Friday, 23 September 2016

Scientists Created Invisibility Cloak With The Help Of Photonic Crystals

Almost as mysterious as unicorns, searching practical materials for invisibility cloaking is difficult. Scientists have new concepts how to solve that. They are using crystal "atoms" made of dielectric bars called photonic crystals. Meta-materials made from metal components originally planned for constructing invisibility cloaks didn’t solve some significant cloaking problems. There are three difficulties remaining. The first is guiding and controlling anisotropy, the flexible behavior of transmitting waves in many directions of the cloak medium. It is also essential to make sure that the cloak materials can function at microwave and optical wave frequencies. Finally, scientists have to reduce losses that control the size of hidden substances.
Photonic crystals open options for using new practical materials for invisibility cloaking. 
Credit: Michigan Tech, Elena Semouchkina

Elena Semouchkina, an associate professor of electrical and computer engineering, and her graduate students have developed numerous novel methods for making invisibility cloaks more practical. Their newest work, printed in a special topic of Journal of Optics on transformation optics, looks at encouraging a new way to handle electromagnetic waves to make things appear invisible. The team developed the method using photonic crystals.

Making objects invisible is no magic trick. Making objects invisible comes down to transmitting electromagnetic waves. The cloak medium needs to turn the routes of waves around an area to hide an object and move waves along the turned paths. Based on the values of transformation optics, there are equations that can calculate what 3-D dispersion of material properties will bend electromagnetic waves accurately.

To deliver the arranged dispersions, Semouchkina and her team began by using meta-materials that were collected not from metal, however from dielectric resonators. Dielectric materials have minor conductivity and low losses; dielectric resonators affect electromagnetic waves to bounce back and forward much like a tuning fork acts as a sound resonator. This allows for guiding wave propagation in the cloak medium.

Semouchkina is now constructing the cloak medium from periodic arrangements known as photonic crystals. Precisely, they are using accurately structured crystals composed of dielectric rods. Unlike meta-materials, the resonances in these crystal "atoms" do not express wave transmission. As such, photonic crystals express a lot of promise for invisibility cloaking. Photonic crystals that Semouchkina and her team made from the cloak medium are able to deliver superluminal phase velocity of spreading waves. That is, the waves move quicker than the speed of light. Such velocity lets for maintaining the original wavefront whereas waves curve past the cloaked object. Like a diamond bending light into many hues, these photonic crystals also take the required anisotropy of their refractive directories. That means wave phase velocities are not similar between several crystal faces. In terms of cloaking, these responding wave speeds would make the illusion of invisibility.

Semouchkina says, "The key point to answering the anisotropy problem is changing the lattice parameters of the crystals in wanted directions."

The uses for cloaking range as far as thoughts can go and would be beneficial for both national security and industry. And while invisibility cloaks appear mystical, the science is just controlling the flow of light.

The above article is re-printed from information and data provided by Michigan Technological University.

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