Near-field Investigation Research Articles

Blending or Bonding? Mechanochromism of an Aggregachromic Mechanophore in a Thermoplastic Elastomer

A straightforward way for the preparation of mechanochromic polymers consists of incorporating a suitable content of a mechanophore in the polymeric matrix either by physical dispersion or via covalent functionalization. Although covalent incorporation may require demanding chemical efforts, this approach can offer significant advantages over physical dispersion. In this work, a common thermoplastic elastomer, styrene-b-(ethylene-co-butylene)-b-styrene triblock copolymer grafted with maleic anhydride (SEBS-MAH), was covalently functionalized with 1-aminomethylpyrene (AMP). MAH functional groups are covalently linked to the ethylene-co-butylene blocks, thus allowing a precise and selective confinement of the chromogenic AMP units in the soft block. Flat, fully conjugated pyrene units undergo the reversible formation of π–π aggregates, readily distinguishable by their red-shifted emission. These aggregates were heavily affected by the application of mechanical stimuli. Despite the low degree of mechanophore functionalization (about 1 wt %), uniaxial deformation of the polymer was reliably monitored via fluorescence and a clear drop in the excimer to monomer emission ratio (IE/IM) was observed starting from 50% of strain. The marked mechanochromism was confirmed by emission lifetime measurements and also by near-field investigations. In addition, the mechanoresponse showed good reversibility after repeated stress–relaxation cycles. Control experiments performed on formulations comprising a physical dispersion of pyrene in unfunctionalized SEBS showed faint excimer emission and a negligible mechanochromic response up to 5 wt % of doping, in substantial agreement with the scanning near-field optical microscopy analysis. An evident drop of the IE/IM ratio occurred for 10 wt % of pyrene, albeit the excimer emission remained predominant even at the highest deformation, being a smaller fraction of pyrene moieties involved. Overall, the covalent approach appeared as an elegant procedure to confine the chromogenic unit in the soft phase of block copolymers and thus to provide an elastomeric film showing a detectable and reversible mechanochromic response with a modest (i.e., ∼1 wt %) amount of pyrene molecules, i.e., 10 times smaller compared to the dispersed system.

Robust Tipless Positioning Device for Near-Field Investigations: Press and Roll Scan (PROscan).

Scanning probe microscopes scan and manipulate a sharp tip in the immediate vicinity of a sample surface. The limited bandwidth of the feedback mechanism used for stabilizing the separation between the tip and the sample makes the fragile nanoscopic tip very susceptible to mechanical instabilities. We propose, demonstrate, and characterize an alternative device based on bulging a thin substrate against a second substrate and rolling them with respect to each other. We showcase the power of this method by placing gold nanoparticles and semiconductor quantum dots on the two opposite substrates and positioning them with nanometer precision to enhance the fluorescence intensity and emission rate. Furthermore, we exhibit the passive mechanical stability of the system over more than 1 h. Our design concept finds applications in a variety of other scientific and technological contexts, where nanoscopic features have to be positioned and kept near contact with each other.

Temporal velocity variations in the northern Hikurangi margin and the relation to slow slip

Slow slip events (SSE) have been studied in increasing detail over the last 20 years, improving our understanding of subduction zone processes. Although the relationship between SSEs and the physical properties of their surrounding materials is still not well-understood, the northern Hikurangi margin in New Zealand is the site of relatively shallow (<10 km deep), frequent SSEs, providing excellent opportunities for near-field investigations. From September to October 2014, an SSE occurred with more than 250 mm slip, and was recorded successfully by the Hikurangi Ocean Bottom Investigation of Tremor and Slow Slip (HOBITSS) deployment. This study applies scattered wave interferometry to ambient noise data acquired by nine HOBITSS ocean bottom seismometers (OBS) to study the seismic velocity variations related to the SSE. Single station cross-component correlations are computed within a period band that focuses on the upper plate in our study region. The average velocity variations display a decrease on the order of 0.05% during the SSE, followed by an increase of similar magnitude afterwards. We suggest two possibilities. The first possibility, which has been suggested by other seismological observations, is that the SSE causes a low-permeability seal on the plate boundary to break. The break allows fluid to migrate into the upper plate, causing a seismic velocity decrease during the SSE because of increased pore fluid volume in the upper plate. Under this model, after the SSE, the fluids in the upper plate diffuse gradually and the velocity increases again. The second possibility is the velocity changes are related to changes in crustal strain during the slow slip cycle, whereby elastic strain accumulates prior to the SSE, causing contraction and reduction of porosity and therefore increase of velocity above the SSE source (the seismic velocity increases between SSEs). During the SSE the upper plate goes into extension as the elastic strain is released, which results in dilation and a porosity increase (seismic velocity reduction). After the SSE, stress and strain accumulate again, causing a porosity decrease and a velocity increase.

An Over-deterministic Method Based on Atomistic Stress Fields: Higher Order Terms of the Williams power expansion

The investigation of the atomistic near crack-tip stress fields under Mixed Mode loading via molecular dynamics method implemented in a flexible classical molecular dynamics method software LAMMPS (Large-scale Atomic/Molecular Massively Parallel Simulator) is presented. The molecular dynamics modelling is applied for evaluation of the coefficients of higher-order terms of the Williams’ expansion -based approximation for the near crack tip fields in isotropic linear elastic materials. Fracture mechanics parameters such as stress intensity factors, higher-order coefficients of the Williams power-series for a copper plate with a central crack under Mode I and Mixed Mode (Mode I+Mode II) loadings are obtained by atomistic simulations. All computational experiments are realized in LAMMPS with the Embedded Atom Method (EAM) potential. The atomistic values of higher-order coefficients of the Williams series expansion are computed by the over-deterministic method and liken to the values obtained from classical continuum fracture mechanics solutions. It is demonstrated that the conventional continuum fracture theory successfully describes fracture quantities at extremely limited singular stress field of only several nanometers.

Transferred energy and acoustic spin density in fluid spherical core-shell scattering scenarios

Sound wave scattering, along with radiation force and torque, are constantly in vogue topics for several distinct applications, such as small object manipulations and Acoustofluidics. Specially for scattering scenarios, far-field approximations are often considered for dealing with the sound waves far beyond the scattering objects. However, a more in-depth investigation of near and internal fields during a scattering process may be able to unravel dynamical and mechanical properties of the scattering objects. In this study, we propose a time-averaged study of the transferred energy and acoustic spin density in a fluid spherical core-shell. We derive the expressions for the internal waves in terms of spherical harmonics and obtaining Mie-like coefficients. Using those expressions, we analytically compute the time-averaged acoustic energy transferred by a plane wave. Furthermore, we use very recent results with regards to acoustic spin to bring a discussion about spin density transfer, properly computing the quantity for this scenario. This is a follow-up study for understanding acoustical properties of compressible fluids with different geometries, searching for electromagnetic parallels, such as Fano resonance regimes, that may be applicable for building or characterizing acoustic metamaterials.

Nanoinfrared Characterization of Bilayer Graphene Conductivity under Dual-Gate Tuning.

Dual-gate tuning on two-dimensional (2D) heterostructures can provide independent control of the carrier concentration and interlayer electrostatic potential, yielding novel electronic and optical properties. In this paper, by utilizing monolayer graphene as both the top gate and a plasmon wavelength magnifier, the optical properties of bilayer graphene (BLG) under dual-gate are quantitatively investigated by nanoinfrared imaging. The hybrid optical modes in the vertically coupled two-layer system are imaged from scattering-type scanning near-field microscopy (s-SNOM). Moreover, plasmon dispersion behaviors under varied dual-gate tuning are explored and explained well with theoretical ones employing tight binding approximation, which reveals the flexibility in individually manipulating the Fermi energy and bandgap. Especially, electron-hole asymmetry in BLG is verified from experiments. Our studies pave route for quantitative near-field investigation of superlattice, topological boundaries, and other emergent phenomena in graphene-based 2D heterostructures.

Electronic bandgap miniaturized UWB antenna for near-field microwave investigation of skin

Near-field microwave investigation and tomography has many practical applications, especially where the trend of fields and signals in different environments is vital. This article shows an elliptical patch ultra-wideband antenna fed by a transmission line for the near-field characterization of cancerous cells in the skin. The antenna comprises an elliptical patch, stub loading to shift the band to lower bands, and an electronic bandgap structure on the ground side. Even though the antenna has a low profile of 15 × 15 mm2, the proposed antenna has more promising results than recent studies. Furthermore, both simulated near-field and far-field results show a broad bandwidth of 3.9–30 GHz and a resonance at 2.4 GHz applicable for industrial, scientific, and medical band applications. The proposed antenna also illustrates a peak gain of 6.48 dBi and a peak directivity of 7.09 dBi. Free space and skin (on a layer of breast fat and a tumor with a diameter of 4 mm at the boundary of skin and breast) are used as test environments during the simulation and measurement of near-field and far-field investigations while considering a phantom breast shape. Both far-field and near-field microwave investigations are performed in Computer Simulation Technology studio, and results are then compared with the measured data. The simulated and measured results are in good agreement, and the focused energy around the tumor is completely reconstructed. Therefore, the proposed antenna can be an adequate candidate for the differentiation of breast skin and tumor to reconstruct the tumor’s image.

On the Application of Endoscopic Background-Oriented Schlieren Technique for Near-Field Investigation of Density Field

Background-Oriented Schlieren (BOS) is a non-intrusive and non-destructive technique for measuring density gradient in transparent media. Given the dependency of the refractive index and density of a medium, its static density field can be determined as a secondary measurand from BOS measurements. For an ideal gas, if the pressure variation is negligible, the static temperature field can be determined as a tertiary measurand. While the BOS technique for far-field measurements has been extensively researched, the implementation of this technique for near-field measurements still needs to be investigated. Near-field BOS is especially important for the measurement and visualization of internal flows, such as turbomachine flows, in which access to flow passage is limited and might be feasible using endoscopes (borescopes). This paper presents the features and challenges of the endoscopic BOS technique and presents methods and solutions for the application of this method in near-field measurements. The features of endoscopic BOS records as well as the comparison of endoscopic BOS and BOS measurements with camera are presented.

Near-field and far-field investigation of miniaturized UWB antenna for imaging of wood

This article presents the design of a droplet shape ultra-wide band antenna for imaging of wood. The proposed antenna is designed on PTFE substrate with a dielectric constant of 2.55, loss tangent of 0.001- and 2.4-mm thickness. The antenna is loaded by a stub to resonate at lower band frequency, strip loading at the back, and a chamfered ground to increase the bandwidth. Despite having miniaturized dimensions of 15 mm × 15 mm, it shows better results compared to recent studies. The simulation results depict a good ultra-wide bandwidth from 3.26 GHz to 20 GHz, and 21.5–25 GHz; Besides, the proposed antenna has two bands at 1.25–1.35 GHz and 1.7–1.81 GHz. In addition to that, the antenna achieved a maximum gain of 5.69 dB and directivity of 7.3 dBi. The measurement results of S-parameters transmitted and received signals performed in air, plywood, and high-density wood show a good agreement with the simulated results. In addition, the measured results illustrate a good isolation and uniform illumination among arrays as well as the received signals’ shapes do not change in different environments, but only the amplitude. Hence, the proposed antenna seems to be adequate for microwave imaging of wood.

Diffraction enhanced transparency in a hybrid gold-graphene THz metasurface

Terahertz (THz) near-field microscopy is used to investigate the underlying physics that leads to diffraction enhanced transparency (DET) in a periodic array of detuned metallic rods. At the transparency frequency, the system is highly dispersive and THz radiation is delayed for several tens of picoseconds before being re-emitted into the forward direction. Using polarization sensitive measurements of the electric THz near-field spectrum, we demonstrate that an out-of-phase field distribution is formed in the unit cell leading to a reduced coupling to the far-field. This quadrupolar field distribution originates from the excitation and interference of surface lattice modes produced by the enhanced radiative coupling through the lattice of localized resonances in the metallic rods. These results represent the first experimental near-field investigation of DET, shedding light onto this phenomenon and providing important information for its further development. Implementing DET in applications requires control over the transparency window. We demonstrate that adding a monolayer of graphene, absorbing at THz frequencies, is sufficient to fully suppress the DET despite its monoatomic thickness. This efficient suppression is due to the diffractive wave character of DET and a metal insulator metal resonance formed between the localized resonance of the gold resonator and the extended graphene layer. The possibility to exert control over the transparency window by changing the conductivity of graphene by altering the Fermi level opens the possibility of active THz devices.

The effect of poly vinyl alcohol matrix on the light absorbance of Ag2S nanoparticles; experimental and discrete dipole approximation results

Well-ordered films of silver sulfide (Ag2S) nanoparticles (NPs) embedded in the matrix of poly vinyl alcohol (PVA) were fabricated using Successive Ionic Layer Adsorption and Reaction (SILAR) technique. Enlarging the size and increasing the concentration of Ag2S NPs by increasing the number of SILAR cycles confirmed by surface scanning electron microscopy (SEM) images of the produced films. The material composition was studied using energy dispersive X-ray (EDX) analysis. UV–visible experimental spectra of samples were compared with discrete dipole approximation (DDA) calculations results and good agreements were achieved. Increasing the number of peaks and valleys and redshift of peaks by increasing the number of SILAR cycles were obviously observed from the results. The most interesting conclusion of this work raised from electric near-field contour distributions investigations and implies that a matrix of insulator material (PVA) can cause an asymmetry in the intensity distribution of electric field around Ag2S NPs and let more dipole oscillations happen and more peaks appear in extinction/absorption spectra.

Boundary layer flow interaction with a permeable wall

The interaction of a zero pressure gradient turbulent boundary layer flow with a rough permeable surface has been investigated experimentally. The flow interaction characteristics have been examined using a long flat plate equipped with several surface pressure transducers and pressure taps. Three types of porous materials with different porosities and permeability constants were used in these investigations. To reveal the behavior of turbulent flows over porous surfaces, measurements were performed for the boundary layer growth, energy content of the turbulent structure within the boundary layer, and surface pressure fluctuations, before, over, and after the porous test-section. The interaction of the flow with the porous substrate was found to significantly alter the energy cascade within the boundary layer. Results have also shown that the boundary layer interaction with the rough porous surfaces leads to an increase in the pressure fluctuations exerted on the wall, particularly at low frequencies. The near-field investigations have shown that the penetration of the boundary layer flow into the porous medium can generate an internal hydrodynamic field within the porous medium. This, in turn, reduces the frequency-energy content of the large boundary layer coherent structures and their spanwise correlation length. This study paves the way for further investigation into the interaction of the porous media with different flow fields and development of tailored porous treatments for improving the aerodynamic and aeroacoustic performance of different aero- and hydro-components.

Numerical analysis of end-fire coupling of surface plasmon polaritons in a metal-insulator-metal waveguide using a simple photoplastic connector

We propose a design for efficient end-fire coupling of surface plasmon polaritons in a metal-insulator-metal (MIM) waveguide with an optical fiber as part of a simple photoplastic connector. The design was analyzed and optimized using the three-dimensional finite-difference time-domain method. The calculated excitation efficiency coefficient of the waveguide is 83.7% (−0.77 dB) at a wavelength of 405 nm. This design enables simple connection of an optical fiber to a MIM waveguide and highly efficient local excitation of the waveguide. Moreover, the length of the metallic elements of the waveguide, and thus the dissipative losses, can be reduced. The proposed design may be useful in plasmonic-type waveguide applications such as near-field investigation of live cells and other objects with super-resolution.

Near-field spectroscopic investigation of dual-band heavy fermion metamaterials

Broadband tunability is a central theme in contemporary nanophotonics and metamaterials research. Combining metamaterials with phase change media offers a promising approach to achieve such tunability, which requires a comprehensive investigation of the electromagnetic responses of novel materials at subwavelength scales. In this work, we demonstrate an innovative way to tailor band-selective electromagnetic responses at the surface of a heavy fermion compound, samarium sulfide (SmS). By utilizing the intrinsic, pressure sensitive, and multi-band electron responses of SmS, we create a proof-of-principle heavy fermion metamaterial, which is fabricated and characterized using scanning near-field microscopes with <50 nm spatial resolution. The optical responses at the infrared and visible frequency ranges can be selectively and separately tuned via modifying the occupation of the 4f and 5d band electrons. The unique pressure, doping, and temperature tunability demonstrated represents a paradigm shift for nanoscale metamaterial and metasurface design.

Near-field investigation of the explosive dispersal of radioactive material based on a reconstructed spherical blast-wave flow

A “dirty bomb” is a type of radiological dispersal device (RDD) that has been the subject of significant safety and security concerns given the disruption that would result from a postulated terrorist attack. Assessing the risks of radioactive dose in a hypothetical scenario requires models that can accurately predict dispersion in a realistic environment. Modelling a RDD is complicated by the fact that the most important phenomena occur over vastly disparate spatial and temporal length scales. Particulate dispersion in the air is generally considered on scales of hundreds to thousands of meters, and over periods of minutes and hours. Dispersion models are extremely sensitive, however, to the particle size and source characterization, which are determined in distances measured in micrometers to meters, over milliseconds or less.This study examines the extent to which the explosive blast determines the transport of contaminant particles relative to the atmospheric wind over distances relevant to “near-field” dispersion problems (i.e., hundreds of meters), which are relevant to urban environments. Our results indicate that whether or not the effect of the blast should be included in a near-field dispersion model is largely dependent on the size of the contaminant particle. Relatively large particles (i.e., >40 μm in diameter), which are most likely to be produced by a RDD, penetrate the leading shock front, thereby avoiding the reverse blast wind. Consequently, they travel much farther than suspended aerosols (<10 μm) before approaching the ambient wind velocity. This suggests that, for these “near-field” dispersion problems in urban environments, the transport of contaminants from the blast wave may be integral to accurately predicting their dispersion.

Characteristics of Horizontal and Vertical Near-Field Ground Motions and Investigation of Their Effects on the Dynamic Response of Bridges

Recently, concerning failure of structures due to earthquakes, special investigations of near fault ground motions have been noticed. Hence, in this paper, characteristics of near field ground motions have been considered in horizontal and vertical directions. In this consideration, the record averages have been compared with Uniform Building Code and Eurocode8 spectra in two levels. In addition, the ratio of vertical to horizontal spectra has been computed and compared with the assumed value of two thirds in some code provisions. Finally, the response of near field records on five artificial bridges that have covered all 0.5-2.5 seconds periods, have been investigated for comparing the ratio of responses in near field to far field, and forward to backward directivity effects. In addition, the results of the response spectrum analyses of six different bridges subjected to vertical excitations are presented.

Near-field investigation of the effect of the array edge on the resonance of loop frequency selective surface elements at mid-infrared wavelengths.

Mid-infrared scattering scanning near-field optical microscopy, in combination with far-field infrared spectroscopy, and simulations, was employed to investigate the effect of mutual-element coupling towards the edge of arrays of loop elements acting as frequency selective surfaces (FSSs). Two different square loop arrays on ZnS over a ground plane, resonant at 10.3 µm, were investigated. One array had elements that were closely spaced while the other array had elements with greater inter-element spacing. In addition to the dipolar resonance, we observed a new emergent resonance associated with the edge of the closely-spaced array as a finite size effect, due to the broken translational invariance.

1205 共焦点マイクロPIVと超解像度顕微鏡の組み合わせによる血管内皮細胞グリコカリックス層近傍の速度分布計測

1205 共焦点マイクロPIVと超解像度顕微鏡の組み合わせによる血管内皮細胞グリコカリックス層近傍の速度分布計測

What Does One Measure When One Measures the Arrival Time of a Quantum Particle?

What is the right statistics for the measurements of arrival times of a quantum particle? Although this question is very old, it is still open. The usual experiments are performed in the far-field regime and this question becomes unimportant, as a semiclassical analysis suffices. Nevertheless, the development in the detector technology will soon allow for near-field investigations; thus, a better understanding of arrival time measurements is needed. Since outcomes of quantum measurements are always described by positive-operator-valued measures (POVMs), various arrival time POVMs have been proposed. On the other hand, many physicists would agree that the arrival time statistics is given by the quantum flux. This urges the question whether a POVM exists, which agrees approximately with the quantum flux values on a reasonable set of wave functions. We answer this question negatively for a very natural set of wave functions, but we remark that the answer is very sensitive to the choice of the set and provide evidence for the existence of a POVM that agrees with the quantum flux on a more restrictive set.

Laser-induced selective crosslinking for scaling the heterointerfacial domain in polymer blends

Organic blends containing heterojunction structures at the interfacial phase have been applied extensively in organic optoelectronic devices to modify charge transfer, separation, and recombination processes. Scaling and controlling the transition domains at the hetero-interface are of crucial importance for deep insights into the involved physics and for architecturing the devices with improved performance. However, it is difficult to recognize and characterize these transition domains directly using the conventional microscopic techniques, in particular when different molecules are dissolved in the same solvent with equal solubility. In this work, we introduce a technique defined as laser-induced selective cross-linking to isolate the interfacial phase from other phases into a directly measurable practicity. Thus, the hetero-domains become visualized and directly measurable. Based on the insolubility of the selectively cross-linked molecules in organic solvents, a lift-off process may remove the uncross-linked or incompletely cross-linked molecules, so that the hetero-domain is more clearly visualized and more precisely measured. A transition domain in a scale of about 200 nm is resolved in the F8BT/PFB blend film between their respectively rich phases after the selective cross-linking of the F8BT molecules by a blue laser. Furthermore, hetero-crosslinking between F8BT and PFB molecules was also resolved by both microscopic and near-field spectroscopic investigations.