Kung Hau Ding

Monte Carlo simulations of the extinction rate of dense media with randomly distributed dielectric spheres based on solution of Maxwell's equations.

We present Monte Carlo simulations of the extinction rate of dense media with randomly distributed dielectric spheres that occupy up to 25% by volume and size parameter ka = 0.2. Maxwell’s equations in multiple-scattering form are solved iteratively for each realization. Convergence is demonstrated numerically by varying the number of iterations, the number of spheres up to 4000, and the number of realizations. Results are compared with that of the independent-scattering approximation, Foldy’s approximation, the quasi-crystalline approximation, and the quasi-crystalline approximation with coherent potential. The simulations are in good agreement with the last two approximations.

Tapered wave with dominant polarization state for all angles of incidence

Typical applications of the method of moments (MoM) to rough surface three-dimensional (3-D) electromagnetic scattering require a truncation of the surface considered and call for a tapered incident wave. It is shown how such a wave can be constructed as a superposition of plane waves, avoiding problems near both normal and grazing incidence and providing clean footprints and clear polarization at all angles of incidence. The proposed special choice of polarization vectors removes an irregularity at the origin of the wavenumber space and leads to a least squared error property of the wave. Issues in the application to 3-D scattering from an object over a rough surface are discussed. Approximate 3-D scalar and vector tapered waves which can be evaluated without resorting to any numerical integrations are derived and important limitations to the accuracy and applicability of these approximations are pointed out.

Electromagnetic Scattering by Bicontinuous Random Microstructures With Discrete Permittivities

For electromagnetic (EM) scattering by dense media, the traditional approach is to use particles of spheres or ellipsoids that are densely and randomly packed in a background medium. The particles have discrete permittivities that are different from the background medium. The dense-medium model has been applied to the microwave remote sensing of terrestrial snow. In this paper, we propose a different approach of using a bicontinuous medium with discrete permittivities and study the EM scattering properties using analytical and numerical methods. The bicontinuous medium is a continuous representation of interfaces between inhomogeneities within the medium. Discrete permittivities are then assigned to the inhomogeneities of the structure. The analytical approach is based on the Born approximation using the derived analytical correlation functions. The numerical method is based on the numerical Maxwell model of 3-D (NMM3D) approach. In particular, the discrete-dipole approximation and the conjugate gradient-squared method accelerated by the fast Fourier transform technique are used in solving the volume integral equation. Scattering results of analytical and numerical approaches are compared. Numerical results are illustrated using parameters in microwave remote sensing of terrestrial snow. In the NMM3D simulations, three kinds of convergence tests are conducted, viz., convergence with respect to the discretization size, convergence with respect to the sample size, and convergence with respect to the number of realization. The NMM3D results indicate that the scattering by the bicontinuous medium with a broader size distribution has a weaker frequency dependence than that by the medium with a more narrow size distribution. The frequency-dependence power law index can be lower than two, which is very much lower than the power of four in Rayleigh scattering. The NMM3D results also exhibit fairly large cross-polarization returns which account for the local nonisotropic microstructures of bicontinuous media, although the medium is statistically isotropic.

Electromagnetic Scattering Model for Rice Canopy Based on Monte Carlo Simulation

A scattering model for rice canopy based on Monte Carlo simulations is applied to interpret RADARSAT data and to predict the temporal response of rice growth. The model takes into account the coherent wave interactions among vegetative elements which usually occur in clusters with closely spaced elements. The model was also used to analyze the structural effect of rice fields on the scattering † Also with Lawrence Livermore National Laboratory, L-645, Livermore, CA 94550, USA

Multiple scattering of waves by dense random distributions of sticky particles for applications in microwave scattering by terrestrial snow

[1]xa0We investigate the multiple scattering of waves by dense random distribution of particles. Maxwell equations are put in the form of Foldy-Lax multiple-scattering equations which are solved numerically. The positions of the particles are generated by random shuffling and bonding. Simulations are performed for applications in microwave scattering by terrestrial snow. The results are illustrated for the copolarization and cross-polarization scattering phase matrices and the extinction coefficients for sticky particles. We consider concentrations of particles up to 40% by volume. Results of dense media simulations depart from the predictions on the basis of classical theory of independent scattering and are applicable for very low concentrations. The simulation results agree with those of quasi-crystalline approximation (QCA) for concentration up to 20%. However, they start to deviate from those of the QCA for higher concentrations as QCA underestimates the extinction. Simulation results also predict strong cross polarization in the phase matrix of densely packed spheres, a result that is not predicted by classical independent scattering nor by QCA.

Electromagnetic Computation in Scattering of Electromagnetic Waves by Random Rough Surface and Dense Media in Microwave Remote Sensing of Land Surfaces

Active and passive microwave remote sensing has been used for monitoring the soil moisture and snow water equivalent. In the interactions of microwaves with bare soil, the effects are determined by scattering of electromagnetic waves by random rough surfaces. In the interactions of microwaves with terrestrial snow, the effects are determined by volume scattering of dense media characterized by densely packed particles. In this paper, we review the electromagnetic full-wave simulations that we have conducted for such problems. In volume scattering problems, one needs many densely packed scatterers in a random medium sample to simulate the physical solutions. In random rough surface scattering problems, one needs many valleys and peaks in the sample surface. In random media and rough surface problems, the geometric characterizations of the media and computer generations of statistical samples of the media are also challenges besides electromagnetic computations. In the scattering of waves by soil surfaces, we consider the soil to be a lossy dielectric medium. The random rough surface is characterized by Gaussian random processes with exponential correlation functions. Surfaces of exponential correlation functions have fine-scale structures that cause significant radar backscattering in active microwave remote sensing. Fine-scale features also cause increase in emission in passive microwave remote sensing. We apply Monte Carlo simulations of solving full 3-D Maxwells equations for such a problem. A hybrid UV/PBTG/SMCG method is developed to accelerate method of moment solutions. The results are illustrated for coherent waves and incoherent waves. We also illustrate bistatic scattering, backscattering, and emissivity which are signatures measured in microwave remote sensing. For the case of scattering by terrestrial snow, snow is a dense medium with densely packed ice grains. We have used two models: densely packed particles and bicontinuous media. For the case of densely packed particles, we used the Metropolis shuffling method to simulate the positions of particles. The particles are also allowed to have adhesive properties. The Foldy-Lax equations of multiple scattering are used to study scattering from the densely packed spherical particles. The results are illustrated for the coherent waves and incoherent waves. For the case of bicontinuous media, the method developed by Cahn is applied to construct the interfaces from a large number of stochastic sinusoidal waves with random phases and directions. The volume scattering problem is then solved by using CGS-FFT. We illustrate the results of frequency and polarization dependence of such dense media scattering.

Monte Carlo Simulations of Pair Distribution Functions of Dense Discrete Random Media With Multiple Sizes of Particles

In a dense discrete random medium, the propagation and scattering of waves are affected by the statistics of the particle positions. For the case of particles of finite size, the positions of the particles relative to each other in the presence of other particles are correlated and the second order statistics are described by the pair distribution functions. In this paper, we perform Monte Carlo simulations of pair distribution functions of dense discrete random media consisting of particles of multiple sizes. The Metropolis technique and the sequential random addition of particles methods are used to generate a series of configurations through random processes. The pair distribution functions are calculated by counting the average occurrence of pair separation of particles. The Monte Carlo results of the particle pair distribution functions are illustrated and are compared with the results of the Percus-Yevick approximation. The results from the two Monte Carlo methods are found to be in good agreement.

Subwavelength imaging enhancement through a three-dimensional plasmon superlens with rough surface.

In this Letter we investigate the subwavelength imaging of a three-dimensional plasmon superlens based on the full vector wave simulations of optical wave propagation and transmission. The optical transfer functions are computed. Comparisons are made between the results of lenses with flat and periodic/random rough surfaces. We also study the problem of practical imaging system geometry using laser as an illumination source. Results show that the lens with periodic or random roughness can reduce the field interference effects, and provide improved focus on the transmission field and the Poynting flux. We illustrate that the subwavelength roughness in a plasmon lens can enhance the image resolution over a flat lens for both matched and unmatched permittivity conditions. The enhancement of resolution occurs because the introduced subwavelength roughness can amplify the evanescent wave components and suppress the surface plasmon resonance peaks.

Effective Propagation Constants and Attenuation Rates in Media of Densely Distributed Coated Dielectric Particles with Size Distributions

The effective propagation constants and attenuation rates in dense nontenuous media with coated dielectric particles governed by size distributions are studied. A coated particle consists of two layers with different materials. In a dense medium, particles do not scatter independently and the effects of correlated scattering have to be included. The correlated scattering can be taken into account by using the quasicrystalline approximation (QCA) or the quasicrystalline approximation with coherent potential (QCA-CP). The Lippmann-Schwinger equation for coated particles is solved analytically to calculate the T-matrix for both QCA and QCA-CP. We also consider the dense medium with particle sizes governed by size distribution. The pair distribution functions for particles of different sizes are computed by using the Percus-Yevick approximation. For small particles, closed form analytic expressions are obtained for the complex effective propagation constants. For moderate size particles, the complex effective...

Image enhancement for flat and rough film plasmon superlenses by adding loss

Though surface plasmon resonances can reduce image beam width of a plasmon superlens, they distort the transfer function in the spectrum and introduce large image sidelobes as well. In this paper, image enhancement of a plasmon film superlens is investigated by adding permittivity loss. First, we add the loss in the film of the superlens and observe the sidelobes suppression. Second, we introduce loss in the image region of the superlens device and observe a flatter transfer function and obtain improved image resolution. For the silver film superlens at a free space wavelength of 337.5 nm, a beam width reduction of 69% is observed. Previously, we found that introducing roughness in the superlens can reduce the beam width. In this paper, we combine surface roughness with the method of adding loss in the image region and observe a further beam width reduction. The lossy sinusoidal surface superlens at a wavelength of 351 nm gains a beam width reduction of 86% compared to the lossless flat superlens. Moreover, in this paper we provide a model for calculating the superlens near-field image intensity when the objects are illuminated by a laser source, and the more general five slits example is shown to further demonstrate the advantage of adding loss.