Based on a coordinate transformation approach, Pendry et al. have reported electromagnetically anisotropic and inhomogeneous shells that, in theory, completely shield an interior structure of arbitrary size from electromagnetic fields without perturbing the external fields. We report full-wave simulations of the cylindrical version of this cloaking structure using ideal and nonideal (but physically realizable) electromagnetic parameters in an effort to understand the challenges of realizing such a structure in practice. The simulations indicate that the performance of the electromagnetic cloaking structure is not especially sensitive to modest permittivity and permeability variations. This is in contrast to other applications of engineered electromagnetic materials, such as subwavelength focusing using negative refractive index materials. The cloaking performance degrades smoothly with increasing loss, and effective low-reflection shielding can be achieved with a cylindrical shell composed of an eight (homogeneous) layer approximation of the ideal continuous medium.PACS numbers: 41.20.Jb, 42.25.Fx, 42.25.Gy Pendry et al. [1] have reported a coordinate transformation approach for designing an electromagnetic material through which electromagnetic fields can be excluded from a penetrable object without perturbing the exterior fields, thereby rendering the interior effectively "invisible" to the outside. Related work has shown how small reflections can be realized through engineered coatings for objects of restricted size and shape [2], and refractive index profiles have been derived that bend light to produce 2D invisibility based on a conformal mapping approach and assuming the short wavelength geometrical optics limit [3]. The approach in [1] is far more general: it can be applied to problems of any dimension, and it applies under any wavelength condition, not just geometrical optics. It requires anisotropic media with each permittivity and permeability element independently controlled, but this is within the realm of the metamaterial approach for engineering electromagnetic materials [4]. This approach also requires permittivity and permeability elements with relative magnitudes less than one, and consequently the bandwidth of a passive cloaking material will be limited.This cloaking structure shares many qualities with another application based on exotic electromagnetic materials-the negative refractive index perfect lens [5]. Both are surprising, novel, and of significant theoretical and practical interest. The physical realizability of each is also not immediately obvious from the original analytical derivation. The realization of subwavelength focusing is constrained by strict limits on the precise properties of the medium [5,6], although the effect has been demonstrated in experiment in a number of forms [7,8,9]. Full-wave electromagnetic simulations provided substantial insight into understanding the physical realizability and limitations of this phenomenon. For example, initial simulations were unable...

Metasurfaces are a family of novel wavefront-shaping devices with planar profile and subwavelength thickness. Acoustic metasurfaces with ultralow profile yet extraordinary wave manipulating properties would be highly desirable for improving the performance of many acoustic wave-based applications. However, designing acoustic metasurfaces with similar functionality to their electromagnetic counterparts remains challenging with traditional metamaterial design approaches. Here we present a design and realization of an acoustic metasurface based on tapered labyrinthine metamaterials. The demonstrated metasurface can not only steer an acoustic beam as expected from the generalized Snell's law, but also exhibits various unique properties such as conversion from propagating wave to surface mode, extraordinary beam-steering and apparent negative refraction through higher-order diffraction. Such designer acoustic metasurfaces provide a new design methodology for acoustic signal modulation devices and may be useful for applications such as acoustic imaging, beam steering, ultrasound lens design and acoustic surface wave-based applications.

Through acoustic scattering theory we derive the mass density and bulk modulus of a spherical shell that can eliminate scattering from an arbitrary object in the interior of the shell--in other words, a 3D acoustic cloaking shell. Calculations confirm that the pressure and velocity fields are smoothly bent and excluded from the central region as for previously reported electromagnetic cloaking shells. The shell requires an anisotropic mass density with principal axes in the spherical coordinate directions and a radially dependent bulk modulus. The existence of this 3D cloaking shell indicates that such reflectionless solutions may also exist for other wave systems that are not isomorphic with electromagnetics.

1The control of sound propagation and reflection has always been the goal of engineers involved in the design of acoustic systems. A recent design approach based on coordinate transformations, which is applicable to many physical systems [1][2][3][4][5][6][7][8][9][10][11][12][13]15 , together with the explosive development of a new class of engineered materials called metamaterials, has opened the road to the unconstrained control of sound. However, the ideal material parameters prescribed by this methodology are complex and challenging to obtain experimentally, even using metamaterial design approaches. Not surprisingly, experimental demonstration of devices obtained using transformation acoustics is difficult, and has been implemented only in two-dimensional configurations 10,16 . Here, we demonstrate the design and experimental characterization of an almost perfect threedimensional, broadband, and, most importantly, omnidirectional acoustic device that renders a region of space three wavelengths in diameter invisible to sound.It is well understood that, given an arbitrary geometric transformation of a sound field, the effective mass density and the bulk modulus required to implement that transformation is determined as 5,17 : ρ r = det(A)(A −1 ) T ρ v A −1 and B r = det(A)B v , where ρ is the mass density tensor, B is the bulk modulus, A is the Jacobian matrix of the transformation and r and v denotes the real and virtual space, respectively. One application of the coordinate transformation method that received significant attention, and that we focus on here, is the so called "ground cloak". The ground cloak is a material shell that when placed over arbitrary objects sitting on reflecting surfaces, i.e. ground, makes the object undetectable using sound radiation. The concept has been introduced in the context of electromagnetics 18-20 , but has rapidly been extended to other physical systems, including acoustics 10 .The coordinate transformation technique enabling these cloaking devices is especially suitable for acoustics. Developed by noticing the similarity between the acoustic wave equation and the conduction equation 5 , the method requires a wide range of anisotropic and inhomogeneous material parameters. Unlike electromagnetics, however, these are easier to realize in acoustics in a broadband manner using metamaterial methods because conventional materials have a broad range of acoustic material parameters spanning multiple orders of magnitude.There have been attempts to avoid the difficulties associated with the coordinate transformation approach by using more conventional techniques. However, these entail lowering 2 design requirements, such as replacing omnidirectionality with unidirectionality, and have been proven very challenging as well when applied experimentally 14,26 . Here we show that omnidirectional three-dimensional ground cloaks obtained using coordinate transformation methods are feasible in practice.There are several options for geometric transformations that will map the volume occu...

We present the design, fabrication, and performance analysis for a class of two-dimensional acoustic cloaking coatings in air. Our approach takes advantage of transformation acoustics and linear coordinate transformations that result in shells which are homogeneous, broadband, and compact. The required material parameters are highly anisotropic; however, we show that they are easily achievable in practice in metamaterials made of perforated plastic plates. The good performance of the fabricated design is assessed from measurements of the sound field produced around the cloak by a broadband source. The remarkably low complexity of the device made of perforated plastic plates shows that sound in air can be fully and effectively manipulated using realizable transformation acoustics devices.