Plasmonics-Based Light Detector Could Support Precision Agriculture

A new, broad-spectrum photodetector that can be implemented on a single chip has been developed at Duke University. The photodetector spans a range of light frequencies by using on-chip spectral filters created with electromagnetic materials. The camera’s technology is based on plasmonics — the use of nanoscale physical phenomena to trap specific frequencies of light.

For their plasmonics-based approach to building a thermal camera with an on-chip filter, professor Maiken Mikkelsen and her team created silver, 100-nm-wide cubes and placed them on a transparent film positioned a few nm above a thin layer of gold. When light hit the surface of a nanocube, it excited the electrons in the silver, trapping the light’s energy at a specific frequency. This frequency was determined by the size of the silver nanocube and its distance from the base layer of gold. By precisely tailoring the sizes and spacings in their setup, the researchers were able to control the amount of light that was absorbed and compel the system to respond to any electromagnetic frequency they chose.

An artistic rendering of a new type of multispectral imaging detector. Depending on their size and spacing, nanocubes sitting on top of a thin layer of gold trap specific frequencies of light, which heats up the materials beneath to create an electronic signal. Courtesy of Ella Maru Studio.

The researchers’ next goal is to harness this phenomena to build a commercial hyperspectral camera. To do so, they believe a grid of tiny, individual detectors, each tuned to a different frequency and made into a larger “superpixel,” will be required.

In a step toward that end, the team demonstrated four individual photodetectors tailored to wavelengths between 750 and 1900 nm. The plasmonic metasurfaces of the detectors absorbed energy from specific frequencies of incoming light and heated up. The heat induced a change in the crystal structure of a thin layer of pyroelectric material (aluminium nitride) sitting directly below the metasurfaces. That structural change created a voltage, which was then read by a bottom layer — a silicon semiconductor layer that transmitted the signal to a computer for analysis.

“It wasn’t obvious at all that we could do this,” Mikkelsen said. “Not only do our photodetectors work, but we’re seeing new, unexpected physical phenomena that will allow us to speed up how fast we can do this detection by many orders of magnitude.”

The new photodetectors are built from three layers. The size and spacing of silver nanocubes on a thin layer of gold dictates what frequency they absorb, causing them to heat up. A thin layer of a pyroelectric material called aluminum nitride then converts the heat to an electric signal, which is picked up and carried by a layer of silicon semiconductor on the bottom. Courtesy of Jon Stewart, Duke University.

The researchers used pyroelectric material to make their detectors. Previous photodetectors have been made with pyroelectrics, but they have not been able to focus on specific electromagnetic frequencies. Also, the thick layers of pyroelectric material that were required to create an adequate electric signal caused these photodetectors to operate at very slow speeds.

“Our plasmonic detectors can be turned to any frequency and trap so much energy that they generate quite a lot of heat,” researcher Jon Stewart said. “That efficiency means we only need a thin layer of material, which greatly speeds up the process.”

The previous record for detection times in any type of thermal camera with an on-chip filter, whether it used pyroelectric materials or not, was 337 microseconds. The Duke team’s plasmonics-based approach sparked a signal in just 700 picoseconds, which is roughly 500,000 times faster. Because the detection times were limited by the experimental instruments used to measure them, the new photodetectors could work even faster in the future, the researchers believe.

Mikkelsen sees several potential uses for commercial cameras based on the technology, because the process required to manufacture the photodetectors is relatively fast, inexpensive, and scalable. The combination of multiple photodetectors with different frequency responses on a single chip could enable lightweight, inexpensive multispectral cameras for applications such as cancer surgery, food safety inspection, and the team’s initial focus, precision agriculture. Mikkelsen envisions a cheap, hand-held detector that could image crop fields from the ground or from inexpensive drones.

A new type of lightweight, inexpensive hyperspectral camera could enable precision agriculture. This graphic shows how different pixels can be tuned to specific frequencies of light that indicate the various needs of a crop field. Courtesy of Maiken Mikkelsen and Jon Stewart, Duke University.

“Obtaining a ‘spectral fingerprint’ can precisely identify a material and its composition,” Mikkelsen said. “Not only can it indicate the type of plant, but it can also determine its condition, whether it needs water, is stressed, or has low nitrogen content, indicating a need for fertilizer.” Hyperspectral imaging could enable precision agriculture by allowing fertilizer, pesticides, herbicides, and water to be applied only where needed, saving water and money and reducing pollution.

The research was published in Nature Materials (www.doi.org/10.1038/s41563-019-0538-6).