Emission control: Infrared metamaterial

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  • Published: May 1, 2017
  • Author: David Bradley
  • Channels: Infrared Spectroscopy
thumbnail image: Emission control: Infrared metamaterial

Efficiency boost

This illustration shows the room temperature MEMS meta material for reconfigurable infrared emission equivalent to a temperature change of almost 20 degrees Celsius (Credit: Xinyu Liu, Duke University)

A meta material that can be tuned to emit different wavelengths of infrared light in a controlled manner having absorbed energy could be used to make a device for collecting and so using waste heat.

Willie Padilla of Duke University, in North Carolina, and colleagues suggest that this new technology might be exploited in boosting the efficiency of thermophotovoltaics. In these solar cells infrared is the working portion of the electromagnetic spectrum absorbed and converted into electricity rather than the visible light from the sun. Research has been inching ever closer to viable thermophotovoltaics because they could be used not only as solar panels but to absorb and use waste heat from furnaces, kilns, power plants and other industrial installations that generate huge amounts of waste energy. They might even be used to harvest heat energy from a vehicle exhaust and use it to charge on-board batteries.

"Because the infrared energy emission, or intensity, is controllable, this new infrared emitter could provide a tailored way to collect and use energy from heat," Padilla explains. "There is a great deal of interest in utilizing waste heat, and our technology could improve this process."

Meta synthetic

The new device is based on a meta material, a class of synthetic material that has properties not available to conventional nor natural materials. Padilla and his graduate student Xinyu Liu used a metamaterial engineered to absorb and emit infrared wavelengths with very high efficiency. They combined the possibility of electronically controlled movement possible with microelectromechanical systems (MEMS) and built the first metamaterial device with infrared emission properties that can be quickly changed on what is essentially a "pixel-by-pixel" basis.

Writing in The Optical Society's journal, Optica, the team describes the new infrared-emitting device as consisting of an 8 x 8 array of individually controllable pixels, each measuring 120 X 120 micrometres across. They have demonstrated their ability to control emission from the MEMS metamaterial device by making it display a letter "D" visible to an infrared camera. The researchers report that their infrared emitter can achieve a range of infrared intensities and can display patterns at rates of up to 110 kilohertz. The possibility of scaling up this technology might allow it to be used to create dynamic infrared patterns for body or vehicle armour displays in a war zone for instance.

Contrasting approach

The big advantage of this system is that in contrast to methods previously used to generate variable infrared emission, the fact that this meta material be tuned to a particular infrared emission energy without any change in temperature is important. Since the material does not need to be heated nor cooled it can operate at room temperature whereas other systems only work at high temperature. There have been previous success with conventional materials but those have been limited to narrow infrared spectral ranges.

"In addition to allowing room-temperature operation, using meta materials makes it simple to scale throughout the infrared wavelength range and into the visible or lower frequencies," Padilla adds. "This is because the device's properties are achieved by the geometry, not by the chemical nature of the constituent materials that we're using." As such, the infrared emitter is entirely reconfigurable. It consists of a movable top layer of patterned metallic metamaterial and a bottom metallic layer that remains stationary. The device absorbs infrared photons and emits them with high efficiency when the two layers are touching but emits less infrared energy when the two layers are held apart. When a voltage is applied the movement of the top layer can be controlled and so the amount of infrared energy emitted tuned by changing the voltage.

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

Related Links

Optica 2017, 4, 430: "Reconfigurable room temperature metamaterial infrared emitter"

Article by David Bradley

The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd.

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