Showing papers on "Near and far field published in 2005"

Experimental Verification of Designer Surface Plasmons

Abstract: We studied the microwave reflectivity of a structured, near perfectly conducting substrate that was designed to verify the existence of a theoretically proposed new class of surface mode. Measurements of the mode's dispersion curve show that it correctly approaches the predicted asymptotic frequency; the curve also agrees well with that derived from a computer simulation. Modeling of the field distribution on resonance provides evidence of strong localization of the electric field at the interface and substantial power flow along the interface, thus verifying the surface plasmon-like nature of the mode.

Guided subwavelength plasmonic mode supported by a slot in a thin metal film

Abstract: We demonstrate the existence of a bound optical mode supported by a slot in a thin metallic film deposited on a substrate, with slot dimensions much smaller than the wavelength. The modal size is almost completely dominated by the near field of the slot. Consequently, the size is very small compared with the wavelength, even when the dispersion relation of the mode approaches the light line of the surrounding media. In addition, the group velocity of this mode is close to the speed of light in the substrate, and its propagation length is tens of micrometers at the optical communication wavelength.

2-D periodic leaky-wave antennas-part I: metal patch design

Abstract: The far-field radiation characteristics of a two-dimensional (2-D) periodic leaky-wave antenna (LWA) constructed from a periodic array of metal patches on a grounded dielectric substrate is investigated. A simple dipole source is used as the excitation. Reciprocity together with a periodic spectral-domain method of moments is used to calculate the far-field pattern. Design rules for the scan angle, the substrate dielectric constant, and the periodicity are provided. Finally, a comparison of the 2-D periodic LWA and a dielectric-layer LWA is given to show the similar performance of the two antennas.

A planar triangular monopole antenna for UWB communication

Abstract: A planar triangular monopole antenna (PTMA) is presented for high-frequency structure simulator ultra-wideband (UWB) communication. The high-frequency structure simulator three-dimensional electromagnetic solver is employed for design simulation. A printed PTMA has been realized by using the FR-4 printed circuit board substrate. The measured voltage standing wave ratio is less than 3 from 4 to 10 GHz. In the UWB communication frequency range, the measured phase distribution of the input impedance is quite linear and the H-plane patterns are almost omni-directional. The Kirchhoff's surface integral representation was adopted in the developed finite-difference time-domain code to compute the far field distributions from the near filed ones in time-domain. This is to investigate the radiated power density spectrum (PDS) shaping to comply with FCC emission limit mask. The effect of various source pulses (first-order Rayleigh pulses with /spl sigma/ of 20, 30, and 50 ps) on the radiated PDS shaping is also studied.

Far-field optical microscopy of single metal nanoparticles.

Abstract: Individual noble-metal particles, with sizes ranging from a few tenths to some hundreds of nanometers, can now be detected by far-field optics. Single-particle microscopy gives access to inhomogeneity, distributions, and fluctuations, which were previously hidden in ensemble experiments. Scattering methods rely on dark-field illumination, spectral signatures of the metal particles, or both. More advanced techniques provide high sensitivity and improved selectivity with respect to other scatterers by isolating metal-specific signals, for example the refractive index change due to heating of the environment by a pump beam or the time-resolved optical response of the particle to a short pump pulse. We review and compare linear and nonlinear methods in far-field optical microscopy that have reached the single-particle regime by means of scattered light, thermal effects, photoluminescence, or nonlinear frequency generation.

2-D periodic leaky-wave Antennas-part II: slot design

Abstract: The far-field radiation patterns of a two-dimensional (2-D) periodic slot leaky-wave antenna (LWA) are studied. The antenna consists of a two-dimensional periodic array of slots in a conducting plane that is printed on top of a grounded dielectric slab. The antenna is excited by a simple source such as a dipole inside the slab. Reciprocity along with the spectral-domain method is used to calculate the far-field pattern, and the radiation characteristics of the structure are investigated. A comparison between the present periodic slot LWA and a 2-D periodic patch LWA discussed in Part I is given to show the advantages of the slot antenna for certain applications. The slot LWA can achieve high directivity patterns, and a circularly-polarized version of the antenna can achieve good circular-polarization at broadside.

Method of superposition applied to patch near-field acoustic holography

Abstract: The method of superposition may be applied to reconstruct the field on a partial surface on a radiating structure from measurements made on a nearby limited surface. Unlike conformal near-field holography, where the measurement surface surrounds the entire structure, in patch holography the measurement surface need only be approximately as large as the patch on the structure surface where the reconstruction is required. Using the method of superposition, the field on and near the measurement surface may be approximated by the field produced by a source distribution placed on a surface inside the structure. The source strengths are evaluated by applying boundary conditions on the measurement surface. The algorithm requires the inversion of the Green’s function matrix which may be ill-conditioned. Truncated singular value decomposition is used to invert it. The field on the structure surface is then approximated by the field produced by the source distribution. The algorithm is easier to implement than the ...

Radiation forces on a Rayleigh dielectric sphere in a patterned optical near field

Abstract: We report on the study of the radiation forces exerted on a Rayleigh dielectric particle by a patterned optical near-field landscape at an interface decorated with resonant gold nanostructures. This configuration allows for the generation of a large array of surface subwavelength optical traps from an extended collimated beam, which may be of interest for parallel optical manipulation and sorting of submicrometer objects.

Lightning electromagnetic environment in the presence of a tall grounded strike object

Abstract: [1] We have analyzed and compared distance dependences of electric and magnetic fields due to a lightning strike to a tall object and due to the same lightning strike to flat ground. In both cases, lightning was represented by a transmission line energized by a lumped voltage source connected at the channel attachment point. The resultant total charge transfer to ground was the same regardless of the presence of strike object. The electric field for the strike-object case is reduced relative to the flat-ground case at closer distances from the object. If we assume, in an idealized case, that the return stroke wave front speed is equal to the speed of light, v = c, the current reflection coefficient at the bottom of the strike object ρbot = 1 (grounding impedance Zgr = 0), and that at the top of the object for upward-propagating waves ρtop = 0 (characteristic impedance of the object is equal to that of the channel Zob = Zch), the ratio of the vertical electric fields on ground for the strike-object and flat-ground cases (electric field attenuation factor) will be d/√(d2 + h2), where h is the height of the strike object and d is the horizontal distance from the object. The corresponding ratio for the azimuthal magnetic field is equal to unity. We show that the ratio for either electric or magnetic field increases with decreasing ρbot (ρbot < 1), decreasing ρtop (ρtop < 0 except for the case of ρbot = 0), and decreasing v (v < c), and at larger distances can become greater than unity. We additionally show that the ratio of the far fields for the strike-object and flat-ground cases is given by (1 − ρtop) (c/v + 1)/(1 + ρgr), where ρgr is the current reflection coefficient at the lightning channel base when the channel terminates directly on ground. For realistic values of ρtop = −0.5, ρgr = 1, and v = 0.5c, this ratio (far field enhancement factor) is equal to 2.3.

Far-field-current relationship based on the TL model for lightning return strokes to elevated strike objects

Abstract: New general expressions relating lightning return stroke currents and far radiated electric and magnetic fields are proposed, taking into account the effect of an elevated strike object, whose presence is included as an extension to the transmission line (TL) model. Specific equations are derived for the case of tall and electrically short objects. The derived expressions show that, for tall structures (when the round-trip propagation time from top to bottom within the tower is greater than the current zero-to-peak risetime), the far field is enhanced through a factor with respect to an ideal return stroke initiated at ground level. The enhancement factor can be expressed in terms of the return stroke wavefront speed v, the speed of light in vacuum c, and the current reflection coefficient at the top of the elevated strike object. For typically negative values of this top reflection coefficient, lightning strikes to tall towers result in a significant enhancement of the far electromagnetic field. Expressions relating the far electromagnetic field and the return stroke current are also presented for electrically short towers and for very long return stroke current wavefronts. For the case of return strokes initiated at ground level (h=0), these expressions represent a generalization of the classical TL model, in which the reflections at the ground are now taken into account. We describe also simultaneous measurements of return stroke current and its associated electric and magnetic fields at two distances related with lightning strikes to the 553-m-high Toronto Canadian National (CN) Tower performed during 2000 and 2001. The derived expressions for tall strike objects are tested versus obtained sets of simultaneously measured currents and fields associated with lightning strikes to the CN Tower, and a reasonable agreement is found. Additionally, it is shown that the peak of the electromagnetic field radiated by a lightning strike to a 553-m-high structure is relatively insensitive to the value of the return stroke velocity, in contrast to the lightning strikes to ground.

High-power 808 nm lasers with a super-large optical cavity

Abstract: We present a detailed design and experimental study of diode laser structures emitting at 808 nm based on the combination of a GaAsP quantum well with well-established AlGaAs waveguide structures By increasing the thickness of the confinement layers of the laser structure, its vertical far field divergence is reduced down to 15° with only a small increase of the threshold current and small loss of efficiency 200 µm aperture 'broad area' devices achieve at a heat sink temperature of 25 °C a continuous wave (CW) output power of more than 15 W with a wall-plug efficiency of 50% with a vertical far field divergence of 18° This output power illustrates the excellent high-power performance by using super-large optical-cavity structures with improved beam characteristics in comparison to the conventional broad waveguide lasers

Comments on "Scattering by a finite set of perfectly conducting cylinders buried in a dielectric half-space: a spectral-domain solution

Abstract: An analytical-numerical technique, for the solution of the two-dimensional electromagnetic plane-wave scattering by a finite set of perfectly conducting circular cylinders buried in a dielectric half-space, is presented. The problem is solved for both the near- and the far-field regions, for TM and TE polarizations. The diffracted field is represented in terms of a superposition of cylindrical waves and use is made of the plane-wave spectrum to take into account the reflection and transmission of such waves by the interface. The validity of the approach is confirmed by comparisons with results available in the literature, with very good agreement. The multiple interactions between two buried cylinders have been studied by considering both the induced currents and the scattered field diagrams. Applications of the method to objects of arbitrary cross-section simulated by a suitable configuration of circular cylinders are shown.

Near-Earth wave propagation characteristics of electric dipole in presence of vegetation or snow layer

Abstract: The problem of near-earth wave propagation in the presence of a dielectric layer such as a vegetation or snow covering is considered in this paper by modeling the propagation environment as a homogeneous two-layer medium (air/dielectric/ground). A number of studies have demonstrated the relevancy of the lateral wave for the case when both the transmitter and receiver are located within a simple half-space dielectric medium . Unfortunately, for the generalized two-layer model, for configurations in which the transmitter or receiver (or both) is located above the dielectric layer, far-field analytical expressions that include all propagation features do not exist. In this paper, in order to arrive at a computational efficient solution for the two-layer model, a second-order asymptotic evaluation for the electric fields of an arbitrarily oriented, infinitesimal electric dipole-for source and observation points located in the vicinity of the air/dielectric interface-is carried out through the method of steepest descents. The formulations are valid in the far field, with the limitation that the exponentially decaying pole and branch cut contributions have been ignored. It is observed that the Norton wave, though it is highly localized near the air/dielectric interface, is a significant contribution either when the dipole and observation points are both located above the dielectric layer or when one is above and the other is within the layer.

Near field propagation law & a novel fundamental limit to antenna gain versus size

Abstract: The paper presents a theoretical analysis of the near field channel in free space The theoretical model is then validated by comparison to data measured in an open field The results are important for low frequency RF systems, such as those operating at short range in the AM broadcast band (525-1715 kHz) Finally, the paper establishes a novel fundamental limit for antenna gain versus size

Dispersion-enhanced photonic crystal fiber array for a true time-delay structured X-band phased array antenna

Abstract: Tunable optical true time-delay modules based on highly dispersive photonic crystal fibers (PCFs) are demonstrated to provide continuous radio-frequency squint-free beam scanning for an X-band (8-12 GHz) phased array antenna system. The dispersion of the fabricated PCF is as high as -600 ps/nm /spl middot/ km at 1550 nm. The time delay is continuously tunable from -31 to 31 ps between adjacent delay lines by tuning the laser wavelength continuously from 1528 to 1560 nm. The far field radiation patterns of a 1/spl times/4 subarray were measured from -45/spl deg/ to 45/spl deg/ scanning angles. Squint-free operation is experimentally confirmed.

Heterodyne detection of guided waves using a scattering-type Scanning Near-Field Optical Microscope

Abstract: An inherent problem to the study of waveguides with strong propagation losses by Scattering-type Scanning Near field Optical Microscopy is the coherent optical background field which disrupts strongly the weak detected near-field signal. We present a technique of heterodyne detection allowing us to overcome this difficulty while amplifying the near field signal. As illustrated in the case of a highly confined SOI structure, this technique, besides the amplitude, provides the local phase variation of the guided field. The knowledge of the complex field cartography leads to the modal analysis of the propagating radiation.

Generation of Laguerre-Gaussian modes in Nd:YAG laser using diffractive optical pumping

Abstract: A beam of pulsed laser radiation at the wavelength λ = 0.532 μm was diffracted at a circular diaphragm to produce a hollow-shape intensity distribution in the near field. This light distribution was used to pump an Nd:YAG laser, resulting in the formation of an inversion profile with a minimum at the laser resonator axis and enabling the suppression of the fundamental mode. Under this condition, the oscillation at λ = 1.064 μm of Laguerre-Gaussian modes (LG0m) with low and high values of the azimuthal index m was produced in the gain-switching regime. By changing the geometry of the resonator, the size of its waist parameter was changed, enabling the selection of the mode with an index m that best overlaps with the inversion profile. Oscillation of LG0m modes with indices ranging from m=1 to more than m=200 was obtained. LG0m modes with m ≤ 50 were produced using the dependence of the waist parameter on the length of the resonator near the boundary of its stability region. A LG0m mode of the highest order, m ≈ 240 was obtained by building a miniature laser resonator and using a pair of diaphragms in order to form a sharper ring-shape pumping distribution. Applications of diffractive optical pumping and ``hollow'' LG0m laser beams are discussed.

Effects of leaky-wave propagation in metamaterial grounded slabs excited by a dipole source

Abstract: In this paper, dispersive propagation and radiation properties of leaky waves on metamaterial grounded slabs are investigated. The proper or improper nature of leaky modes supported by such structures is shown to be related to the metamaterial being /spl epsi/-negative, /spl mu/-negative, or double-negative, and to field polarization, giving rise to backward or forward radiation depending on the frequency range of operation. These spectral features and the associated frequency scan of the radiated beam are illustrated by considering the field excited by a dipole source in the presence of an infinite metamaterial grounded slab. The possibility to achieve nearly equal values for the phase constants of a TE and a TM leaky mode on a large frequency range is shown; this allows us to obtain a conical radiation pattern and, also, for suitable values of the attenuation constants, the radiation of a pencil beam at broadside. Conditions for achieving maximum power density at broadside are derived, when one constitutive parameter is much smaller than the other. In order to illustrate these novel features, numerical results based on experimentally tested dispersion models for permittivity and permeability of the metamaterial media are provided, concerning leaky-wave modal properties and near and far fields excited by a dipole source.

Compact size highly directive antennas based on the SRR metamaterial medium

Abstract: In this work, we studied the far-field properties of the microwave radiation from sources embedded inside the split-ring resonator (SRR) metamaterial medium. Our results showed that the emitted power near the resonance frequency of the SRR structure was confined to a narrow angular region in the far field. The measured radiation patterns showed half-power beamwidths around 14 ◦ .The highly directive radiation is obtained with a smaller radiation surface area when compared to the previous results obtained by using photonic crystals. The reduction in the surface area is ten-fold in the case of the SRR metamaterial medium when compared to the photonic crystals. Our results provide means to create compact size highly directive antennas.

Design and fabrication of low beam divergence and high kink-free power lasers

Abstract: We report the design and fabrication of high performance high power lasers with emission wavelength from 800 to 1000 nm using a novel wafer structure, in which a graded V-shape layer was incorporated, to reduce the vertical far field (wafer growth direction) and to suppress higher order mode lasing. The structure offers the freedom to independently design the vertical far field and optical overlap with the quantum wells. An extremely low far field can be achieved, which still retains high optical overlap, allowing a low threshold current to be maintained. In addition, the structure can greatly enhance the laser kink-free power by suppressing or even completely eliminating higher order mode lasing, an extremely desirable property for high power single mode lasers.

Analysis of optical interferometric displacement detection in nanoelectromechanical systems

Abstract: Optical interferometry has found recent use in the detection of nanometer scale displacements of nanoelectromechanical systems (NEMS). At the reduced length scale of NEMS, these measurements are strongly affected by the diffraction of light. Here, we present a rigorous numerical model of optical interferometric displacement detection in NEMS. Our model combines finite element methods with Fourier optics to determine the electromagnetic field in the near-field region of the NEMS and to propagate this field to a detector in the far field. The noise analysis based upon this model allows us to elucidate the displacement sensitivity limits of optical interferometry as a function of device dimensions as well as important optical parameters. Our results may provide benefits for the design of next generation, improved optical NEMS.

Calibration and compensation of near-field scan measurements

Abstract: A procedure for the calibration and compensation of near-field scanning is described and demonstrated. Ultimately, the objective is to quantify the individual field components associated with electromagnetic interference (EMI) from high speed circuitry and devices. Specific examples of these methods are shown. The effects of compensation are small but noticeable when the uncompensated output signal from near field scanning is already a very good representation of the field being measured. In other cases, the improvement provided by compensation can be significant when the uncompensated output signal bears little resemblance to the underlying field.

The Convex Scattering Support in a Background Medium

Abstract: We discuss inverse problems for the Helmholtz equation at fixed energy, specifically the inverse source problem and the inverse scattering problem from a medium or an obstacle. In (S. Kusiak and J. Sylvester, Comm. Pure Appl. Math., 56 (2003), pp. 1525-1548), we introduced the convex scattering support of a far field, a set which will be a subset of the convex hull of the support of any source or scattering inhomogeneity which can produce it. We extend these results and modify the methods to locate a source within a known inhomogeneous background medium, or a deviation from that medium, using observations of a single far field. We also describe some numerical examples that illustrate the robustness of the method. 1. Introduction. We study an inverse problem for the Helmholtz equation at fixed energy. Our aim is to deduce the location of the source or scatterer from observa- tions of scattered waves made at a distance, which are called far fields. Typically, one has access to several far fields. For the inverse medium problem, the index of refrac- tion is uniquely determined by the full scattering kernel, i.e., the observed scattered field for every possible incident wave. In special cases (9, 5, 6), substantial information about the support of the scatterer has been obtained from the scattered field of a few, or even only one, incident wave. In (8), we showed that, in a homogeneous background medium, we could associate the convex scattering support with a single far field. This set is the smallest convex set which supports a source that can produce that far field. We also produced a test, the circular Paley-Wiener theorem, for computing the convex scattering support in two dimensions. In (10), we introduced a different numerical method, called the range test, for computing this support in a two-dimensional homogeneous medium. Our work was motivated by the linear sampling method of Colton and Kirsch (see (2)). They first developed a Picard test, which determines whether a far field belongs to the range of the (compact) scattering operator, as a tool for inverse scattering. This method, and the subsequent factorization method of Kirsch (7), differ from what we present here in that they require much more data (the full scattering map) and compute much more (the exact support of the scatterer). In section 2 of this paper, we introduce the necessary scattering formalism and restate the circular Paley-Wiener theorem as a Picard test. This restatement, though less explicit, generalizes directly to inhomogeneous media and higher dimensions. In section 3 we produce this general test. The general test tells us if a far field could have been produced by a source or a scatterer located within a specific domain,

Invisible plasmonic meta-materials through impedance matching to vacuum

Abstract: We report on perfect transmission in two-dimensional plasmonic matamaterials in the terahertz frequency range, in which zeroth order transmittance becomes essentially unity near specific resonance frequencies. Perfect transmission may occur when the plasmonic metamaterials are perfectly impedance matched to vacuum, which is equivalent to designing an effective dielectric constant around epsilonr = -2. When the effective dielectric constant of the metamaterial is tuned towards epsilonr and the hole coverage is larger than 0.2, strong evanescent field builds up in the near field, making perfect transmission possible.

Localized terahertz generation via optical rectification in ZnTe

Abstract: We report the measurement of terahertz (THz) radiation, generated via optical rectification in a 20 µm thick ZnTe crystal, as a function of the size of optical excitation. The result shows that, before the onset of significant higher-order nonlinear processes, the THz emission obtained with a fixed excitation power is largely size independent for excitation sizes smaller than the THz wavelength. This experimental finding is well described by a theoretical model including the generation of THz radiation through optical rectification from a subwavelength source and its propagation into the far field. The characteristic size dependence of the radiation from a subwavelength THz source is advantageous for use in apertureless near-field microscopy.

A uni-directional ring-slot antenna achieved by using an electromagnetic band-gap surface

Abstract: Slot antennas offer a number of advantages including low-profile, ease of fabrication, and ease of integration with electronics. Their main drawback, however, is that they are inherently bi-directional radiators. One common technique to redirect the back radiation forward is to place a conducting reflector at a fixed distance away from the antenna. This distance is usually chosen to be a quarter wavelength so that the reflected back radiation incurs an additional phase of 360/spl deg/ and thus adds in phase with the forward directed radiation. However, in this case the parallel-plate geometry permits the excitation of the dominant transverse electromagnetic mode, which drastically reduces the overall radiation efficiency. This work describes a solution to this problem and shows how to achieve uni-directional slot antenna elements by backing them with an electromagnetic band-gap surface, instead of the uniform conducting reflector. This structure, when operated in its band-gap, inhibits the propagation of electromagnetic waves over all possible azimuthal directions, in the region between the antenna ground plane and the periodic surface.

Far-field radiation of photonic crystal organic light-emitting diode

Abstract: Utilizing the near- to far-field transformation based on the 3-D finite difference time domain (FDTD) method and Fourier transformation, the far-field profile of a photonic crystal organic light emitting diode is studied to understand the viewing angle dependence. The measured far-field profiles agree well with those of the simulation. The enhancement of the extraction efficiency in excess of 60% is observed for the optimized photonic crystal pattern.

Analysis of the near-field ultrasonic scattering at a surface crack

Abstract: The near-field scattering of a Rayleigh wave at a surface crack is analyzed with analytical and numerical calculations complemented by detailed experimental observations. These investigations are directed towards the development of inversion schemes for surface crack sizing in mechanical components. The near-field analysis is based on a procedure that allows filtering the Rayleigh wave from the other wave modes scattered at the defect. Pointwise measurements of the out-of-plane displacement using a laser interferometer lead to a complete displacement field image in the crack near field with high spatial resolution. The numerical calculations are compared with the analytical solution and the experimental data in order to validate specific crack modeling approaches and to identify parameters for surface crack characterization. The evaluation is carried out with respect to the stress fields at the crack tip, the surface displacement near field, Rayleigh wave transmission and reflection coefficients, and the time delay of Rayleigh wave transmission. The latter appears as the most suitable parameter for crack characterization due to the relative insensitivity of the measured values on the variability of the experimental conditions. The results of the present work also highlight differences between the scatterings at cracks and narrow slots (“artificial cracks”).

The determination of the surface conductivity of a partially coated dielectric

Abstract: A variational method is given for determining the essential supremum of the surface conductivity of a partially coated anisotropic dielectric medium from a knowledge of the far field pattern of the time-harmonic electric field at fixed frequency corresponding to an incident plane wave. It is assumed that the shape of the scatterer has been determined (e.g., by solving the far field equation and using the linear sampling method). Numerical examples are given for the scalar case with constant surface conductivity.