Localized Inhomogeneities Research Articles

Nonuniform doping of graphene for plasmonic tapers

A numerical study of graphene plasmons (GPs) propagating along various doping profiles of the graphene sheet is proposed. We show that the GP reflection at an abrupt doping interface is a simple function of the doping difference, consistent with a Fresnel model. In addition, improved transmission is obtained with a tapered interface and we find an optimal size for the transition zone. By considering localized inhomogeneities we describe a Fabry–Perot type cavity leading to an absorption above 60%. Furthermore, an alternative design of a 6 nm length cavity is proposed achieving 100% absorption. These results may lead to novel optical switches, circuits or nano-sensors.

Correlation between the beam profile from a curing light and the microhardness of four resins

ObjectiveTo demonstrate the effect of localized irradiance and spectral distribution inhomogeneities of one LED-based dental light-curing unit (LCU) on the corresponding microhardness values at the top, and bottom surfaces of four dental resin-based composites (RBCs), which contained either camphorquinone (CQ) alone or a combination of CQ and monoacylphosphine oxide (TPO) as photoinitiators. MethodsLocalized irradiance beam profiles from a polywave LED-based LCU were recorded five times using a laser beam analyzer, without and with either a 400nm or 460nm narrow bandpass filter placed in front of the camera lens. Five specimens of each of the four RBCs (two containing CQ/TPO and two containing CQ-only) were exposed for 5-, 10-, or 30-s with the light guide directly on the top surface of the RBC. After 24h, Knoop microhardness values were measured at 45 locations across the top and bottom surfaces of each specimen. Microhardness readings for each RBC surface and exposure time were correlated with localized patterns of the LCU beam profile, measured using the 400nm and 460nm bandpass filters. Spearman rank correlation was used to avoid relying on an assumption of a bivariate normal distribution for the KHN and irradiance. ResultsThe local irradiance and spectral emission values were not uniformly distributed across the light tip. There was a strong significant positive correlation with the irradiance beam profile values from the LCU taken through bandpass filters and the microhardness maps of the RBC surfaces exposed for 5 and 10s. The strength of this correlation decreased with increasing exposure time for the RBCs containing CQ only, and increased for the RBCs containing both CQ and TPO. ConclusionsLocalized beam and spectral distributions across the tip end of the light guide strongly correlated with corresponding areas of microhardness in both the top and bottom surfaces among four RBCs with different photoinitiator contents. SignificanceA light-curing unit with a highly inhomogeneous light output can adversely affect localized microhardness of resin-based composites and this may be a contributing factor for premature failure of a restoration.

Feasibility of interstitial near-infrared radiance spectroscopy platform for ex vivo canine prostate studies: optical properties extraction, hemoglobin and water concentration, and gold nanoparticles detection.

The canine prostate is a close match for the human prostate and is used in research of prostate cancers. Determining accurately optical absorption and scattering properties of the gland in a wide spectral range (preferably in a minimally invasive way), linking optical properties to concentrations of major endogenous chromophores, and detecting the presence of localized optical inhomogeneities like inclusions of gold nanoparticles for therapeutic and diagnostic purposes, are among the major challenges for researchers. The goal of the article is to demonstrate a feasibility of the multifunctional radiance spectroscopy platform in providing the required information. For ex vivo canine prostate, extraction of the effective attenuation and diffusion coefficients using relative cw radiance measurements was demonstrated in the 650- to 900-nm range. The derived absorption coefficient was decomposed to contributions from 9.0 μM HbO₂, 29.6 μM Hb, and 0.47 fractional volume of H₂O. Detection of a localized inclusion containing ∼1.5·1010 gold nanorods (0.8 μg Au) at 10 mm distance from the urethra was achieved with the detector in the urethra and the light source in a virtual rectum position. The platform offers the framework for a systematic study of various chromophores in the prostate that can be used as comprehensive diagnostic markers.

Attractive and repulsive contributions of localized excitability inhomogeneities and elimination of spiral waves in excitable media

The attracting and repelling of spiral waves in a two-dimensional excitable medium in the presence of localized excitability inhomogeneities are studied. The choice of two effects depends on the comparison of excitabilities inside and outside the localized obstacle. We inspect the changes in attracting and repelling behaviors with respect to the size of the obstacle and the initial distance between the center of the spiral core and the obstacle. To understand the occurrence of these phenomena, we investigated the small v-value areas near the tip and the function of the wave velocity as the excitability parameter ε. Considering the attributes of the attractive obstacle, an eliminating scheme of spiral waves is proposed in which the attractive obstacle is rapidly moved at several fixed times. This method can avoid the high-amplitude and high-frequency stimulus in most conventional methods.

The influence of substrate morphology on thickness uniformity and unintentional doping of epitaxial graphene on SiC

A pivotal issue for the fabrication of electronic devices on epitaxial graphene on SiC is controlling the number of layers and reducing localized thickness inhomogeneities. Of equal importance is to understand what governs the unintentional doping of the graphene from the substrate. The influence of substrate surface topography on these two issues was studied by work function measurements and local surface potential mapping. The carrier concentration and the uniformity of epitaxial graphene samples grown under identical conditions and on substrates of nominally identical orientation were both found to depend strongly on the terrace width of the SiC substrate after growth.

Entropy-Based Clutter Rejection for Intrawall Diagnostics

The intrawall diagnostic problem of detecting localized inhomogeneities possibly present within the wall is addressed. As well known, clutter arising from masonry structure can impair detection of embedded scatterers due to high amplitude reflections that wall front face introduces. Moreover, internal multiple reflections also can make it difficult ground penetrating radar images (radargramms) interpretation. To counteract these drawbacks, a clutter rejection method, properly tailored on the wall features, is mandatory. To this end, here we employ a windowing strategy based on entropy measures of temporal traces “similarity.” Accordingly, instants of time for which radargramms exhibit entropy values greater than a prescribed threshold are “silenced.” Numerical results are presented in order to show the effectiveness of the entropy-based clutter rejection algorithm. Moreover, a comparison with the standard average trace subtraction is also included.

The method of multifrequency near-field acoustical tomography of bulk inhomogeneities of the sea bottom

In this work, we propose a method of near-field acoustical multi-frequency coherent tomography of spatially localized inhomogeneities of the shallow-sea bottom. In the framework of the developed method, two-dimensional spatial scanning of the radiating-receiving system along the bottom over the region of location of the inhomogeneities is performed at multiple frequencies. Using the Born approximation, the initial three-dimensional integral equation for the scattered field is reduced to one-dimensional Fredholm equation of the first kind with respect to the depth profile of the transverse spatial spectrum of the inhomogeneities. In the solution of this integral equation for each pair of spectral components, we use the method of generalized residual and obtain the sought three-dimensinal distribution via the inverse Fourier transform of the reconstructed spectrum. Results of numerical simulation of the tomography scheme and visualized inhomogeneities of shallow-sea bottom are presented. Acoustical methods of remote sensing of the sea are an efficient tool in solving various practical problems related to the development of sea-shelf regions [1–3]. In recent years, remote acoustic sensing of the sea bottom has become especially topical. This is related to solving many practical problems of engineering seismics, detection of buried objects in sedimentary layers, and ecological monitoring in the regions of ports, underwater pipelines, and oil-producing platforms. For solving such problems, pulse-type methods using non-coherent seismo-acoustic sources of explosive type became widely spread. Along with this, promising methods of coherent sounding of the sea bottom using towed hydroacoustic radiators are also studied. Utilization of such sources makes it possible to coherently accumulate signals, which significantly helps to increase sensitivity and resolution in the sounding problems [4–8]. However, usually the spatial processing is performed only for synthesized apertures which are formed by translational one-directional motion of the radiating and receiving systems. Creation and testing of coherent hydroacoustic radiators for sea-bottom sounding makes it possible to apply even more efficient methods using the data of two- and three-dimensional scanning [1, 2]. We note that many of such diagnostic problems based on data of multi-dimensional observations (see, for example, [8– 10]) belong to the class of ill-posed inverse problems, the solution of which can be based on application of regularization methods [9, 10] and use of ap rioriinformation [11]. Such problems are topical for underwater engineering prospecting and detection of dangerous objects buried in sedimentary layers. The tomographic method proposed in the present work, i.e., the method of reconstruction of threedimensional structure of inhomogeneities located in the sea bottom based on solving the corresponding backscattering problem, is a natural development of the above-mentioned research. Studies and applications of backscattering problems have a long history not only in acoustics, but also in electrodynamics and quantum mechanics. The simplest problem of such kind, for an infinite one-dimentionally inhomogeneous

Nucleation and evolution of magnetic inhomogeneities of the pulson and 2D soliton type in magnets with local anisotropy inhomogeneities

The dynamics of a 180° domain wall in a magnet with two-dimensional inhomogeneities of the parameters of magnetic anisotropy and the conditions of excitation of nonlinear magnetization waves have been investigated using numerical methods. Three types of localized magnetic inhomogeneities that appear in the region of anisotropy inhomogeneities have been revealed, namely, a pulson, a pulson transforming into a 2D soliton, and a 2D soliton. Regions of the values of the parameters that describe the magnetic-anisotropy inhomogeneities, which determine the existence of each of these inhomogeneities, have been found.

Oblique incidence for broad monoenergetic proton beams

The depth dose of a monoenergetic broad parallel proton beam has been modeled in a number of ways, but evidently not yet for oblique incidence. The purpose of this investigation is to find an accurate analytic formula for this case, which can then be used to model the depth dose of a broad beam with an initial Gaussian angular distribution. The Bortfeld model of depth dose in a broad normally incident proton beam has been extended to the case of oblique incidence. This extension uses an empirically determined Gaussian parameter sigma(x) which (roughly) characterizes the off-axis dose of a proton pencil beam. As with Bortfeld's work, the modeling is done in terms of parabolic cylinder functions. To obtain the depth dose for an initial angular distribution, the result is integrated over the angle of incidence, weighted by a Gaussian probability function. The predictions of the theory have been compared to MCNPX Monte Carlo calculations for four phantom materials (water, bone, aluminum, and copper) and for initial proton energies of 50, 100, 150, 200, and 250 MeV. Comparisons of the depth dose predicted by this theory with Monte Carlo calculations have established that with very good accuracy, sigma(x) can be taken to be independent both of the depth and of the angle of incidence. As a function of initial proton range or of initial proton energy, sigma(x) has been found to obey a power law to very high accuracy. Good fits to Monte Carlo calculations have also been found for an initial Gaussian angular distribution. This investigation is the first step in the accurate modeling of a proton pencil beam with initial Gaussian angular distribution. It provides the longitudinal factor, with its Bragg peak buildup and sharp distal falloff. A transverse factor must still be incorporated into this theory and this will give the lateral penumbra of a collimated proton beam. Also, it will be necessary to model the dose of product particles from nuclear interactions of the proton beam. With the accurate modeling of a pencil beam, it will be possible to accurately take into account the effect of localized tissue inhomogeneities.

Symmetry-enhanced supertransfer of delocalized quantum states

Coherent hopping of excitation relies on quantum coherence over physically extended states. In this work, we consider simple models to examine the effect of symmetries of delocalized multi-excitation states on the dynamical timescales, including hopping rates, radiative decay and environmental interactions. While the decoherence (pure dephasing) rate of an extended state over N sites is comparable to that of a non-extended state, superradiance leads to a factor of N enhancement in decay and absorption rates. In addition to superradiance, we illustrate how the multi-excitonic states exhibit ‘supertransfer’ in the far-field regime—hopping from a symmetrized state over N sites to a symmetrized state over M sites at a rate proportional to MN. We argue that such symmetries could play an operational role in physical systems based on the competition between symmetry-enhanced interactions and localized inhomogeneities and environmental interactions that destroy symmetry. As an example, we propose that supertransfer and coherent hopping play a role in recent observations of anomalously long diffusion lengths in nano-engineered assembly of light-harvesting complexes.

Tomography in observation of inhomogenenities in shallow sea during sounding by a focused high-frequency acoustic field: II. Study of the observation efficiency

We study the efficiency of high-frequency acoustic sounding of small-size spatially localized inhomogeneities in shallow sea by a tomographic method. The method is based on that the focusing of the received and radiated fields to the region of the assumed location of the inhomogeneity is matched to the waveguide. During the observation, the statistical check is performed for hypotheses on the object location that are formed using a priori information on the waveguide model and data on the observation-system structure. To decrease the influence of the interference structure of the acoustic fields in the waveguide, we use accumulation of partial images corresponding to separate ray tomographic projections. Using a simulation model of the considered method, we analyze its accuracy and sensitivity under different observation conditions in shallow sea.

Effects of the Incidence of a Gaussian Temporal Short-Pulse Laser of Different Spatial Profiles on a Two-Dimensional, Rectangular, Inhomogeneous Participating Medium

This article deals with the effects of a short-pulse laser of Gaussian temporal and different spatial profiles on the signals emanating from a two-dimensional rectangular participating medium containing localized inhomogeneities. An incident short-pulse laser of Gaussian temporal profile has spatial loading on the south boundary either as a Gaussian or as a step function. The absorbing and scattering participating medium consists of a localized circular or elliptical inhomogeneity. The extinction coefficient of the inhomogeneity is the same as the containing medium, while its scattering albedo is different. Transmittance and reflectance signals are analyzed for the effects of the extinction coefficient and the spatial profile of the incident short-pulse laser. Spatial distributions of the signals along the boundaries and temporal evolution of the heat flux distributions inside the medium are also studied. The problem is analyzed using the finite-volume method.

Unraveling subtle details of expendable bottom penetrometer (XBP) deceleration profiles

The expendable bottom penetrometer (XBP), a free-falling probe which can be deployed by a single person, is commonly used to rapidly classify seafloor sediment strength as hard, medium or soft. Assessments of deceleration-depth profiles of soft sediments (maximum deceleration smaller than 20 g) suggest that a finer subdivision of this class is possible. Thus, XBP datasets were recorded during two cruises aboard the R/V Planet in 2003 and 2006 in four study areas in the western Baltic Sea. The profiles can be distinguished in terms of gradient and curve progression. Comparing these data with descriptions of gravity cores taken at the same localities during the research cruises reveals a typical profile for western Baltic Sea mud, and distinct profiles for sediments containing localized inhomogeneities such as gravel, plant fragments, layers of sand or silt, and shell fragments. This good agreement between deceleration and sediment core information means that, once calibrated, XBPs can be used for a quick and suitably precise characterization of seabed sediments for research or engineering purposes. Notably, existing models to predict geotechnical properties such as shear strength and bearing capacity still show the disadvantage of involving excessively broad assumptions or estimates based on the bearing capacity factor or strain rate multiplier.

Spatially resolved photocurrent mapping of operating organic photovoltaic devices using atomic force photovoltaic microscopy

A conductive atomic force microscopy (cAFM) technique, atomic force photovoltaic microscopy (AFPM), has been developed to characterize spatially localized inhomogeneities in organic photovoltaic (OPV) devices. In AFPM, a biased cAFM probe is raster scanned over an array of illuminated solar cells, simultaneously generating topographic and photocurrent maps. As proof of principle, AFPM is used to characterize 7.5×7.5μm2 poly(3-hexylthiophene):[6,6]-phenyl-C61-butyric acid methyl ester OPVs, revealing substantial device to device and temporal variations in the short-circuit current. The flexibility of AFPM suggests applicability to nanoscale characterization of a wide range of optoelectronically active materials and devices.

Coherent Structures Generated by Inhomogeneities in Oscillatory Media

We investigate the effect of spatially localized inhomogeneities on a spatially homogeneous oscillation in a reaction‐diffusion system. In dimension up to two, we find sources and contact defects, that is, the inhomogeneity may either send out phase waves or act as a weak sink. We show that small inhomogeneities cannot act as sources in more than two space dimensions. We also derive asymptotics for wavenumbers and group velocities in the far field. The results are established rigorously for radially symmetric inhomogeneities in reaction‐diffusion systems, and for arbitrary inhomogeneities in a modulation equation approximation.

Autoresonance parametric excitation of localized oscillations of magnetization in a ferromagnet by an AC field of a variable frequency

Dynamics of localized magnetic inhomogeneities in the form of breathers has been studied in an easy-plane ferromagnet in an applied ac magnetic field of a variable frequency. Conditions have been determined for an autoresonance parametric excitation of a breather in the presence of dissipation with its amplitude growing in time from small values to approximately unity. The possibility has been shown of not only its resonance growth but also of a resonance depression of nonlinear oscillations of localized magnetic inhomogeneities.

Low-mode tomography of inhomogeneities in a shallow sea

Potentialities of observing spatially localized inhomogeneities by the low-frequency low-mode pulsed diffraction tomography in a shallow sea are analyzed. A numerically simulated model is used to study the procedure of reconstructing the parameters of inhomogeneities by coprocessing the signals of the tomographic projections formed at the multistatic excitation and reception of low-frequency low-mode acoustic pulses.

Hamiltonian formalism for two magnon scattering microwave relaxation: Theory and applications

A two magnon scattering theory for microwave relaxation in magnetic systems is formulated in the framework of the Hamiltonian formalism. The paper provides general expressions for inhomogeneity coupling coefficients in the case of localized inhomogeneities. An approximate solution for the relaxation rate of the ferromagnetic resonance uniform mode relaxation rate is presented. Two examples of the application of the theory are presented, one for bulk polycrystalline ferrites and one for polycrystalline metallic thin films.

Enhanced photoemission from localized inhomogeneities on Cu(001) characterized by laser-assisted photoemission electron microscopy and low-energy electron microscopy

Multiphoton photoelectron emission from a state-of-the-art prepared Cu(001) surface has been investigated with photoemission electron microscopy. Randomly distributed, spatially confined regions (about $1\phantom{\rule{0.3em}{0ex}}\ensuremath{\mu}\mathrm{m}$ in diameter) of intense electron yield (hot spots) are observed. The hot spots give rise to distortions of the corresponding low-energy electron microscopy image. They are identified as polycrystallinelike protrusions embedded in the surface. The density of these inhomogeneities is about $2\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}4}\phantom{\rule{0.3em}{0ex}}\ensuremath{\mu}{\mathrm{m}}^{2}$ and cannot be manipulated by ion bombardment or by homoepitaxial growth. The response of hot spots to illumination under ultraviolet light produced by an arc lamp and illumination by blue and infrared femtosecond laser light for different polarizations is recorded from the same surface region, allowing for a direct comparison. Hot spots respond very efficiently and even stronger to $s$-polarized light as compared to $p$-polarized light, and show apparently a nonresonant behavior with respect to the exciting wavelength. We discuss some of the conceivable mechanisms underlying this anomalous photoemission, also in view of the particular characteristics that are specific to our experimental setup. Our data are not compatible with an interpretation which relies exclusively on the excitation of (localized) surface plasmon modes, but rather with a nonresonant near-zone light field enhancement.

Coherent Structures Generated by Inhomogeneities in Oscillatory Media

We investigate the effect of spatially localized inhomogeneities on a spatially homogeneous oscillation in a reaction‐diffusion system. In dimension up to two, we find sources and contact defects, that is, the inhomogeneity may either send out phase waves or act as a weak sink. We show that small inhomogeneities cannot act as sources in more than two space dimensions. We also derive asymptotics for wavenumbers and group velocities in the far field. The results are established rigorously for radially symmetric inhomogeneities in reaction‐diffusion systems, and for arbitrary inhomogeneities in a modulation equation approximation.