Elastic transformation optics devices using smart metamaterials

다국어 초록 (Multilingual Abstract)

Transformation optics and artificial metamaterials can make possible to design and realize new phenomena such as negative refraction and optical cloaking. As invariance of Maxwell’s equations in coordinate transformation, transformation optics can not only control the behavior of electromagnetic wave but also be the theoretical basis for optical cloak which bend the light around an object to conceal. Metamaterials are artificial periodic structure represented by physical properties designed by transformation optics. The metamaterials are satisfied effective medium theory and regard as effective medium not each components because the period of the metamaterials is much smaller than operating wavelength.
In this dissertation, I suggest a theoretical approach to elastic transformation optics as a design tool of smart metamaterials that overcome the limit of the conventional metamaterials, then demonstrate these concepts as an electromagnetic cloak which is the most popular transformation optics device. Smart metamaterials have changeable property distribution from altered internal composition of medium by external elastic deformation. So far existing metamaterials with rigid shape and fixed property distribution are fragile to external deformation and impact and hard to fabricate when complicated material properties are required. Smart metamaterials are self-adjustable to the external stimulus, while the distribution of the internal structural compositions and physical properties satisfy the transformation optics during elastic deformation. To design smart metamaterials, it is required that elastic transformation optics integrating elastics and conventional transformation optics. Based on quasi-conformal mapping for isotropic dielectric materials, elastic transformation optics is proved that deformation of linear elastic solid with negative Poisson’s ratio satisfies transformation optics. Then one versatile elastic transformation optics device consist of smart metamaterials with negative Poisson’s ratio is fabricated and experimentally demonstrated in microwave regime(X-band) as a self-adjustable smart cloak for any deformable shape to conceal and a bendable smart waveguide maintaining phase of internal wave. This smart metamaterials with negative Poisson’s ratio and high dielectric constant called auxetic smart metamaterials.
Negative Poisson’s ratio and high dielectric component in smart metamaterials make it possible to satisfy transformation optics. However negative Poisson’s ratio requires complex structures that are hard to fabricate and high dielectric constant cannot support broad operating band. Therefore I present another smart metamaterials whose composite structure has ordinary Poisson’s ratio and dielectric constant of silicone rubber. This non-auxetic smart metamaterials without negative Poisson’s ratio satisfy transformation optics only after elastic deformation, not during deformation. However, it is easy to fabricate initial periodic structure then simple deformation complete a transformation optics device. So that non-auxetic smart metamaterial scheme is amenable to easier fabrication of large area metamaterial device. Carpet cloak of non-auxetic smart metamaterials is fabricated and verified in microwave regime, then it is confirmed that the cloak’s broadband performance maintains regardless of incident angles.