Nanoantennas Improve Infrared Sensing

Patterning nanoantennas with “slots” that correspond to wavelengths in the mid-IR has yielded a new way to tune IR light into mechanical action, paving the way for more sensitive IR cameras and more compact chemical-analysis techniques.

Detecting light in the mid-IR range is important for applications ranging from night-vision cameras to spectroscopy. Existing IR detectors use cryogenically cooled semiconductors, or thermal detectors known as microbolometers, which correlate changes in electrical resistance to temperatures. These techniques have many advantages, but both need expensive and bulky equipment to be sensitive enough for spectroscopy applications.

A diagram depicting how the University of Pennsylvania optomechanical IR-detecting structure works. Images courtesy of University of Pennsylvania.

The optomechanical thermal IR detector developed at the University of Pennsylvania, however, works by connecting mechanical motion to changes in temperature rather than changes in resistance. This approach could reduce the footprint of an IR sensing device to something that would fit on a disposable silicon chip.

The engineers fabricated a proof-of-concept device in their study. At its core is a nanoscale structure — about one-tenth of a millimeter wide and five times as long — comprised of a gold layer bonded to a layer of silicon nitride. These materials were used because of their different thermal expansion coefficients. Because metals will naturally convert some energy from IR light into heat, scientists can connect the amount the material expands to the amount of IR light hitting it.

“A single layer would expand laterally, but our two layers are constrained because they’re attached to one another,” said Ertugrul Cubukcu, assistant professor in the department of material science and engineering at Penn’s School of Engineering and Applied Science. “The only way they can expand is in the third dimension. In this case, that means bending toward the gold side, since gold has the higher thermal expansion coefficient and will expand more.”

The investigators used a fiber interferometer to measure this movement. A fiber-optic cable pointed upward at this system bounces light off the underside of the silicon nitride layer, enabling the engineers to determine how far the structure has bent upward.

“We can tell how far the bottom layer has moved based on this reflected light,” Cubukcu said. “We can even see displacements that are thousands of times smaller than a hydrogen atom.”

Other researchers have developed optomechanical IR sensors based on this principle, but their sensitivities have been comparatively low. The Penn team’s device is an improvement in this regard because of its inclusion of slot nanoantennas, cavities that are etched into the gold layer at intervals that correspond to wavelengths of mid-IR light.

“The infrared radiation is concentrated into the slots, so you don’t need any additional material to make these antennas,” Cubukcu said. “We take the same exact platform and, by patterning it with these nanoscale antennas, the conversion efficiency of the detector improves 10 times.”

The nanoantennas provide the device with an additional advantage: the ability to tailor which type of light it is sensitive to by etching a different pattern of slots on the surface.

“Other techniques can only work at the maximum absorption determined by the material itself,” said postdoctoral researcher Fei Yi. “Our antennas can be engineered to absorb at any wavelength.”

Future research will demonstrate the device’s capabilities as a low-cost way to analyze individual proteins and gas molecules.

The research — supported by the National Science Foundation, Penn’s Materials Research Science and Engineering Center, Penn’s Nano/Bio Interface Center and the Penn Regional Nanotechnology Facility — appeared in Nano Letters (doi: 10.1021/nl400087b).

For more information, visit: www.upenn.edu