ABSTRACT
Double layer structures consisting of stainless steel and polymer rods are designed to blur and attenuate Bragg and Bloch-Floquet scattering from a periodically ribbed plate in a given frequency bandwidth. These structures can be considered as ribbed plate-spring-mass systems, the resonance frequencies of which are obtained from random and circular permutations of five basic oscillators. Analytical and finite element methods are used to find their parameters and tank tests have been carried out to ensure the accuracy of the numerical results and validate the relevance of such a model. It has been found that the typical features associated with the periodicity of the ribs are replaced by a "Christmas tree" appearance when the far field pressure backscattered from the model is displayed in the (frequency, aspect angle) plane. An analysis of backscattering patterns is presented and comparisons with Naval Research Laboratory results are made.
ABSTRACT
A piezoelectric plate, poled along its thickness and supporting on its top and bottom surfaces a periodic grating of electrodes, is considered. An analytical model allowing band structure calculation is derived for the first symmetrical mode propagating along the length of the plate. Analytical results show that an electrical Bragg (EB) bandgap can be observed for this mode, depending on the electrical boundary conditions applied on the electrodes. This "EB bandgap" is associated with a discontinuity of the electric field between two successive unit cells. These results are validated with the numerical simulations based on the finite element method. Analytical and numerical results prove that the EB bandgap is highly tunable and can be optimized by changing the crystallographic orientation of the material. A simple tunable filter exploiting this bandgap is designed and fabricated. Experimental results of electrical impedance and electrical potential at the output together with a scanning laser vibrometer analysis are presented, which confirm the theoretical predictions.
ABSTRACT
Two ways of controlling the acoustic waves propagation by external inductance or capacitance in a one-dimensional (1-D) piezomagnetic phononic crystal are investigated. The structure is made of identical bars, constituted of a piezomagnetic material, surrounded by a coil and connected to an external impedance. A model of propagation of longitudinal elastic waves through the periodic structure is developed and the dispersion equation is obtained. Reflection and transmission coefficients are derived from a 2 × 2 transfer matrix formalism that also allows for the calculation of elastic effective parameters (density, Young modulus, speed of sound, impedance). The effect of shunting impedances is numerically investigated. The results reveal that a connected external inductance tunes the Bragg band gaps of the 1-D phononic crystal. When the elements are connected via a capacitance, a hybridization gap, due to the resonance of the LC circuit made of the piezomagnetic element and the capacitance, coexists with the Bragg band gap. The value of the external capacitance modifies the boundaries of both gaps. Calculation of the effective characteristics of the phononic crystal leads to an analysis of the physical mechanisms involved in the wave propagation. When periodically connected to external capacitances, a homogeneous piezomagnetic stack behaves as a dispersive tunable metamaterial.
ABSTRACT
Phononic crystals made of piezoelectric composites with 1-3 connectivity are studied theoretically and experimentally. It is shown that they present Bragg band gaps that depend on the periodic electrical boundary conditions. These structures have improved properties compared to phononic crystals composed of bulk piezoelectric elements, especially the existence of larger band gaps and the fact that they do not require severe constraints on their aspect ratios. Experimental results present an overall agreement with the theoretical predictions and clearly show that the pass bands and stop bands of the device under study are easily tunable by only changing the electrical boundary conditions applied on each piezocomposite layer.
ABSTRACT
The dispersion curves of a phononic crystal (PC) based on a hollow metallic structure are presented. They exhibit a negative refraction dispersion branch and perfect refractive index matching with the surrounding water, leading to focusing capability. Numerical and experimental results are reported for a flat PC lens. The characteristics of the focal spot (intensity, dimensions, etc.) are numerically and experimentally investigated with the goal of finding the frequency of the optimal imaging performance.
ABSTRACT
A two-dimensional phononic crystal (PC) made of a square lattice of air holes in an aluminum matrix is studied. The band structure calculated in the irreducible Brillouin zone of the PC exhibits a branch with a negative slope that allows negative refraction. This phenomenon has been numerically verified using a prism-shaped PC for plane waves entering the PC with two different incidences. A detailed study of the waves at the exit of the PC shows that the plane wave is reconstructed after several wavelengths. Finally, the description of the refracted waves is interpreted using a point source array, giving information about the angular spreading and the relative amplitude of each refracted beam.
ABSTRACT
An original all-dielectric design that performs cloaking at 0.58 THz is demonstrated. The cloak consists of radially positioned micrometer-sized ferroelectric cylinders which exhibit under Mie theory a strong magnetic resonance. Full-wave simulations coupled with a field-summation retrieval technique were employed to adjust the rods magnetic plasma frequency; hence, the radial distribution in the permeability of the cloak. The behavior of the complete micro-structured device was simulated and results unambiguously show good reconstruction of the E-field wavefronts behind the cloak with high power transmission. This all-dielectric configuration provides an attractive route for designing cloaking devices at microwave and terahertz frequencies.
