Asymmetric Metal-Dielectric Metacylinders and Their Potential Applications From Engineering Scattering Patterns to Spatial Optical Signal Processing

We propose a bianisotropic hybrid metal-dielectric structure comprising dielectric and metallic cylindrical wedges wherein the composite metacylinder enables advanced control of electric, magnetic, and magnetoelectric resonances. We establish a theoretical framework in which the electromagnetic response of this meta-atom is described through the electric and magnetic multipole moments. The complete dynamic polarizability tensor, expressed in a compact form, is derived as a function of the Mie-scattering coefficients. Further, the constitutive parameters—determined analytically—illustrate the tunability of the structure’s frequency and strength of resonances in light of its high degree of geometric freedom. Flexibility in the design makes the proposed metacylinder a viable candidate for various applications in the microscopic (single meta-atom) and macroscopic (metasurface) levels. We show that the highly versatile bianisotropic meta-atom is amenable to being designed for the desired electromagnetic response, such as electric dipole-free and zero or near-zero (backward and forward) scattering at the microscopic level. In addition, we show that the azimuthal asymmetry gives rise to normal polarizability components, which are vital elements in synthesizing asymmetric optical transfer function at the macroscopic level. We conduct a precise inspection, from the microscopic to the macroscopic level, of the metasurface synthesis for emphasizing on the role of normal polarizability components for spatial optical signal processing. It is shown that this simple two-dimensional asymmetric meta-atom can perform first-order differentiation and edge detection at normal illumination. The results reported herein contribute toward improving the physical understanding of wave interaction with artificial materials composed of asymmetric elongated metal-dielectric inclusions and open the potential of its application in spatial signal and image processing.

Acoustic metamaterial properties of a 2D closed cellular solid with entrained fluid

Vladimir Dorodnitsyn

Metamaterials are often defined as artificial compositions designed to exhibit desired physical properties. These materials attract a lot of research attention due to unusual behavior that may not yet have been seen in nature. Although there is no commonly accepted definition for metamaterials, they are typically associated with peculiar macroscale properties resulting from their substructure. The electromagnetic metamaterial concept was first developed in 1968. As the wave theory is similar in every field, the achievements in optics were reflected in acoustics several decades later. This allowed developing acoustic metamaterials with such extraordinary properties as negative refractive index, negative bulk modulus and mass density, acoustic lensing, sound wave spectral decomposition, and acoustic bandgaps. All of those features are not only attractive scientifically, but are of interest for plenty of potential applications, including sound and vibration insulation, waveguiding, audible and high-frequency filtering, and even seismic absorption. On the other hand, cellular solids and saturated porous media have been studied for a long time. These media are abundant in nature as granular soils, wood, rocks, bones, and foams. Wave analysis in such environments typically requires some crucial assumptions which do not allow extending a theory to other configurations. An example of such a constraint is the openness of the cells in a medium. Many porous media applications are found in geophysics - particularly gas and oil extractions - the permeability of the cells plays an important role. The ad-hoc dynamic models for such media operate only with open-cell configuration. Moreover, the study is limited to the low-frequency analysis, omitting the influence of wave scattering. The latter, however, is the key source of dispersion in acoustic metamaterials. Scattering may have two origins, geometrical or resonant. Bragg's scattering is determined by the geometrical configuration, such that constructive interference occurs only when the incident wave matches the characteristic size of a unit cell. This makes such systems practically inconvenient. The concept of resonant scattering, introduced about a decade ago, has much fewer limitations and is mostly determined by the dynamics of matrix inclusions. In this thesis, a closed-cell cellular solid with thin vibrating walls and fluid-filled cells is proposed as a new class of acoustic metamaterials. First, the dynamics of a prototypical square cell is investigated numerically considering periodic boundary conditions and taking into account fluid-structure interaction. The results are compared to Biot's theory of saturated porous media in the limit of a closed-cells system. The proposed configuration is studied with respect to dispersion sources, showing the presence of local resonant behavior for different combinations of relative density and entrained fluid. Surprisingly, semi-analytical models can be used to provide a bottom-up explanation of the structure's dynamic behavior. The presence of two pressure waves, slow and fast, is confirmed numerically and analytically. Finally, an experimental proof-of-concept was carried out. Periodic cellular solids represent a versatile acoustic metamaterial platform characterized by low cost, simple, scalable design, which makes possible achieving the desired macroscopic behavior using different types of fluids and bulk materials.

EPFL2016

Ferroelectricity from spin supercurrents in LiCuVO4

Mechthild Enderle, Martin Mourigal

We have studied the magnetic structure of the ferroelectric frustrated spin-1/2 chain material LiCuVO4 in applied electric and magnetic fields using polarized neutrons. A symmetry and mean-field analysis of the data rules out the presence of static Dzyaloshinskii-Moriya interaction, while exchange striction is shown to be negligible by our specific-heat measurements. The experimentally observed magnetoelectric coupling is in excellent agreement with the predictions of a purely electronic mechanism based on spin supercurrents.

2011

Order and Dynamics of Model Quantum Antiferromagnets

Martin Mourigal

The spin-1/2 Heisenberg antiferromagnets in one and two dimensions are important models in quantum magnetism in which many-body effects play a crucial role. Their collective excitations are associated with quasiparticles the interaction of which are difficult to describe. Neutron scattering offers a very powerful experimental technique to study these excitations. In addition, it can reveal the nature of exotic ground-states favored by competing frustrated interactions or by an applied magnetic field. This thesis presents neutron scattering studies of four materials, each representing a model magnetic system. • Cu(DCOO)2·4D2O (CFTD) is a model realization of the S =1/2 square-lattice antiferromagnet. It displays Néel order and its long-wavelength excitations are well described by renormalized spin-wave theory. In contrast, the excitations at the magnetic zone-boundary display anomalous dispersion and intensity close to (π, 0). A polarized neutron study of the anomaly additionally reveals the presence of a transverse continuum associated with a lineshape that extends to high energies. Results for the transverse and longitudinal continua are compared with Quantum Monte Carlo and fermionic Resonating Valence Bond theories. • (5CAP)2CuCl4 (CAPCC) is a S = 1/2 square-lattice antiferromagnet with modest saturation magnetic field Hs = 3.6 T and ∼ 20% antiferromagnetic interlayer coupling. Its spin excitations, measured above saturation are used to determine the microscopic exchange parameters of its Hamiltonian, while the presence of spontaneous magnon decay is investigated above the theoretically predicted threshold field H*= 0.76Hs. • CuSO4·5D2O is a model realization of the S = 1/2 antiferromagnetic Heisenberg chain. Its excitations are fully characterized in various regimes of applied magnetic field. In zero field, the presence of two- and four-spinon states is quantitatively confirmed, while the development of long-range Néel order is shown to preserve this picture qualitatively. In applied magnetic field, the excitations are studied with polarized neutrons and the existence of exotic psinons, anti-psinons and string-solutions continua is discussed. • LiCuVO4 is a S = 1/2 frustrated multiferroic chain material with competing ferromagnetic nearest and antiferromagnetic next-nearest neighbor interactions. Its magnetic structure is found to be a perfect circular cycloid the chirality of which can be tuned with an applied electric field. In crossed electric and magnetic fields, the behavior of the cycloid closely matches with that theoretically predicted from the spin supercurrent scenario. A magnetic-field induced transition to short-range longitudinal order is also evidenced with polarized neutrons. The present work was carried out in the Triple-Axis Spectrometers group at the Institut Laue Langevin in Grenoble, France and in the Laboratory for Quantum Mangetism at the EPFL, Switzerland.