Showing papers on "Near and far field published in 2004"
TL;DR: In this paper, the optical properties of metallodielectric nanostructures, variations of a core−shell geometry, are investigated using the finite difference time domain method, which provides a convenient, systematic and general approach for calculating the optical response of a nanostructure of arbitrary symmetry and geometry to an incident light wave.
Abstract: The optical properties of metallodielectric nanostructures, variations of a core−shell geometry, are investigated using the finite difference time domain method. This method provides a convenient, systematic, and general approach for calculating the optical response of a nanostructure of arbitrary symmetry and geometry to an incident light wave. Properties such as the optical absorption and scattering cross sections as well as the local electromagnetic fields and induced charge densities at the surfaces of the nanostructures can be obtained by this method. Issues of convergence with grid size and other simulation parameters are discussed in detail. The method is applied to uniform single nanoshells, nanoshells with surface defects, and nanoshells with shape distortions from a spherical geometry. The results show that, while defects can significantly affect local surface field enhancements, far field properties such as optical absorption and scattering spectra can be remarkably insensitive to defects and d...
325 citations
TL;DR: In this paper, it was shown that the laser induced ultrafast demagnetization of ferromagnetic films results in the emission of a terahertz electromagnetic pulse, which was detected from Ni films using free-space electro-optic sampling.
Abstract: It is shown that the laser induced ultrafast demagnetization of ferromagnetic films results in the emission of a terahertz electromagnetic pulse. This emission has been detected from Ni films using free-space electro-optic sampling. The radiated electric field E(t) is explained by Maxwell equations (radiation from a time dependent magnetic dipole), and is expected to be proportional to the second time derivative of the magnetization d2M/dt2, as measured in the far field. This technique opens appealing perspectives in the context of measuring and understanding the ultrafast spin dynamics as well as the interaction of electrons (both charge and spin) with electromagnetic fields.
245 citations
TL;DR: In this paper, the authors present numerical simulations of tsunami run-up in the near field for a data set of 72 models of sources, involving both seismic dislocations and landslides.
Abstract: SUMMARY We present numerical simulations of tsunami run-up in the near field for a data set of 72 models of sources, involving both seismic dislocations and landslides. By varying one by one the parameters describing the source and the receiving beach, we are able to separate their individual influence on the amplitude and distribution of run-up, which we characterize by forming several dimensionless parameters, principally the ratio I 2 of its maximum amplitude to its lateral extent along the beach. We find that I 2 remains less than 10 −4 for seismic dislocation sources but is greater than this threshold for all physically realistic models of underwater landslides. Thus, it can be used as a discriminant for the nature of the source of a near-field tsunami. For seismic dislocations, we also consider the ratio I 1 of maximum run-up to seismic slip on the fault plane, and validate numerically the previously suggested ‘rule of thumb’ that this ratio cannot be much greater than 1. In the case of underwater landslides, we show that the distribution of near-field run-up is primarily controlled by the 2-D ‘wall of water’ displaced on the ocean surface as an initial condition to the simulation, and is practically independent of its extension in the third dimension, perpendicular to the beach. We apply this approach to nine profiles of tsunami run-up obtained experimentally during recent field surveys, and show that our method successfully identifies the 1998 Papua New Guinea tsunami as having been generated by an underwater landslide, thus confirming the results of shipboard and hydroacoustic investigations. It also strongly suggests a similar mechanism for the generation of the near-field Aleutian tsunami of 1946 April 1.
208 citations
Patent•
06 Oct 2004
TL;DR: In this paper, the authors present an antenna system, comprising a plurality of non-uniformly spaced antenna elements arranged substantially linearly, a power divider for dividing transmit power thereby coupling signals to the plurality of antenna elements, a phase shifter capable of phase shifting the signals between the power dividers and the antenna elements such that radiated signals from each antenna element add coherently such that the radiated electromagnetic energy may be focused at a focal point in the near field region or in the Fresnel region of the antenna system.
Abstract: An embodiment of the present invention provides an antenna system, comprising a plurality of non-uniformly spaced antenna elements arranged substantially linearly, a power divider for dividing transmit power thereby coupling signals to the plurality of antenna elements, a phase shifter capable of phase shifting the signals between the power divider and the plurality of antenna elements such that radiated signals from each antenna element add coherently such that the radiated electromagnetic energy may be focused at a focal point in the near field region or in the Fresnel region of the antenna system. An embodiment of the present invention may also provide at least one additional receive antenna element capable of receiving signals backscattered from at least one RFID tag located in the near field of the focused array and the divider may create substantially equal power levels feeding each antenna element. The antenna elements may be similar or identical and each element may be oriented such that the individual element main beam may point in a unique direction.
166 citations
TL;DR: In this article, the authors show that the properties of a random electromagnetic field are equivalent to those of a fully coherent electric field, in the sense of the recently introduced electromagnetic degree of coherence and the electric cross-spectral density tensor factor.
Abstract: We show that the following properties of a random electromagnetic field are equivalent: (i) the field is spatially completely coherent in the sense of the recently introduced electromagnetic degree of coherence and (ii) the electric cross-spectral density tensor factors in the two spatial variables.
131 citations
TL;DR: In this paper, the temporal shape of the observed near-field signals is approximately proportional to the time-integral of the incident field, which is observed using both a near field detection technique and a far-field detection technique.
Abstract: We have performed measurements on terahertz (THz) apertureless near-field microscopy that show that the temporal shape of the observed near-field signals is approximately proportional to the time-integral of the incident field. Associated with this signal change is a bandwidth reduction by approximately a factor of 3 which is observed using both a near-field detection technique and a far-field detection technique. Using a dipole antenna model, it is shown how the observed effects can be explained by the signal filtering properties of the metal tips used in the experiments.
130 citations
TL;DR: In this paper, the authors employed the finite difference time domain (FDTD) method to numerically study the transmission characteristics of an H-shaped nano-aperture in a metal film in the optical frequency range.
Abstract: The finite-difference time-domain (FDTD) method is employed to numerically study the transmission characteristics of an H-shaped nano-aperture in a metal film in the optical frequency range. It is demonstrated that the fundamental TE10 mode concentrated in the gap between the two ridges of the H-shaped aperture provides a high transmission efficiency above unity and the size of the gap determines the sub-wavelength resolution. Fabry–Perot-like resonance is observed. Localized surface plasmon (LSP) is excited on the edges of the aperture in a silver film but has a negative effect on the signal contrast and field concentration, while aluminum acts similar to an ideal conductor if the film thickness is several times larger than the finite skin depth. In addition, it is shown that two other ridged apertures, C-shaped and bowtie-shaped apertures, can also be used to achieve a sub-wavelength resolution in the near field with a transmission efficiency above unity and a high contrast.
121 citations
TL;DR: In this paper, two methods, namely quasi-images formula and finite-difference time-domain (FDTD) method, were proposed to evaluate the electromagnetic fields very close to lightning channel, which are applicable for poorly conducting ground case.
Abstract: In this short paper, we present two methods: Quasi-images formula and finite-difference time-domain (FDTD) method to evaluate the electromagnetic fields very close to lightning channel, which are applicable for poorly conducting ground case, and the numerical results are consistent with each other. Moreover, the vertical electric fields at 15 m obtained by the two proposed methods is in good agreement with the measured result, and the horizontal electric field at a distance of 100 m above finely conducting ground obtained by the FDTD method is identical to the field obtained by accurate Cooray-Rubinstein approximation. With the proposed quasi-images formula, the effects of electrical dispersion of the ground on the lightning generated electromagnetic fields are analyzed and some significant results are obtained.
116 citations
TL;DR: A mathematical model to simulate the radio frequency behavior of the antenna is described, the equivalent weight vector used for formulation of the characteristics, the admittance matrix including varactors, the effective element length and the equivalent steering vector method.
Abstract: The electronically steerable parasitic array radiator antenna consists of one feed radiating element and parasitic radiating elements placed in the near field of the active radiator. A beam is formed due to spatial electromagnetic field coupling among radiating elements. The radiation pattern is electronically controlled by means of the variable capacitance devices (varactors) loading the parasitic elements. Unlike a conventional phased array, only one transmitter and receiver are needed for system configuration. Therefore, adaptive beamforming of low dissipation power and low fabrication cost can be achieved. On the other hand, there is only one output port to observe the signal and the weights can be controlled indirectly via reactors instead of direct control. In addition, due to interelement mutual coupling and the parasitic element being directly connected to the reactive device, the linear adaptive array theory developed to date cannot be applied straightforward. In this paper, the configuration of this antenna, its operating principle and formulation, and its measurement method, control scheme, and applications to signal processing are presented. We describe a mathematical model to simulate the radio frequency behavior of the antenna, the equivalent weight vector used for formulation of the characteristics, the admittance matrix including varactors, the effective element length, the equivalent steering vector method, the method for effectively extending the variable range of the capacitance of the varactor, the reactance circuit to cancel nonlinear distortions, the method for calibration of varactors and radiation pattern by measurement of near field of the radiating element, the learning criteria used to control radiation patterns autonomously in adaptation to the electromagnetic environment, the reactance optimization algorithm, the concept of the reactance domain signal processing and the direction of arrival estimation based on such a process, and diversity reception and spatial correlation. © 2004 Wiley Periodicals, Inc. Electron Comm Jpn Pt 2, 87(10): 25–45, 2004; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/ecjb.20081
102 citations
TL;DR: In this article, a single subwavelength circular aperture of diameter d = 2.5mm in a metallic plate was used to investigate the electromagnetic fields on both the illuminated and the exit side of the structure.
Abstract: Strongly enhanced transmission of microwave radiation (λ0∼5 mm) is observed through a single subwavelength circular aperture of diameter d=2.5 mm in a metallic plate. The phenomenon is caused by resonant excitation of electromagnetic surface waves supported by four concentric grooves surrounding the aperture on the illuminated side of the sample. It is also shown that similar surface patterning on the output face of the sample results in very strong angular confinement (directivity) of the transmitted beam. A finite element code is used to investigate the electromagnetic fields on both the illuminated and the exit side of the structure, the predictions from which show excellent agreement with the experimental results.
101 citations
TL;DR: In this paper, a system of two coupled planar material sheets possessing surface mode (polariton) resonances can be used for the purpose of evanescent field restoration and, thus, for sub-wavelength near-field imaging.
Abstract: It is shown that a system of two coupled planar material sheets possessing surface mode (polariton) resonances can be used for the purpose of evanescent field restoration and, thus, for sub-wavelength near-field imaging. The sheets are placed in free space so that they are parallel and separated by a certain distance. Due to interaction of the resonating surface modes (polaritons) of the sheets an exponential growth in the amplitude of an evanescent plane wave in the system can be achieved. This effect was predicted earlier for backward-wave (double-negative or Veselago) slab lenses. The alternative system considered here is proved to be realizable at microwaves by grids or arrays of resonant particles. The necessary electromagnetic properties of the resonating grids and the particles are investigated and established. Theoretical results are supported by microwave experiments that demonstrate amplification of evanescent modes.
TL;DR: The polarization properties and angular distribution of intensity of the far fields from a nanowire laser are investigated and can be applied for experimental determination of a lasing mode by its far fields as well as for optimization of laser emission.
Abstract: The polarization properties and angular distribution of intensity of the far fields from a nanowire laser are investigated. The far-field emission depends strongly on the mode type (HE11, TE01, TM01) and the radius of the nanowire. The emission is weakly directional, and a large part of it can be emitted in the backward direction. Our results can be applied for experimental determination of a lasing mode by its far fields as well as for optimization of laser emission.
TL;DR: In this paper, the authors demonstrate a technique for optical trapping of single particles in a fluid, which is based on the intense near-field gradients around small apertures in a metal film.
Abstract: In recent years, optical micromachines based on forces exerted by strongly focused beams of light have started to provide unprecedented access to nonintrusive measurement and manipulation of matter on submicron length scales. However, the sharpness of the present optical tweezers is restricted by the spatial gradients of light attainable with diffraction-limited optics. Here, we demonstrate a technique for optical trapping of single particles in a fluid, which is based on the intense near-field gradients around small apertures in a metal film. Our scheme should be able to trap smaller particles with a fraction of the laser intensity required by conventional optical tweezers. Detailed simulations of the electromagnetic fields near apertures and the resulting forces they can produce are described. We also present a proof-of-principle experiment in which the trapping of latex beads is demonstrated by following the time evolution of their fluorescence. Our scheme allows containment of particles in free soluti...
TL;DR: In this article, a numerical simulation of the absorptivity of a grating was used to explore the validity of the ray tracing approach and to predict the absorption and thermal emission of electromagnetic radiation by a rough surface.
Abstract: In this paper we revisit the absorption and thermal emission of electromagnetic radiation by a rough surface. We use a numerical simulation of the absorptivity of a grating to explore the validity of the ray tracing approach. We show that it often predicts correctly the absorptivity and emissivity of a surface with characteristic lengths on the order of a wavelength. Recent advances in the understanding of the microscopic mechanism of thermal emission in the near field are used to discuss the data and to explain this surprising result. We also identify three different regimes depending on the ratio of the period to the wavelength: the homogenization regime, the resonance regime, and the geometrical optics regime.
TL;DR: In this article, the performance of the ridge waveguide as a near-field aperture in data storage systems is investigated using finite element method (FEM) and finite difference time-domain (FDTD) based software.
Abstract: The performance of the ridge waveguide as a near-field aperture in data storage systems is investigated. Finite element method (FEM) and finite-difference time-domain (FDTD) based software are used in the numerical simulations. To verify their accuracy at optical frequencies, the FEM and FDTD are first compared to analytical results. The accuracy of these techniques for modeling ridge waveguides at optical frequencies is also evaluated by comparing their results with each other for a plane wave illumination. The FEM, which is capable of modeling focused beams, is then used to simulate various geometries involving ridge waveguides. Near-field radiation from ridge waveguide transducer is expressed in terms of power density quantities. Previous studies in the literature consider the performance of the transducer in free space, rather than in the presence of a recording magnetic medium. The effect of the recording magnetic medium on the transmission efficiency and spot size is discussed using numerical simula...
TL;DR: A variational method is given for determining the essential supremum of the surface impedance of a partiallycoated perfect conductor from a knowledge of the far field pattern of the time-harmonic electric field at fixed frequency.
Abstract: A variational method is given for determining the essential supremum of the surface impedance of a partiallycoated perfect conductor from a knowledge of the far field pattern of the time-harmonic electric field at fixed frequency. It is assumed that the shape of the scatterer has been determined (e.g., bysolving the far field equation and using the linear sampling method). Numerical examples are given for the scalar case with constant surface impedance.
TL;DR: In this paper, the authors studied the thermal emission of highly doped silicon surfaces and showed that by modifying the doping, they can tune the frequency of emission, which can be used to realize a spatially partially coherent thermal source and a quasi-isotropic source.
TL;DR: In this paper, the second-order kinematic parameters (spatial distribution of slip, rupture velocity and more basically the discrimination between the fault plane and the other nodal plane) of distant events are found.
Abstract: SUMMARY Since the end of the 1970s, teleseismic waves are used routinely to infer the first-order characteristics of the event (location, depth, duration, focal mechanism), but finding the second-order kinematic parameters (spatial distribution of slip, rupture velocity and more basically the discrimination between the fault plane and the other nodal plane) of distant events remains a difficult task. Classically, these events are studied by two different methods; either they are seen as a succession of subevents, each of which is considered as a point source, or like in near field, as extended sources where the kinematic parameters are retrieved on a grid. The first approach is not physically satisfactory and can lead to erroneous interpretations of the
TL;DR: The near field fluctuating pressure of a cold subsonic jet (Mach 0.85) issuing from a contoured convergent nozzle was studied in this paper, where conventional time series analysis (i.e., correlation and spectral analysis) as well as a wavelet analysis were used to characterize time-dependent events.
Abstract: The near field fluctuating pressure of a cold subsonic jet (Mach 0.85) issuing from a contoured convergent nozzle was studied. Conventional time series analysis (i.e., correlation and spectral analysis) as well as a wavelet analysis were used to characterize time-dependent events thereby revealing features consistent with current understandings of jet noise while also uncovering modal features consistent with previous measurements of the jet flow field itself. Specifically, the measurements reveal extensive sources of multiple frequencies in the streamwise direction. The near field information of the present investigation should help to bridge our understanding of the relationship between the velocity field and the far field acoustic emission associated with subsonic jet noise as well as supply a data set that can be used for verification of computational efforts.
01 Nov 2004
TL;DR: In this article, the authors use near-field scanning data, performed in a typical laboratory bench top environment, to predict radiated electromagnetic interference (EMI) in typical product environment.
Abstract: Near-field scanning has often been used to measure and characterize magnetic fields surrounding individual integrated circuits (IC) and high speed digital electronic circuits. The paper describes the use of near-field scanning data, performed in a typical laboratory bench top environment, to predict radiated electromagnetic interference (EMI) in a typical product environment. The product environment may include enclosures and apertures. The approach begins by acquiring sufficient near-field scanning data to allow representation of an unintentional radiating source by an equivalent surface current distribution. The equivalent current distribution is used as a source in numerical full wave modeling. The agreement between direct full wave simulation results and full wave simulation results using equivalent sources works well under certain assumptions.
TL;DR: In this article, a free-space broadband terahertz pulse is coupled into a propagating mode along the shaft of an optical antenna acting as an apertureless near-field probe.
Abstract: We report a direct observation of the electromagnetic wave propagation on optical antennas. A free-space broadband terahertz pulse is coupled into a propagating mode along the shaft of an optical antenna acting as an apertureless near-field probe. We determine the spatial extent of this guided mode and its velocity. In addition, we consider the possibility of multiply reflected modes propagating along the near-field probe, an effect which will be significant in near-field spectroscopic measurements.
TL;DR: In this article, a miniaturized sensor capable for measuring high magnetic flux densities is presented, where the magnetic flux density is converted into a movement of a micro machined U-shaped cantilever, which bears a thin film lead.
Abstract: A miniaturized sensor capable for measuring high magnetic flux densities is presented. The magnetic flux density is converted into a movement of a micro machined U-shaped cantilever, which bears a thin film lead. The cantilever movement is accomplished by the Lorentz force acting on the electrical current. The cantilever poses as a deflecting mirror in an optical readout system. The ratio of the intensity of the light reflected by the front side of the cantilever to the intensity of the incident light is analyzed. The optical conversion principle was proven experimentally. It is well described by an optical near field model. To overcome measurement problems caused by the deformation due to mechanically prestressed cantilevers, an ac excitation was used for sensor characterization. A measurement range of the current–flux density product of 2.2 mA T was achieved by evaluating the peak light intensity. Beyond this limit more sophisticated signal analysis has to be applied. Depending on the rest position, a magnetic flux density of about 10 mT can be resolved with a current of 10 mA. The target of an upper limit of 50 T can be achieved by reducing the electrical current.
TL;DR: Methods based on multi-station recordings are presented for constructing the experimental dispersion curve of Rayleigh waves, and a method based on the linear regression of phase angles measured at multiple stations is introduced for determining data quality and filtering criteria.
Patent•
25 Mar 2004
TL;DR: In this article, an electromagnetic field generating element restrains magnetic field attenuation or magnetic field delay in a high frequency recording/reproducing head for thermally assisted magnetic field recording and re-production using a near field.
Abstract: An electromagnetic field generating element restrains magnetic field attenuation or magnetic field delay in a high frequency recording/reproducing head for thermally assisted magnetic field recording/reproduction using a near field. An information recording/reproducing head and an information recording/reproducing apparatus carry out high frequency magnetic recording/reproduction. The electromagnetic field generating element includes: (i) a substrate, (ii) conductors each provided on the substrate and each serving as a supporting section, (iii) a plate-like-shaped conductor provided on the conductors and (iv) a semiconductor laser element provided on the substrate. The semiconductor laser element irradiates laser light to the plate-like-shaped conductor substantially parallel to an extending flat surface of the plate-like-shaped conductor. This causes generation of a near field in the plate-like-shaped conductor. In addition, a magnetic field is generated by supplying a current to the plate-like-shaped conductor.
TL;DR: This work solves the two-dimensional problem of acoustic scattering by a semi-infinite array of identical isotropic point scatterers and confirms that a number of phenomena reported for specific geometries are in fact present in the general case.
Abstract: We solve the two-dimensional problem of acoustic scattering by a semi-infinite pe- riodic array of identical isotropic point scatterers, i.e., objects whose size is negligible compared to the incident wavelength and which are assumed to scatter incident waves uniformly in all directions. This model is appropriate for scatterers on which Dirichlet boundary conditions are applied in the limit as the ratio of wavelength to body size tends to infinity. The problem is also relevant to the scattering of an E-polarized electromagnetic wave by an array of highly conducting wires. The actual geometry of each scatterer is characterized by a single parameter in the equations, related to the single-body scattering problem and determined from a harmonic boundary-value problem. Using a mixture of analytical and numerical techniques, we confirm that a number of phenomena reported for specific geometries are in fact present in the general case (such as the presence of shadow boundaries in the far field and the vanishing of the circular wave scattered by the end of the array in certain specific directions). We show that the semi-infinite array problem is equivalent to that of inverting an infinite Toeplitz matrix, which in turn can be formulated as a discrete Wiener-Hopf problem. Numerical results are presented which compare the amplitude of the wave diffracted by the end of the array for scatterers having different shapes.
01 Jan 2004
TL;DR: A review of the different schemes for generating terahertz (THz) radiation using photoconductive emitters excited by femtosecond lasers is provided in this article. But the authors focus on large-area emitters because of their relative simplicity, both in fabrication and analysis, and their capabilities for high power THz generation.
Abstract: We provide a review of the different schemes for generating terahertz (THz) radiation using photoconductive emitters excited by femtosecond lasers. The discussion is concerned principally with large-area emitters because of their relative simplicity, both in fabrication and analysis, and their capabilities for high-power THz generation. In addition to discussing the principal characteristics of these emit- ters, we present a simple, but unified description of their behavior. The principal ingredients in this description are a Drude-Lorentz model for the carrier dynamics combined with a suitable solution of the radiation problem for a current sheet. This formalism permits one to examine the nature of the generated THz waveforms in the near and far field, as well as to consider the effect of the material properties and excitation conditions on the THz emission process. Within this picture, we describe the origin of the strong enhancement in THz emission that is observed upon appli- cation of an external magnetic field to emitters relying on transient photocurrents flowing perpendicular to the surface. Saturation processes that limit the efficiency of THz emission are also an important feature of these devices. These effects are dis- cussed in terms of perturbation of the bias field induced by space-charge screening and the THz radiation field.
TL;DR: In this paper, a numerical analysis of the local electric field enhancement of nanosized silver ellipsoids placed in the evanescent field near a glass surface is presented.
Abstract: We report on the numerical analysis of the local electric field enhancement of nanosized silver ellipsoids placed in the evanescent field near a glass surface. Across the visible spectrum the enhancement factor for silver particles varies by more than one order of magnitude because of surface-plasmon resonance. Because of the spatially inhomogeneous excitation, higher-order modes additionally contribute and modify the spectral dependence of the electric field compared with plane-wave excitation. Moving the metal particle toward the glass surface increases the field enhancement and shifts the plasmon resonance, which in addition splits between both ends of the particle. Besides the near-field properties of such a probe we also discuss to what extent these local properties can be measured in the far field.
TL;DR: In this article, a class of electromagnetic materials on a scale of much less than a wavelength is described. But the properties of these materials are not seen in nature, such as negative refractive index, which are the key to controlling the near field.
Abstract: New technology is enabling scientists to engineer a class of electromagnetic materials on a scale of much less than a wavelength. The new materials have properties not seen in nature - such as a negative refractive index - which are the key to controlling the near field.
TL;DR: Evidence is presented that the s-SNOM enables optical imaging at a spatial resolution on a 10nm scale, independent of the wavelength used (lambda=633 nm and 10 microm), and a tip-induced phonon-polariton resonance on flat polar crystals such as SiC and Si3N4 is found.
TL;DR: A method is presented to reconstruct the structure of a scattering object from data acquired with a photon scanning tunneling microscope, which may be understood to form a Gabor type near-field hologram.
Abstract: A method is presented to reconstruct the structure of a scattering object from data acquired with a photon scanning tunneling microscope . The data may be understood to form a Gabor type near-field hologram and are obtained at a distance from the sample where the field is defocused and normally uninterpretable. Object structure is obtained by the solution of the inverse scattering problem within the accuracy of a perturbative, two-dimensional model of the object.