Almost as unicorns as elusive, looking for stealth material that can be applied in practice is challenging. Now, researchers at the Michigan Tech University have some new ways to do this.

Metallurgical metamaterials were initially proposed for the construction of stealth cloak material but did not address some of the key invisibility issues.

Three remaining challenges. The first is to control the variability of the behavior of propagating waves in different directions of an anisotropic medium on stealth cloaks. It is also important to ensure that stealth materials can work at microwave and light wave frequencies. Finally, researchers have to reduce the loss and limit the size of hidden objects.

Elena Semouchkina, an associate professor in electrical and computer engineering, and his graduate students have developed several new ways of making invisible capes more practical. Their latest work, published in the optical section of Optical magazine's special theme section, focuses on a promising new way to manipulate electromagnetic waves to render objects invisible. The team developed a method using photonic crystals.

Invisible science

Being invisible is not a trick of magic. The object invisible to boil down to the electromagnetic reorientation. Invisibility cloak media needs to bend the path of electromagnetic waves around an area to hide an object and accelerate the waves along a curved path. According to the principle of transformation optics, the use of equations can predict which material's spatial dispersion will distort electromagnetic waves.

In order to provide the prescribed dispersion, Semouchkina and her team started to use metamaterials that are not composed of metals but rather dielectric resonators. Dielectric materials have negligible conductivity and low loss characteristics; dielectric resonators cause the electromagnetic waves to reflect back and forth, much like a tuning fork acts as a resonator. This allows the control of the propagation of electromagnetic waves in stealth cloaks.

Based on this understanding, Semouchkina's laboratory developed invisibility cloaks for use in the microwave and infrared frequency ranges, and cloak designs utilizing ceramic and chalcogenide glass resonators, respectively. Later, they proposed stealth cloaks based on multilayer films based on common dielectric materials. In order to suppress waves scattered from invisible objects, they optimize dielectric properties and layer thickness. In order to hide a larger object, the team proposed another method to create a larger cloak of space using a specially designed dielectric lens.

Now they are turning to stealth media from materials whose periodic structure is called photonic crystals. Specifically, they use a dielectric rod made of a suitably structured crystal. Unlike metamaterials, the resonance of these "atoms" of crystals does not define the transmission of waves. Therefore, photonic crystals for the production of stealth clothing has great potential.

Superluminal material

Photonic crystal materials used by Semouchkina and her team to make cloak media can provide phase velocities that propagate the wave's speed of light beyond the speed of light. In other words, the waves move faster than the light.

This speed preserves the original wavefront as the wave curve passes through the hidden object. Like a diamond refracted light is divided into many colors, these photonic crystals also have the required refractive index anisotropy. This means that the wave velocity is different on different planes. Invisibly, these invisible wave velocities give rise to invisible delusions.

"The key point to solve the anisotropy problem is the different crystal lattice parameters in different directions," Semouchkina said.

Invisibility cloaks are just as promising as the range imaginable and will be useful for both national security and industrial applications. Stealth cloak may sound mysterious, and science simply controls the flow of light.

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