Plasmonics Enables Sensing on Demand

Plasmonic nanostructures let infrared sensors turn themselves on and off

3 min read
Pseudo-coloured scanning electron microscope images of a fabricated PMP
Image: Northeastern University/Nature Nanotechnology

Researchers at Northeastern University in Boston have developed an infrared sensor based on plasmonics that is capable of turning itself on when it needs to perform its sensing duties and then turns itself off when not needed to decrease energy demands and increase its lifetime.

The result is a sensor that performs only event-driven sensing. The sensor is dormant, yet is always alert and it awakes only in the presence of a signal of interest. As a result, the sensor only consumes power when there is something to be detected.

In research described in the journal Nature Nanotechnology, the Northeastern researchers used plasmonic nanostructures that take the form of nanoscale gold patches to act as tiny mechanical switches that take energy from the signal of interest—in this case a specific wavelength of infrared light—and mechanically close the contacts of the switches to create a low-resistance electrical connection.

“When infrared light hits the device, the optical energy is absorbed by an integrated ultra-thin plasmonic infrared absorber and converted into heat,” explained Matteo Rinaldi, associate professor at Northeastern and co-author of the paper, in an e-mail interview with IEEE Spectrum. “The induced heat increases the temperature of a pair of bi-material beams in the device, which then bend due to thermal expansion and bring one piece of metal into contact with another.”

Plasmons, the waves of electrons that move along the surface of a metal after it’s been struck by photons, are the key to engineering the infrared (IR) light absorption in these tiny mechanical switches. The plasmonic phenomenon make it possible to achieve strong and spectrally selective absorption of light in very tiny structures that would not be possible otherwise given the relatively long length of wavelengths of light. 

“Thanks to such a spectrally selective absorption, our devices are triggered only by light in a predetermined narrow spectral band,” said Rinaldi. “Furthermore, two switches fabricated on the same chip can have different triggering wavelengths thanks to the plasmonic-enabled capability of lithographically defining light absorption properties, resulting in the capability of detecting and discriminating different spectral signatures (specific wavelengths of light) by using multiple spectrally selective tiny switches to form a passive hardware logic.”

The Northeastern team’s plasmonic absorber is composed of a three-material stack: a 100 nanometer dielectric layer sandwiched by an array of 50 nanometer gold nano-patches on the top and 100 nanometer platinum plate on the bottom.

In this arrangement, highly localized gap plasmons are excited when electromagnetic radiation of a specific spectral band impinges on the array of nanostructures. These localized gap plasmons effectively traps the light within the thin dielectric gap between the nanostructures and the continuous metal layer, inducing a relatively large and swift increase of temperature in the absorber.

A critical aspect to the function of the device is the nanoscale air gap separating the contacts of the switches. The smaller the gap is the smaller the amount of absorbed light power is needed to close it. 

Rinaldi and his colleagues have dubbed their sensors infrared digitizing sensors since they tell if a certain intensity and wavelength of infrared radiation is present or not, instead of giving a reading of the power as common sensors would do.

“They basically detect and discriminate the IR radiation of interest and they convert it into a wake-up bit (i.e. digitizing the impinging radiation), without consuming any electrical power while in standby,” said Rinaldi. 

This sensor could be useful for detecting approaching infrared sources such as human bodies and fuel-burning cars, according to Rinaldi. He adds that they could also be used for monitoring the appearance of hot spots such as flames and explosions to trigger an alarm in the event of a disaster.

“When paired with laser sources, this technology could be utilized for remote control and communication,” said Rinaldi. “In all of these scenarios, our infrared digitizing sensors can be used as a zero-power trigger to power on next-stage electronics (could be a wireless module or a more sophisticated sensing and signal processing system) for a confirmed event while consuming zero-power in standby.”

Vacuum packaging of the sensors is key to commercialization, according to Rinaldi. These devices require a vacuum operate and achieve their high the thermal isolation for better performance. 

Despite still believing that there remain some engineering issues to be addressed with the devices, Rinaldi and his colleagues have demonstrated a complete battery-powered palm-sized IR wireless sensor node with near-zero (2.6 nano Watts) standby power consumption.

“Our devices currently are already sensitive enough (detection threshold of ~500 nW) for many applications, said Rinaldi. “Nevertheless, the achievement of an even lower IR power threshold would further increase the possible application scenarios.

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