Electrical engineers at Duke University have created the fastest pyroelectric photodetector ever demonstrated, a device that detects light by sensing the tiny amount of heat it produces when absorbed.
The ultrathin sensor can capture light across the entire electromagnetic spectrum. It operates at room temperature, requires no external power source, and can be integrated directly into on-chip systems. The technology could eventually enable a new generation of multispectral cameras with applications in areas such as skin cancer detection, food safety monitoring, and large scale agriculture.
The findings were reported in the journal Advanced Functional Materials.
Why Traditional Photodetectors Have Limits
Most digital cameras rely on semiconductor photodetectors that produce an electrical current when struck by visible light. Computers then convert that signal into the images we see.
However, semiconductors can detect only a small portion of the electromagnetic spectrum. In that sense, they are similar to the human eye, which is also limited to visible wavelengths of light.
To detect light outside that range, researchers often turn to pyroelectric detectors. These devices produce an electrical signal when they warm up after absorbing incoming light. But generating enough heat from harder to capture wavelengths has traditionally required thick absorbing materials or very bright illumination, making such detectors bulky and slow.
"Commercial pyroelectric detectors aren't very responsive, so they need a very bright light or very thick absorbers to work, which naturally makes them slow because heat doesn't move that fast," said Maiken Mikkelsen, professor of electrical and computer engineering at Duke. "Our approach cleverly integrates near-perfect absorbers and super-thin pyroelectrics to achieve a response time of 125 picoseconds, which is a huge improvement for the field."
Metasurface Design Traps Light Efficiently
The device developed by Mikkelsen's lab relies on a specially engineered structure known as a metasurface. It consists of precisely arranged silver nanocubes positioned on a transparent layer located just 10 nanometers above a thin sheet of gold.
When light hits a nanocube, it excites electrons in the silver. This interaction traps the light's energy through a process called plasmonics. The exact frequency of light captured depends on the size of the nanocubes and the spacing between them.
Because this light trapping is extremely efficient, only a very thin layer of pyroelectric material is needed underneath the structure to generate an electrical signal. Mikkelsen's team first demonstrated the concept in 2019, although the original setup was not designed to measure how quickly the device could respond.
"Thermal photodetectors are supposed to be slow, so this was mind-boggling to the entire community," Mikkelsen said. "We were taken off guard that it seemed to be working on time scales similar to that of silicon photodetectors."
Optimizing the Device for Speed
Over the past several years, Eunso Shin, a PhD student in Mikkelsen's laboratory, has worked to refine the design while also developing a method to measure the device's speed without relying on extremely expensive equipment.
In the newest version of the detector, the metasurface that absorbs light was redesigned into a circular shape rather than a rectangular one. This configuration increases the surface area exposed to incoming light while reducing the distance electrical signals must travel. The researchers also incorporated even thinner pyroelectric layers supplied by collaborators and improved the electronic circuitry used to capture and transmit the signals.
To measure the detector's performance, Shin devised an experimental setup using two distributed feedback lasers. The lasers intensified when their frequencies approached the operating speed of the device, allowing the researchers to determine how quickly the detector could respond.
Their measurements showed that the thermal photodetector can operate at speeds up to 2.8 GHz. At that rate, incoming light produces an electrical signal in only 125 picoseconds.
"Pyroelectric photodetectors commonly operate in the nano-to-microsecond range, so this is hundreds or thousands of times faster," Shin said. "These results are really exciting, but we're still working to make them even faster while figuring out the kinetic limit of pyroelectric photodetectors."
Future Applications From Agriculture to Medicine
The researchers believe the device could become even faster by placing the pyroelectric material and electronic readout components in the narrow gap between the nanocubes and the gold layer. They are also exploring ways to expand the system's capabilities, including designs that use multiple metasurfaces to detect several wavelengths of light and their polarity at the same time.
As development continues and manufacturing challenges are addressed, the technology could open the door to powerful new imaging systems. Because the detectors do not need external power, they could be deployed in drones, satellites, and spacecraft.
Such systems could support precision agriculture by revealing in real time which crops require additional water or fertilizer.
"When you get into the ability to detect lots of frequencies at once, you open the door to so many different things," Mikkelsen said. "Cancer diagnosis, food safety, remote sensing vehicles. Those are all still pretty far down the line, but that's the direction we're heading in."
This research was supported by the Air Force Office of Scientific Research (FA9550-21-1-0312) and the Gordon and Betty Moore Foundation (GBMF8804).
