A team of Penn engineers has used a pattern of nanoscopic grooves on a tiny strip of gold to turn infrared light into mechanical action, a technique that could lead to more sensitive night-vision cameras and additional compact chemical-analysis techniques.

The research was conducted by assistant professor Ertugrul Cubukcu and postdoctoral researcher Fei Yi, along with graduate students Hai Zhu and Jason C. Reed, all of the Department of Materials Science and Engineering in the School of Engineering and Applied Science.  

Existing infrared detection techniques can be used to infer the chemical composition of materials by measuring the way light scatters off of them, but these techniques need expensive, bulky equipment that is sensitive enough to differentiate one element from another. The advantage of the Penn team’s mechanical approach is that it could reduce the footprint of an infrared sensing device to something that would fit on a disposable silicon chip.

The small size and simplicity of this approach relies on an elegant use of a basic physical principle: Materials naturally convert some energy from infrared light into heat, so the amount a material expands can be connected to the amount of infrared light hitting it. In this case, the materials were a layer of gold bonded to a layer of silicon nitride, a tenth of a millimeter wide and half a millimeter long.

“A single layer would expand laterally, but our two layers are constrained because they’re attached to one another,” Cubukcu says. “The only way they can expand is in the third dimension. In this case, that means bending toward the gold side.”

A fiber optic cable pointed upward at this system bounces light off the underside of the silicon nitride layer, enabling the researchers to determine how far the structure has bent.

Other researchers have attempted to make use of this technique, but the Penn team’s device is much more sensitive. This is due to the inclusion of “slot” nanoantennas, cavities that concentrate the infrared light, which are etched into the gold layer at intervals that correspond to its wavelength.

“We take the same exact platform and, by patterning it with these nanoscale antennas, the conversion efficiency of the detector improves 10 times,” Cubukcu says.

While only a proof-of-concept technique at this stage, the researchers say future study will demonstrate the device’s capabilities as a low-cost way of analyzing individual proteins and gas molecules.