Colloquium

The concept of transformation optics as applied to cloaking is to map a region of space into another region with the same outer boundary in such a way that both regions display the same optical properties as far as the exterior world is concerned. The talk will discuss how the original ideas for transformation optics translate to acoustic and elastic waves. Surprisingly, the acoustic transformation is not unique, unlike the case for electromagnetic waves. The non-uniqueness means that the same effects can be achieved using different types of metamaterial in the transformed domain, ranging from materials with anisotropic density to pentamode materials. Both of these unusual types of constitutive properties will be defined and related to the transformation (mapping). Recent work on the fabrication of pentamode materials (PM) based devices will be described, and some unexpected properties explained, such as negative index of refraction. The ideas of transformation optics do not go over completely for elasticity – the difficulty arises from the presence of two wave types with distinct speeds. Some recent progress will be discussed.