Showing papers on "Near and far field published in 2011"
Book•
01 Jan 2011
TL;DR: In this paper, the authors propose a method to solve the problem of homonymity in homonym identification, which is called homonym-based homonymization, or homonymisation.
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207 citations
TL;DR: A novel experimental technique that uses a high-energy electron beam as broad band point dipole source of visible radiation, to study the emission properties of a Yagi-Uda antenna composed of a linear array of Au nanoparticles, and establishes angle-resolved cathodoluminescence spectroscopy as a powerful technique tool to characterize single optical nanoantennas.
Abstract: Optical nanoantennas mediate optical coupling between single emitters and the far field, making both light emission and reception more effective. Probing the response of a nanoantenna as a function of position requires accurate positioning of a subwavelength sized emitter with known orientation. Here we present a novel experimental technique that uses a high-energy electron beam as broad band point dipole source of visible radiation, to study the emission properties of a Yagi–Uda antenna composed of a linear array of Au nanoparticles. We show angle-resolved emission spectra for different wavelengths and find evidence for directional emission of light that depends strongly on where the antenna is excited. We demonstrate that the experimental results can be explained by a coupled point dipole model which includes the effect of the dielectric substrate. This work establishes angle-resolved cathodoluminescence spectroscopy as a powerful technique tool to characterize single optical nanoantennas.
188 citations
TL;DR: In this paper, the design, fabrication, and characterization of an ultrawideband (UWB) antenna for near-field microwave imaging of dielectric objects are presented together with the imaging setup.
Abstract: The design, fabrication, and characterization of an ultrawideband (UWB) antenna for near-field microwave imaging of dielectric objects are presented together with the imaging setup. The focus is on an application in microwave breast tumor detection. The new antenna operates as a sensor with the following properties: 1) direct contact with the imaged body; 2) more than 90% of the microwave power is coupled directly into the tissue; 3) UWB performance; 4) excellent de-coupling from the outside environment; 5) small size; and 6) simple fabrication. The antenna characterization includes return loss, total efficiency, near-field directivity, fidelity, and group velocity. The near-field imaging setup employs planar aperture raster scanning. It consists of two antennas aligned along each other's boresight and moving together to scan two parallel apertures. The imaged object lies between the two apertures. With a blind de-convolution algorithm, the images are de-blurred. Simulation and experimental results confirm the satisfactory performance of the antenna as an UWB sensor for near-field imaging.
132 citations
TL;DR: In this article, a new physical-geometric optics hybrid (PGOH) method is developed to compute the scattering and absorption properties of ice particles, which is suitable for studying the optical properties of icicles with arbitrary orientations, complex refractive indices (i.e., particles with significant absorption), and size parameters (proportional to the ratio of particle size to incident wavelength) larger than ∼20.
Abstract: A new physical-geometric optics hybrid (PGOH) method is developed to compute the scattering and absorption properties of ice particles. This method is suitable for studying the optical properties of ice particles with arbitrary orientations, complex refractive indices (i.e., particles with significant absorption), and size parameters (proportional to the ratio of particle size to incident wavelength) larger than ∼20, and includes consideration of the edge effects necessary for accurate determination of the extinction and absorption efficiencies. Light beams with polygon-shaped cross sections propagate within a particle and are traced by using a beam-splitting technique. The electric field associated with a beam is calculated using a beam-tracing process in which the amplitude and phase variations over the wavefront of the localized wave associated with the beam are considered analytically. The geometric-optics near field for each ray is obtained, and the single-scattering properties of particles are calculated from electromagnetic integral equations. The present method does not assume additional physical simplifications and approximations, except for geometric optics principles, and may be regarded as a “benchmark” within the framework of the geometric optics approach. The computational time is on the order of seconds for a single-orientation simulation and is essentially independent of the size parameter. The single-scattering properties of oriented hexagonal ice particles (ice plates and hexagons) are presented. The numerical results are compared with those computed from the discrete-dipole-approximation (DDA) method.
130 citations
TL;DR: A terahertz near-field microscope with a high dynamic range that can capture images of a 370 x 740 μm2 area at 35 frames per second is reported, and the field enhancement at the gap position of a dipole antenna after the irradiation of a terAhertz pulse is revealed.
Abstract: We report a terahertz near-field microscope with a high dynamic range that can capture images of a 370 x 740 μm2 area at 35 frames per second. We achieve high spatial resolution (14 μm corresponding to λ/30 for a center frequency at 0.7 THz) on a large area by combining two novel techniques: terahertz generation by tilted-pulse-front excitation and electro-optic balanced imaging detection using a thin crystal. To demonstrate the microscope capability, we reveal the field enhancement at the gap position of a dipole antenna after the irradiation of a terahertz pulse.
122 citations
TL;DR: A three-dimensional plasmonic nanostructure that exhibits a strong and isotropic magnetic response in the visible spectral domain and is sufficiently small to be perceived as an individual object in the far field is theoretically analyzed and characterized.
Abstract: We theoretically analyze, fabricate, and characterize a three-dimensional plasmonic nanostructure that exhibits a strong and isotropic magnetic response in the visible spectral domain. Using two different bottom-up approaches that rely on self-organization and colloidal nanochemistry, we fabricate clusters consisting of dielectric core spheres, which are smaller than the wavelength of the incident radiation and are decorated by a large number of metallic nanospheres. Hence, despite having a complicated inner geometry, such a core–shell particle is sufficiently small to be perceived as an individual object in the far field. The optical properties of such complex plasmonic core–shell particles are discussed for two different core diameters.
116 citations
TL;DR: This work demonstrates the possibility to characterize multielement nanoantennas by electromagnetic antenna near-field scanners by investigating the dynamics of the local out-of-plane electric field components and to visualize the temporal evolution of this time-harmonic reception process.
Abstract: We present near-field measurements of optical Yagi-Uda nanoantennas that are used in receiving mode. The eigenmode imaging of amplitude and phase by apertureless scanning near-field optical microscopy allows us to investigate the dynamics of the local out-of-plane electric field components and to visualize the temporal evolution of this time-harmonic reception process. The antenna directionality manifests itself by the dependence of the local field enhancement at the feed element on the illumination direction. Simulations taking into account the substrate confirm our observation of the directionality. Our work demonstrates the possibility to characterize multielement nanoantennas by electromagnetic antenna near-field scanners.
113 citations
TL;DR: A superlens for electric evanescent fields with low losses using perovskites in the mid-infrared regime using near-field microscopy with a tunable free-electron laser to address precisely the polariton modes, which are critical for super-resolution imaging.
Abstract: A planar slab of negative-index material works as a superlens with sub-diffraction-limited resolution, as propagating waves are focused and, moreover, evanescent waves are reconstructed in the image plane. Here we demonstrate a superlens for electric evanescent fields with low losses using perovskites in the mid-infrared regime. The combination of near-field microscopy with a tunable free-electron laser allows us to address precisely the polariton modes, which are critical for super-resolution imaging. We spectrally study the lateral and vertical distributions of evanescent waves around the image plane of such a lens, and achieve imaging resolution of λ/14 at the superlensing wavelength. Interestingly, at certain distances between the probe and sample surface, we observe a maximum of these evanescent fields. Comparisons with numerical simulations indicate that this maximum originates from an enhanced coupling between probe and object, which might be applicable for multifunctional circuits, infrared spectroscopy and thermal sensors.
105 citations
TL;DR: In this paper, a procedure is developed to predict electromagnetic interference from electronic products using near-field scan data, which is used to define equivalent electric and magnetic current sources characterizing the electromagnetic emissions from an electronic circuit.
Abstract: A procedure is developed to predict electromagnetic interference from electronic products using near-field scan data. Measured near-field data are used to define equivalent electric and magnetic current sources characterizing the electromagnetic emissions from an electronic circuit. Reconciliation of the equivalent sources is performed to allow the sources to be accurately applied within full-wave numerical modeling tools like finite-difference time domain (FDTD). Results show that the radiated fields must typically be represented by both electric and magnetic current sources if scattering and multiple-reflections from nearby objects are to be taken into account. The accuracy of the approach is demonstrated by predicting the fields generated by a microstrip trace within and outside of a slotted enclosure, and by predicting the fields generated by the microstrip trace close to a long wire. Values predicted from near-field scan data match those from full-wave simulations or measurements within 6 dB.
103 citations
TL;DR: The far field diffraction pattern for incident fields incident upon a triangular aperture possessing non-integer values of the azimuthal index ℓ is explored and the birth of a vortex is inferred and its evolution is explored by observations of the diffraction patterns.
Abstract: The study and application of optical vortices have gained significant prominence over the last two decades. An interesting challenge remains the determination of the azimuthal index (topological charge) l of an optical vortex beam for a range of applications. We explore the diffraction of such beams from a triangular aperture and observe that the form of the resultant diffraction pattern is dependent upon both the magnitude and sign of the azimuthal index and this is valid for both monochromatic and broadband light fields. For the first time we demonstrate that this behavior is related not only to the azimuthal index but crucially the Gouy phase component of the incident beam. In particular, we explore the far field diffraction pattern for incident fields incident upon a triangular aperture possessing non-integer values of the azimuthal index l. Such fields have a complex vortex structure. We are able to infer the birth of a vortex which occurs at half-integer values of l and explore its evolution by observations of the diffraction pattern. These results demonstrate the extended versatility of a triangular aperture for the study of optical vortices.
100 citations
Patent•
23 Feb 2011TL;DR: In this article, a near field transducer, a first magnetic pole, a heat sink, and a diffusion barrier are placed between the first magnetic poles and the transducers.
Abstract: An apparatus includes a near field transducer positioned adjacent to an air bearing surface, a first magnetic pole, a heat sink positioned between the first magnetic pole and the near field transducer, and a diffusion barrier positioned between the near field transducer and the first magnetic pole. The diffusion barrier can be positioned adjacent to the magnetic pole or the near field transducer.
TL;DR: In this article, the Fourier analysis of the wideband transmission and reflection signals recorded by two antennas scanning together along two rectangular parallel apertures on both sides of the inspected region is proposed to reconstruct targets in the near-field range.
Abstract: A new 3-D holographic microwave imaging technique is proposed to reconstruct targets in the near-field range. It is based on the Fourier analysis of the wideband transmission and reflection signals recorded by two antennas scanning together along two rectangular parallel apertures on both sides of the inspected region. The complex scattering parameters of the two antennas are collected at several frequencies and then processed to obtain a representation of the 3-D target in terms of 2-D slice images at all desired range locations. No assumptions are made about the incident field and Green's function, which are derived either by simulation or by measurement. Furthermore, an approach is proposed to reduce the image artifacts along range. To validate the proposed technique, predetermined simulated targets are reconstructed. The effects of random noise, number of sampling frequencies, and dielectric contrast of the targets are also discussed.
TL;DR: A novel NSOM aperture probe is reported that gives a 100× higher throughput and 40× increased damage threshold than conventional near-field aperture probes and is employed by employing a novel variant of extraordinary optical transmission, relying solely on a single aperture and a coupled waveguide.
Abstract: Near-field scanning optical microscopy (NSOM) offers high optical resolution beyond the diffraction limit for various applications in imaging, sensing, and lithography; however, for many applications the very low brightness of NSOM aperture probes is a major constraint. Here, we report a novel NSOM aperture probe that gives a 100× higher throughput and 40× increased damage threshold than conventional near-field aperture probes. These brighter probes facilitate near-field imaging of single molecules with apertures as small as 45 nm in diameter. We achieve this improvement by nanostructuring the probe and by employing a novel variant of extraordinary optical transmission, relying solely on a single aperture and a coupled waveguide. Comprehensive electromagnetic simulations show good agreement with the measured transmission spectra. Due to their significantly increased throughput and damage threshold, these resonant configuration probes provide an important step forward for near-field applications.
TL;DR: In this paper, the focusing properties of FZP lens antennas in the near-field region are presented at the ka-band, where they show displacement of the maximum intensity of the electric field along the axial direction from the focal point toward the antenna aperture.
Abstract: Some focusing properties of Fresnel zone plate (FZP) lens antennas in the near-field region are presented at the ka-band. Simulated and measured results of the FZP antenna show displacement of the maximum intensity of the electric field along the axial direction from the focal point toward the antenna aperture. This displacement increases as the antenna's focal length increases. In addition, the focused beam scanning of the FZP lens antennas in the radiation near-field is examined.
TL;DR: In this article, the authors proposed a topology optimization strategy for the systematic design of a conductor-based sub-wavelength antenna, which is shown to be self-resonant, efficient and exhibit distorted omnidirectional, elliptically polarized far field radiation patterns.
Abstract: We propose a topology optimization strategy for the systematic design of a three-dimensional (3D), conductor-based sub-wavelength antenna. The post-processed finite-element (FE) models of the optimized structure are shown to be self-resonant, efficient and exhibit distorted omnidirectional, elliptically polarized far-field radiation patterns. The computed approximate Q value for this antenna is QZ(ω0)≈ 7.74 for ω0=2π × 350.8 MHz and it is 1.64 times larger than the theoretical lower bound value.
TL;DR: In this article, the rotational symmetry breaking of the scalar Greens function was studied from a source point of view, and a suitable mathematical machinery for dealing with the symmetry breaking procedure from the source point-of-view was developed in detail.
Abstract: We continue in this paper a comprehensive theory of antenna near fields inaugurated in Part I. The concept of near-field streamlines is introduced using the Weyl expansion in which the total field is decomposed into propagating and nonpropagating parts. This process involves a breaking of the rotational symmetry of the scalar Greens function that originally facilitated the derivation of the Weyl expansion. Such symmetry breaking is taken here to represent a key to understanding the structure of the near fields and how antennas work in general. A suitable mathematical machinery for dealing with the symmetry breaking procedure from the source point of view is developed in detail, and the final results are expressed in clear and compact form susceptible to direct interpretation. We then investigate the concept of energy in the near field where the localized energy (especially the radial localized energy) and the stored energy are singled out as the most important types of energy processes in the near-field zone of general antenna systems. A new devolvement is subsequently undertaken by generalizing the Weyl expansion in order to analyze the structure of the near field, but this time from the far-field point of view. A hybrid series combining the Weyl and Wilcox expansions is derived, after which only the radial streamline picture turns out to be compatible with the far-field description via Wilcox series. We end up with an explication of the general mechanism of far-field formation from the source point of view. It is found that the main formative processes in the antenna near-field zone are reducible to simple geometrical and filtering operations.
TL;DR: A numerical comparison of a regularized Newton-type method and a direct method for reconstructing the surface impedance function of a three dimensional acoustic scatterer with known shape from the full far field pattern for scattering of one incident time-harmonic plane wave are presented.
TL;DR: A photodetector platform is demonstrated that uses a silicon-on-insulator substrate to spectrally and spatially map the absolute values of enhanced fields near any type of optical antenna by transducing local electric fields into photocurrent.
Abstract: A silicon-on-insulator photodetector substrate can be used to produce absolute and quantitative maps of nanoscale optical antenna resonances in the near field.
TL;DR: In this paper, a near-field scanning thermal microscope is used to detect the local density of states of the thermally excited electromagnetic modes at nanometer distances from some material, which can be employed for nanoscale imaging of structures on that material's surface.
Abstract: We show that a near-field scanning thermal microscope, which essentially detects the local density of states of the thermally excited electromagnetic modes at nanometer distances from some material, can be employed for nanoscale imaging of structures on that material's surface. This finding is explained theoretically by an approach which treats the surface structure perturbatively.
TL;DR: In this paper, a subdiffractive, subwavelength focal spot of full width half maximum 222 nm at an operating wavelength of 633 nm was generated using an optical eigenmode approach.
Abstract: We report the focusing of light to generate a subdiffractive, subwavelength focal spot of full width half maximum 222 nm at an operating wavelength of 633 nm using an optical eigenmode approach. Crucially, the spot is created in the focal plane of a microscope objective thus yielding a practical working distance for applications. The optical eigenmode approach is implemented using an optimal superposition of Bessel beams on a spatial light modulator. The effects of partial coherence are also discussed. This far field method is a key advance toward the generation of subdiffractive optical features for imaging and lithographic purposes.
TL;DR: Theoretical expressions for the incident and scattered field from a rigid immovable sphere are derived in this paper, where the near and far-field acoustic scattering fields are expressed using partial wave series involving the spherical harmonics, the scattering coefficients of the sphere, the half-conical angle of the wave number components of the beam, its order and the beam shape coefficients.
TL;DR: It is shown that it is possible to create a superoscillatory diffraction free beam by superimposing already known diffraction-free solutions (Bessel beams for example) of Helmholtz equation.
Abstract: It is theoretically shown that by superimposing diffraction-free solutions of the Helmholtz equation, one can construct localized diffraction-free beams that pass through predetermined points on subwavelength distances. These beams are based on the phenomenon of superoscillations and thus do not contain any evanescent waves. The effect of an aperture and noise is examined in specific examples where truncated beams with λ/3 subwavelength features can propagate into the far field.
TL;DR: The results reveal the potential of wavelength-sized spheroids to manipulate light beyond the limitations of macroscopic geometrical optics, and can be of interest for several applications, such as light management in photovoltaics.
Abstract: This paper presents a numerical study of the light focusing properties of dielectric spheroids with sizes comparable to the illuminating wavelength. An analytical separation-of-variables method is used to determine the electric field distribution inside and in the near-field outside the particles. An optimization algorithm was implemented in the method to determine the particles’ physical parameters that maximize the forward scattered light in the near-field region. It is found that such scatterers can exhibit pronounced electric intensity enhancement (above 100 times the incident intensity) in their close vicinity, or along wide focal regions extending to 10 times the wavelength. The results reveal the potential of wavelength-sized spheroids to manipulate light beyond the limitations of macroscopic geometrical optics. This can be of interest for several applications, such as light management in photovoltaics.
TL;DR: In this article, a fast analytical solution for the radiation field of a microstrip antenna loaded with a generalized superstrate is proposed using the cavity model of microstrip antennas in conjunction with the reciprocity theorem and the transmission line analogy.
Abstract: A fast analytical solution for the radiation field of a microstrip antenna loaded with a generalized superstrate is proposed using the cavity model of microstrip antennas in conjunction with the reciprocity theorem and the transmission line analogy. The proposed analytical formulation for the antenna's far-field is much faster when compared to full-wave numerical methods. It only needs 2% of the time acquired by full-wave analysis. Therefore the proposed method can be used for design and optimization purposes. The method is verified using both numerical and experimental results. This verification is done for both conventional dielectric superstrates, and also for artificial superstrates. The analytical formulation introduced here can be extended for the case of a patch antenna embedded in a multilayered artificial dielectric structure. Arguably, the proposed analytical technique is applied for the first time for the case of a practical microstrip patch antenna working in the Universal Mobile Telecommunications System (UMTS) band and covered with a superstrate made of an artificial periodic metamaterial with dispersive permeability and permittivity.
TL;DR: Thermal radiation from samples of Au layers patterned on GaAs, SiO(2), and SiC at 300 K are studied with a scattering-type scanning near-field optical microscope, accounting for by theoretically expected surface evanescent waves, which are thermally excited in the close vicinity of material surfaces.
Abstract: Thermal radiation from samples of Au layers patterned on GaAs, SiO2, and SiC at 300 K are studied with a scattering-type scanning near-field optical microscope (wavelength: ~14.5 μm), without applying external illumination. Clear near-field images are obtained with a spatial resolution of ~60 nm. All the near field signals derived from different demodulation procedures decrease rapidly with increasing probe height h with characteristic decay lengths of 40 ~60 nm. Near-field images are free from any signature of in-plane spatial interference. The findings are accounted for by theoretically expected surface evanescent waves, which are thermally excited in the close vicinity of material surfaces. Outside the near-field zone (1 μm < h), signals reappear and vary as a sinusoidal function of h, exhibiting a standing wave-like interference pattern. These far-field signals are ascribed to the effect of weak ambient radiation.
TL;DR: In this paper, the authors focus on the physics of a medium with finite dimensions consisting of a square lattice of parallel conducting wires arranged on a sub-wavelength scale, and they show that such modes are dispersive due to the finiteness of the medium.
Abstract: This article is the first one in a series of two dealing with the concept of a ‘resonant metalens’ we introduced recently. Here, we focus on the physics of a medium with finite dimensions consisting of a square lattice of parallel conducting wires arranged on a sub-wavelength scale. This medium supports electromagnetic fields that vary much faster than the operating wavelength. We show that such modes are dispersive due to the finiteness of the medium. Their dispersion relation is established in a simple way, a link with designer plasmons is made, and the canalization phenomenon is reinterpreted in the light of our model. We explain how to take advantage of this dispersion in order to code sub-wavelength wavefields in time. Finally, we show that the resonant nature of the medium ensures an efficient coupling of these modes with free space propagating waves and, thanks to the Purcell effect, with a source placed in the near field of the medium.
TL;DR: Using terahertz near-field imaging, the interaction between split-ring resonators (SRRs) in metamaterial arrays is experimentally investigated and hybridization between plasmonic and lattice modes can be clearly identified in the experimentally obtained near- field maps.
Abstract: Using terahertz near-field imaging we experimentally investigate the interaction between split-ring resonators (SRRs) in metamaterial arrays. Depending on the inter-SRR spacing two regimes can be distinguished for which strong coupling between SRRs occurs. For dense arrays SRRs couple via their electric and magnetic near-fields. In this case distinct deformations of the SRRs’ characteristic near-field patterns are observed as a signature of their strong interaction. For larger separations with a periodicity matching the resonance wavelength, the SRRs become diffractively coupled via their radiated fields. In this regime hybridization between plasmonic and lattice modes can be clearly identified in the experimentally obtained near-field maps.
TL;DR: In this paper, an RFID-compatible focused circular phase-array antenna working at 5.8 GHz was theoretically analyzed and experimentally demonstrated, which consists of 24 half-wavelength dipole antennas placed on three circles.
Abstract: An RFID-compatible focused circular phase-array antenna working at 5.8 GHz is proposed, which is theoretically analyzed and experimentally demonstrated. It consists of 24 half-wavelength dipole antennas placed on three circles. Using only two phase-shifters, a tunable focal spot in the range of 0.4-1 m (10λ) is obtained. The antenna system is analyzed using the Huygens-Fresnel theory, which is compared to the experimental results. At a focal distance of 0.9 m, the measured beam width and focal depth are given by 0.75λ and 5λ, respectively, which are in agreement with the calculated results.