MIT slows light to absorb and generate energy

Reducing the speed of light using nanotech could lead to new ways to generate electricity, say MIT researchers.

A paper about to be published in the forthcoming issue of the journal Nano Letters describes the creation of a “metamaterial” that promises much more efficient absorption of a wide range of light, which the researchers say could lead to a new generation of highly efficient solar cells and light bulbs as well as devices for generating electricity from heat—by actually reducing the speed of light.

The intriguingly named metamaterials are a new class of extremely thin artificial nanotech substances with properties unlike anything found in the natural world. Because they are created from the atom up, they can be perfectly engineered for any purpose by designing functional materials that interact with light in unconventional ways.

MIT Department of Mechanical Engineering lead author Nicholas X. Fang says that his international team was able to slow light to less than one-hundredth of its normal speed in a vacuum, making it much easier to trap inside the material. “When something is going very fast, it’s difficult to catch it,” he says, “so we slow it down so it’s easier to absorb.”

A lot of work has been done in engineering nanomaterials to more fully capture light to improve solar absorption. Some we’ve covered recently include Anorexic Silicon Wafer Could Slash Solar Cost, and Solar ‘Nanotrees’ Key To Clean Hydrogen Fuel? and Nanowire Mesh Could Be Solar Window Coating, but most have been limited to a very narrow range of wavelengths and angles of incidence, says MIT.

Fang’s design uses a pattern of wedge-shaped ridges whose widths are precisely tuned to slow and capture light of a wide range of wavelengths and angles of incidence.

Because their material would be both a very efficient absorber and emitter of photons, it could be used to either capture or emit electromagnetic radiation for very particular wavelengths, such as microwave and terahertz frequencies. It could be be made sensitive to just one specific range of wavelengths.

This means that, in addition to more efficient solar cells that so many are working on, it could also be used make devices for generating electricity from heat, and to make lightbulbs that use a fraction of the energy used today.

“We can selectively enhance the material’s interaction with infrared light at the wavelengths we want,” Fang says. The metamaterial wedges harvest photons at different depths, in rather the same way that our hearing works to sort sound frequencies.

“Our ears separate different frequencies and gather them at different depths,” he explains.

Metamaterials also have the potential to be very cheap. They are super thin, saving both materials and cost, and yet can easily be fabricated using equipment that is already standard in conventional photovoltaic-cell manufacturing.

The paper by Nicholas X. Fang, the Brit and Alex d’Arbeloff Career Development Associate Professor in Engineering Design in MIT’s Department of Mechanical Engineering, was co-authored with researchers at Zhejiang University and Taiyuan University in China, and the University of Illinois at Urbana-Champaign, and was funded by both the Chinese and the U.S. governments.

Their preliminary version of the study at Nano Letters describes their initial computer simulations, and the team is now working on lab experiments to confirm their findings.