A groundbreaking ultrathin sensor has been developed that captures light across the entire electromagnetic spectrum. This innovative technology operates at room temperature, requires no external power, and can be seamlessly integrated into on-chip systems. Its potential applications are vast, including advancements in multispectral cameras for skin cancer detection, food safety monitoring, and large-scale agricultural practices.
Challenges with Conventional Photodetectors
Traditional digital cameras utilize semiconductor photodetectors, which generate an electrical current upon exposure to visible light. However, these semiconductors are limited in their ability to detect only a fraction of the electromagnetic spectrum, akin to the human eye's visible light restriction.
To extend detection capabilities, researchers often resort to pyroelectric detectors. These devices produce electrical signals by heating up after absorbing light. Yet, capturing harder-to-detect wavelengths typically necessitates bulky materials or intense illumination, resulting in slower response times.
Maiken Mikkelsen, a professor of electrical and computer engineering at Duke, noted, "Commercial pyroelectric detectors lack responsiveness, requiring bright light or thick absorbers, which inherently slows them down due to heat transfer limitations. Our method ingeniously combines near-perfect absorbers with ultra-thin pyroelectrics to achieve an impressive response time of just 125 picoseconds."
Efficient Light Trapping with Metasurface Design
The innovative device relies on a meticulously engineered metasurface, featuring precisely arranged silver nanocubes situated only 10 nanometers above a thin gold sheet. When light interacts with the nanocubes, it excites electrons in the silver, effectively trapping the light's energy through a process known as plasmonics. The light's frequency is determined by the nanocubes' size and spacing.
This efficient light trapping allows for a minimal layer of pyroelectric material beneath the structure to generate an electrical signal. Mikkelsen's team first showcased this concept in 2019, although the initial design did not focus on response speed.
Enhancing Speed and Performance
Over recent years, PhD student Eunso Shin has refined the design and established a method to assess the device's speed without expensive equipment. The latest iteration features a circular metasurface, maximizing light exposure while minimizing the distance electrical signals must travel. The team also utilized thinner pyroelectric layers and improved electronic circuitry for signal capture.
Shin created an experimental setup with two distributed feedback lasers that intensified as they approached the device's operational speed, enabling precise measurements. Their findings revealed that the thermal photodetector operates at speeds up to 2.8 GHz, generating electrical signals in just 125 picoseconds.
"Pyroelectric photodetectors typically function in the nano-to-microsecond range, making this advancement hundreds or thousands of times faster," Shin remarked. "While these results are thrilling, we continue to explore ways to enhance speed further and understand the kinetic limits of pyroelectric detectors."
Expanding Future Applications
The researchers anticipate even greater speeds by positioning pyroelectric materials and electronic components within the narrow gap between the nanocubes and the gold layer. They are also investigating designs that employ multiple metasurfaces for simultaneous detection of various light wavelengths and polarities.
As development progresses and manufacturing hurdles are overcome, this technology could revolutionize imaging systems. Its lack of reliance on external power sources makes it suitable for deployment in drones, satellites, and spacecraft, paving the way for innovations in precision agriculture and beyond.
"The ability to detect multiple frequencies simultaneously opens up numerous possibilities," Mikkelsen stated. "Applications in cancer diagnosis, food safety, and remote sensing are on the horizon." This research received support from the Air Force Office of Scientific Research and the Gordon and Betty Moore Foundation.
