J.A. Kong

Identification of terrain cover using the optimum polarimetric classifier

A systematic approach for the identification of terrain media such as vegetation canopy, forest, and snow-covered fields is developed using the optimum polarimetric classifier. The covariance matrices for various terrain cover are computed from theoretical models of random medium by evaluating the scattering matrix elements. The optimal classification scheme makes use of a quadratic distance measure and is applied to classify a vegetation canopy consisting of both trees and grass. Experimentally measured data are used to validate the classification scheme. Analytical and Monte Carlo simulated classification errors using the fully polarimetric feature vector are compared with classification based on single features which include the phase difference between the VV and HH polarization returns. It is shown that the full polarimetric results are optimal and provide better classification performance than single feature measurements.

A Finite-Difference Time-Domain Analysis of Wave Scattering from Periodic Surfaces: Oblique Incidence Case

Scattering of electromagnetic waves from periodic surfaces is considered in time-domain for an oblique angle of incidence. The finite-difference time-domain (FDTD) method is used to obtain numerical solutions without resorting to frequency-domain analysis and Fourier transformation. For the application of FDTD method to the oblique incidence case, Maxwells equations are transformed such that the computational domain can be truncated by using periodic boundary conditions. The FDTD method is then used to solve the transformed equations. In solving the transformed equations by the FDTD method, the absorbing boundary conditions are modified and the eigenvalues of the system are determined for the stability analysis. The final results are obtained by using the inverse transformation. Since the transformation is very simple, the computational time is primarily determined by the FDTD solution of the transformed equations. The theoretical results are illustrated by calculating the scattered fields in the computa...

Trion-Induced Negative Photoconductivity in Monolayer MoS 2

Optical excitation typically enhances electrical conduction and low-frequency radiation absorption in semiconductors. We, however, observe a pronounced transient decrease of conductivity in doped monolayer molybdenum disulfide (MoS(2)), a two-dimensional (2D) semiconductor, using ultrafast optical-pump terahertz-probe spectroscopy. In particular, the conductivity is reduced to only 30% of its equilibrium value at high pump fluence. This anomalous phenomenon arises from the strong many-body interactions in the 2D system, where photoexcited electron-hole pairs join the doping-induced charges to form trions, bound states of two electrons and one hole. The resultant increase of the carrier effective mass substantially diminishes the conductivity.

NUMERICAL STUDIES OF LEFT HANDED METAMATERIALS

We use the three dimensional Finite Difference Time Domain (FDTD) technique to study metamaterials exhibiting both negative permittivity and permeability in certain frequency bands. The structure under study is the well-known periodic arrangement of rods and split-ring resonators, previously used in experimental setups. The three parameters we study are the transmission coefficient of a slab, the phase variation of the propagating fields within the metamaterial, and the refraction of a wave through a prism. To our knowledge, this is the first time that the last two parameters are studied rigorously using a numerical method. The results of this work show that fields propagating inside the metamaterial with a forward power direction exhibit a backward phase velocity and negative index of refraction.

Electromagnetic Wave Interaction of Conducting Object With Rough Surface By Hybrid Spm/Mom Technique - Abstract

An electromagnetic wave scattering model based on a hybrid SPM/MoM technique has been developed for the study of the EM wave interaction of a conducting object above a rough surface. In this hybrid technique, the Greens function and surface variables are expanded in terms of the surface height function on the mean surface, so that the integral equations based on the extinction theorem and boundary conditions can be decomposed into different orders. Each order represents the problem of EM wave scattering from an object on a flat interface excited by a wave from an equivalent source. With the introduction of the dyadic Greens function for layered media, the integral equations are reformulated and solved without involving the surface variables on the interface. Since the returned fields using the hybrid technique are expressed term by term, the EM wave interaction of the object with the rough surface can be clearly identified and characterized. The total returned fields calculated by the hybrid technique, ...

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.

Scattering of electromagnetic waves from dense distributions of spheroidal particles based on Monte Carlo simulations

In a dense discrete random medium, the propagation and scattering of waves are affected not only by the individual properties of the particles such as size, shape, and permittivity, but also by group properties such as the statistics of relative particle positions and relative orientations. We use Monte Carlo simulations to investigate the interactions of electromagnetic waves with a dense medium consisting of spheroidal particles for cases of random orientation and for cases of aligned orientation. A shuffling process is used to generate the positions of densely packed spheroids. Multiple-scattering equations are formulated by means of the volume integral equation and are solved numerically. The scattering results are averaged over realizations. Numerical results are presented for the extinction rates and the phase matrices. Salient features of the numerical results indicate that (1) the extinction rates of densely packed small spheroids are smaller than those of independent scattering; (2) for aligned spheroids, the extinction rates are polarization dependent; and (3) the co-polarized part of the phase matrix for densely packed spheroids is smaller than that of independent scatering, while the cross-polarized part is larger than that for independent scattering. This means that the ratio of cross-polarization to co-polarization is significantly higher than that of independent scattering.

Propagation of Electromagnetic Waves in a Slab with Negative Permittivity and Negative Permeability

In this paper, we study the electromagnetic fields propagating through a slab which permittivity and permeability are simultaneously negative. We show that symmetry properties of the wave solution remove all ambiguity in the choice of the sign of the wave numbers inside the slab. Upon developing the Green’s functions in terms of plane waves, growing “evanescent” waves in the direction of power flow are shown to exist inside the slab. As an illustration, the perfect imaging property of a slab where 21 = −20 and μ1 = −μ0 is verified.

Electromagnetic Scattering Model for Soybean Canopy

An electromagnetic scattering model for soybean canopy is presented in this paper. For the inter-plant structure, we propose a semi-deterministic description using the antenna array theory to account for the prevailing agriculture practice of soybean. For the underlying rough surface contribution, we employ a recently developed statistical multiple scattering model. For single plant scattering, we adopt the conventional decomposition of the scattering mechanisms. However, in dealing with the ground bounce scattering mechanisms, we use an approach to calculate the roughness modified Fresnel reflection coefficients similar to that of Rodriguez. To validate this proposed model, a comparison is made between the theoretical predictions and polarimetric backscattering measurements. The results show very good agreements between the two and thus clearly demonstrate the validity of this model.

K-Distribution and Multi-Frequency Polarmetric Terrain Radar Clutter

A multivariate K-distribution, well supported by experimental data, is proposed to model the statistics of fully polarimetric radar clutter of earth terrain. In this approach, correlated polarizations of backscattered radar returns are characterized by a covariance matrix and homogeneity of terrain scatterers is characterized by a parameter . As compared with C-, L- and P-band polarimetric SAR image simultaneously measured by Jet Propulsion Laboratory (JPL) on Mt. Shasta, it is found that appears to decrease from C- to P-band for both the forest and burned areas.