The coordinate-transformation-based technique of transformation acoustics allows unprecedented flexibility in manipulating acoustic waves with artificially structured metamaterials. And recent advances in acoustic metamaterials have promised a way to realize unique material parameters unavailable in nature. This provides the possibility of designing unconventional devices such as superlens, concentrators and invisibility cloaks. Among them, the invisibility cloak that can create an “illusion” of the original space with the cloaked object concealed, may be one of the most analyzed and fascinating innovations.
In previous designs, the experimental realization of an illusion cloak using existing materials becomes extremely difficult, especially when the original object has a complicated configuration. Besides, the previous schemes to realize the illusion cloak are supposed to be in free-spaces, but camouflaging an object near boundaries of curved geometry is more common in practice.
As a result, a new scheme to realize the acoustic illusion for an arbitrary object located near a concave surface in any shape is proposed and experimentally verified in this research. Inherently different from the previous designs, here the scattering field of the object is manipulated by utilizing the interesting features of anisotropic metamaterials instead of resorting to "complementary materials"which necessarily require negative-index materials.
And then, the resulting illusion cloak has a simple structure comprising anisotropic materials with positive index, for which the material parameters are non-singular, homogeneous and, moreover, independent of the properties of either the original object or the concave boundary. These features of anisotropic metamaterials should significantly facilitate practical applications of the designed structure. And this unique manipulation should also benefit the development of waveguide, concentrating and imaging devices.
The first experimental demonstration of acoustic illusion is presented. It is characterized by the phenomenon that the scattering waves from the original object are transformed as if the waves are scattered from another “illusion object” which can be freely designed. In a special case where this illusion object is chosen as a bulk of background medium, the illusion cloak becomes an invisibility cloak that can hide an arbitrary object near boundaries of curved geometry.
This research was supported by the National Basic Research Program of China (973 Program), National Natural Science Foundation of China, Research Fund for the Doctoral Program (for new scholar) of Higher Education of China, and A Project Funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions.
The relevant research article entitled “Acoustic Illusion near Boundaries of Arbitrary Curved Geometry” was released online: http://www.nature.com/srep/2013/130312/srep01427/full/srep01427.html and on the Scientific Reports 3(article number: 1427, doi:10.1038/srep01427).