Invisibility Cloak With Photonic Crystals

The performance of the superluminal medium formed by rod array sets was demonstrated on an example of a cloaking shell developed for microwave frequency range (Source: MTU)

The performance of the superluminal medium formed by rod array sets was demonstrated on an example of a cloaking shell developed for microwave frequency range (Source: MTU)

Almost as elusive as unicorns, finding practical materials for invisi­bility cloaking is challenging. Michigan Techno­logical University researchers have new ideas how to solve that. 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.

Elena Semouchkina, an associate professor of electrical and computer engi­neering, and her graduate students 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.

Making objects invisible comes down to redirecting electro­magnetic waves. The cloak medium needs to bend the paths of waves around an area to hide an object and accelerate waves along the bent tra­jectories. Based on the principles of trans­formation optics, there are equations that can predict what spatial dispersion of material pro­perties will warp electro­magnetic waves properly. To provide the prescribed dispersions, Semouch­kina and her team started by using meta­materials that were composed not from metal, but di­electric resonators. Dielectric materials have negligible conduc­tivity and low losses; dielectric resonators cause electro­magnetic waves to bounce back and forth much like a tuning fork acts as a sound resonator. This allows for controlling wave pro­pagation in the cloak medium.

Based on this insight, Semouch­kina’s lab developed cloak designs for microwave and infrared frequency ranges using, respec­tively, ceramic and chalco­genide glass resonators. Later, they proposed another approach to cloaking objects by using multilayer coatings formed from ordinary dielectrics. In order to suppress wave scattering from a cloaked object, they optimized the di­electric properties and thick­nesses of layers. Then to hide a larger object, the group came up with another approach using specially designed dielectric lenses to cloak a bigger space.

Now, they’re turning to building the cloak medium from perio­dically structured photonic crystals. Specifically, they are using properly structured crystals composed of dielectric rods. Unlike meta­materials, the resonances in these crystal subunits do not define wave transmission. As such, photonics crystals show a lot of promise for invisi­bility cloaking. Photonic crystals that Semouch­kina and her team employ for the cloak medium are able to provide super­luminal phase velocity of propa­gating waves. That is, the waves move faster than the speed of light.

Such velocity allows for preserving the original wave front while waves curve past the cloaked object. Like a diamond refracting light into many hues, these photonic crystals also possess the required aniso­tropy of their refrac­tive indices. That means wave phase velo­cities are different between the various crystal faces. In terms of cloaking, these counter­acting wave speeds would create the illusion of invisi­bility.

“The key point to solving the aniso­tropy problem is varying the lattice para­meters of the crystals in desirable directions,” Semouch­kina says. The appli­cations for cloaking range as far as imagi­nations can go and would be useful for both national security and industry. And while invisi­bility cloaks seem mystical, the science is simply controlling the flow of light. (Source: MTU)

Reference: E. Semouchkina et al.: Superluminal media formed by photonics crystals for transformation optics-based invisibility cloaks, J. Opt. 18, 044007 (2016); DOI: 10.1088/2040-8978/18/4/044007

Link: Dept. of Electrical and Computer Engineering, Michigan Technological University, Houghton, USA

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