Becoming invisible is no magic trick, although many magicians do employ tricks to ‘disappear’. Redirecting electromagnetic waves so that they bend around an object will make it invisible. Invisibility cloaks have been made from metamaterials using metal elements, but these still have three unresolved problems with cloaking.
The first problem is that this technology does not always work at optical and microwave wave frequencies. Losses incurred by using metal-based metamaterials also limits the size of the object that can be hidden. The final problem is that the variable behavior of spreading waves in different directions of the cloak medium needs to be controlled – this is called anisotropy.
Several fresh approaches to making invisibility cloaks practical have been developed by Elena Semouchkina, an associate professor of electrical and computer engineering, and a group of her graduate students. Instead of using metal-based metamaterials as the cloaking medium, the team is using photonic crystals.
Equations based on the principles of transformation optics can be used to predict what spatial dispersion of material properties will warp electromagnetic waves. The waves do not only have to be bent, but also need to be accelerated along the bent trajectories.
Dielectric materials have insignificant conductivity and low losses. Resonators built with these materials cause electromagnetic waves to bounce back and forth. The principle is similar to how a tuning fork acts as a sound resonator. In their first attempt, Semouchkina and her team used dielectric resonators as this allowed them to control wave propagation in the cloak medium.
Semouchkina’s next step was to use periodic structures known as photonic crystals to build the cloak medium. The team is specifically using properly structured crystals made up of dielectric rods. The resonances in these crystal “atoms” do not define wave transmission as metamaterials do, and therefore have a lot of potential for invisibility cloaking.
The exact type of photonic crystals that the team is using for the cloak medium are able to deliver superluminal phase velocity of propagating waves, meaning that the waves move faster than light speed.
This speed allows the original wave front to be preserved while waves curve past the cloaked object. The crystals also have the essential anisotropy of their refractive indices and act much as a diamond does when it refracts light into many hues. The result is that the wave phase velocities between the various crystal faces are different. The counteracting wave speeds create the illusion of invisibility, thereby acting as a very effective cloaking device.
Semouchkina notes that the pivotal point to solving the anisotropy problem is achieved by varying the lattice parameters of the crystals in required directions.
Cloaking would be useful for both national industry and security, and its applications are only limited by the imagination. Though invisibility cloaks seem mystical, the science behind it is simply being able to control the flow of light.
Full study has been published in the Journal of Optics.