Showing papers on "Near and far field published in 1999"

Near-Field Effects in Spatial Coherence of Thermal Sources

Abstract: We present an exact calculation of the cross-spectral density tensor of the near field thermally emitted into free space by an opaque planar surface. The approach, based on fluctuational electrodynamics and the fluctuation-dissipation theorem, yields novel near-field correlation properties. We show that the spatial coherence length of the field close to the surface at a given wavelength $\ensuremath{\lambda}$ may be much smaller than the well-known $\ensuremath{\lambda}/2$ of blackbody radiation. We also show that a long-range correlation may exist, when resonant surface waves, such as surface-plasmon or surface-phonon polaritons, are excited. These results should have important consequences in the study of coherence in thermal emission and in the modeling of nanometer scale radiative transfer.

Three-dimensional orientation measurements of symmetric single chromophores using polarization microscopy

Abstract: A complete understanding of any complex molecular system generally requires a knowledge of the three-dimensional (3D) orientation of its components relative both to each other, and to directional perturbations such as interfaces and electromagnetic fields. Far-field polarization microscopy is a convenient and widespread technique for detecting and measuring the orientation of single chromophores. But because the polarized electromagnetic field that is used to probe the system lacks a significant longitudinal component, it was thought that, in general, only 2D orientation information could be obtained1,2,3. Here we demonstrate that far-field polarization microscopy can yield the 3D orientation of certain highly symmetric single chromophores (CdSe nanocrystal quantum dots in the present case). The key requirement is that the chromophores must have a degenerate transition dipole oriented isotropically in two dimensions, which gives rise to a perpendicular ‘dark axis’ that does not couple to the light field. By measuring the fluorescence intensity from the dipole as a function of polarization angle, it is possible to calculate both the tilt angle between the dark axis and the sample plane, as well as the in-plane orientation, and hence obtain the 3D orientation of the chromophore

Near-field to near/far-field transformation for arbitrary near-field geometry utilizing an equivalent electric current and MoM

Abstract: Presented here is a method for computing near- and far-field patterns of an antenna from its near-field measurements taken over an arbitrarily shaped geometry. This method utilizes near-field data to determine an equivalent electric current source over a fictitious surface which encompasses the antenna. This electric current, once determined, can be used to ascertain the near and the far field. This method demonstrates the concept of analytic continuity, i.e., once the value of the electric field is known for one region in space, from a theoretical perspective, its value for any other region can be extrapolated. It is shown that the equivalent electric current produces the correct fields in the regions in front of the antenna regardless of the geometry over which the near-field measurements are made. In this approach, the measured data need not satisfy the Nyquist sampling criteria. An electric field integral equation is developed to relate the near field to the equivalent electric current. A moment method procedure is employed to solve the integral equation by transforming it into a matrix equation. A least-squares solution via singular value decomposition is used to solve the matrix equation. Computations with both synthetic and experimental data, where the near field of several antenna configurations are measured over various geometrical surfaces, illustrate the accuracy of this method.

A two probes scanning phaseless near-field far-field transformation technique

Abstract: An innovative and effective technique to determine the far-field of a radiating system from near-field intensity data is introduced, analyzed, and tested. The approach is based on the simultaneous measurement of the amplitude of the voltages received by two different probe antennas moving over a single scanning surface in the near zone and performs the phase retrieval of the near-field by assuming as unknown the plane wave spectrum of the field. The radiated field is then straightforwardly evaluated. As compared to the existing phaseless measurement techniques, the use of two different probes makes it possible to avoid the need for a second scanning surface and thus allows the use of smaller (and cheaper) anechoic chambers. Furthermore, the measurement time is essentially equal to that required by conventional techniques based on the measurement of the complex near-field. The reliability and the effectiveness' of the approach are investigated and discussed and the key factors affecting its behavior are highlighted. In particular, the relevance of the difference between the plane wave spectra (PWS) of the two probe antennas in ensuring an acceptable reliability of the solution, with respect to the starting point of the procedure, is outlined. Finally, the effectiveness of the approach is confirmed by an extensive numerical analysis, which also shows the stability of the solution against data noise.

Far-field and near-field material processing with. femtosecond laser pulses

Abstract: Recently, it has been proven that femtosecond lasers are ideal tools for the microstructuring of solid targets. Since thermal and mechanical influences are minimized, diffraction-limited structures can be generated in the far field. The diffraction limit can be overcome when one works in the near field. In this paper, concrete applications and new developments in both regimes are highlighted.

Near-field and far-field scattering of surface plasmon polaritons by one-dimensional surface defects

Abstract: A rigorous formulation for the scattering of surface plasmon polaritons (SPP's) from a one-dimensional surface defect of any shape that yields the electromagnetic field in the vacuum half-space above the vacuum-metal interface is developed by the use of an impedance boundary condition. The electric and magnetic near fields, the angular distribution of the far-field radiation into vacuum due to SPP-photon coupling, and the SPP reflection and transmission coefficients are calculated by numerically solving the k-space integral equation upon which the formulation is based. In particular, we consider Gaussian-shaped defects (either protuberances or indentations) and study the dependence of the above-mentioned physical quantities on their $1/e$ half-width a and height h. SPP reflection is significant for narrow defects $(a\ensuremath{\lesssim}\ensuremath{\lambda}/5,$ for either protuberances or indentations, where $\ensuremath{\lambda}$ is the wavelength of the SPP); maximum reflection (plasmon mirrors) is achieved for $a\ensuremath{\approx}\ensuremath{\lambda}/10.$ For increasing defect widths, protuberances and indentations behave differently. The former give rise to a monotonic increase of radiation at the expense of SPP transmission for increasing defect half-width. However, indentations exhibit a significant increase of radiation (decrease of SPP transmission) for half-widths of the order of or smaller than the wavelength, but tend to total SPP transmission in an oscillatory manner upon further increasing the half-width. Both the position of the maximum radiation and the oscillation period depend on the defect height, which in all other cases only affects the process quantitatively. Light emitters might thus be associated with either wide indentations or protuberances with widths that are of the order of or smaller than the wavelength.

A system and method for analyzing topological features on a surface

Abstract: Disclosed is a method and apparatus for using far field scattered and diffracted light (412) to determine whether a collection of topological features on a surface (408), (e.g. a semiconductor wafer) conforms to an expected condition or quality. This determination is made by comparing the far field diffraction pattern of a surface under consideration with a corresponding diffraction pattern (a 'baseline'). If the baseline diffraction pattern and far field diffraction pattern varies by more than a prescribed amount or in characteristic ways, it is inferred that the surface (408) features are defective. The method may be implemented as a die-to-die comparison of far field diffraction patterns of two dies on a semiconductor wafer. The portion of the far field scattered and diffracted light (412) sensitive to a relevant condition or quality can also be reimaged to obtain an improved signal-to-noise ratio.

Basic introduction to bioelectromagnetics

Abstract: Electric and Magnetic Fields: Basic Concepts Electric Field Concepts Magnetic Field Concepts Sources of Electric Fields (Maxwell's Equations) Sources of Magnetic Fields (Maxwell's Equations) Electric and Magnetic Field Interactions with Materials Other Electromagnetic Field Definitions Waveforms Used in Electromagnetics Sinusoidal EM Functions Root Mean Square or Effective Values Wave Properties in Lossless Materials Boundary Conditions for Lossless Materials Complex Numbers in Electromagnetics (the Phasor Transform) Wave Properties in Lossy Materials Boundary Conditions for Lossy Materials Energy Absorption Electromagnetic Behavior as a Function of Size and Wavelength Electromagnetic Dosimetry EM Behavior When the Wavelength Is Large Compared to the Object Size Low-Frequency Approximations Fields Induced in Objects by Incident E Fields in Free Space E Field Patterns for Electrode Configurations Electrodes for Reception and Stimulation in the Body Fields Induced in Objects by Incident B Fields in Free Space E Field Patterns for In Vitro Applied B Fields Measurement of Low-Frequency Electric and Magnetic Fields EM Behavior When the Wavelength Is About the Same Size as the Object Waves in Lossless Media Wave Reflection and Refraction Waves in Lossy Media Transmission Lines and Waveguides Resonant Systems Antennas Diffraction Measurement of Mid-Frequency Electric and Magnetic Fields EM Behavior When the Wavelength Is Much Smaller Than the Object Ray Propagation Effects Total Internal Reflection and Fiber Optic Waveguides Propagation of Laser Beams Scattering from Particles Photon Interactions with Tissues X-Rays Measurement of High-Frequency Electric and Magnetic Fields (Light) Bioelectromagnetic Dosimetry Polarization Electrical Properties of the Human Body Human Models Energy Absorption (SAR) Extrapolating from Experimental Animal Results to Those Expected in Humans Numerical Methods for Bioelectromagnetic Stimulation Electromagnetic Regulations Electromagnetics in Medicine: Today and Tomorrow Fundamental Potential and Challenges Hyperthermia for Cancer Therapy Magnetic Effects Proposed Bioelectromagnetic Effects Emerging Bioelectromagnetic Applications Appendices Index

Time-domain near-field analysis of short-pulse antennas .I. Spherical wave (multipole) expansion

Abstract: The radiation from a time-dependent source distribution in free-space is analyzed using time-domain (TD) spherical wave (multipole) expansion. The multipole moment functions are calculated from the time-dependent source distribution. The series convergence rate in the near and far zone and the bounds on the near-zone reactive field are determined as functions of the source support and of the pulse length. The formulation involves a spherical transmission line representation that can be extended to more general spherical configurations. This formulation also describes the field and energy transmission mechanisms in a physically transparent fashion that will be used in a companion paper to define and explore fundamental concepts such as TD reactive energy and Q and to derive bounds on the antenna properties. Finally, the concepts discussed above are demonstrated numerically for pulsed radiation by a circular current disk.

The inverse source problem of electromagnetics: linear inversion formulation and minimum energy solution

Abstract: We address the inverse source problem of finding the time-harmonic current distribution (source) with minimum L/sup 2/ norm (minimum energy) that generates a prescribed electromagnetic field outside the source's region of support. Using the well-known multipole expansion of the electromagnetic field we compute (via a linear operator formalism) the sought-after minimum L/sup 2/ norm-current distribution consistent with the data.

A Simulation Study of the Beam Steering Characteristics for Linear Phased Arrays

Abstract: Ultrasonic beam steering characteristics for linear phased array transducers are simulated numerically by visualizing the full-field acoustic pressure field of the waves radiated from a linear phased array transducer. The influences of various transducer parameters on the beam steering properties are studied, including number of elements, inter-element spacing, element size, frequency of the transducer and the steering angle. In addition, the effects of these parameters on the near field characteristics are investigated by analyzing the acoustic pressure profile in the steering direction. The simulation results agree well with the analytical solutions which are valid only in the far field. A suggested scheme for optimal transducer design is presented.

Nonactive antenna compensation for fixed-array microwave imaging. I. Model development

Abstract: Fixed-array microwave imaging with multisensor data acquisition can suffer from nonactive antenna element interactions which cause distortions in the measurements. In Part I of a two-part paper, the authors develop a nonactive antenna compensation model for incorporation in model-based near-field microwave image reconstruction methods. The model treats the nonactive members of the antenna array as impedance boundary conditions applied over a cylindrical surface of finite radius providing two parameters, the effective antenna radius and impedance factor, which can be determined empirically from measured data. Results show that the effective radius and impedance factor provide improved fits to experimental data in homogeneous phantoms where measurements are obtained with and without the presence of the nonactive antenna elements. Once deduced, these parameters are incorporated into the nonactive antenna compensation model and lead to systematic data-model match improvements in heterogeneous phantoms. While the improvements afforded by the nonactive antenna model are small on a per measurement basis, they are not insignificant. As shown in Part II (see ibid., vol. 18, no. 6, p. 508, 1999), inclusion of this new model for nonactive antenna compensation produces significantly higher quality image reconstructions from measurements obtained with a fixed-array data acquisition system over the frequency band 500-900 MHz.

Imaging microwave electric fields using a near-field scanning microwave microscope

Abstract: By scanning a fine open-ended coaxial probe above an operating microwave device, we image local electric fields generated by the device at microwave frequencies. The probe is sensitive to the electric flux normal to the face of its center conductor, allowing different components of the field to be imaged by orienting the probe appropriately. Using a simple model of the microscope, we are able to interpret the system’s output and determine the magnitude of the electric field at the probe tip. We show images of electric field components above a copper microstrip transmission line driven at 8 GHz, with a spatial resolution of approximately 200 μm.

Spatiotemporal evolution of focused single-cycle electromagnetic pulses

Abstract: We analyze exact solutions of Maxwell's equations that are capable of describing focused single-cycle electromagnetic pulses. These finite energy solutions are a subset of Ziolkowski's ``modified power spectrum'' pulse solutions [Phys. Rev A 39, 2005 (1989)]. They display substantial temporal reshaping, time reversal, and polarity reversals as they pass through the focus. The temporal profiles at the focus and in the far field are related by a Hilbert transform in time. These results are explained in terms of the Gouy phase shift of focused beams. We also show that these pulse solutions are natural spatiotemporal modes of an open resonator and propose methods for their practical realization.

Near-field measurements of VLSI devices

Abstract: This short paper describes a technique for the measurement of the electric near field at the package surface of microprocessors and other VLSI devices. The technique uses precision stepper motors for highly accurate placement of an electric field probe at the surface of the device to be measured. Structural resolution across the device is on the order of 400-600 /spl mu/m. Typical scans accumulate 10000 data points across a variable scan area, which can be defined by device package dimensions or by the die dimensions. Characterizing a device involves a repeated series of surface scans at harmonics of the fundamental clock frequency. This paper describes the electric near field at the surface of a multichip module (MCM) composed of a processor, a flash memory, and application specific integrated circuit (ASIC). The MCM was measured while in operation in the actual circuit application.

Transmitted signal detection of optical disks with a superresolution near-field structure

Abstract: We observed transmitted signals of phase change marks recorded in super-resolution near-field structure (super-RENS) disks under high speed disk rotation. The observed minimum mark size was 81 nm and the carrier to noise ratio was about 6 dB at a constant linear velocity of 4.8 m/s. The mark size was far beyond the diffraction limit of an optical pickup with a wavelength of 635 nm and a numerical aperture of 0.6. It was clear that near-field scattering actually occurred in super-RENS disks.

Heating of tissue by near-field exposure to a dipole: a model analysis

Abstract: We report a numerical study of the induced electric fields and specific absorption rate (SAR) produced by microwave radiation from a half-wavelength dipole near tissue models, and the resulting transient and steady-state temperature rises. Several models were explored, including a uniform semi-infinite plane of tissue, uniform sphere, a phantom model of the head filled with tissue-equivalent material, a numerical model of the head with uniform dielectric properties (obtained from a digitized computed tomography image), and a numerical model of the head with different dielectric properties corresponding to various tissues. The electromagnetic calculations were performed for half-wave dipoles radiating at 900 and 1900 MHz at various distances from the model, using the finite-difference-time-domain (FDTD) method. The resulting temperature rises were estimated by finite element solution of the bioheat equation. The calculated SAR values agree well with an empirical correlation due to Kuster. If the limiting hazard of such exposures is associated with excessive temperature increase, present exposure limits are very conservative and guidelines that are easier to implement might provide adequate protection.

Prediction of Near Field Plume Characteristics Using Far Field Circulation Model

Abstract: 3-D numerical models are being used more commonly to predict changes in coastal water quality associated with point discharges such as sewage outfalls. Because these “far field” models use grid sizes which are orders of magnitude larger than the scale of near field entrainment processes, it is of interest to compare their predictive capability with that of initial mixing models and to identify ways in which the two model types can be coupled. Comparisons between the 3-D circulation model ECOMsi and the Environmental Protection Agency’s mixing model RSB suggest that the former does a reasonable job predicting plume trap height and volumetric dilution but often overpredicts plume width. Results are sensitive to source representation and parameterization of horizontal and vertical diffusion. The success results from the fact that initial dilution is governed in part by gravitational exchange flow (a large-scale phenomenon that can be resolved in a far field model) in addition to plume entrainment (which is clearly subgrid scale), as well as the self-regulating relationship between plume trap height and initial dilution. Overprediction of plume width is attributed to numerical diffusive effects. Several procedures for improving predictions by coupling near and far field models are explored, ranging from the use of the near field model to dynamically adjust far field mixing parameters so that the far field model simulates the correct trap height, to simply using the near field to assign source location and dimension for the far field.

Analysis of terahertz waveforms measured by photoconductive and electrooptic sampling

Abstract: Terahertz (THz) waveform measurements by photoconductive (PC) sampling and free-space electrooptic sampling (FS-EOS) are analyzed and quantitatively compared. Our data suggest that a short dipole antenna used in a PC receiver contributes a flat frequency response when used without a substrate lens and a j/spl omega/ response when used with a substrate lens, for the specific THz frequency range and optical system investigated in our experiments. These findings are explained using results from basic antenna theory. Experiments testing our theory for a variety of THz waveforms (obtained by using different THz emitters and simple as well as shaped optical excitation pulses) and for different carrier lifetimes are also presented. Finally, we demonstrate near-field effects in the PC sampling measurements of broad-band THz waveforms and explore the evolution of THz radiation from the near field into the far field.

Time-domain near-field analysis of short-pulse antennas .II. Reactive energy and the antenna Q

Abstract: For pt. I see ibid., vol.47, no.2, p.271-79 (1999). The time-domain (TD) multipole expansion, developed in the first part of this two-part sequence, is extended here to analyze the power-flow and energy balance in the vicinity of a pulsed antenna. Using the spherical transmission line formulation, we derive expressions for the pulsed power-flow and energy and identify the radiative and the reactive constituents. For time-harmonic fields, the reactive concepts are well understood in terms of the stored energy, but this interpretation is not applicable for short-pulse fields where there is no stored energy. By considering the TD energy balance, we clarify the transition of the near-zone pulsed reactive energy to the radiation power and show that the pulsed reactive energy discharges back to the source once the pulse has been radiated. We thus introduce a TD Q factor that quantifies the radiation efficiency. In particular, we show that super resolution using short-pulse fields involves large TD reactive energies and Q and is, therefore, not feasible. The general TD concepts discussed are demonstrated through a numerical example of radiation from a circular disk carrying a pulsed current distribution.

Attached and Radiated Electromagnetic Fields of an Electric Point Dipole

Abstract: The standard dyadic Green function description of the electromagnetic field generated by an electric point dipole is modified (and corrected) so that a rigorous classical theory for the attached and radiated parts of the near field appears. The present propagator formalism follows from analysis of the transverse and longitudinal dipole electrodynamics. Elimination of both the transverse and the longitudinal self-fields leads to a description of the radiated dipole field that enables one to obtain the associated energy flux in the near- and mid-field zones also and that is correctly retarded (with the vacuum speed of light) everywhere in space. The related retarded transverse propagator exists in the time (space) domain, whereas the standard propagator exists only in the frequency (space) domain. As a forerunner to an analysis of the Weyl expansions for the standard, longitudinal self-field and retarded transverse propagators, the plane-wave mode expansions of these propagators are investigated, and contour integrations are specified in such a manner that the rigorous Green function description is regained. It is found that, in order for the retarded transverse propagator description to be consistent in the near-field zone, the Weyl expansion for this propagator has to contain evanescent components not only for wave numbers larger than the vacuum wave number but in the entire angular spectrum. The present theory may influence our view of optical near-field phenomena and (classical) photon tunneling because in both of these fields a proper identification of attached and radiated fields seems needed.

Electromagnetic Scattering from an Orthotropic Medium

Abstract: We investigate electromagnetic wave propagation in an inhomogeneous anisotropic medium. For the case of an orthotropic medium we derive the Lippmann-Schwinger equation, which is equivalent to a system of strongly singular integral equations. Uniqueness and existence of a solution is shown and we examine the regularity of the solution by means of integral equations. We prove the innnite Fr echet diierentiability of the scattered eld in its dependence on the refractive index of the anisotropic medium and we derive a characterization of the Fr echet derivatives as a solution of an anisotropic scattering problem.

Near-field alignment of phased-array antennas

Abstract: This paper describes the algorithms and equipment used to apply near-field scanning techniques to the phase alignment of phased-array antennas. This procedure achieves a level of precision not previously available. The electronic scanning property of the antenna is used to bring different sections of the antenna spectrum within range of the near-field scanning process. These partial spectra are then merged to define the entire spectrum of the antenna. This process provides the resolution needed to determine the excitation at individual elements by the inverse Fourier transformation operation. The process described here has been used in the production of a very large number of phased-array antennas currently in service.

Near-field calibration system for phase-array antennas

Abstract: A near-field calibration arrangement for a phased-array antenna determines the phase shifts or attenuation of the elemental control elements of the array. The calibration system includes a probe located in the near field, and a calibration tone generator. The tone generator is coupled to the probe in the case of calibration of a receive antenna, and to the signal port of the array in the case of calibration of a transmit antenna. A code generator generates sets of values, with each set being orthogonal to the others. The codes are applied to the control input port of the array antenna, so that the codes encode the tone passing through the antenna. The encoded tones are applied to a decoder and processor, which processes the signals to determine the phase shift or attenuation associated with each bit of the control signal.