Metamaterial Engineers Blackbody Radiation

Thermophotovoltaics may get a boost from a new designer metamaterial that can engineer emitted blackbody radiation with an efficiency beyond the natural limits imposed by the material's temperature.

A blackbody object represents a theorized ideal (also known as the blackbody law) that the energy absorbed is equal to the energy emitted in equilibrium.

"For the first time, metamaterials are shown to be able to engineer blackbody radiation, and that opens the door for a number of energy harvesting applications," said Willie Padilla, the physicist at Boston College who designed the metamaterial, along with colleagues from Duke University and SensorMetrix Inc. "The energy [that] a natural surface emits is based on its temperature and nothing more. You don't have a lot of choice. Metamaterials, on the other hand, allow you to tailor that radiation coming off in any desirable manner, so you have great control over the emitted energy."

A designer metamaterial has shown it can engineer emitted blackbody radiation with an efficiency beyond the natural limits imposed by the material’s temperature, a team of researchers report in Physical Review Letters. The illustration shows the design of the infrared metamaterial absorber. (a) Top view of a single-band metamaterial absorber unit cell. (b) Schematic of a dual-band metamaterial absorber. (c, d) Perspective view for single- and dual-band metamaterial absorbers. (Image: Physical Review Letters)

Constructed from artificial composites, metamaterials are designed such that their new properties exceed the performance limits of their actual physical components and allow them to produce "tailored" responses to radiation. Metamaterials have exhibited effects such as a negative index of refraction, and researchers have combined metamaterials with artificial optical devices to demonstrate the invisibility cloak effect, essentially directing light around a space and masking its existence.

Three years ago, the team developed a "perfect" metamaterial absorber capable of absorbing all of the light that strikes it, thanks to its nanoscale geometric surface features. Knowing that, the researchers sought to exploit Kirchoff's law of thermal radiation, which holds that the ability of a material to emit radiation equals its ability to absorb radiation.

Working in the mid-infrared range, the thermal emitter achieved experimental emissivity of 98 percent. A dual-band emitter delivered emission peaks of 85 and 89 percent. The results confirmed performance consistent with Kirchoff's law, the researchers report.

"We also show by performing both emissivity and absorptivity measurements that emissivity and absorptivity agree very well," Padilla said. "Even though the agreement is predicted by Kirchoff's law, this is the first time that Kirchoff's law has been demonstrated for metamaterials."

The researchers said that altering the composition of the metamaterial can result in single-, dual-band and broadband metamaterials, which could allow greater control of emitted photons to improve energy conversion efficiency.

"Potential applications could lie in energy harvesting area such as using this metamaterial as the selective thermal emitter for thermophotovoltaic cells," he said. "Since this metamaterial has the ability to engineer the thermal radiation so that the emitted photons match the bandgap of the semiconductor – part of the [thermophotovoltaic] cell – the converting efficiency could be greatly enhanced."

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