Field In Free Space Research Articles

Wigner functions for nonparaxial, arbitrarily polarized electromagnetic wave fields in free space

New representations are defined for describing electromagnetic wave fields in free space exactly in terms of rays for any wavelength, level of coherence or polarization, and numerical aperture, as long as there are no evanescent components. These representations correspond to tensors assigned to each ray such that the electric and magnetic energy densities, the Poynting vector, and the polarization properties of the field correspond to simple integrals involving these tensors for the rays that go through the specified point. For partially coherent fields, the ray-based approach provided by the new representations can reduce dramatically the computation times for the physical properties mentioned earlier.

Coupled dipole method for scatterers with large permittivity

In the coupled dipole method, a three-dimensional scattering object is discretized over a lattice into a set of polarizable units that are coupled self-consistently. Starting from the volume integral equation for the field, we show that performing the integration of the free-space field susceptibility tensor over the lattice cell dramatically improves the accuracy of the method when the permittivity of the object is large. This integration, done without any approximation, allows us to define a prescription for the polarizability used in the coupled dipole method. Our derivation is not restricted to any particular shape of the scatterer or to a cubic discretization lattice.

The role of quantum jumps in the squeezing of resonance fluorescence from short-lived and long-lived atoms

Phase-sensitive squeezing in the resonance fluorescence of two-level atoms, that are coherently driven by a near-resonant laser field in free space, was observed recently (Lu et al 1998 Phys. Rev .L ett. 81 3635). This was accomplished via homodyne detection at a phase near ±45 ◦ relative to the driving field for strong off-resonant excitation of ‘long-lived’ atoms (where th ea tomic lifetime far exceeded the laser–atom interaction time, meaning that relaxation effects could be ignored). On the other hand, traditional theoretical predictions of phase-sensitive squeezing in the resonance fluorescence from two-level atoms have emphasized in- and out-of-phase (i.e., 0 ◦ and 90 ◦ ) quadratures, and weak, on-resonant excitation of ‘short-lived’ atoms (where the observation time for laser–atom interaction fa re xceeded the natural atomic lifetime, meaning that relaxation effects dominate). Here, we calculate the probability of a delayed-coincidence detection in the interference field of a fluorescing dipole with a local oscillator (LO). We show that, despite the strikingly different conditions in which squeezing occurs in short- and long-lived atoms, squeezing in both cases can be shown to arise from a joint detection of two photons which are related by a quantum jump in the following way: the first photodetection precipitates a quantum jump of the atom to the ground state, and the second measures the mean amplitude of the fluorescent field subsequent to the quantum jump.

Quantization of Gauss–Hermite and Gauss–Laguerre beams in free space

Starting from the continuous quantization of the transverse electromagnetic field in free space, we construct wavepackets from their Fourier components and form boson annihilation and creation operators for them. Then using a derived equivalence property, we express the ten conservation quantities from the Poincare invariance of space–time which are the energy, momentum, angular momentum and boost by the introduced boson operators and by the integrals over the Fourier components. After deriving the equations of the paraxial approximation with transverse and longitudinal beam shapes, we specialize this to Gauss–Hermite and Gauss–Laguerre beams where we apply the Hermite 2D and Laguerre 2D polynomials and functions. This provides a class of nonstationary beam solutions which includes the continuous transition between Gauss–Hermite and Gauss–Laguerre beams and which we discuss in detail. The quantized form in the Heisenberg representation which needs longitudinal shape propagation with group velocity is obtained by comparing the general quantum-mechanical formulae for the energy and momentum of wavepackets with the special formulae for the beam solutions considered with coefficients to be determined. The calculation of the Fourier spectrum of the quantized Gauss–Hermite and Gauss–Laguerre beam solutions leads back to the starting point of the quantization.

GTEM cell facility use during project development phases for automotive

The purpose of this paper is to present the results obtained with a gigahertz transverse electromagnetic mode (GTEM) cell for each phase of the project development, which consists of measuring and quantifying the integrated circuit's emissions and immunity compliance with automotive standards. These standards are defined using half anechoic chambers (HAC). Studies have already been conducted to compare trans electromagnetic (TEM) cell and HAC, or far-field emissions, and the test results show a limited correlation between TEM cells and the free space field or HAC. In this paper, it is shown that the correlation between the measurements made in a GTEM cell and HAC can be good enough in allowing us to use the GTEM facility during the research and development (R&D) phase (considering the facts that the HAC facility is often quite overloaded and of a high cost), that knowledge of the electromagnetic compatibility behavior of the integrated circuit is important for predicting the whole system's results as well as how to use an integrated circuit electromagnetic model to make the prediction of emissions.

Description of bipolar charge transport in polyethylene using a fluid model with a constant mobility: model prediction

We present a conduction model aimed at describing bipolar transport and space charge phenomena in low density polyethylene under dc stress. In the first part we recall the basic requirements for the description of charge transport and charge storage in disordered media with emphasis on the case of polyethylene. A quick review of available conduction models is presented and our approach is compared with these models. Then, the bases of the model are described and related assumptions are discussed. Finally, results on external current, trapped and free space charge distributions, field distribution and recombination rate are presented and discussed, considering a constant dc voltage, a step-increase of the voltage, and a polarization–depolarization protocol for the applied voltage. It is shown that the model is able to describe the general features reported for external current, electroluminescence and charge distribution in polyethylene.

Comment on “Some implications of the quantum nature of laser fields for quantum computations”

A recent discussion of quantum limitations to the fidelity with which superpositions of internal atomic energy levels can be generated by an applied, quantized, laser pulse is shown to be based on unrealistic physical assumptions. This discussion assumed the validity of Jaynes-Cummings dynamics for an atom interacting with a laser field in free space, that is, when the atom is not surrounded by a resonant cavity. If the laser field is a multimode quantum coherent state, and the Rabi frequency is much greater than the spontaneous decay rate, then the total atomic decoherence rate is on the order of the spontaneous decay rate. With the use of a unitary transformation of the field states due to Mollow, it can be shown that the atomic decoherence rate is the same as if the laser field were treated classically, without any additional contribution due to the quantum nature of the laser field.

The influence of the measurement probe on the evaluation of electromagnetic fields

In this paper, the influence of the measurement probe on the evaluation of the far and near field of an electromagnetic source is characterized. While measuring, the electromagnetic field will be disturbed by the measurement probes themselves. Therefore, not the true, free-space field but the disturbed field will be measured. The disturbance can not be fully taken into account by the calibration, especially in the near field. It is found that in the near field, these disturbances are much higher than in the far field for a large sensitive measurement probe. This paper will show that when the disturbance must be limited to a predefined value (e.g., 5%), a suitable measurement probe with maximum sensitivity can be selected. The characterization was performed using a numerical electromagnetic computational program.

A new method for three-dimensional numerical simulation of electromagnetic and fluid-flow phenomena in electromagnetic separation of inclusions from liquid metal

The magnetohydrodynamic (MHD) flow around a suspended particle in a liquid metal subjected to electric and magnetic fields can affect the force exerted by the applied electromagnetic field on the particle. In this article, a novel approach to the computational simulation of three-dimensional nonlinear MHD flow in two-phase systems is proposed. The electromagnetic field in the conducting fluid, including the particle, is represented using the current-vector potential (T) and reduced magnetic scalar potential (Ψ) to avoid the discontinuity of the electric field at the fluid-particle interface. To avoid the solution of the electromagnetic field in free space and to account exactly for the electromagnetic field interactions with the fluid and the particle, the electric and magnetic fields are specified at the boundary of the fluid-flow domain using Ampere’s law. This formulation permits the numerical solution of the coupled electromagnetic and fluid-flow equations on a common mesh. The discretized equations are derived using a finite-element formulation, and an iterative procedure is described for the efficient solution of these equations. This method is used to investigate the electromagnetic and fluid-flow phenomena in electromagnetic separation of a nonconducting spherical particle in crossed uniform electric and magnetic fields at intermediate Hartmann numbers. The computed results show that the magnetic field has no effect on either the velocity field or the net force on the particle when the Hartmann number is less than 1. Beyond this threshold value of the Hartmann number, the velocity decreases almost linearly with increasing magnetic-field strength. The damping of the flow by the magnetic field manifests itself in a reduction of the separation force, even though it is relatively small for this system.

Turbulence induced beam spreading of higher order mode optical waves

It is well known that laser beams spread as they propagate through free space due to natural diffraction, and that there is additional spreading when optical waves propagate through atmospheric turbulence. Previous studies on Gaussian beams have mainly involved the lowest order mode (zero order). The study of higher order mode Gaussian beams has involved Hermite-Gaussian and Laguerre-Gaussian beams for rectangular and cylindrical geometry, respectively. These studies have developed expressions for the field and intensity in free space, in addition to developing new definitions of beam size in the receiver plane for the higher order modes. We calculate the mean intensity of higher order mode Gaussian beams propagating through atmospheric turbulence, and, based on previously developed definitions for beam radius, we calculate the additional beam spreading due to random media. It is shown that higher order mode Gaussian beams experience less percentage of additional broadening due to atmospheric fluctuations than the zero-order mode beams.

Measurement of object vibrations using the theory of speckle pattern decorrelation

This paper deals with noncontact measurement of object vibrations using the theory of speckle decorrelation based on the statistical properties of a speckle pattern propagating through free-space and image field. It concentrates on the object vibrations in the direction of the normal to the object surface plane. It follows a paper published in this journal, which presented the theory and experimental results of measurement of an object in-plane and normal rotations by means of the theory of speckle pattern decorrelation. First the basic relations for free-space and image field are introduced, next an experimental arrangement for the measurement of the normal vibrations of an object with a rough surface is designed. Finally, experimental results for some harmonic and anharmonic object vibrations are presented.

Toward a semiconductor-based terahertz nonlinear medium

The recent growth of interest in the terahertz (THz) region of the spectrum has spurred the development of new THz elements in analogy to elements used in optical and microwave systems. Nonlinear as well as linear elements are needed. In this study we explore theoretically the use of valence-subband nonparabolicity in strained p-type quantum wells (QWs) for third-harmonic generation. All fields—the fundamental as well as the harmonic—are polarized in the quantum-well plane, thus facilitating integration with lateral high-frequency electronic devices as well as for coupling to free-space fields. For strained InAs quantum wells, we find that the corresponding value of χ (3) can be as large as ∼10 −12 ( m/ V) 2 . The predicted values of χ (3), though not extremely large, are nevertheless in the range of those associated with intersubband transitions in the mid-infrared region of the spectrum.

A micromachined photoconductive near-field probe for picosecond pulse propagation measurement on coplanar transmission lines

For the first time, a micromachined near-field probe has been developed, based on picosecond photoconductive sampling using low-temperature-grown GaAs (LT-GaAs). By changing the direction of the probe, it is capable of scanning/mapping independent orthogonal components of free-space electric fields. The micromachined 1-/spl mu/m-thick LT-GaAs epilayer substrate of the probe provides a minimal loading effect for measuring picosecond electric fields. The use of optical fibers, for guiding laser pulses to the photoconductive switch on the probe and for electrical connections, enables the probe to be positioned freely with uniform sensitivity. We have demonstrated by finite-difference time-domain simulation that the measurement process of the developed probe is dominated by the switching action of the carriers generated in the photoconductive switch of the probe. Using the probe, propagating picosecond pulse waveforms and field distribution images on coplanar transmission lines were successfully obtained. The probe measurement provides a useful understanding, which cannot be obtained from conventional external-port access test instruments, of electromagnetic phenomena related to wave propagation.

Radiometry and wide-angle wave fields II Coherent fields in three dimensions

The wave-based generalized radiance definitions presented in a previous manuscript [J. Opt. Soc. A18, 902 (2001)] for two-dimensional coherent monochromatic fields in free space are extended here to the three-dimensional case. These new definitions preserve all the properties of their two-dimensional analogs. Notably, they are exactly conserved along rays and well suited for the description of fields traveling in all directions. The different members of this set of functions are seen to correspond to weighted radial projections in momentum of the Wigner function of the field.

Pulse-coupling measurement of coupled microstrip lines using a micromachined picosecond optical near-field probe

By measuring the transverse electric-field distributions using a newly developed micromachined optical near-field mapping probe, pulse-coupling phenomena on coupled microstrip lines are reported for the first time. The measured field distribution of the propagating coupled pulse provides useful information to aid understanding of the coupling phenomena; this cannot be obtained by conventional external-port access test instruments. The measurement is performed based on the picosecond photoconductivity of low-temperature-grown GaAs (LT-GaAs). A system for the measurement of the internal electric field distribution using the optical near-field probe is described and characterized. It is capable of measuring independent orthogonal components of free-space electric fields with less than 2-ps temporal resolution and with minimal loading effects. The loading effects of the probe are minimized by adopting a micromachining technique for the use of a 1-/spl mu/m-thick LT-GaAs epilayer as a substrate, and by using silver-paint-coated optical fibers for electrical connections.

Electromagnetic wave generation by an electron layer rotating in free space

The generation of axially nonuniform electromagnetic waves by electrons rotating in crossed external fields in free space is studied theoretically. It is demonstrated that the wave increment increases with frequency under the Cherenkov resonance conditions, provided the radial electrostatic field is sufficiently strong. It is inferred that the generated frequency can be significantly increased by applying a strong radial electrostatic field.

Generation and amplification of electromagnetic waves by an annular electron beam in a radial electric field in free space

Mechanisms for the generation and amplification of electromagnetic waves by a thin-walled annular beam of electrons rotating in a radial electric field in free space are studied theoretically. It is shown that electromagnetic waves can be generated and amplified under the Cherenkov resonance conditions. The frequencies and growth rates of the generated waves are determined, and the propagation characteristics and amplification coefficients of the amplified waves are found.

Electro-optic field mapping system utilizing external gallium arsenide probes

External electro-optic probes fabricated from two different crystal orientations of GaAs have been implemented in an electro-optic sampling system that is capable of mapping three independent orthogonal components of free-space electric fields. The results obtained for the radiated field from a microstrip patch antenna by the GaAs probes are compared with results on the same antenna obtained using bismuth silicate and lithium tantalate probes. An 8 μm spatial resolution has also been demonstrated for the electro-optic field-mapping system, and the capability for the system to measure field patterns at frequencies up to 100 GHz has been shown.

Contribution of evanescent waves to the far field: the atomic point of view

Evanescent modes of the electromagnetic field are seldom invoked in conventional far-field optics, as their contribution far from the source (a few wavelengths) is negligible. Contradicting this fact, in recent theoretical works, based on a particular decomposition of the free-space Green tensor, it has been asserted that evanescent waves do indeed contribute to the far field, where they appear as an additional ~1/r component of the field. We provide an explicit demonstration that evanescent modes do not contribute to the power radiated to the far field by any dipolar source. First we derive an expression for the free-space field susceptibility in which contributions from evanescent and homogeneous modes are separated, and then we use linear response theory to compute the decay rate for an atomic dipole in vacuum.

Evaluation of electromagnetic interference from a cellular telephone with a hearing aid

In a collaborative effort, electromagnetic interference (EMI) is evaluated from a global system for mobile communication telephone with one model of a hearing aid used in the ear canal. Since the electromagnetic fields cannot be measured in the ear canal, a reliable method of their modeling with the finite-difference time-domain method is established. Very good agreement has been achieved between the measured and computed electric and magnetic fields in free space in very close proximity to the telephone. Subsequently, electric and magnetic fields in the ear canal are computed for two models of the ear, and three positions of the telephone. The computed fields are compared with the acoustic measurements for a small number of humans subjected to the EMI test.