Metamaterials - From new concepts to applications

For a long time, the natural materials have been classified into two types: crystals and noncrys-tals, until Daniel Shechtman discovered quasicrystals in 1982, who won the Nobel chemistry prize in 2011 for this work. In fact, crystals and noncrystals are composed of periodically-distributed and randomly-distributed atoms, while quasicrystals have a third material state between crystals and noncrystals: which are non-periodic structures of atoms with certain rules instead of random. Hence the two factors to affect natural material properties are the atoms themselves and the spatial arrangements of atoms. Quasicrystals have brought a lot of new features of materials and found applications in steel armour, non-stick frying pans, and devices in cars for recycling waste heat into electricity. However, it is very hard to control atoms themselves and their spatial arrangements to get more material properties. Metamaterials provide us a freedom to tailor the material properties, both for electric and magnetic. Metamaterials are composed of periodic or non-periodic structures of artificial “atoms” or “particles”, which have a size of subwavelength scale. The flexible design of single artificial particles, the feasible arrangements of such particles, and the high anisotropy make it possible to control the material properties as desired: metamaterials can be used to realize the effective permittivity and/or permeability which cannot be achieved in nature. Hence they have either unique features with unusual physical phenomena (such as negative refraction, invisibility cloak, optical illusion, etc.) or superior performance than the natural materials. In this talk, I will focus on microwave metamaterials and introduce their counterparts to crystals, noncrystals, and quasicrystals: homogeneous metamaterials, random metamaterials, and inhomogeneous metamaterials. For all three cases, I will introduce the new concepts and important experiments and ap- lications in microwave frequencies conducted in my group, including the invisibility cloaks, electromagnetic black hole, radar illusion devices, power combination for omnidirectional radiations, planar gradient-index lenses, flattened Luneburg lens, Maxwell fisheye lens, high-gain Vivaldi antennas, and decoupling device for MIMO system.