We report a metamaterial which simultaneously possesses a negative bulk modulus and mass density. This metamaterial is a zinc blende structure consisting of one fcc array of bubble-contained-water spheres (BWSs) and another relatively shifted fcc array of rubber-coated-gold spheres (RGSs) in epoxy matrix. The negative bulk modulus and mass density are simultaneously derived from the coexistent monopolar resonances from the embedded BWSs and dipolar resonances from the embedded RGSs. The Poisson ratio of the metamaterial also turns negative near the resonance frequency.

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 lights on designing novel devices of acoustic metamaterials. a)To whom all correspondence should be addressed, e-mail address is zyliu@whu.edu.cn ε = − and permeability 1 EM μ = − can focus both the propagating and evanescent waves of a point source into a perfect image. 5 Thereby such a slab device has been referred as the perfect lens. Pendry's perfect lens stimulated lots of research interests due to the great significance of subwavelength imaging in various applications (see the ref. 6 and references therein).Acoustic MMs have also attracted much attention more recently. 7-14 Till now most concerns have been focused on their realization. MMs with negative effective mass density has been realized by a class of three-component phononic crystal, 7,8 and more recently by a membrane-type structure. 9 An ultrasonic MM consisting of arrays subwavelength Helmholtz resonators has been demonstrated to exhibit a negative effective bulk modulus. 10 MMs with simultaneously negative mass density and bulk modulus have also been realized with liquid 11 and solid 12 matrix. These offer promising opportunities to design novel acoustic devices. 13,14 However, studies on such subjects are still relatively lacking.Recently, Zhang et al investigated the surface waves between a fluid half-space and an acoustic MM half-space. 13 They concluded that the negative mass density is the necessary condition for the existence of surface states on acoustic MMs. In addition, a

We realize a flat lens with graded negative refractive index by a two-dimensional phononic crystal.The index-grade is achieved by gradual modification of the filling fraction along the transverse direction to propagation. We demonstrate that this lens can realize the focusing and amplification of parallel incident acoustic waves. A formula for the refractive index profile is derived by which the lens with desired focal length can be precisely designed and the aberrations of image can be effectively reduced. This designable lens is expected to bear significance in applications such as coupling or integration with various types of acoustic devices.PACS numbers: 43.20.+g, 43.35.+d, 43.40.+s The ability to manipulate waves has been greatly enhanced with the recent advances in metamaterials (MMs) due to the many unusual properties they possess [1]. One of these predominant properties is the negative refraction which has been utilized to image with flat MM lenses [2]. Alternatively, negative refraction can also be realized by photonic/phononic crystals (PCs) which behave like materials with negative refractive index within some frequency regions 3

Acoustic resonant modes in a slab with a periodic array of holes are analyzed by using three-dimensional finite-difference time-domain method. We show that there exist two different types of resonant modes in this slab: the coupled Stoneley wave resonant modes and the waveguide resonant modes. Both resonant modes can lead to complex resonant line shapes in the transmission spectrum. By using the distinct property of these resonant modes, a tunable phononic crystal structure consisting of coupled phononic crystal slabs is introduced for achieving acoustic filters and sensors.

In this letter, the subwavelength imaging of acoustic waves is reported based on a mechanism that the evanescent modes of a source are canalized by the Bloch modes of a two-dimensional phononic crystal that served as the lens. The phononic crystal was designed to have a thickness that meets the condition of Fabry–Pérot resonance in order to enhance wave transmission and hence to improve imaging performance. Numerical simulations demonstrated that for a point acoustic source an image as small as 0.16λ can be formed.

We report the realization of acoustic collimating beams by a two-dimensional phononic crystal, which has a negative effective refractive index of small magnitude. The radiation far field of a point source through the phononic crystal with a divergence angle of about 5° is obtained. The measured intensity field distribution in experiment further confirms this collimating property.

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