Trapping light on thermal photodetectors shatters speed records

(Press-News.org) Electrical engineers at Duke University have demonstrated the fastest pyroelectric photodetector to date that works by absorbing heat generated by incoming light.

Capable of capturing light from the entire electromagnetic spectrum, the ultrathin device requires no external power, operates at room temperature and can be readily integrated into on-chip applications. The advance could form the basis of a new class of multispectral cameras capable of impacting a wide range of fields such as skin cancer detection, food safety inspection and large-scale agriculture.

The results appear online December 11 in Advanced Functional Materials.

As the foundation of traditional digital cameras, semiconductor photodetectors work by sparking an electric current when struck by visible light that is interpreted by a computer into a cohesive image. But semiconductors, like human eyes, can only view a narrow range of frequencies on the electromagnetic spectrum.

A common approach to capturing more exotic frequencies of light uses pyroelectric detectors, which generate electric signals when heated up after absorbing light. But these types of devices have long lagged behind the effectiveness of traditional digital cameras in a myriad of ways, as producing enough heat in difficult-to-capture frequencies has made them 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.”

Mikkelsen’s lab’s approach, called a “metasurface,” uses precisely tailored silver nanocubes placed on a transparent film only 10 nanometers above a thin layer of gold. When light strikes the surface of a nanocube, it excites the silver’s electrons, trapping the light’s energy through a phenomenon known as plasmonics—but only at a specific frequency controlled by the nanocubes’ sizes and spacings.

This phenomenon is so efficient at trapping light and absorbing its energy that it only requires an extremely thin layer of pyroelectric material beneath it to create an electric signal. This approach was initially demonstrated by Mikkelsen’s lab in 2019, but at the time, the experiment wasn’t set up in a way that allowed them to measure its speed.

“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.”

For the past few years, Eunso Shin, a PhD student working in Mikkelsen’s lab, has been working to optimize this approach while also figuring out a way to measure its speed without spending hundreds of thousands of dollars on fancy equipment.

In the new iteration, the light-absorbing metasurface is circular rather than rectangular to maximize its exposure while minimizing the distance the signal must travel. They also procured even thinner layers of pyroelectric materials from their collaborators to integrate into the device and upgraded the circuit design for reading and relaying the electric signals.

Then, to measure just how fast the setup was capable of detecting light, Shin used a clever set up involving two distributed feedback lasers that brightened when their frequencies became close to the same as the device’s operating speed.

The team found that their new thermal photodetector operates at record-breaking speeds up to 2.8 GHz, which corresponds to an electrical signal being generated by the incoming light in just 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.”

The team believes they can improve on this already record-setting mark by integrating the pyroelectric materials and electrical readouts into the space between the nanocubes and thin layer of gold. They are also working to add capabilities to setups using this design, such as using multiple metasurfaces to detect several different frequencies of light and their polarity all at once.

As they continue to refine their design and overcome hurdles to fabrication, the photodetectors could prove transformative for a number of applications. Because they do not require external power sources, these imaging devices could be used in drones, satellites and spacecraft. This could, in turn, allow them to be used in precision agriculture by revealing which crops need water and fertilization in real-time.

“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).

“Metasurface-Enhanced Thermal Photodetector Operating at Gigahertz Frequencies.” Eunso Shin, Rachel E. Bangle, Nathaniel C. Wilson, Stefan B. Nikodemski, Jarrett H. Vella, Maiken H. Mikkelsen. Advanced Functional Materials, 2025. DOI: 10.1002/adfm.202420953

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