Showing papers on "Near and far field published in 1997"

Theory of Nanometric Optical Tweezers

Abstract: We propose a scheme for optical trapping and alignment of dielectric particles in aqueous environments at the nanometer scale. The scheme is based on the highly enhanced electric field close to a laser-illuminated metal tip and the strong mechanical forces and torque associated with these fields. We obtain a rigorous solution of Maxwell’s equations for the electromagnetic fields near the tip and calculate the trapping potentials for a dielectric particle beyond the Rayleigh approximation. The results indicate the feasibility of the scheme. [S0031-9007(97)03687-9] Optical trapping by highly focused laser beams has been extensively used for the manipulation of submicronsize particles and biological structures [1]. Conventional optical tweezers rely on the field gradients near the focus of a laser beam which give rise to a trapping force towards the focus. The trapping volume of these tweezers is diffraction limited. Near-field optical microscopy enables the optical measurements at dimensions beyond the diffraction limit and makes it possible to optically monitor dynamics of single biomolecules [2]. The potential application of optical near fields to manipulate atoms or nanoparticles has been discussed in Ref. [3]. In this Letter, we present a new methodology for calculating rigorously and self-consistently the trapping forces acting on a nanometric particle in the optical near field and propose a novel high-resolution trapping scheme. The proposed nanometric optical tweezers rely on the strongly enhanced electric field at a sharply pointed metal tip under laser illumination. The near field close to the tip mainly consists of evanescent components which decay rapidly with distance from the tip. The utilization of the metal tip for optical trapping offers the following advantages: (1) The highly confined evanescent fields significantly reduce the trapping volume; (2) the large field gradients result in a larger trapping force; and (3) the field enhancement allows the reduction of illumination power and radiation damages to the sample. High resolution surface modification based on the field enhancement at laser-illuminated metal tips has been recently demonstrated [4]. It is essential to perform a rigorous electromagnetic analysis to understand the underlying mechanism for the field enhancement. Our analysis is therefore relevant not only to optical tweezers, but also to other applications, such as surface modification, nonlinear spectroscopy and near-field optical imaging. To solve Maxwell’s equations in the specific geometry of the tip and its environment, we employ the multiple multipole method (MMP) which recently has been applied to various near-field optical problems [5]. In MMP, electromagnetic fields are represented by a series expansion of known analytical solutions of Maxwell’s equations. To determine the unknown coefficients in the series expansion, boundary conditions are imposed at discrete points on the interfaces between adjacent homogeneous domains. Once the resulting system of equations is solved and the coefficients are determined, the solution is represented by a self-consistent analytical expression. Figure 1 shows our three dimensional MMP simulation of the foremost part of a gold tip (5 nm tip radius) in water for two different monochromatic plane-wave excitations. The wavelength of the illuminating light is l › 810 nm (Ti:sapphire laser), which does not match the surface plasmon resonance. The dielectric constants of tip and water were taken to be « › 224.9 1 1.57i and « › 1.77, respectively [6]. In Fig. 1(a), a plane wave is incident from the bottom with the polarization perpendicular to the tip axis, whereas in Fig. 1( b) the tip is illuminated from the side with the polarization parallel to the tip axis. A striking difference is seen for the two different polarizations: in Fig. 1( b), the intensity

Uni-vector-sensor ESPRIT for multisource azimuth, elevation, and polarization estimation

Abstract: This paper introduces a novel eigenstructure-based algorithm uni-vector-sensor ESPRIT that yields closed-form direction-of-arrival (DOA) estimates and polarization estimates using one electromagnetic vector sensor. A vector sensor is composed of six spatially co-located nonisotropic polarization-sensitive antennas, measuring all six electromagnetic field components of the incident wave field. Uni-vector-sensor ESPRIT is based on a matrix-pencil pair of temporally displaced data sets collected from a single electromagnetic vector sensor. The closed-form parameter estimates are obtained through a vector cross-product operation on each decoupled signal-subspace eigenvector of the data correlation matrix. This method exploits the electromagnetic sources' polarization diversity in addition to their spatial diversity, requires no a priori knowledge of signal frequencies, suffers no frequency-DOA ambiguity, pairs automatically the x-axis direction cosines with the y-axis direction cosines, eliminates array interelement calibration, can resolve up to five completely polarized uncorrelated monochromatic sources from near field or far field. It impressively out-performs an array of spatially displaced identically polarized antennas of comparable array-manifold size and computational load.

Electromagnetic scattering by an aggregate of spheres: far field

Abstract: In electromagnetic multisphere-scattering calculations the reexpansion method for seeking a single-field representation of the total scattered field is found impracticable because of severe numerical problems. We present a simple single-field expansion of the total scattered far field based on an asymptotic form of vector translational addition theorems. With this asymptotic expansion of the far field, we derive analytical expressions for the scattering properties of an arbitrary aggregate of spheres. Resulting formulas are free from numerical problems in practical applications. Theoretical predictions from this far-field solution for various aggregates of spheres that we tested agree favorably with laboratory microwave scattering measurements. Some numerical results are presented and compared with experimental data.

An Introduction to Classical Electromagnetic Radiation

Abstract: 1. Basic theory of classical electromagnetism 2. Electromagnetic plane waves in free space - polarized waves 3. Inhomogeneous plane waves and the plane-wave spectrum 4. Electromagnetic analogues of some optical principles 5. Radiation from distributions of charge and current - general formulation 6. Electromagnetic field of a moving point charge 7. Dipole radiation 8. Radiation from thin-wire antennas Appendix A. Units and dimensions Appendix B. Review of vector analysis.

Light scattering by hexagonal ice crystals: solutions by a ray-by-ray integration algorithm

Abstract: A ray-by-ray integration (RBRI) method has been developed for the solution of light scattering by nonspherical dielectric particles. The principles of geometric optics are applied to solve the internal electric field within the scattering particles (near field) with the inclusion of complete phase and polarization configurations. The scattered field at the radiation zone (far field) and the extinction and absorption cross sections are obtained by integrating the near field along the propagation paths of geometric rays inside the scatterers by using a number of rigorous electromagnetic integral equations. In the computations of extinction cross section and single-scattering albedo, we demonstrate that the well-known anomalous diffraction approximation is a special case of the RBRI method when the scatterers are optically tenuous. The RBRI method is employed to compute the single-scattering properties of hexagonal ice crystals at visible and near-infrared wavelengths. Based on the reference results computed by the finite-difference time domain (FDTD) technique, we show that the RBRI method is more accurate than the conventional geometric ray-tracing technique and the anomalous diffraction approximation. The extinction efficiency and the single-scattering albedo computed by the RBRI method converge to the reference results when the size parameters along the ice crystal maximum dimension are larger than approximately 15. Substantial differences in terms of relative errors, in comparison with the FDTD solutions, are still noted in the phase function and polarization patterns computed by the RBRI method for size parameters of the order of 10.

Contrast of microwave near-field microscopy

Abstract: Constant-height scanning is demonstrated to improve near-field microscopy by eliminating artifacts connected with topography scanning, hence, to image the inherent electromagnetic contrast. Microwaves are chosen for this study because the long wavelength eliminates coherence artifacts, owing to a scale separation of wave and image frequencies. Measured amplitude and phase images of conductive films are quantitatively analyzed by considering the longitudinal electric near field. The observed spatial resolution of 200 nm equals the probing tip size and proves that the skin depth δ of the tip material (here, 1600 nm) presents no resolution limit to scanning optical microscopy.

Modeling near-field GPR in three dimensions using the FDTD method

Abstract: Complexities associated with the theoretical solution of the near‐field interaction between the fields radiated from dipole antennas placed near a dielectric half‐space and electrical inhomogeneities within the dielectric can be overcome by using numerical techniques. The finite‐difference time‐domain (FDTD) technique implements finite‐difference approximations of Maxwell's equations in a discretized volume that permit accurate computation of the radiated field from a transmitting antenna, propagation through the air‐earth interface, scattering by subsurface targets and reception of the scattered fields by a receiving antenna. In this paper, we demonstrate the implementation of the FDTD technique for accurately modeling near‐field time‐domain ground‐penetrating radar (GPR). This is accomplished by incorporating many of the important GPR parameters directly into the FDTD model. These variables include: the shape of the GPR antenna, feed cables with a fixed characteristic impedance attached to the terminals...

A phase-locked shear-force microscope for distance regulation in near-field optical microscopy

Abstract: A nonoptical phase-locked shear-force microscope utilizing a quartz crystal tuning fork acting as a voltage-controlled oscillator in a phase-locked loop has been implemented. A tapered optical fiber is rigidly mounted on one of the prongs of the fork to serve as both a shear-force pickup and a near-field optical probe. The crystal is driven at its resonance frequency through positive feedback of the monitored current through the crystal. This signal is used as the voltage-controlled oscillator in a phase-locked loop. The scheme allows for scan speeds far beyond the Q-limited amplitude sensor bandwidth and exhibits excellent sensitivity for a high-Q resonator. Furthermore, given the small vibration amplitude of the tip ( 6 nm), it is unlikely that the tip is making direct contact with the sample surface as has been suggested for the optical shear-force detection scheme.

Combined topography and electromagnetic field scanning probe microscope

Abstract: A combined near-zone electromagnetic field and topography probe based on a scanning-probe microscope (SPM). One or a plurality of sub-wavelength electromagnetic antennas and waveguides are integrated with tip-cantilever assemblies commonly used in scanning probe microscopy, combining the contact- or non-contact-mode tip-sample distance control and topography-sensing capacity of the SPM with the ability to measure or excite local electromagnetic fields on the sample. A wide range of topography-sensing forces acting on the tip-cantilever assembly can be employed, such as capillary, van der Waals, electrostatic and magnetic. Simultaneous measurement and/or excitation of electromagnetic fields in the frequency range from near zero to tens of THz can be performed, limited only by the design and ability of the antenna and waveguide material(s) to localize and guide the electromagnetic energy, and by suitable means for energy detection and/or generation connected to the waveguide. In one embodiment, the topography probe tip also acts as the center conductor of a tapered coaxial tip which is connected to a shielded transmission line waveguide running the length of the cantilever to a coaxial cable feeding an oscilloscope. In another embodiment, the coaxial tip is replaced by a magnetic-field loop antenna or electric-field gap antenna.

Propagation of half-cycle far infrared pulses

Abstract: We have studied the propagation of half-cycle (i.e., unipolar) electromagnetic pulses centered at terahertz frequencies, in free space, through apertures, and through focusing optics. The temporal pulse shape of an apertured half-cycle pulse is significantly altered during propagation, but retains much of its unipolar character after traveling more than 20 times the aperture dimension. When focused by an achromatic lens, a half-cycle pulse evolves into a single-cycle pulse at the focal waist and an inverted half-cycle pulse in the far field.

Reflection-mode scanning near-field optical microscopy using an apertureless metallic tip.

Abstract: Recently, a reflection-mode near-field optical microscope with an apertureless tungsten tip has been introduced and 100-nm resolution has been achieved [ R. Bachelot P. Gleyzes A. C. Boccara , Microsc. Microanal. Microstruc.5, 389–397 (1994)]. The optical signal is recorded in parallel with a tapping-mode atomic force microscope signal. By showing several images here, we confirm the capabilities of this device and clearly demonstrate a 20-nm (∼λ/35) resolution that has been achieved with smaller tips. A study of these images shows that both the topography and the near electromagnetic field of the sample can be independently probed by this device. Additionally, we discuss the principle of our approach, notably on the basis of interference phenomena between a Rayleigh scatterer and its image through the reflecting surface, and some of the setup’s experimental characteristics are presented.

Coherent radar imaging: Signal processing and statistical properties

Abstract: The recently developed technique for imaging radar scattering irregularities has opened a great scientific potential for ionospheric and atmospheric coherent radars. These images are obtained by processing the diffraction pattern of the backscattered electromagnetic field at a finite number of sampling points on the ground. In this paper, we review the mathematical relationship between the statistical covariance of these samples, ( †), and that of the radiating object field to be imaged, ( †), in a self-contained and comprehensive way. It is shown that these matrices are related in a linear way by ( †) = aM(FF†)M†a*, where M is a discrete Fourier transform operator and a is a matrix operator representing the discrete and limited sampling of the field. The image, or brightness distribution, is the diagonal of (FF†). The equation can be linearly inverted only in special cases. In most cases, inversion algorithms which make use of a priori information or maximum entropy constraints must be used. A naive (biased) “image” can be estimated in a manner analogous to an optical camera by simply applying an inverse DFT operator to the sampled field and evaluating the average power of the elements of the resulting vector . Such a transformation can be obtained either digitally or in an analog way. For the latter we can use a Butler matrix consisting of properly interconnected transmission lines. The case of radar targets in the near field is included as a new contribution. This case involves an additional matrix operator b, which is an analog of an optical lens used to compensate for the curvature of the phase fronts of the backscattered field. This “focusing” can be done after the statistics have been obtained. The formalism is derived for brightness distributions representing total powers. However, the derived expressions have been extended to include “color” images for each of the frequency components of the sampled time series. The frequency filtering is achieved by estimating spectra and cross spectra of the sample time series, in lieu of the power and cross correlations used in the derivation.

Near-field optical thermometry

Abstract: Far-field optical thermometry techniques have spatial resolution limited by diffraction to the order of the radiation wavelength. We report progress on near-field optical thermometry (NFOT) that targets spatial resolution better than 50 nm. A tapered, single-mode optical fiber scans nanometers above electronic microstructures, which are heated using transient electrical currents. The fiber tip releases about 1 nW of radiation power from a steady probe laser, and the reflected radiation is used to measure the local temperature. Simultaneous electrical resistance thermometry is used to estimate the relative importance of temperature dependent optical properties of the sample and thermal expansion of the sample and tip. This work provides guidance for implementing other NFOT techniques using radiation transmission and infrared emission.

Near-field second-harmonic imaging of ferromagnetic and ferroelectric materials.

Abstract: A novel scanning probe technique that is able to image surface magnetic and electric properties has been developed. It is based on the near-field microscopy of surface second-harmonic generation. We have demonstrated the capability of the technique by imaging the domain structure of a Ni single crystal and a piezoelectric ceramic.

Plane-wave scattering by a dielectric circular cylinder parallel to a general reflecting flat surface

Abstract: We present a generalization of a method developed for treating the plane-wave scattering by a perfectly conducting circular cylinder in front of a plane surface to the case of a generic dielectric circular cylinder. Thanks to this formulation, the problem can be treated in a very efficient way for both the near and the far field, and an accurate determination of the field inside the cylinder is possible. Numerical results and comparisons with other methods are presented.

Theoretical treatment for scattering scanning near-field optical microscopy

Abstract: A scanning probe interacts with the optical near field near a sample surface. The whole system radiates, and the radiation is collected in the far-field domain. The collected far field contains subwavelength information about the sample surface. A microscopic description of the device is presented. A self-consistent procedure is employed that takes into account the optical coupling effects between the microscopic objects in the probe–surface system to calculate the induced field at the probe and at the surface microscopic features. The radiation stemming from the system can then be described in the half-space. The intensity of the field radiation at the entrance of the far-field detector determines the received signals. Two incident fields, total internal reflection and external reflection, are examined numerically. Subwavelength resolutions of the device are clearly shown. The results are used to discuss other relevant aspects: the resolution of the device, the position of the far-field detector, and the probe material.

Localized picosecond resolution with a near-field microwave/scanning-force microscope

Abstract: We modify a scanning-force microscope tip/cantilever with a coaxial metal shield for use as an ultra-small field probe, observing 30 ps waveforms on an integrated circuit at a spatial resolution set by the tip radius, the first direct measurements of electric field at these levels of temporal and spatial resolution. This new ability to acquire topography while maintaining constant tip–sample distance for calibrated measurement or excitation of near-zone picosecond time-resolved electric fields is useful not only for probing advanced circuits but also for localized broadband spectroscopy of condensed matter and biological samples.

Far-field optical degradation due to near-field transmission through a turbulent heated jet.

Abstract: When a laser beam traverses an optically active, turbulent flow field, the laser wave front is aberrated by the flow. Density variations in a heated two-dimensional jet, for example, correspond to index-of-refraction variations, and this modulation of the index in the fluid can imprint an optical phase disturbance, or phase error, onto the laser wave front. Adaptive-optic systems seek to correct the phase error of the wave front, and thus restore the integrity of the far-field irradiance pattern. Given a near-field spatial mapping of a phase disturbance, the far-field irradiance pattern of the affected wave front can be calculated with Fourier-optics techniques. A Fourier-optics computer code was used to study the far-field irradiance patterns arising from actual time-varying measurements of a fluid-induced phase error. The time-averaged Strehl ratio was studied to provide insight into the spatial and temporal design requirements for adaptive-optic systems applied to the time series of near-field spatial phase-error maps.

Near field terahertz imaging

Abstract: A THz imaging system with the emitter region of the THz generator designed so that the sample to be analyzed is placed in the near field of the generator allows radiation to impact the sample without intervening optics.

A probe for making near-field measurements with minimal disturbance: the optically modulated scatterer

Abstract: We describe an optically modulated scatterer as an electric-field probe for measuring radio-frequency and microwave fields. It has a high spatial resolution and the ability to operate very close to conducting and dielectric objects without appreciable distortion of the field to be measured. Thus, it can scan close to antennas and diffracting metal structures. We describe how the electric field is deduced from the measurements and present gain measurements and far-field patterns deduced from near-field scans of antennas. The results are tested by comparing them with those obtained by established measurement techniques.

Limited diffraction array beams

Abstract: Limited diffraction beams have a large depth of field infinite aperture. Because of their high side lobes and complex and could have many applications. In this article, new limited diffrac- beam patterns near the cone surface, these devices are not cur- tion beams are developed. They are composed of multiple parallel rently used in clinics. Ring antenna (42) or transducer (43,44) is beams and are thus called array beams. A broadband synthetic array another type of device that increases the depth of field. However, experiment is used to produce these beams of a finite aperture, and the depth of field is increased only in the far field of the rings results are in excellent agreement with theory over a large depth of (45,46) where beams diffract significantly. Near the surfaces of field. In addition, potential applications of the new beams to real-time the rings, beams do not even have a central peak. In addition, volumetric imaging and measurement of transverse velocity of blood ring devices have a low energy efficiency because they must be flow are described. q 1997 John Wiley & Sons, Inc. Int J Imaging Syst thin. Dynamic focusing (47) is another method to increase the Technol, 8, 126-136, 1997 depth of field, in which the focal length of a receiver increases

Near-diffraction-limited laser beam shaping with diamond-turned aspheric optics.

Abstract: We used a pair of diamond-turned CaF2 aspheres to convert the pure TEM00 Gaussian spatial profile output of a diode-pumped Nd:YAG laser oscillator into a super-Gaussian intensity profile with a nearly flat phase front. The resulting super-Gaussian beam was nearly diffraction limited with an M2 of 1.75; in the near field the 5-mm diameter beam retained a nominally flat-top intensity distribution without significant diffraction peaks for an excellent working distance of more than 50 cm. A 10% improvement in amplifier-energy extraction obtained by use of the reshaped beam is demonstrated.

A new algorithm in electromagnetic inverse scattering theory with an application to medical imaging

Abstract: We consider the electromagnetic inverse scattering problem of determining the index of refraction of an isotropic inhomogeneous medium from a knowledge of the total electric near field data. We consider both the case of scattering by an infinite inhomogeneous cylinder and a bounded inhomogeneous region in space. We conclude by giving an application of our method to the problem of imaging leukemia in the upper leg by means of microwaves.

Optical magnetic near-field intensities around nanometer-scale surface structures

Abstract: Recently, local probes used in optical experiments added a new dimension to the study of the optical properties of small particles lying on a surface. Until now, several theoretical frameworks, developed to understand the interaction of optical fields with mesoscopic and nanoscopic objects, emphasized mainly the prediction of the electric near-field distributions generated by these structures. This paper demonstrates how such subwavelength dielectric surface structures also produce a particular confinement of the optical magnetic near field when the sample is illuminated by a surface wave.

High-power ultrawideband electromagnetic pulse radiation

Abstract: Basing on energetic processes studying in the near-field radiator zone, a new concept of antenna synthesizing for ultrawideband electromagnetic pulse radiation has been suggested. The results of experimental investigations of the antennae developed with using of this concept for high-power applications are presented. The antennae have small dimensions, high electrical strength, cardioid pattern with linear polarization of the pulse radiated and they are ideally adapted to be used as a steering antenna array element. A high-voltage nanosecond bipolar pulse generator design to excite antennae is described.

Polarization quadrature measurement of subwavelength diffracting structures

Abstract: The amplitude and the phase of the diffracted far field depends on polarization when the diffracting structure is comparable to or less than the wavelength. When the far-field amplitude and the phase of one polarization with respect to the orthogonal polarization is measured, small changes in the structure can be measured. To make the far-field polarization measurements, we design a detector that measures the relative polarization amplitude and the phase in quadrature. We predict numerically and verify experimentally the polarization amplitude and the phase for an optical disc and a set of gratings with varying depth. Our results show that this measurement technique is sensitive to small variations in the diffracting structure and that it can be useful in applications such as critical dimension and overlay metrology in microelectronics fabrication.

Numerical computation of fat layer effects on microwave near-field radiation to the abdomen of a full-scale human body model

Abstract: Numerical computation results of fat layer effects on the microwave near field radiation to the abdomen of a three-dimensional (3-D) full-scale human body model are presented. The human body is modeled as a 3-D homogeneous muscle phantom with a fat layer covering the abdomen part. The dipole wire-antenna located proximate to the abdomen is used as the microwave radiation source at 915 MHz. This is to study the effects on hyperthermia heating by using the microwave applicator (at 915 MHz) or the near-field exposure from the proximate handset antenna to the human body at ISM band wireless communication band (902-928 MHz). Coupled integral equations (CIE) and the method of moments (MoM) are employed to numerically compute electromagnetic (EM) energy deposition specific absorption rate (SAR) from the radio frequency (RF) antenna applicator into the proximate fat layer covered abdomen. The antenna input impedance (proximate to the body), return loss (RL), and the resonant antenna length (proximate to the body) will also be numerically determined to increase the microwave power delivered into the body. The study of fat layer effects is important for microwave hyperthermia applications. It is also important for the investigation of the potential health hazard from the near-field radiation of a wireless communication antenna.

Beam Recombination Characteristics in Array Laser Amplification Using Stimulated Brillouin Scattering Phase Conjugation

Abstract: Beam recombination characteristics were numerically investigated in array laser amplification using stimulated Brillouin scattering phase conjugation. To clarify the effect of piston errors due to imperfect phase locking, spatial intensity profiles of the beam recombination output were calculated in both the near and the far field on the basis of Rayleigh-Sommerfeld diffraction theory. The analyses indicate that piston errors are seriously detrimental to the quality of a beam recombination output and should be eliminated by a proper phase locking. It is also found that the gap between the beam splitting-combining wedges has a negligible effect.

Enhanced compressibility tomography

Abstract: We are interested here in the inversion of data leading to high quality imaging of physical parameters. We deal with ultrasonic diffraction tomography and first show how far field diffraction measurements of an incident plane wave give access to the spatial Fourier transform of a composite object. In the case of an acoustic model (soft tissues), this object is characterized by two parameters, e.g., the compressibility and density, each being affected by its own point spread function. The development of quantitative imaging proceeds from the separation of each parameter contribution. This can be done by measuring the scattered field over an arc for several transmitter positions around the object. This allows us, under specified conditions, to reconstruct either the compressibility, or the velocity, or the impedance maps. We have focused on compressibility imaging for which we propose a novel algorithm based on a redundant reconstruction procedure. We present tomograms of biological phantoms obtained with our experimental set-up.