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Nanoantennas improve infrared sensing

Photonics Spectra
Aug 2013

PHILADELPHIA – Nanoantennas with “slots” that correspond to mid-IR wavelengths are a new way to tune IR light into mechanical action – which could lead to more sensitive IR cameras and more compact chemical-analysis techniques.

Existing IR detectors use cryogenically cooled semiconductors or microbolometers to correlate changes in electrical resistance to temperatures. Both techniques require expensive, bulky equipment to be sensitive enough for spectroscopy applications.

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 an area that would fit on a disposable silicon chip.


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


The engineers fabricated a proof-of-concept device in their study. At its core is a nanoscale structure – about 0.1 mm wide and five times as long – comprising 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 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 the type of light to which it is sensitive 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.” The research appeared in Nano Letters (doi: 10.1021/nl400087b).


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