We have fabricated sonic crystals, based on the idea of localized resonant structures, that exhibit spectral gaps with a lattice constant two orders of magnitude smaller than the relevant wavelength. Disordered composites made from such localized resonant structures behave as a material with effective negative elastic constants and a total wave reflector within certain tunable sonic frequency ranges. A 2-centimeter slab of this composite material is shown to break the conventional mass-density law of sound transmission by one or more orders of magnitude at 400 hertz.

http://science.sciencemag.org/content/sci/289/5485/1734.full.pdf

Locally Resonant Sonic Materials

The term photonic crystals appears because of the analogy between electron waves in crystals and the light waves in artificial periodic dielectric structures. During the recent years the investigation of one-, two-and three-dimensional periodic structures has attracted a widespread attention of the world optics community because of great potentiality of such structures in advanced applied optical fields. The interest in periodic structures has been stimulated by the fast development of semiconductor technology that now allows the fabrication of artificial structures, whose period is comparable with the wavelength of light in the visible and infrared ranges.

http://ieeexplore.ieee.org/iel5/6354/16969/00781867.pdf

Photonic crystals

저작근 근전도에 관한 정상교합자와 ii급 부정교합자의 비교 연구

The present study demonstrates the creation of a stable, superhydrophobic surface using the nanoscale roughness inherent in a vertically aligned carbon nanotube forest together with a thin, conformal hydrophobic poly(tetrafluoroethylene) (PTFE) coating on the surface of the nanotubes. Superhydrophobicity is achieved down to the microscopic level where essentially spherical, micrometer-sized water droplets can be suspended on top of the nanotube forest.

/pdf/superhydrophobic-carbon-nanotube-forests-4wzbqm5f24.pdf

Superhydrophobic Carbon Nanotube Forests

The emergence of artificially designed subwavelength electromagnetic materials, denoted metamaterials, has significantly broadened the range of material responses found in nature. However, the acoustic analogue to electromagnetic metamaterials has, so far, not been investigated. We report a new class of ultrasonic metamaterials consisting of an array of subwavelength Helmholtz resonators with designed acoustic inductance and capacitance. These materials have an effective dynamic modulus with negative values near the resonance frequency. As a result, these ultrasonic metamaterials can convey acoustic waves with a group velocity antiparallel to phase velocity, as observed experimentally. On the basis of homogenized-media theory, we calculated the dispersion and transmission, which agrees well with experiments near 30 kHz. As the negative dynamic modulus leads to a richness of surface states with very large wavevectors, this new class of acoustic metamaterials may offer interesting applications, such as acoustic negative refraction and superlensing below the diffraction limit.

Ultrasonic metamaterials with negative modulus

This work reports the excellent dielectric properties of polyimide (PI) embedded with CaCu3Ti4O12(CCTO)/Ag nanoparticles (CCTO@Ag). By functionalizing the surface of CCTO nanoparticles with Ag coating, the dielectric permittivity of PI/CCTO@Ag composites is significantly increased to 103 (100 Hz) at 3 vol% filler loading. The enhancement of dielectric permittivity is attributed to the increment of conductivity of the interlayer between CCTO and PI by Ag, which enhances the space charge polarization and Maxwell–Wagner–Sillars (MWS) effect. The experimental results fit well with percolation theory. Moreover, the low loss (0.018 at 100 Hz) achieved is attributed to the blockage of charge transfer by insulating polyimide chains. It is shown that the electrical field distortion is significantly improved by decorating the surface of CCTO nanoparticles with Ag using Comsol Multiphysics. This plays an important role in the enhancement of the dielectric properties.

High performance of polyimide/CaCu3Ti4O12@Ag hybrid films with enhanced dielectric permittivity and low dielectric loss

The static mass density of a composite is simply the volume average of its constituents' densities. The dynamic density of a composite is defined to be the quantity that enters in evaluating the elastic wave velocity at the low-frequency limit. We show through a rigorous derivation that the effective dynamic mass density of an inhomogeneous mixture can differ from its static counterpart when the composite matrix is a fluid or, more generally, when there are relative motions between the matrix and inclusions. Derivation of the dynamic mass density expressions, involving taking the long wavelength limit of the rigorous multiple scattering theory, is detailed for the two-dimensional case. We also extend the effective dynamic mass density expression to finite frequencies where there can be low-frequency resonances. By combining both analytical and numerical approaches, negative or complex dynamic mass density is obtained for composites that contain a sufficient fraction of locally resonant inclusions. Thus, the dynamic mass density of a composite can differ from the static (volume-averaged) value even in the zero frequency limit, although both must be positive in that limit. Negative or complex dynamic mass density can occur at finite frequencies. These two results are shown to be consistent with each other, as well as related by the same underlying physics. As by-products of our rigorous derivation, we also verify some prior known results on the effective elastic moduli of composites.

/pdf/effective-dynamic-mass-density-of-composites-2rfsmx23wf.pdf

Effective Dynamic Mass Density of Composites

Point defect modes of acoustic wave in two-dimensional square arrays of square water rods in a mercury host were studied. The defects are created by three kinds of geometry, namely, square defect, circular defect, and rectangular defect, respectively. The results show that for both square defect and circular defect, the defect modes are only related to the defect filling fraction F(d), but not with the geometry of defects (square or circular), as well as the orientations of the square defect. For the rectangular defect, the defect modes could be tuned by changing the ratio of edge width of the defect, moreover, the double degenerate one will split into two nondegenerate modes when the ratio of edge widths Lx / L(y) >9.0. Meanwhile the corresponding pressure distributions also will be changed.

Splitting and tuning characteristics of the point defect modes in two-dimensional phononic crystals.

We investigate theoretically subwavelength imaging by acoustic metamaterial slabs immersed in the liquid matrix. A near-field subwavelength image formed by evanescent waves is achieved by a designed metamaterial slab with negative mass density and positive modulus. A subwavelength real image is achieved by a designed metamaterial slab with simultaneously negative mass density and modulus. These results are expected to shed some light on designing novel devices of acoustic metamaterials.

Theoretical study of subwavelength imaging by acoustic metamaterial slabs

In this letter, we investigated the resonant tunneling of elastic waves through double phononic potential barriers formed by two slabs of two-dimensional phononic crystals consisting of pure solid components. It is found that the resonant tunneling longitudinal waves can be distinguished easily from the resonant tunneling transverse waves. Thus, such double-barrier structures can be served as a mode-selecting acoustic filter, used to pick out the single longitudinal wave component or transverse wave component at a certain frequency.

Zhengyou Liu

Papers

High performance of polyimide/CaCu3Ti4O12@Ag hybrid films with enhanced dielectric permittivity and low dielectric loss

Effective Dynamic Mass Density of Composites

Splitting and tuning characteristics of the point defect modes in two-dimensional phononic crystals.

Theoretical study of subwavelength imaging by acoustic metamaterial slabs

Mode-selecting acoustic filter by using resonant tunneling of two-dimensional double phononic crystals