ENG Lab Makes Metamaterial Advances

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Xin Zhang, Hu 'Tiger' Tao and Richard Averitt
Xin Zhang, Hu 'Tiger' Tao and Richard Averitt

Metamaterials, synthetic materials crafted in the laboratory and not available in nature, are providing the building blocks for future technologies, from detectors of dangerous chemicals to invisibility cloaks. Associate Professor Xin Zhang (ME) and doctoral student Hu “Tiger” Tao are leading the way, with their expertise in complex, highly technical fabrication techniques, to create metamaterials that can absorb, bend and detect electromagnetic radiation.

“We are gaining capability in building these flexible multi-layered metamaterials which have so many possible applications,” said Tao, whose recent advances in the field include a detector combining several different metamaterials to absorb multiple wavelengths of radiation simultaneously and creating a metamaterial that is plastic-like and flexible.

The potential applications of any metamaterial depend on the properties researchers design metamaterials to have, including which wavelengths of radiation the metamaterial can slurp up or push around. Zhang and Tao, in collaboration with Assistant Professor Richard Averitt (CAS, Physics), focus on inventing metamaterials that manipulate electromagnetic waves in the terahertz range.

In a past publication, they created a specialized metamaterial that absorbs terahertz radiation, a previously hard-to-detect range of wavelengths longer than visible light but shorter than microwaves. The group’s recent article in Optics Express builds on this work, constructing detectors of hexagonal metamaterial tiles, rather than the usual square cells. Each hexagon is 30 microns wide with a pattern of two-micron wide gold lines printed on it. The spacious interior of a hexagonal shaped cell – more roomy than the squares’— gives the researchers room to make variations of the gold pattern in the cells. Each different pattern can detect a specific terahertz frequency.

“Our metamaterial combines multiple designs into one sample. That has never been done before,” said Tao. “Here, we’ve matched the performance of the metamaterials to a target.”

By putting together three differently patterned hexagonal cells, the team created an array that detects three terahertz wavelengths at once. The three designs can be tailored to match the signature set of wavelengths radiated by any specific molecule. This mosaic detection method minimizes the chances of a false positive reading because it matches three characteristic wavelengths, not just one.

“In the future, we can detect any biological molecule or other things such as dangerous chemicals, as long as we find their frequency spectrum features, then we can use metamaterials to match and search for those,” said Tao.

Tao, Zhang and Averitt continued to tinker with metamaterial fabrication techniques to make them flexible and make them better absorbers of incoming electromagnetic radiation. This work is described in Physical Review B.

Tao changed the way he builds metamaterials to make them absorb nearly 100 percent of incoming light from all angles, not just light headed straight for the detector. This puts a metamaterial detector one step closer to commercial viability, since in real world scenarios, radiation will likely hit the detector at many different angles. 

Tao also made a switch from using stiff wafers of gallium arsenide to polyimide, a flexible plastic-like material, so his metamaterials are flexible and can wrap around any surface –a nice feature for his terahertz detectors and absorbers and an essential property for invisibility cloaks.

Building metamaterial devices in the terahertz range has its own applications, and it helps bring the field closer to bending visible light around objects. This task – creating invisibility —continues to intrigue researchers and has been the subject of many theoretical papers, said Tao, but, as yet, no real-world demonstrations with visible light waves. One research group built a device that shunts microwaves around an object. Microwaves are very long compared to light waves, however, and the job gets increasingly difficult the shorter the wavelengths involved. Metamaterials that can wrangle visible light like this don’t yet exist, but as researchers create metamaterials to work with shorter and shorter wavelengths, they step closer to manipulating visible light wavelengths, and possibly, someday achieving invisibility.

“We can’t make an invisibility cloak yet, but this is a really big step. We’ve demonstrated the potential toward a functional invisibility cloak,” said Tao.  “It’s kind of funny that on one hand, I am trying to build an invisibility cloak to hide objects, while, on the other hand, I am trying to build a terahertz detector to make any hidden object visible to terahertz frequencies.”
 

Three hexagonal metamaterial designs. Each different pattern of two-micron-wide gold lines confers the ability to detect a specific terahertz frequency.
Three hexagonal metamaterial designs. Each different pattern of two-micron-wide gold lines confers the ability to detect a specific terahertz frequency.