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arXiv2018

Fluidic bypass structures for improving the robustness of liquid scanning probes

Objective: We aim to improve operational robustness of liquid scanning probes. Two main failure modes to be addressed are an obstruction of the flow path of the processing liquid and a deviation from the desired gap distance between probe and sample. Methods: We introduce a multi-functional design element, a microfluidic bypass channel, which can be operated in dc and in ac mode, each preventing one of the two main failure modes. Results: In dc mode, the bypass channel is filled with liquid and exhibits resistive behavior, enabling the probe to passively react to an obstruction. In the case of an obstruction of the flow path, the processing liquid is passively diverted through the bypass to prevent its leakage and to limit the build-up of high pressure levels. In ac mode, the bypass is filled with gas and has capacitive characteristics, allowing the gap distance between the probe and the sample to be monitored by observing a phase shift in the motion of two gas-liquid interfaces. For a modulation of the input pressure at 4 Hz, significant changes of the phase shift were observed up to a gap distance of 25 μm. Conclusion: The presented passive design element counters both failure modes in a simple and highly compatible manner. Significance: Liquid scanning probes enabling targeted interfacing with biological surfaces are compatible with a wide range of workflows and bioanalytical applications. An improved operational robustness would facilitate rapid and widespread adoption of liquid scanning probes in research as well as in diagnostics.

physics.app-ph↗

arXiv2018

500 GHz plasmonic Mach-Zehnder modulator enabling sub-THz microwave photonics

Broadband electro-optic intensity modulators are essential to convert electrical signals to the optical domain. The growing interest in THz wireless applications demands modulators with frequency responses to the sub-THz range, high power handling and very low nonlinear distortions, simultaneously. However, a modulator with all those characteristics has not been demonstrated to date. Here we experimentally demonstrate that plasmonic modulators do not trade off any performance parameter, featuring - at the same time - a short length of 10s of micrometers, record-high flat frequency response beyond 500 GHz, high power handling and high linearity, and we use them to create a sub-THz radio-over-fiber analog optical link. These devices have the potential to become a new tool in the general field of microwave photonics, making the sub-THz range accessible to e.g. 5G wireless communications, antenna remoting, IoT, sensing, and more.

physics.app-ph↗

arXiv2019

Modeling of Radiation-Induced Defect Recovery in 3C-SiC Under High Field Bias Conditions

In this work, the implications of high field bias conditions in radiation-induced defect recovery in 3C-SiC crystals is studied. It is well known that transient heating effects (or thermal spikes) occur when energetic swift heavy ions (SHIs) deposit energy to the surrounding medium via ionization. Here, we explore the dynamics of this transient event under high background electric fields in 3C-SiC, which is what occurs when an ion strike coincides with field-sensitive volumes. In this study, we use the Ensemble Monte Carlo method to quantify how the energy deposition of the ionized regions change in response to high background electric fields. Subsequently, we study the relationship between the radiation-induced thermal spike and defect recovery using molecular dynamics simulations. We find that field strengths below the critical breakdown of wide bandgap devices are sufficient to exacerbate the localized heating, which subsequently enhances the defect recovery effect. This work is beneficial for 3C-SiC electronics and materials used in high radiation environments.

physics.app-ph↗

arXiv2019

An Irregular-shaped Ring-Pair Magnet Array with a Monotonic Field Gradient for 2D Head Imaging in Low-field Portable MRI

We present a design and the optimization of an irregular-shaped ring-pair magnet array that generates a 1D monotonic field pattern for 2D head imaging in a low-field portable MRI system. The magnet rings are discretized into fan-shaped ring segments with varying outer diameters for the design and optimization. Besides, the inner radii of ring-pairs are tapered from outside in to provide the controlled field inhomogeneity. Genetic algorithm (GA) was used, and a current model for a fan-shaped ring segment was derived to have a fast forward calculation in the optimization. A monotonic field pattern is successfully obtained along the x-direction in a cylindrical field of view (FoV), with a relatively strong magnetic field (132.98 mT) and the homogeneity of 151840 ppm. The proposed array was further evaluated by applying its field as a spatial encoding magnetic field (SEM) for imaging numerically. Due to the field monotonicity, the reconstructed image by applying the fields of the proposed array shows clearer features (a higher structural similarity index) with a reduced error rate compared to that using a sparse dipolar Halbach array. The proposed magnet array is a promising alternative to supply SEM for imaging in a permanent-magnet-based low-field portable MRI system.

physics.med-ph↗

arXiv2019

Efficiency enhancement on the solar steam generation by wick materials with wrapped graphene nanoparticles

Solar steam generation technology can utilize abundant and renewable solar energy for many applications. In this work, we proposed a solar steam generator using wick material with wrapped graphene nanoparticles, and the energy efficiency can reaches up to 80%. Instead of traditional smearing method, the chemical wrapping method was used to better adhere the graphene nanoparticles on the wick materials. Through the SEM morphological results, the graphene nanoparticles are shown to be evenly wrapped across the fibres of the wick material, which have better dispersity and stability. The evaporation rate, instantaneous energy efficiency and the absorptivity of three wick materials with/without nanoparticles under different conditions were compared and analyzed. Among the three different wick materials, the flannel cloth with dense fine hairs can provide three-dimensional contact area for wrapping graphene nanoparticles and thus contribute to better evaporation. Additionally, the influence of two different reduction methods and different concentrations of graphene oxide solution on the energy efficiency was also discussed. Our work offers a simple and effective way of using nanotechnology in practical application for solar steam generation.

physics.app-ph↗

arXiv2019

Chalcogen assisted contact engineering: towards CMOS integration of Tungsten Diselenide Field Effect Transistors

One of the major roadblocks for the establishment of 2D semiconductor technology for CMOS integrated circuits is lack of industry scalable doping techniques that lead to 2D FETs with comparable n-type and p-type behavior. Here we demonstrate a Chalcogen based technique to alter the surface of WSe2 to realize enhanced ambipolar behavior along with a complete transition from n to p-type behavior of WSe2 FETs. The technique involves dry chemistry between Chalcogen atom and TMDC surface which leads to surface states that cause improved hole and electron injection through the FETs. We propose such a technique for realization of all WSe2 based CMOS integrated circuits and therefore unveil its potential towards technology.

physics.app-ph↗

arXiv2019

H2S assisted contact engineering: a universal approach to enhance hole conduction in all TMD Field-Effect Transistors and achieve ambipolar CVD MoS2 Transistors

Unlike Si, 2-dimensional (2D) Transition Metal Dichalcogenides (TMDs) offer atomically thin channels for carrier transport in FETs. Despite advantages like superior gate control, steep sub-threshold swing and high carrier mobility offered by 2D FET channels, process related challenges like lack of selective doping techniques like implantation and CMOS compatible process for fabrication of 2D TMD based FETs hinder the anticipated viability of 2D semiconductor technology for future electronic applications. In this letter, we demonstrate a process oriented approach to realize superior ambipolarity in 2D FETs based on TMDs like Molybdenum disulfide (MoS2), Tungsten disulfide (WS2), Molybdenum diselenide (MoSe2) and Tungsten diselenide (WSe2) by enhancing hole current by multiple orders of magnitude in otherwise strong N-type transistors. The method involves Hydrogen Sulfide (H2S gas) assisted contact engineering of N-type FETs to introduce surface states that alter device behavior. Based on material characterization and bandstructure calculations, physical insights have been developed to understand the effect of such a contact engineering technique. Subsequently, this technique has been demonstrated to alter device behavior by enhancing hole conduction in originally N-type exfoliated (MoS2, WS2, MoSe2 and WSe2) and CVD grown (MoS2) TMD samples to confirm its potential towards enabling the feasibility of 2D semiconductor device technology.

physics.app-ph↗

arXiv2019

Space-coiling Acoustic Metasurface with Independent Modulations of Phase and Amplitude

In this work, we propose a design of acoustic meta-surfaces in sub-wavelength scale enabling independent modulations of phase and amplitude. Each unit cell of the acoustic meta-surface consists of simple conventional space-coiling structure added with an air layer, which can be analyzed as two equivalent slabs with non-dispersion effective parameters. The amplitude depends on the space-coiling structure regardless of the air layer, and the phase can be further adjusted by the air layer independent to the amplitude. The acoustic meta-surface covers an entire phase change of 2-pi and amplitude change of one. We demonstrate an acoustic illusion effect by using the acoustic meta-surface screen, which works well as a full-control discontinuous boundary to support phase and amplitude differences between the original and illusion patterns. The incident field of a point source is transformed into a target field of the point source scattered by an object with shadow area behind it.

physics.app-ph↗

arXiv2019

Acquisition of time-frequency localized mechanical properties of biofilms and single cells with high spatial resolution

Biofilms are a cluster of bacteria embedded in extracellular polymeric substances (EPS) that contain a complex composition of polysaccharides, proteins, and extracellular DNA (eDNA). Desirable mechanical properties of the biofilms are critical for their survival, propagation, and dispersal, and the response of mechanical properties to different treatment conditions also sheds light on biofilm control and eradication in vivo and on engineering surfaces. However, it is challenging yet important to interrogate mechanical behaviors of biofilms with a high spatial resolution because biofilms are very heterogeneous. Moreover, biofilms are viscoelastic, and their time-dependent mechanical behavior is difficult to capture. Herein, we developed a powerful technique that combines the high spatial resolution of the atomic force microscope (AFM) with a rigorous history-dependent viscoelastic analysis to deliver highly spatial-localized biofilm properties within a wide time-frequency window. By exploiting the use of static force spectroscopy in combination with an appropriate viscoelastic framework, we highlight the intensive amount of time-dependent information experimentally available that has been largely overlooked. It is shown that this technique provides a detailed nanorheological signature of the biofilms even at the single-cell level. We share the computational routines that would allow any user to perform the analysis from experimental raw data. The detailed localization of mechanical properties in space and in time-frequency domain provides insights on the understanding of biofilm stability, cohesiveness, dispersal, and control.

physics.app-ph↗

arXiv2019

Enhanced sensitivity of silicon-photonics-based ultrasound detection via BCB coating

Ultrasound detection via silicon waveguides relies on the ability of acoustic waves to modulate the effective refractive index of the guided modes. However, the low photo-elastic response of silicon and silica limits the sensitivity of conventional silicon-on-insulator (SOI) sensors, in which the silicon core is surrounded by a silica cladding. In this paper, we demonstrate that the sensitivity of silicon waveguides to ultrasound may be significantly enhanced by replacing the silica over-cladding with bisbenzocyclobutene (BCB) - a transparent polymer with a high photo-elastic coefficient. In our experimental study, the response to ultrasound, in terms of the induced modulation in the effective refractive index, achieved for a BCB-coated silicon waveguide with TM polarization was comparable to values previously reported for polymer waveguides and an order of magnitude higher than the response achieved by an optical fiber. In addition, in our study the susceptibility of the sensors to surface acoustic waves and reverberations was reduced for both TE and TM modes when the BCB over-cladding was used.

physics.optics↗

arXiv2019

Addition Theorem Revisiting for Phase/Amplitude-Encoded Metasurfaces: Asymmetric Spatial Power Dividers

Recent years have witnessed an extraordinary spurt in attention toward the wave manipulating strategies revealed by coding metasurfaces as they build up a bridge between the physical and digital worlds. Newly, it has been shown that when two different coding patterns responsible for doing separate missions are added together based on the superposition theorem, the mixed coding pattern will perform both missions at the same time. In this paper, via a semi-analytical procedure, we demonstrate that such a theorem is not necessarily valid for all possible functionalities with considering phase-only coding distributions and ignoring the element pattern function. By revisiting the addition theorem, we introduce the concept of asymmetric spatial power divider (ASPD) with arbitrary power ratio levels in which modulating both amplitude and phase of the meta-atoms is inevitable to fully control the power intensity pattern of the metasurface. Numerical simulations illustrate that the proposed ASPD driven by proper coding sequences can directly generate a desired number of beams with pre-determined orientations and power budgets. The phase/amplitude-encoded Pancharatnam-Berry meta-atoms realize the required coding pattern in each case and good conformity between simulations and theoretical predictions verifies the presented formalism. This work exposes a new opportunity to implement spatial power dividers for various applications such as multiple-target radar systems, beamforming networks, and multiple-input multiple-output (MIMO) communication.

physics.app-ph↗

arXiv2019

A Study Of a Wide-Angle Scanning Phased Array Based On a High-Impedance Surface Ground Plane

This paper presents a two-dimensional infinite dipole array system with a mushroom-like high-impedance surface (HIS) ground plane with wide-angle scanning capability in the E-plane. The unit cell of the proposed antenna array consists of a dipole antenna and a four-by-four HIS ground. The simulation results show that the proposed antenna array can achieve a wide scanning angle of up to 65$^{\circ}$ in the E-plane with an excellent impedance match and a small $S11$. Floquet mode analysis is utilized to analyze the active impedance and the reflection coefficient. Good agreement is obtained between the theoretical results and the simulations. Using numerical and theoretical analyses, we reveal the mechanism of such excellent wide scanning properties. For the range of small scanning angles, these excellent properties result mainly from the special reflection phase of the HIS ground, which can cause the mutual coupling between the elements of the real array to be compensated by the mutual coupling effect between the real array and the mirror array. For the range of large scanning angles, since the surface wave (SW) mode could be resonantly excited by a high-order Floquet mode $\textrm{TM}_{-1,0}$ from the array and since the SW mode could be converted into a leaky wave (LW) mode by the scattering of the array, the radiation field from the LW mode is nearly in phase with the direct radiating field from the array. Therefore, with help from the special reflection phase of the HIS and the designed LW mode of the HIS ground, the antenna array with an HIS ground can achieve wide-angle scanning performance.

physics.app-ph↗

arXiv2019

Parallel-Plate Waveguides Formed by Arbitrary Impedance Sheets

In this work, we introduce and study parallel-plate-waveguide structures that are formed by two penetrable metasurfaces having arbitrary sheet impedances. We investigate the guided modes which can propagate in such structures and derive the corresponding dispersion relations. Different scenarios including series- and parallel-resonant impedance sheets are considered. The obtained theoretical formulas are applied to predict the dispersion properties for different separations between the two symmetric or asymmetric metasurfaces.

physics.app-ph↗

arXiv2018

3-D Radiation Mapping in Real-Time with the Localization and Mapping Platform LAMP from Unmanned Aerial Systems and Man-Portable Configurations

Real-time, meter-resolution gamma-ray mapping is relevant in the detection and mapping of radiological materials, and for applications ranging from nuclear decommissioning, waste management, and environmental remediation to homeland security, emergency response, and international safeguards. We present the Localization and Mapping Platform (LAMP) as a modular, contextual and radiation detector sensor suite, which performs gamma-ray mapping in three dimensions (3-D) and in real time, onboard an unmanned aerial vehicle (UAV) or in a man-portable configuration. The deployment of an unmanned aerial system (UAS) for gamma-ray mapping can be advantageous, as the UAS provides a means of measuring large areas efficiently and improving accessibility to some environments, such as multi-story structures. In addition, it is possible to increase measurement robustness through autonomous navigation, and to reduce radiation exposure to users as a result of the remote measurement. LAMP enables meter-resolution gamma-ray mapping through Scene Data Fusion (SDF) [1], a capability that fuses radiation and scene data via voxelized 3-D Maximum Likelihood Expectation Maximization (MLEM) to produce 3-D maps of radioactive source distributions in real-time. Results are computed onboard LAMP while it is flying on the UAV and streamed from the system to the user, who can view the 3-D map on a tablet in real-time. We present results that demonstrate the SDF concept, including a set of UAS flights where a 133Ba source is localized at a test site in Berkeley, CA and a handheld measurement in Fukushima Prefecture, Japan where the distribution of radiocesium(137,134Cs) released from the accident of the Fukushima Daiichi Nuclear Power Plant is mapped. The reconstruction parameters used for each measurement were identical, indicating that the same algorithm can be used for both point or distributed sources.

physics.app-ph↗

arXiv2019

SiC-YiG X band quantum sensor for sensitive surface paramagnetic resonance applied to chemistry, biology, physics

Here I present the SiC-YiG Quantum Sensor, allowing electron paramagnetic resonance (EPR) studies of monolayer or few nanometers thick chemical, biological or physical samples located on the sensor surface. It contains two parts, a 4H-SiC substrate with many paramagnetic silicon vacancies (V2) located below its surface, and YIG ferrimagnetic nanostripes. Spins sensing properties are based on optically detected double electron-electron spin resonance under the strong magnetic field gradient of nanostripes. Here I describe fabrication, magnetic, optical and spins sensing properties of this sensor. I show that the target spins sensitivity is at least five orders of magnitude larger than the one of standard X band EPR spectrometer, for which it constitutes, combined with a fiber bundle, a powerful upgrade for sensitive surface EPR. This sensor can determine the target spins planes EPR spectrum, their positions with a nanoscale precision of +/- 1 nm, and their 2D concentration down to 1/(20nm.20nm).

physics.app-ph↗

arXiv2019

AlN/beta-Ga2O3 based HEMT: a potential pathway to ultimate high power device

Gallium Oxide (Ga2O3) has a huge potential on the power device for its high breakdown filed and good transport properties. beta-Ga2O3 as the thermodynamics stable phase, has been demonstrated to form high electron mobility transistor (HEMT) through delta-doping in the barrier due to its none-polar property. Following the development in III-V HEMT which turns from delta-doping-induced to polarization-induced 2DEG, an alternative method based on III-N materials/beta-Ga2O3 heterostructure is proposed that utilizing the polarization difference on the interface. Further requirements of electric field and conduction band difference show that only nitrogen (N)-polar AlN on beta-Ga2O3 can form the channel and hold large 2DEG concentration on the interface. Compared with conventional metal-polar AlN/GaN HEMT, the proposed N-polar AlN/beta-Ga2O3 HEMT show a much larger 2DEG concentration, the spontaneous-polarization-dominated electric field, better DC output performance, as well as higher breakdown voltage. This study provides a new research approach that shifting from delta-doping-induced to polarization-induced on beta-Ga2O3-based HEMT, which can also be a guideline for community excavating the application potential of Ga2O3.

physics.app-ph↗

arXiv2019

Ultrasonic Nanofabrication with an AFM

Ultrasonic AFM may improve fabrication technologies on the nanometer scale. In the presence of ultrasonic vibration, hard surfaces can be indented and scratched with the tip of a soft cantilever, due to its inertia. Ultrasound reduces or even eliminates friction, and hence modifies the tip-nanoparticle-surface interactions in AFM manipulation. The subsurface sensitivity of the technique makes feasible the purposed manipulation of subsurface nanoscale features by ultrasonic actuation.

physics.app-ph↗

arXiv2019

Calculating the Band Structure of 3C-SiC Using sp3d5s*+$Δ$ Model

We report on a semi-empirical tight binding model for 3C-SiC including the effect of sp3d5s* orbitals and spin-orbital coupling. In this work, we illustrate in detail the method to develop such a model for semiconductors with zincblende structure, based on Slater-Koster integrals, and we explain the optimization method used to fit the experimental results with such a model. This method shows high accuracy for the evaluation of 3C-SiC band diagram both in terms of the experimental energy levels at high symmetry points and the effective masses.

physics.app-ph↗