Bending a periodically layered structure for transformation acoustics

Abstract

Anisotropic acoustic metamaterials have been proved very useful for their high potential in guiding and manipulating sound energy. In this paper, we further develop the idea by using periodically layered structures for transformational acoustics. Such a simple scheme periodically inserts identically bent solid plates in a background fluid. It forms a metamaterial with high refractive index normal to the curved plates and an index near to one along the plates. We show that the periodically layered structure, combined with transformation approach, can be cut into particular device shapes for acoustic cloaking and illusion. * Electronic address: jensen.li@cityu.edu.hk 1 Metamaterials are bringing new opportunities in manipulating electromagnetic, acoustic, and water waves, particularly through the framework of transformation optics [1][2][3][4][5][6][7][8][9][10]. In acoustics, we can design such metamaterials which consist of artificial periodic subwavelength structures to make their description using effective densities and moduli valid. Moreover, due to the engineering flexibility associated with these artificial materials, extreme values in their constitutive parameters can be obtained by using resonating subwavelength units [11][12][13][14]. Nevertheless, there is always a tradeoff between extreme constitutive parameters near resonance and a large working frequency bandwidth with low loss. It is therefore reasonable to investigate moderate acoustic metamaterials which can work in broadband with low loss. By introducing asymmetry in the shape of unit cells or in the constructing units, we can create acoustic metamaterials with broadband anisotropy [15,16].In particular, a one-dimensional lattice of alternating layers of a solid and a fluid has been found to possess large anisotropy. Such a simple periodically layered structure has been used to guide sound energy along the direction of fluid perforation for different applications. Acoustic superlenses, hyperlenses, acoustic devices with extraordinary transmission and recently an acoustic cavity have been fabricated utilizing this guiding capabilty of anisotropic acoustic metamaterials [17][18][19][20][21][22][23][24].In this work, we further extend the usage of such a simple periodically layered structure into the domain of transformation acoustics. In particular, we bend a periodic stack of planar solid plates with a percolating fluid. The periodically layered structure can be used to guide sound waves as a transformational acoustic device through cutting a particular shape out of the periodic medium. Instead of the isotropic multilayer approach in constructing cylindrical cloaks [25] with the isotropic constitutive parameters varying in space, we consider only two types of constant materials: a solid and a fluid with constant material parameters and constant filling ratio. Let us begin with the planar solid plates, with volume filling fraction η in a fluid, stacking along the ydirection with a subwavelength lattice constant. Such a metamaterial (with l...

Experimental realization of broadband acoustic omnidirectional absorber by homogeneous anisotropic metamaterials

“…In practice, a homogeneous anisotropic metamaterial can be conveniently mimicked by using periodically layered structure. 5,18 In the current work, we will demonstrate an implementation of the proposed scheme by using a fin-like structure, which can mimic a metamaterial with extremely anisotropic parameters. 5 The designed model is composed of fins placed periodically at the angular direction in the host medium (chosen as air here), as shown in Fig.…”

An analogy strategy for transformation optics

“…Similar to an invisibility cloak that can conceal isolated objects as if nothing is present in the space, a carpet cloak hides a bump below it and renders the uneven surface flat to observers. Although some previous work demonstrated that employing anisotropy is a possible route as well [34][35][36], we focus on isotropic design as most authors have done [11,[37][38][39][40][41][42][43]. Taking advantage of the light-fluid analogy, the principle of cloaking can be interpreted with a very intuitive picture, that light is guided to wrap around an object like water flows around a stone [2,27].…”