↗

arXiv2019

Organic Thermoelectric Textiles for Harvesting Thermal Energy and Powering Electronics

Wearable thermoelectric devices show promises to generate electricity in a ubiquitous, unintermittent and noiseless way for on-body applications. Three-dimensional thermoelectric textiles (TETs) outperform other types in smart textiles owing to their out-of-plane thermoelectric generation and good structural conformability with fabrics. Yet, there has been lack of efficient strategies in scalable manufacture of TETs for sustainably powering electronics. Here, we fabricate organic spacer fabric shaped TETs by knitting carbon nanotube yarn based segmented thermoelectric yarn in large scale. Combing finite element analysis with experimental evaluation, we elucidate that the fabric structure significantly influences the power generation. The optimally designed TET with good wearability and stability shows high output power density of 51.5 mW/m2 and high specific power of 173.3 uW/(g.K) at delta T= 47.5 K. The promising on-body applications of the TET in directly and continuously powering electronics for healthcare and environmental monitoring is fully demonstrated. This work will broaden the research vision and provide new routines for developing high-performance and large-scale TETs toward practical applications.

physics.app-ph↗

arXiv2019

A new compact, portable 2-LTD-brick x-pinch driver at the Idaho Accelerator Center: design, fabrication, testing and x-ray performance

We propose to construct a compact and portable x-pinch driver with x-ray radiation performance, comparable to standard x-pinch drivers. Such a new x-pinch driver was recently designed, fabricated and tested at the Idaho Accelerator Center. The generator is based on two slow LTD bricks combined into one solid unit, and can be described by a simple RLC circuit with four fast 140-nF, 100-kV capacitors that store up to 2.8 kJ. The test data reveals that when charged to 80 kV, the driver supplies 185-kA peak-current into a short Ni-wire load with 220-ns, 10-90%, rise time. The total internal inductance of our driver was measured to be about 60 nH. The revised driver model shows that when fully charged to 100 kV, the driver will supply 180-kA peak-current with 150-ns rise-time into the x-pinch load. The corresponding current rise rate is about 1.2 kA/ns. To prove the driver x-pinch efficiency and to estimate the x-ray radiation performance, we could, for example, image an exploding wires, placed in a separate HV pulser, with our x-pinch x-ray radiation source. The study of exploding wires helps to understand the behavior of a warm dense matter, and our x-pinch driver can be part of the diagnostics needed for this study which is currently under progress at the IAC. Our driver contains no oil inside, is very compact and portable, and can be easily relocated to practically anywhere, which makes it an ideal backlighting diagnostic tool in many areas of plasma physics, biology, and industry where a bright, fast, and small x-pinch radiation source is required.

physics.plasm-ph↗

arXiv2019

Mixing properties of room temperature patch-antenna receivers in a mid-infrared (9um) heterodyne system

A room-temperature mid-infrared (9 um) heterodyne system based on high-performance unipolar optoelectronic devices is presented. The local oscillator (LO) is a quantum cascade laser, while the receiver is an antenna coupled quantum well infrared photodetector optimized to operate in a microcavity configuration. Measurements of the saturation intensity show that these receivers have a linear response up to very high optical power, an essential feature for heterodyne detection. By an accurate passive stabilization of the local oscillator and minimizing the optical feed-back the system reaches, at room temperature, a record value of noise equivalent power of 30 pW at 9um. Finally, it is demonstrated that the injection of microwave signal into our receivers shifts the heterodyne beating over the bandwidth of the devices. This mixing property is a unique valuable function of these devices for signal treatment.

physics.app-ph↗

arXiv2019

A novel, compact and portable 2-LTD-Brick x-pinch radiation source: its development and radiation performance

Almost all well-known x-pinch x-ray radiation machines are large, based on a conventional Marx generator, and lack portability. The literature suggests that a current rate of rise of 1 kA/ns or more is required for "good" x-pinch radiation performance, which, for reasonable current rise times, translates to a current requirement of 100 kA or more. Those requirements are difficult to achieve in a limited volume, if one wants to build a compact machine without the use of traditional Marx generators, pulse-forming lines, and transmission lines. In this work we describe a new, compact and portable x-pinch driver based on two "slow" LTD bricks combined into one solid unit. The short-circuit tests demonstrated the required 1-kA/ns current rate-of-rise and x-pinch shots confirmed "good" x-pinch radiation performance and revealed the potential for many x-pinch applications.

physics.plasm-ph↗

arXiv2019

Releasable micro-waveplates

A simple procedure is demonstrated for fabrication of waveplates which can be released from substrate by laser cutting. Oblique angle deposition, chemical etching and laser inscription steps were used for the final lift-off and release of micro-waveplates in HCl solution.

physics.app-ph↗

arXiv2019

A Combined Computational and Experimental Investigation on Evaporation of a Sessile Water Droplet on a Heated Hydrophilic Substrate

We numerically and experimentally investigate evaporation of a sessile droplet on a heated substrate. We develop a finite element (FE) model in two-dimensional axisymmetric coordinates to solve coupled transport of heat in the droplet and substrate, and of the mass of liquid vapor in surrounding ambient while assuming diffusion-limited, quasi-steady evaporation of the droplet. The two-way coupling is implemented using an iterative scheme and under-relaxation is used to ensure numerical stability. The FE model is validated against the published spatial profile of the evaporation mass flux and temperature of the liquid-gas interface. We discuss cases in which the two-way coupling is significantly accurate than the one-way coupling. In experiments, we visualized side view of an evaporating microliter water droplet using a high-speed camera at different substrate temperatures and recorded temperature of the liquid-gas interface from the top using an infrared camera. We examine the dependency of inversion of the temperature profile across the liquid-gas interface on the ratio of the substrate thickness to the wetted radius, the ratio of the thermal conductivity of the substrate to that of the droplet and contact angle. A comparison of measured evaporation mass rate with the computed values at different substrate temperature show that the evaporation mass rate increases non-linearly with respect to the substrate temperature, and FE model predicts these values close to the experimental data. Comparisons of time-averaged evaporation mass rate obtained by the previous and present models against the measurements suggest that the evaporative cooling at the interface and variation of diffusion coefficient with temperature should be taken into account in the model in order to accurately capture the measurements.

physics.app-ph↗

arXiv2019

All-optical tuning of a diamond micro-disk resonator on silicon

High quality integrated diamond photonic devices have previously been demonstrated in applications from non-linear photonics to on-chip quantum optics. However, the small sample sizes of single crystal material available, and the difficulty in tuning its optical properties, are barriers to the scaling of these technologies. Both of these issues can be addressed by integrating micron scale diamond devices onto host photonic integrated circuits using a highly accurate micro-assembly method. In this work a diamond micro-disk resonator is integrated with a standard single mode silicon-on-insulator waveguide, exhibiting an average loaded Q-factor of 3.1x10^4 across a range of spatial modes, with a maximum loaded Q-factor of 1.05x10^5. The micron scale device size and high thermal impedance of the silica interface layer allow for significant thermal loading and continuous resonant wavelength tuning across a 450 pm range using a mW level optical pump. This diamond-on-demand integration technique paves the way for tunable devices coupled across large scale photonic circuits.

physics.optics↗

arXiv2019

Single-cell micro- and nano-photonic technologies

Since the advent of optogenetics, technology development has focused on new methods to optically interact with single nervous cells. This gave rise to the field of photonic neural interfaces, intended as the set of technologies that can modify light radiation in either a linear or non-linear fashion to control and/or monitor cellular functions. These include the use of plasmonic effects, up-conversion, electron transfer and integrated light steering, with some of them already implemented in vivo. This article will review available approaches in this framework, with a particular emphasis on methods operating at the single-unit level or having the potential to reach single-cell resolution.

q-bio.CB↗

arXiv2019

Generation of Schubert polynomial series by nanophotonics

Generation of irregular time series based on physical processes is indispensable in computing and artificial intelligence. In this report, we propose and experimentally demonstrate the generation of Schubert polynomials, which is the foundation of versatile permutations in mathematics, via optical near-field processes introduced in a photochromic crystal of diarylethene, which optical near-field excitation on the surface of a photochromic single crystal yields a chain of local photoisomerization, forming a complex pattern on the opposite side of the crystal. The incoming photon travels through the nanostructured photochromic crystal, and the exit position of the photon exhibits a versatile pattern. We experimentally generated Schubert matrices, corresponding to Schubert polynomials, via optical near-field density mapping. The versatility and correlations of the generated patterns could be reconfigured in either a soft or hard manner by adjusting the photon detection sensitivity. This is the first study of Schubert polynomial generation by physical processes or nanophotonics, paving the way toward future nano-scale intelligence devices and systems.

physics.optics↗