Looking Inside Refraction of Metamaterials

The shape and positioning of the rods in this metamaterial cause light to bend at a negative angle. Better understanding of this dynamic will speed the development of new metamaterials such as perfect lenses and invisibility cloaks. (Source: N. Ganji)

Meta­materials offer the very real possibility that our most far-fetched fancies could one day become real as rocks. From invisi­bility cloaks and perfect lenses to immensely powerful batteries, their super-power appli­cations tantalize the imagination. “Not many real meta­material devices have been developed,” says Elena Semouch­kina, an associate professor of electrical engi­neering at Michigan Techno­logical Univer­sity. Soldiers can’t throw invisi­bility cloaks over their shoulders to elude sniper fire, and no perfect lens app lets you see viruses with your smartphone. In part, that’s because tradi­tionally, researchers overly simplify how meta­materials actually work. Semouchkina says their compli­cations often have been ignored.

So she and her team set about inves­tigating those compli­cations and dis­covered that the magic of meta­materials is driven by more than just one mechanism of physics. Meta­materials may seem complex and futuristic, but the opposite is closer to the truth, says Semouch­kina. Met­amaterials are engineered materials that have proper­ties not found in nature. They are typically built of multiple identical elements fashioned from conven­tional materials, such as metals or noncon­ductive materials. These designer materials work by bending the paths of electr­omagnetic radiation – from radio waves to visible light to high-energy gamma rays – in new and different ways. How meta­materials bend those paths drives their peculiar appli­cations. For example, a meta­material invisi­bility cloak would bend the paths of light waves around a cloaked object, accelera­ting them on their way, and reunite them on the other side. Thus, an onlooker could see what was behind the object, while the object itself would be invisible.

The conventional approach among meta­materials researchers has been to relate a meta­material’s refrac­tive properties to resonance. Each tiny building block of the meta­material vibrates like a tuning fork as the electromagnetic radiation passes through, causing the desired type of refraction. “Meta­materials seem simple, but their physics is more complicated,” Semouch­kina says, explaining that she and her team focused on dielectric meta­materials, which are built of elements that don’t conduct elec­tricity. The team ran numerous computer simu­lations and made a surprising discovery: it was the shape and repetitive organi­zation of the building blocks within the metamaterial that affected the refrac­tion. Resonance seemed to have little or nothing to do with it.

The meta­materials they studied had charac­teristics of another type of artificial material, photonic crystals. Like meta­materials, photonic crystals are made of many identical cells. In addition, they behave like the semi­conductors used in electronics, except they transmit photons instead of electrons. “We found that the proper­ties that go along with being a photonic crystal can mask the resonance of met­amaterials, to the point they can cause unusual refraction including negative refrac­tion, which is necessary for the development of a perfect lens,” Semouch­kina says.

So what does this mean for the scientists and engineers designing tomorrow’s super materials? “Basically, we need to recognize that some of these structures can exhibit proper­ties of photonic crystals, and we need to take their physics into account,” Semouch­kina says. “It’s an evolving field, and it’s a lot more compli­cated than we’ve given it credit for.” Semouch­kina’s team is working on developing using photonic crystals, but she stresses that meta­materials research can have other real-world appli­cations.

One of her projects focuses on using meta­material concepts to improve the sensi­tivity of magnetic resonance imaging (MRI), which could lead to better medical diag­nostics and advances in biolo­gical research. “This is a very practical outcome, compared to the Harry Potter stuff,” she says. Under­standing the under­lying physics of meta­materials will speed up the develop­ment of such devices. (Source: Michigan Tech)

Reference: N. P. Gandji et al.: All-dielectric metamaterials: irrelevance of negative refraction to overlapped Mie resonances, J. Phys. D: Appl. Phys. 50, 455104 (2017); DOI: 10.1088/1361-6463/aa89d3

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

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