Hongru Ma

Extending the bandwidth of electromagnetic cloaks

Using the idea of transformation medium, a cloak can be designed to make a domain invisible for one target frequency. In this article, we examine the possibility to extend the bandwidth of such a cloak. We obtained a constraint of the band width, which is summarized as a simple inequality that states that limits the bandwidth of operation. The constraint originates from causality requirements. We suggest a simple strategy that can get around the constraint.

Third-order optical nonlinearity enhancement through composite microstructures

Formulas for the evaluation of third-order optical susceptibility of nonlinear composites are derived in the mean-field approximation for four different microstructures corresponding to those described by the Maxwell–Garnett theory, the Bruggeman self-consistent theory, the Sheng theory, and the differential effective medium theory. A commonly encountered error in the literature is pointed out, with the correct formulation given. Anomalous dispersion, i.e., surface plasmon resonance of coated spheres, is identified as the source of large optical nonlinear susceptibility enhancement. Examples are given that demonstrate this microstructural enhancement effect. Comparison with experimental data on AuSiO2 granular films shows that large enhancement in the third-order Kerr-type nonlinear susceptibility can indeed be realized at compositions below the percolation threshold, with prediction of even larger enhancement possible.

Dielectric electrorheological fluids: theory and experiment

Electrorheological (ER) fluids are a class of materials whose rheological properties are controllable by the application of an electric field. A dielectric electrorheological (DER) fluid is the simplest type of ER fluid, in which the material components follow a linear electrostatic response. We review and discuss the progress of the studies on physics of this type of material. A first-principles theory of DER fluids, along with relevant experimental verifications, are presented in some detail. In particular, the properties presented include static equilibrium structure, shear modulus, static yield stress and its variation with applied electric field frequency, and structure-induced dielectric nonlinearity.

Conceal an entrance by means of superscatterer

By using the novel property of the rectangular superscatterer, we propose a design which can conceal an entrance from electromagnetic wave detection. Such a superscatterer is realized by coating a negative index material shell on a perfect electrical conductor rectangle cylinder. The results are numerically confirmed by full-wave simulations both in the far-field and near-field.

Reshaping the perfect electrical conductor cylinder arbitrarily

A general method is proposed to design a cylindrical cloak, concentrator and superscatterer with an arbitrary cross section. The method is demonstrated by the design of a perfect electrical conductor (PEC) reshaper which is able to reshape a PEC cylinder arbitrarily by combining the concept of cloak, concentrator and superscatterer together. Numerical simulations are performed to demonstrate its properties.

Low frequency elastic wave propagation in two dimensional locally resonant phononic crystal with asymmetric resonator

The resonance modes and the related effects to the transmission of elastic waves in a two dimensional phononic crystal formed by periodic arrangements of a two block unit cell in one direction are studied. The unit cell consists of two asymmetric elliptic cylinders coated with silicon rubber and embedded in a rigid matrix. The modes are obtained by the semianalytic method in the least square collocation scheme and confirmed by the finite element method simulations. Two resonance modes, corresponding to the vibration of the cylinder along the long and short axes, give rise to resonance reflections of elastic waves. One mode in between the two modes, related to the opposite vibration of the two cylinders in the unit cell in the direction along the layer, results in the total transmission of elastic waves due to zero effective mass density at the frequency. The resonance frequency of this mode, which has not yet been identified before, changes continuously with the orientation angle of the elliptic resonator.

Transformation media that turn a narrow slit into a large window

Based on the transformation media theory, the authors propose a way to replace a wide window with a narrow slit filled with designed metamaterial to achieve the same transmittance as the one of the window. Numerical simulations for a two dimensional case are given to illustrate the ideas and the performance of the design.

Frequency-induced structure variation in electrorheological fluids

We show that by changing the frequency of the externally applied electric field there can be significant variation in the column structure of the electrorheological (ER) fluids, with attendant yield stress implications. This phenomenon is quantitatively explained by the Debye-type relaxation associated with ER fluids’ solid particles. First-principle predictions based on the Debye-type dielectric constant are in excellent agreement with the experiments.

Scattering of elastic waves by elastic spheres in a NaCl-type phononic crystal

Based on the formalism developed by Psarobas et al (Phys. Rev. B 62, 278(2000)), which using the multiple scattering theory to calculate properties of simple phononic crystals, we propose a very simple method to study the NaCl-type phononic crystal. The NaCl-type phononic crystal consists of two kinds of non-overlapping elastic spheres with different mass densities, Lcoefficients and radius following the same periodicity of the ions in the real NaCl crystal. We focus on the (001) surface, and view the crystal as a sequence of planes of spheres, each plane of spheres has identical 2D periodicity. We obtained the complex band structure of the infinite crystal associated with this plane, and also calculated the transmission, reflection and absorption coefficients for an elastic wave (longitudinal or transverse) incident, at any angle, on a slab of the crystal of finite thickness.

Third-Order Nonlinear Properties of Au Clusters Containing Dielectric Thin Films

We show experimentally and theoretically that very large enhancement of χ 3 can be obtained in gold-cluster-containing dielectric thin films. At a high volume fraction of gold clusters, the concentration dependence of χ (3) cannot be accounted for by the Maxwell Garnett theory. To go beyond the MG theory, we have derived the spectral densities of different microgeometries with related formulae for calculating the effective dielectric constant and the enhancement factors for third-order nonlinearities. It is shown that the largest enhancement source is the anomalous dispersion, realized by the MG and the Sheng theories. Substantially improved agreement between theory and experiment is obtained. Even larger enhancement is predicted as possible by the theory.