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Metamaterial MEMS device offers tuned thermal emission

20 Apr 2017

Duke University system emits infrared energy in reconfigurable patterns.

The ability to dynamically tune a device's thermal emission properties could be of benefit in a number of applications, from temperature control to the harvesting of energy from waste heat.

To date, most research has focused on natural materials that change their thermal behavior at elevated temperatures, and the effects have tended to be relatively slow and constrained.

A team at Duke University has developed a promising alternative approach, and designed an infrared emitter that can be spatially and temporally controlled in real-time based on electrically actuated MEMS metamaterials. The research was published in Optica.

The project's emitter was created by tessellating a surface with individually reconfigurable pixels of a MEMS metamaterial manufactured from gold and germanium. Each pixel has a layered structure, including a movable top layer above a stationary lower layer. When the layers are in contact, the device absorbs infrared photons and emits them with high efficiency, but it emits less infrared energy when the two layers are apart.

An applied voltage controls the movement of the layers in each individual 120 x 120-micron pixel in the device, which the Duke team terms a MEMS metamaterial emitter (MME). The amount of infrared energy emitted therefore depends on the exact voltage applied, rather than any change in the external temperature of the device.

Room temperature effects

"In addition to allowing room-temperature operation, using metamaterials makes it simple to scale throughout the infrared wavelength range and into the visible or lower frequencies," said Willie Padilla of Duke University. "This is because the device’s properties are achieved by the geometry, not by the chemical nature of the constituent materials that we are using."

Tests showed that the system could dynamically modify the number of infrared photons coming off the surface of the MEMS metamaterial over a range of intensities equivalent to a temperature change of nearly 20 degrees Celsius.

One potential application of the device is in combat identification and camouflage. In the Optica paper, the team used the technique to "write" a D-shaped pattern in an 8 by 8-pixel MME, readable by an IR camera. The pattern could be made to appear and disappear by appropriate pixel control, even though the device remained at a consistent room temperature.

According to the project team, its next steps could include modifying the metamaterial patterns in the top layer to create differently colored infrared pixels that would be each be tunable in intensity. This could allow the creation of infrared pixels that are similar to the RGB pixels used in a TV. Other avenues involve manufacture of a larger system with a greater number of pixels, and increasing the size of each one.

Once optimized, the new MME technology could improve the performance of thermophotovoltaic cells, or offer a way to harvest waste heat from vehicle engines, converting it into energy to charge a car battery.

"In principle, an approach similar to ours could be used to create many kinds of dynamic effects from reconfigurable metamaterials," said Padilla. “This could be employed to achieve a dynamic infrared optical cloak, or a negative refractive index in the infrared, for example."

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