Shining a light on the elusive 'blackbody' of energy research

July 22, 2011
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. Illustration shows 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. Credit: Physical Review Letters

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 led by Boston College physicist Willie Padilla report in the current edition of Physical Review Letters.

A "blackbody" object represents a theorized ideal of performance for a material that perfectly absorbs all radiation to strike it and also emits energy based on the material's temperature. According to this blackbody law, the energy absorbed is equal to the energy emitted in equilibrium.

The breakthrough reported by Padilla and colleagues from Duke University and SensorMetrix, Inc., could lead to used to cull energy from produced by numerous . Furthermore, the man-made metamaterial offers the ability to control emissivity, which could further enhance energy conversion efficiency.

"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 Padilla. "The energy 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."

Researchers have long sought to find the ideal "blackbody" material for use in solar or thermoelectric . So far, the hunt for such a class of thermal emitters has proved elusive. Certain rare earth oxides are in limited supply and expensive, in addition to being almost impossible to control. proved to be inferior emitters that failed to yield significant efficiencies.

Constructed from artificial composites, metamaterials are designed to give them new properties that 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 nano-scale geometric surface features. Knowing that, the researches sought to exploit Kirchoffs'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 percent and 89 percent. The results confirmed achieving 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," said Padilla. "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 altering the composition of the metamaterial can results in single-, dual-band and broadband , which could allow greater control of emitted photons in order to improve .

"Potential applications could lie in harvesting area such as using this metamaterial as the selective thermal emitter for thermophotovoltaic (TPV) cells," said Padilla. "Since this metamaterial has the ability to engineer the thermal so that the emitted photons match the band gap of the semiconductor – part of the TPV cell – the converting efficiency could be greatly enhanced.

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not rated yet Jul 22, 2011
How exactly are these metamaterials "tailored"? Is this something that can be tuned continuously?
not rated yet Jul 22, 2011
judging from the diagram at the top, they have to tailor each material to their desired properties, and therefore not "continuously tunable"

Although the article does not explain the science behind it, my guess is that the metamaterial is designed to be efficiently emissive, and the geometry of the surface determines the wavelength of the emissions. I could certainly be wrong tho.
5 / 5 (1) Jul 22, 2011
I don't know if this is correct, but judging from the article and the diagram they're able to tune the metamaterials by adjusting the length of the "box" in which electrons in the substance are trapped. Make it smaller (the smaller cross) and the gap between energy levels increases, allowing you to absorb shorter wavelength (higher energy) photons; making the arms of the cross larger would have the opposite effect. The cross shape itself, of course, is so that the metamaterials are able to deal with light of any polarization angle/any superposition of circularly polarized photons.

I wonder, is the increased efficiency of emission due only to the particle-inna-box effects, or is it because they've effectively added that to already-existing transitions within individual atoms in the metamaterial? Maybe I wasn't reading carefully enough, but I don't think that that was made clear in the article.
not rated yet Jul 22, 2011
Emissivity is defined for a material as the total energy emitted by the material divided by the total energy emitted by a black body at the same temperature.

Further the emissivity for any band of frequencies is defined in a similar ratio between the two frequencies in consideration.

The article states....

"A designer metamaterial has shown it can engineer emitted "blackbody" radiation with an efficiency beyond the natural limits imposed by the material's temperature" Implying an band emissivity > 1

It then goes on to say...

"achieved experimental emissivity of 98 percent. A dual-band emitter delivered emission peaks of 85 percent and 89 percent."

Which means the emissivity was less than that of a black body.

Science authors should understand what they are writing about.
not rated yet Jul 22, 2011
@ ronan - Now it would just be a shame if such an in depth and logical explanation was wrong...
not rated yet Jul 22, 2011
"Since this metamaterial has the ability to engineer the thermal radiation so that the emitted photons match the band gap of the semiconductor..."

Huge. If (and thats a big if) they can make enough of these cheaply enough, many advances will be made with their help. Band gap matching is a major problem in many energy technologies.
not rated yet Jul 28, 2011
Come on; I think that we all know that what is really holding this thing up is that some bureaucrats still believe that the materials could cascade because of (unproven) dark matter, increasing the energy production 10,000 fold and still not achieving the optimum band gap goals. Some crackpot is delaying this. They have already shown that they can achieve an acceptable emmissity level. The elements that they are using have been chosen specifically because they won't create self sustaining on-going efficiency requirements . When Crouse first laid the groundwork for the perpetual oxidation chamber, the only thing that he lacked was the correct amplitude vector warping frequency modulation device. If he were still alive, he would have made it by now and really showed these guys something.

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