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Man has shown considerable ingenuity in seeking out renewable energy – chasing after wind, tides, biomass, sunshine and more. But there is one source that we have not yet tapped – the 1017 W of infrared thermal radiation emitted by the Earth into outer space as a result of the warmth it receives from the Sun.

"Wherever there is an opportunity to generate energy, scientists should be working on it," says Steve Byrnes of Harvard University in US. "Although there is an enormous amount of infrared energy flowing in the environment, it has not been properly evaluated in the context of energy generation. The rapid improvement in mid-infrared technology over the past 20 years…enables us to imagine new mid-infrared devices and applications."

Byrnes and colleagues at Harvard, including Federico Capasso, co-inventor of the infrared quantum-cascade laser, have investigated two possible ways to make an "emissive energy harvester" (EEH) that could extract some of this infrared power.

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