Choon Mahn Park

Composite acoustic medium with simultaneously negative density and modulus.

We report observation of reverse Doppler effect in a double negative acoustic metamaterial. The metamaterial exhibited negative phase velocity and positive group velocity. The dispersion relation is such that the wavelength corresponding to higher frequency is longer. We observed that the frequency was down-shifted for the approaching source, and up-shifted when the source receded. ∗ To whom correspondence should be addressed. E-mail: ckkim@yonsei.ac.kr 1 ar X iv :0 90 1. 27 72 v2 [ co nd -m at .o th er ] 2 2 Ja n 20 09 Metamaterials with negative constitutive parameters [1, 2, 3, 4, 5] brought many phenomena previously regarded impossible into reality [6, 7, 8, 9]. Electromagnetic waves have been demonstrated to propagate with negative phase velocity in the double negative (DNG) matematerials which have negative electric permittivity and negative magnetic permeability simultaneously. For acoustic waves, negative phase velocity is expected if density and modulus can be made simultaneously negative [10]. We combined negative density material [11, 12] and negative modulus material [13, 14] to fabricate an acoustic metamaterial that exhibits the acoustic DNG property. The negative phase velocity introduces a dispersion relation in which the wavelengths corresponding to higher frequencies become longer. By measuring the frequency shifts in the DNG metamaterial, we observed the reverse Doppler effect of sound, where the frequency was down-shifted for the approaching source and up-shifted when the source receded. In comparison with the numerous theoretical and experimental research results on electromagnetic metamaterials, research efforts on the acoustic metamaterials are relatively few and mainly concentrated on theoretical aspects [10, 15, 16, 17, 18, 19, 20]. Only, recently, Fang et al. reported realization of negative modulus in a metamaterial consisting of Helmholtz resonators [11]. Also negative mass has been realized using a membrane by Yang et al. [13]. The authors also have investigated single negative acoustic metamaterials [12, 14]. Here, we present fabrication of a new acoustic DNG metamaterial by combining the two single negative metamaterials. Fig. 1a shows schematically the structure of a negative density material [12], which has a regular array of tensioned thin membranes placed inside a tube. The metamaterial shown in Fig. 1b exhibits negative modulus due to the motion of air column in the side holes [14]. Fig. 1c shows the acoustic DNG metamaterial with the membranes and side holes alternately placed along the tube. Acoustic double negativity of this structure is due to the actions of the membranes and side holes. The membrane forms a paraboloid when pushed by the fluid, and when the spacing between the membranes is much smaller than the wavelength (the long-wavelength limit), the restoring force from the tension gives the static pressure gradient 5p = −κ~ ξ where p, κ, and ~ ξ are the pressure, the elastic modulus, and the displacement of the fluid, respectively. In the dynamic case, due to the force from the membranes, Newton’s equation becomes −5p = ρ′∂~u/∂t+κ~ ξ, where ~u = ∂~ ξ/∂t is the longitudinal velocity of the fluid. The volume-averaged density ρ′ of the fluid (air in the present case) and the membranes, giving

Acoustic wave propagation in one-dimensional phononic crystals containing Helmholtz resonators

One-dimensional acoustic waveguide containing subwavelength-sized Helmholtz resonators is known to exhibit novel physical phenomena. However, no systematic theoretical study on this system has been carried out so far except on a few limited cases. We present a thorough theoretical calculation on the acoustic wave propagation in phononic crystals containing Helmholtz resonators without any geometrical size restrictions. The band structures, transmission spectra, and defect states are studied for diverse geometries using the interface response theory. It is shown that the acoustic band structure of the model is fundamentally different from the conventional acoustic–elastic cases and richer due to the coexistence of the resonant and the Bragg gaps. It is also shown that the presence of a defect resonator in the system can give rise to a localized mode inside the resonance gaps. The results clearly show that the presence of the Helmholtz resonators singly or periodically can play a prominent role in designing...

Acoustic superlens using membrane-based metamaterials

We report construction of an acoustic superlensing using two dimensional membrane-based negative-density metamaterials. When two point sources separated by a distance of 1/17 of the wavelength are placed near to a surface of the metamaterial slab, well-resolved images are formed on the opposite surface across the slab. The mechanism for the subwavelength resolution is the surface wave stemming from negative density. Potential applications include acoustic imaging and sensing.

Acoustic metamaterial exhibiting four different sign combinations of density and modulus

We fabricated a double negative acoustic metamaterial which consisted of Helmholtz resonators and membranes. Experimental data on the transmission and dispersion relation are presented. The system exhibited three frequencies where the acoustic state makes sharp transitions from density negative ( ρ -NG) to double negative (DNG), modulus negative (B-NG), and double positive (DPS) in sequence with the frequency. We observed a wide range of negative refractive indexes from −0.06 to −3.7 relative to air, which will allow for new acoustic transformation techniques.

Effective medium theory of the one-dimensional resonance phononic crystal

A general theoretical scheme to describe the effective modulus and mass density for acoustic metamaterials is presented. For such a purpose, an effective medium theory of a one-dimensional acoustic waveguide containing subwavelength-sized Helmholtz resonators is formulated. It is shown that, when the wavelength is much larger than the periodic length and the size of the resonators, the whole composite structure can be treated as an effective homogeneous medium in accounting for its acoustic properties. It is also shown that the acoustic characteristics, such as the effective modulus and the effective mass density, can be determined precisely from the transmission and the reflection data. The calculated effective modulus and effective mass density confirm that this structure behaves as a homogeneous metamaterial with a negative effective modulus in a frequency range just above the resonant frequency.

Propagation of acoustic waves in a metamaterial with a refractive index of near zero

A metamaterial system with a refractive index of near zero for acoustic wave is realized. The designed system, which uses Helmholtz resonators, is analyzed using the equivalent circuit theory. Through the simulation and the experiment, we observed a large phase velocity in the metamaterial and a concentration of the energy density in the narrow normal waveguide directly coupled to the metamaterial. This is different to the metamaterial for the electromagnetic wave, because the concentration of energy density of the electromagnetic wave occurs in the metamaterial inside. The acoustic metamaterial system can be applied to the development of device for the acoustic energy concentrator and the acoustic filter.

Hierarchical performance management using OAM MIB in ATM networks

As the network grows bigger and faster, the demand for reliable and rapid network monitoring and control is on the increase. For efficient network performance management, the network manager should be able to not only acquire the overall network status information with minimum management traffic, but also immediately figure out the faulty network elements. We propose a hierarchical performance management scheme with operation and maintenance (OAM) management information base (MIB) by using the SNMP management protocol. With this MIB we can remotely monitor each connection and pinpoint the location of a performance problem.

An acoustic lens built with a low dispersion metamaterial

The realization of a near-frequency-independent acoustic metamaterial and the successful application of this material as a convex acoustic lens are introduced. We design a face-centered-orifice-cubic (FCOC) unit cell that has the characteristics of an acoustic wave retarder. The increment in the acoustic refractive index produced by the FCOC unit cell is observed by reducing the size of the orifice. This also shows that the refractive index remains nearly constant with frequency increments in the frequency regime that satisfies the homogeneous medium condition of the metamaterial. Using these unit cells, we create a two-dimensional acoustic metamaterial convex lens that correspond to the conventional optical convex lens and show that this lens focuses acoustic waves in the same manner that an optical lens focuses light waves.

Suppression of superconductivity in the Er1? x Pr x Ba2Cu3O7??

Measurements of electrical resistivity. X-ray diffraction patterns, magnetic susceptibility and thermoelectric power of the Er1-xPrxBa2Cu3O7-δ system have been made. The superconducting transition temperature was found to decrease monotonically with praseodymium concentration, x. From the susceptibility data, it was determined that the valence of praseodymium lies between +3 and +4. The thermoelectric power was found to increase with x, and the slopes of dS/dT were negative except for the case x= 0. The. tendency of the thermopower to change with increasing praseodymium concentration has been qualitatively explained using the theory for strongly correlated systems.