Metamaterial stays cool while absorbing electromagnetic energy

Monday, 13 February 2017
New metamaterial stays cool while absorbing electromagnetic energy

Electrical engineers at Duke University have created the world's first electromagnetic metamaterial made without any metal, capable of absorbing electromagnetic energy without heating up.

Metamaterials are synthetic materials that feature individual engineered properties that are not found in nature. Metamaterials are tuned to manipulate electromagnetic waves in specific ways, such that a wave passing through the metamaterial behaves in a certain way as a whole.

Traditionally, in order to manipulate electromagnetic waves, engineers have had to use electrically conducting metals. However, metals have fundamental issues -- the higher the electrical conductivity, the better the material conducts heat. This limits their usefulness in temperature-dependent operations.

The Duke University engineers demonstrated the first completely non-metal, dielectric electromagnetic metamaterials. Their metamaterial is created with boron-doped silicon, and its surface is dimpled with cylinders, and designed to absorb terahertz waves.

Its ability to absorb electromagnetic energy without heating up will have direct implications for imaging, sensing and lighting, and according to the researchers, their approach will be applicable not just for terahertz fields, but also for almost any frequency of the electromagnetic spectrum.

"People have created these types of devices before, but previous attempts with dielectrics have always been paired with at least some metal," said Willie Padilla, Professor of Electrical and Computer Engineering at Duke University.

"We still need to optimise the technology, but the path forward to several applications is much easier than with metal-based approaches."

The engineers used computer simulations in their creation of the metamaterial, calculating how terahertz waves would interact with cylinders of varying heights and widths. They then manufactured a prototype consisting of hundreds of the optimised cylinders aligned in rows on a flat surface. Testing this metasurface found that it absorbed 97.5 per cent of the energy produced by waves at 1.011 terahertz.

Efficiently absorbing energy from electromagnetic waves is an important property for many applications. For example, thermal imaging devices can operate in the terahertz range, but because they have previously included at least some metal, the fast heat propagation means it is a challenge to get sharp images.

Another potential application for the new technology is efficient lighting. Incandescent light bulbs make light but also create a significant amount of wasted heat. They must operate at high temperatures to produce light—much higher than the melting point of most metals.

"We can produce a dielectric metasurface designed to emit light, without producing waste heat," Padilla said.

"Although we've already been able to do this with metal-based metamaterials, you need to operate at high temperature for the whole thing to work. Dielectric materials have melting points much higher than metals, and we're now quickly trying to move this technology into the infrared to demonstrate a lighting system."