Finding practical materials for invisibility cloaking is challenging. Metamaterials made from metal elements initially proposed for constructing invisibility cloaks, did not solve some important cloaking problems. There are three challenges remaining. The first is controlling anisotropy—the variable behavior of propagating waves in different directions of the cloak medium. It's also important to make sure that the cloak materials can operate at microwave and optical wave frequencies. Finally, researchers have to decrease losses that restrict the size of hidden objects. Michigan Technological University researchers have developed several novel approaches to making invisibility cloaks more practical. Their latest work looks at a promising new way to manipulate electromagnetic waves to make objects appear invisible. The team developed an approach using photonic crystals. http://phys.org/news/2016-08-invisibility-cloak-photonic-crystals.html Paper: http://iopscience.iop.org/article/10.1088/2040-8978/18/4/044007
