Pulsed Electromagnetic Wave Research Articles

Reduced order computational methods for electromagnetic material interrogation using pulsed signals and conductive reflecting interfaces

We consider the interrogation by means of a pulsed planar electromagnetic wave of a dielectric slab with a supraconductive backing. Previous work using a weak formulation with finite elements (FE) demonstrated the ability to determine material parameters and the slab thickness in the inverse problem. In this work we report on results using Proper Orthogonal Decomposition (POD) to create a more efficient set of basis functions than the standard FE basis functions. We first demonstrate the ability of the reduced basis POD formulation to capture the electromagnetic behavior in the case of the forward problem. We then apply the POD formulation to the inverse scattering problem with unknown parameters and show that the POD formulation provides a considerable reduction in computational time over standard FE methods with comparable ability to recover the unknown parameter values.

Electromagnetic waves focused by a negative-index planar lens

We demonstrate that negative refraction occurs for both cw and pulsed electromagnetic waves when traversing from a "right-handed" (index > 0) to a "left-handed" (index < 0) material (LHM) which has causal dispersive intrinsic properties. We also demonstrate that a divergent line source spaced a distance H in front of a planar LHM slab and excited by either an impulse cw or a Gaussian frequency pulse is imaged at a distance H away, inside the LHM, and at H to the other side of the slab. The image size is approximately lambda consistent with limitations dictated by wave optics. We find no evidence of evanescent mode amplification. The studies were performed using numerical experiments with finite difference time domain solutions and incorporating a causal Lorentzian form for the frequency-dependent material properties.

On compression of electromagnetic pulses in dielectric plates

The results of theoretical investigation of propagation of electromagnetic wave pulses in nonlinear dielectric plates, taking into account the dispersion introduced by the plate to the maximum possible extent, are presented. The investigation is aimed at obtaining the shortest possible pulses of electromagnetic waves. Pulse compression is based on the collapse of electromagnetic waves in transparent dielectrics with electronic non-linearity. The integral equation for describing the pulse compression is derived, and conditions under which a pulse is compressed relative to time and one of the transverse coordinates are determined. Numerical simulation shows that quite short (on the order of a few femtoseconds) pulses can be obtained in this way.

Current and Future Status of Pulse Elecromagnetic Wave Generation Using Plasma

Novel frequency upshift phenomena, a flash ionization and a photon acceleration process, are reviewed. The flash ionization is the frequency upshift that the quick creation of the plasma around a electromagnetic wave, on time scales on the order of a cycle of the wave. The photon acceleration is that the frequency of the electromagnetic wave propagating with the plasma density perturbation is upshifted because of the difference of the phase velocity. Furthermore, the generation of the short pulse electromagnetic wave in the THz region is also introduced.

Generation of Electromagnetic Wave Pulse Used by Discharged Plasmas

Generation of the electromagnetic wave pulse from a periodic electrostatic field via a laser-produced ionization front is reviewed. Experimental results of the generation of electromagnetic wave pulse from a DC to AC radiation converter (DARC) are described.

Numerical solution of time domain integral equations for arbitrarily shaped conductor/dielectric composite bodies

In this work, we present a numerical solution of the coupled time domain integral equations to obtain induced currents and scattered far fields on a three-dimensional, arbitrary shaped conducting/dielectric composite body illuminated by a Gaussian electromagnetic plane wave pulse. The coupled integral equations are derived utilizing the equivalence principle. The solution method is based on the method of moments and involves the triangular patch modeling of the composite body, in conjunction with the patch basis functions. Detailed mathematical steps along with several numerical results are presented to illustrate the efficacy of this approach.

Acoustic wave generation by microwaves and applications to nondestructive evaluation

Although acoustic wave generation by electromagnetic waves has been widely studied in the case of laser-generated ultrasounds, the literature on acoustic wave generation by thermal effects due to electromagnetic microwaves is very sparse. Several mechanisms have been suggested to explain the phenomenon of microwave generation, i.e. radiation pressure, electrostriction or thermal expansion. Now it is known that the main cause is the thermal expansion due to the microwave absorption. This paper will review the recent advances in the theory and experiments that introduce a new way to generate ultrasonic waves without contact for the purpose of nondestructive evaluation and control. The unidirectional theory based on Maxwell's equations, heat equation and thermoviscoelasticity predicts the generation of acoustic waves at interfaces and inside stratified materials. Acoustic waves are generated by a pulsed electromagnetic wave or a burst at a chosen frequency such that materials can be excited with a broad or narrow frequency range. Experiments show the generation of acoustic waves in water, viscoelastic polymers and composite materials shaped as rod and plates. From the computed and measured accelerations at interfaces, the viscoelastic and electromagnetic properties of materials such as polymers and composites can be evaluated (NDE). Preliminary examples of nondestructive testing applications are presented.

Response of a nonlinear magnetic medium to an incident intense electromagnetic pulse

Abstract Reflection of an intense electromagnetic wave pulse is considered from a class of strongly nonlinear magnetic media. It is shown that almost all the incident energy of the electromagnetic pulse can be reflected, and the polarization of the reflected wave will be rotated like magneto-optical Kerr effect. Also constitutive equations for those media are obtained by analytical solution of Maxwell equations. Such media can be of interest as a power limiters against powerful laser pulses.

Optical Response in Amorphous GaAs Thin Films Prepared by Pulsed Laser Deposition

Femtosecond optical response in GaAs thin films has been studied. We prepared GaAs thin films on MgO substrates and on YBa2Cu3O7-δ (YBCO) thin films using pulsed laser deposition (PLD) at temperatures below 250°C. A photocarrier lifetime of less than 1 ps is measured for the prepared GaAs thin films using femtosecond time-domain reflectivity change measurements. Pulsed electromagnetic wave [terahertz (THz) radiaiton] containing a frequency component of up to 1 THz is emitted from fabricated photoconductive switches using the prepared thin films. We also evaluated the THz radiation properties emitted from the photoswitches on the YBCO thin films.

An Asymptotic Diffusion Limit for Electromagnetic Wave Reflection from a Random Medium

We consider a temporally pulsed electromagnetic wave obliquely incident upon a weakly dispersive and dissipative random multilayer. The incident pulse width is chosen to be intermediate to two spatial scales with a deterministic macroscale and a random microscale. An asymptotic diffusion limit theory of stochastic differential equations is extended to a noncentered system and applied to analyze the asymptotic interplay of random scattering and total internal reflection. The perturbation analysis of the Kolmogorov--Fokker--Planck equation is performed for the random reflection coefficient and the pseudodifferential operator theory is used to represent the transition probability density of it.

Propagation and interaction of extremely short electromagnetic pulses in quadratic-nonlinear medium

Propagation of extremely short unipolar pulses of electromagnetic waves was considered in framework of the anharmonic oscillator model. For quasi-harmonic waves, the model leads to the description of quadratic parametric processes, e.g., sum-frequency and difference-frequency mixings. Unipolar steady state pulse was obtained by an analytical method. In numerical simulations it was shown that these steady state pulses are stable under collisions. However, these pulses are not solitons in the strict sense.

Transmission and reflection of normally incident, pulsed electromagnetic plane waves upon a Lorentz half-space

One-dimensional propagation of a normally incident, pulsed electromagnetic plane wave upon an isotropic, spatially homogeneous, Lorentz half-space is investigated analytically. Fourier integral representations of the time-dependent reflected and transmitted fields are obtained for an arbitrary incident pulse, and detailed examinations of these fields are made when the incident field is a finite-cycle-sine function. The inversion integral for the time-dependent reflected field is expressed in terms of the pole contribution and branch-cut integrals, whereas the uniform asymptotic methodology of Oughstun Sherman [J. Opt. Soc. Am. A6, 1394 (1989)] is applied to the transmitted field. Only the contribution from the distant saddle points to the transmitted field is studied in detail. An example is provided that shows that the effects of including the reflection and transmission coefficients may not be ignored when microwave or optical pulses are launched across the interface. Specifically, for Brillouin’s choice of the medium’s physical parameters, the reflected field has a peak value that is 21% of the incident field’s amplitude and that corresponds to a 21% decrease in the main signal (pole contribution) of the transmitted field when the transmission coefficient is unity. This study extends past analytical formulations of the one-dimensional problem by conducting an in-depth analysis of the reflected field for a normally incident finite-cycle-sine wave and by addressing how inclusion of frequency-dependent transmission and reflection coefficients affects the fields. In particular, situations for which the effect of the frequency-dependent transmission and reflection coefficients is significant are discussed. Finally, the analysis of the reflected field should be useful for diagnostic purposes.

PHYSICAL PROCEDURES OF THE PULSE RADIATION FROM THE DIPOLE ANTENNAS AND ITS NUMERICAL SIMULATION

After the discussion of the numerical simulation method based on the FDTD method and the PML technique, the motions of the charges and the electrical field lines were first employed to describe the radiation procedures of the pulse electromagnetic waves. Then the causes of how the charges are accelerated and how the motion status of the charges are maintained were further studied from the angle of interaction of charge and field. After these analyses, it was pointed out that the pulse radiation is due to the suddenly occurred time-varying electrical field (displacement current) in the open space. This view was further evidenced by two examples: one is the partly resistance loaded antenna, the other is the partly curved antenna. The radiation procedures of the pulse electromagnetic waves of many different situations were simulated throughout this paper with the contours and waveforms of electric field given. These figures are very helpful to the understanding of the radiation mechanism of the pulse electromagnetic waves.

Reflection of an electromagnetic pulse incident on a nonlinear medium

The reflection and transmission of an electromagnetic wave pulse incident on a strongly nonlinear medium are considered. An exact analytical solution is found for a rather general medium model that is characterized by two arbitrary parameters. It is shown that almost all the incident energy of some pulse forms can be reflected at large amplitudes, whereas small amplitude fields are transmitted through the medium. For other pulse forms, part of the reflected pulse can reverse its polarization due to the time dependence of the reflection coefficient.

Ionization-induced frequency up-shift of a high-power microwave interacting with a plasma

The propagation of an intense electromagnetic wave (EMW), of frequency 9 GHz, in an underdense, near critical, inhomogeneous plasma (ne⩽1.2×1012 cm−3, Te≃3 eV) leads to an up-shift of the EMW frequency of a few MHz. The injected EMW with power, P0⩽250 kW, and with pulse duration of 1 μs, produces additional ionization of the neutral argon gas (n0≃6.6×1013 cm−3) through the heating of plasma electrons. The increase of plasma density, which can reach during the EMW pulse several tens of percents in relative value, leads to a modulation in time of the spatial phase of the EMW. Experimental results for the evolution in space and in time of density, electric field, and frequency shift are compared to a simple nonlinear model of EMW propagation in a plasma, which takes into account the additional ionization due to the absorption of the EMW.

Large-current radiator with avalanche transistor switch

The development and experimental investigation of a large-current radiator (LCR) with a pulse generator using avalanche transistors is reported. The LCR is intend for radiating nonsinusoidal pulse electromagnetic waves with a duration of 2 ns. The radiated field strengths in both the near and the far field are measured. The LCR described produces magnetic field strengths of about 4.2 mA/m at a distance of 5 m for radiator current amplitudes of about 0.7 A with rise time of 1 ns. The width of the antenna pattern, measured by the drop of the field strength to one half of its peak value is 120/spl deg/.

Electromagnetic wave pulse amplification induced by the reflection from relativistic plasma fronts

For a radiation pulse with incidence normal to an incoming plasma front, the reflective index at the front of the plasma serves as a reflector. A description is given of the up-shift frequency of the radiation pulse and the reflection coefficient developments with different plasma densities from an initial state to saturation and decaying are studied. The results show that the reflection coefficient is closely correlated both to the velocity of the plasma front and the amplitude of the Langmuir wave. We obtained reflection coefficients that are larger than those from a moving mirror at the same speed. However, at special points the reflection coefficient is independent of the amplitude of the Langmuir wave.

Electrooptic sampling of freely propagating terahertz fields

The electrooptic measurement of freely-propagating, subpicosecond pulsed electromagnetic waves (terahertz fields) are presented. A LiTaO3 crystal was used as a sensor head and achieved subpicosecond temporal resolution and 22 mVcm−1Hz−1/2 field sensitivity. The flat frequency response from the electrooptic crystals permitted the detection of radiation with ultrawide bandwidth and overcame previous limitations imposed by the resonant structure of photoconductive dipole antennas.

Methods to generate femto- and attosecond electron and X-ray pulses

Abstract Short electromagnetic wave pulses of wavelength λ can be used to modulate the energy of a charged particle beam on a scale of λ 2 , which in turn can generate particle bunching of dimension ≲ λ 2 , in the usual manner. These short bunches can then radiate correspondingly short photon bunches, including higher energy photons. Four types of devices are discussed which can achieve the requisite energy modulation on a scale of λ 2 : Vacuum, Cherenkov, resonator, and klystron modulators. The first is the simplest, and for short bunches its performance approaches, or is superior to that of the last two. For broadband short pulses it is generally to be preferred.

Dynamic range of an electro-optic field sensor and its imaging applications

We report the measurement of the dynamics of free-space electro-optic field sensors for pulsed electromagnetic wave radiation. With an optical probe power spanning six decades of linearity and excellent signal-to-noise ratio (SNR), it is feasible to convert a far-infrared 2D image into an optical 2D image. A simple estimation indicates that 100 mW of optical probe power can achieve an image of 256×256 pixels with a 50 pA signal current per pixel and a SNR ≳200. We also present a comparison measurement of an ultrafast photoconductive antenna and an electro-optic sensor crystal.