First metal-free metamaterial that can absorb electromagnetic energy

Electrical engineers at Duke University claim to have created the world's first electromagnetic metamaterial made without any metal. The device's ability to absorb electromagnetic energy without heating up has direct applications in imaging, sensing and lighting.

For researchers to manipulate electromagnetic waves they've typically had to use electrically conducting metals. That approach, however, brings with it a fundamental problem of metals - the higher the electrical conductivity, the better the material conducts heat. This limits their usefulness in temperature-dependent applications.

The Duke University engineers have demonstrated the first completely dielectric electromagnetic metamaterial - a surface dimpled with cylinders that is designed to absorb terahertz waves. While this specific frequency range sits between infrared waves and microwaves, the approach should be applicable 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,” explained 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.”

Prof Padilla and his colleagues created their metamaterial with boron-doped silicon. Using computer simulations, the engineers calculated how terahertz waves would interact with cylinders of varying heights and widths.

The researchers then manufactured a prototype consisting of hundreds of these optimised cylinders aligned in rows on a flat surface. Physical tests showed that the new ‘metasurface’ absorbed 97.5% of the energy produced by waves at 1.011THz.

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, getting sharp images has been challenging.

Another potential application for the new technology is efficient lighting. Incandescent light bulbs make light but also create a significant amount of wasted heat.

“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.”

Tom Austin-Morgan

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