Electrical engineers at Duke University have developed the fastest pyroelectric photodetector ever demonstrated. This pyroelectric photodetector is a device that detects light by sensing a minute amount of heat generated when light is absorbed.
The ultra-thin sensor can capture light across the entire electromagnetic spectrum. It operates at room temperature, requires no external power supply, and can be directly integrated into on-chip systems. This technology could ultimately enable a new generation of multispectral cameras with applications in areas such as skin cancer detection, food safety surveillance, and large-scale agriculture.
The findings were reported in the journal Advanced Functional Materials.
Why traditional photodetectors have limitations
Most digital cameras rely on semiconductor photodetectors that generate an electrical current when visible light hits them. A computer then converts that signal into an image that we see.
However, semiconductors can only detect a small portion of the electromagnetic spectrum. In that sense, it is similar to the human eye, which is similarly limited to visible light wavelengths.
To detect light outside that range, researchers often utilize pyroelectric detectors. These devices generate an electrical signal when they absorb incoming light and warm up. But generating enough heat from difficult-to-capture wavelengths has traditionally required thick absorbing materials or very bright illumination, making such detectors large and slow.
“Commercially available pyroelectric detectors are not very responsive, so they require either very bright light or very thick absorbers to work. Heat doesn’t move as fast, so that necessarily slows down the detector,” said Miken Mikkelsen, professor of electrical and computer engineering at Duke University. “Our approach cleverly integrates a near-perfect absorber and an ultra-thin pyroelectric material to achieve a response time of 125 ps, which is a major improvement for the field.”
Metasurface design that efficiently confines light
The device developed by Mikkelsen’s lab relies on specially designed structures known as metasurfaces. It consists of precisely positioned silver nanocubes placed on a transparent layer located just 10 nanometers above a thin sheet of gold.
When light hits the nanocubes, electrons in the silver are excited. This interaction traps the energy of light through a process called plasmonics. The exact frequency of the captured light depends on the size of the nanocubes and the spacing between them.
This light capture is so efficient that only a very thin layer of pyroelectric material is required beneath the structure to generate an electrical signal. Mikkelsen’s team first demonstrated the concept in 2019, but the original setup wasn’t designed to measure how quickly the device could respond.
“This was shocking to the entire community because thermal photodetectors are supposed to be slow,” Mikkelsen said. “We were caught off guard because it seemed to operate on a similar time scale as silicon photodetectors.”
Optimize device speed
Eunseo Shin, a doctoral student in Mikkelsen’s lab, has spent the past few years working to improve the design and develop ways to measure the speed of devices without relying on very expensive equipment.
In the latest version of the detector, the light-absorbing metasurface has been redesigned to be circular instead of rectangular. This configuration increases the surface area exposed to incident light while decreasing the distance over which electrical signals travel. The researchers also incorporated an even thinner pyroelectric layer provided by their collaborators and improved the electronics used to capture and transmit the signal.
To measure the detector’s performance, Singh devised an experimental setup that uses two distributed feedback lasers. As the laser frequency approached the device’s operating speed, the laser became stronger, 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 this speed, incident light generates an electrical signal in just 125 picoseconds.
“Pyroelectric photodetectors typically operate in the nano to microsecond range, so this is hundreds or thousands of times faster,” Singh said. “While these results are very exciting, we are still working to make the pyroelectric photodetector even faster while understanding its motion limits.”
Future applications from agriculture to medicine
The researchers believe the device could be made even faster by placing pyroelectric materials and electronic readout components in the narrow gap between the nanocubes and the gold layer. We are also exploring ways to expand the system’s capabilities, including designs that use multiple metasurfaces to simultaneously detect multiple wavelengths of light and their polarities.
As development continues and manufacturing challenges are resolved, this technology could open the door to powerful new imaging systems. The detector does not require an external power source, so it can be deployed on drones, satellites, and spacecraft.
Such systems could support precision agriculture by revealing in real time which crops require additional water or fertilizer.
“Being able to detect many frequencies at once opens the door to so many different things,” Mikkelsen said. “Cancer diagnostics, food safety, remote sensing vehicles. They’re all still a long way off, but that’s the direction we’re headed.”
This research was supported by the Air Force Office of Scientific Research (FA9550-21-1-0312) and the Gordon and Betty Moore Foundation (GBMF8804).
