Who says the invisibility cloak only exists in works of fiction? A group of physicists has been able to make objects fully invisible in the microwave range.

The current prevailing idea of invisibility is based on metamaterial coatings. Metamaterials, artificially designed structures with optical properties that cannot be found elsewhere in nature, have the capability to change light's direction in exotic ways such as making light curve around a cloaked object.

In 2012, researchers from Duke University used a tweaked metamaterial for a cloak that uses a negative refractive index so microwaves bend around objects. The result is that what's behind the object can be seen instead of the object itself.

One problem with coating layers that are based on metamaterials is that they are very hard to fabricate. They are also not compatible with a number of invisibility ideas. The new results, however, which used a process that made use of a new understanding of electromagnetic wave scattering, were achieved using homogeneous objects that do not rely on additional coating layers. The new method is better in terms of simplicity and cost-effectiveness.  

"Here we demonstrate that the suppression of light scattering for any direction of observation can be achieved for a uniform dielectric object with high refractive index, in a sharp contrast to the cloaking with multilayered plasmonic structures suggested previously," Mikhail Rybin, from the Metamaterials Laboratory at ITMO University in St. Petersburg, Russia, and colleagues reported in their study, which was published in the journal Scientific Reports on March 5. "Our finding is based on the novel physics of cascades of Fano resonances observed in the Mie scattering from a homogeneous dielectric rod." Physicists from Ioffe Institute and Australian National University also participated in the study.

For the experiment, the researchers used a glass cylinder that is filled with water, the refractive index of which can be regulated by altering the temperature. The researchers found that at some frequencies, waves that get scattered via resonant and nonresonant mechanisms have opposing phases. They are also mutually destroyed, which makes the object invisible.

The invisibility phenomenon in an object that is not covered in additional coating layers is significant in engineering in that homogeneous cylinders are easier to produce. The findings could also pave way to developments in nanoantennas where invisible structural elements may help in the reduction of disturbances. Invisible rods, for instance, can be used to support a miniature antenna complex that connects two optical chips.

Photo: ITMO University 

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