Several years ago, Prof. John Pendry proposed a recipe to make an invisibility cloak. The theory behind this proposal, which is now known as transformation optics, is based on a property of Maxwell’s equations to preserve their shape under coordinate transformations. In particular, it was shown that almost any arbitrary deformation of the electromagnetic wave field one can imagine is possible when transformation optics is used to provide a description of the materials that would produce this deformation. One can then readily apply this knowledge to (theoretically) make an invisibility cloak: just create a space which cannot be accessed by electromagnetic fields, so that all waves bend around this space and propagate away from it as if it wasn’t there. This is brilliant, but it turns out that the materials required to make an invisibility cloak do not exist. Therefore, the efforts of many researchers were directed to solve this issue, by either making a simplified cloak, which reduces the visibility of objects for a restricted class of illumination fields, or by applying the transformation optics to creating other fascinating optical devices, such as light concentrators or rotators. Many of these proposed devices rely on a certain polarisation state of light, and they do not work for another polarization.
The authors of this work have proposed how to use transformation optics to create dual function structures, which they call “bicephalous metamaterials”. Through careful design of the material parameters the authors were able to show how metamaterial structures can be used to perform two different sensible operations depending on the polarization of light. They have provided two examples of such structures. The first is a structure that can concentrate light of one polarization, while rotating the field of another polarisation. The second structure acts as a cloak that hides an object from light of one polarisation, while it behaves as a ‘superscatterer’ for another polarisation. Since such strict division of the polarisations is only possible in cylindrical geometry, the proposed designs are limited to cylindrical structures. The authors also suggest that similar approach can be applied to acoustics.
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