Sonic gradient index lens for aqueous applications

Acoustics Division, Naval Research Laboratory, Washington, DC 20375, USA
Applied Physics Letters (Impact Factor: 3.79). 10/2010; DOI: 10.1063/1.3489373
Source: IEEE Xplore

ABSTRACT We study the acoustic scattering properties of a phononic crystal designed to behave as a gradient index lens in water, both experimentally and theoretically. The gradient index lens is designed using a square lattice of stainless-steel cylinders based on a multiple scattering approach in the homogenization limit. We experimentally demonstrate that the lens follows the graded index equations derived for optics by mapping the pressure intensity generated from a spherical source at 20 kHz. We find good agreement between the experimental result and theoretical modeling based on multiple scattering theory.

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    ABSTRACT: In this letter, we numerically demonstrate focusing of the lowest antisymmetric Lamb wave in a gradient-index phononic crystal PC silicon plate and its application as a beam-width compressor for compressing Lamb wave into a stubbed phononic tungsten/silicon plate waveguide. The results show that beam width of the lowest antisymmetric Lamb wave in the PC thin plate can be compressed efficiently and fitted into tungsten/silicon PC plate waveguide over a wide range of frequency. © 2011 American Institute of Physics. doi:10.1063/1.3583660 Over the past decade, propagation of bulk and surface acoustic waves in periodic structures called phononic crystals PCs has attracted a lot of interests due to the re-newed physical properties of complete band gap, 1–5 negative refraction, 6,7 etc. The existence of complete band gaps in such periodic structures has led to a variety of potential ap-plications, such as filters, 8 efficient acoustic waveguides, 9–11 and high frequency resonators. 12–14 Analogous to the nega-tive refraction of electromagnetic waves in photonic crystals, 15–17 the counterpart of bulk acoustic waves in slab of two-dimensional PCs has also triggered intensive researches, 6,7,18–23 and led to the research of PC-based acous-tic flat lenses that can be used to focus narrow band acoustic waves for certain incident angles. By utilizing PC flat lens, reported results have demonstrated theoretically and experi-mentally that the resulting superlensing and imaging effects can break the diffraction limit. In addition to the bulk wave studies, investigations on the focusing of bending waves in perforated PC thin plates due to the negative refraction effect were reported. 24,25 In spite of the complicated wave disper-sion existed in thin plates, the results showed that propaga-tion of the lowest order antisymmetric Lamb wave in PC thin plates still preserve the negative refraction features. Re-cently, in order to obtain focusing of bulk acoustic waves with larger bandwidth, the gradient-index GRIN PC has also been proposed. 26,27 The results demonstrated that GRIN PC allows acoustic wave focusing over a wide range of op-erating frequencies and making it suitable for applications such as flat acoustic lenses 28,29 and acoustic wave couplers. In this letter, we demonstrate focusing of the lowest antisymmetric Lamb wave in a perforated GRIN PC silicon plate and its application as a beam-width compressor for compressing Lamb wave into a stubbed phononic tungsten/silicon plate waveguide. 30 Figure 1 shows the band structure of a thin perforated silicon 31 PC plate with square lattice calculated by the finite element method COMSOL MULTIPHYSICS.
    Applied Physics Letters 01/2011; 98. · 3.79 Impact Factor
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    ABSTRACT: We present a theoretical study on the tunability of phononic band gaps in two-dimensional phononic crystals consisting of various anisotropic cylinders in an isotropic host. A two-dimensional plane-wave expansion method was used to analyze the band diagrams of the phononic crystals; the anisotropic materials used in this work include cubic, hexagonal, trigonal, and tetragonal crystal systems. By reorienting the anisotropic cylinders, we show that phononic band gaps for bulk acoustic waves propagating in the phononic crystal can be opened, modulated, and closed. The methodology presented here enables enhanced control over acoustic metamaterials which have applications in ultrasonic imaging, acoustic therapy, and nondestructive evaluation.
    Physical review. B, Condensed matter 01/2011; 83. · 3.77 Impact Factor
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    ABSTRACT: A structured cylindrical scatterer with low-frequency resonances in both the effective bulk modulus and the dynamical mass density is designed and characterized. The proposed scattering unit is made of a rigid cylinder surrounded by a fluid-like shell embedded in a two-dimensional waveguide of height less than the length of the cylindrical scatterer. It is demonstrated that the acoustic metamaterials based on this building unit have negative acoustic parameters in a broad range of frequencies. It is also shown that double-negative behavior can be tailored by adjusting the dimensions and properties of the materials forming the structured scattering unit.
    New Journal of Physics 10/2012; 14(10):103052. · 4.06 Impact Factor

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