Scientists have designed an organic dye-based device that can see light waves in the shortwave infrared (SWIR) range. The device is easy to make using cheap materials, and is stable at high temperatures, which should lead to its widespread use in imaging and sensing systems.
The human eye detects a very narrow segment of the electromagnetic spectrum, from around 400 to 700nm. The SWIR region, on the other hand, extends from 1,000 to 2,500nm. Specially-designed cameras can take images of objects that reflect waves in the SWIR region. They are used for improving night vision, in airborne remote sensing and deep-tissue imaging. The cameras also help assess the composition and quality of silicon wafers, building structures and even food produce.
“These cameras are typically difficult to manufacture and are quite expensive, as they are made of inorganic semiconductor photodiode arrays interconnected with read-out integrated circuitry,” says Roland Hany of the Swiss Federal Laboratories for Materials Science and Technology. Hany worked with colleagues in Switzerland and Italy to design an organic dye-based ‘SWIR upconversion device’ that efficiently converts shortwave infrared light to visible light. In it, SWIR light is absorbed by the squaraine dye in the photodetector (PD), producing electrical charge, which is directed into organic light-emitting diodes (OLEDs) to recombine under the emission of visible light.
The team had to play with the molecular composition of several squaraine dyes to get them to absorb specific wavelengths. Ultimately, they synthesised squaraine dyes that absorb SWIR light beyond 1,200nm and remained stable to 200°C. The finished dye-based device performed stably for several weeks under normal laboratory conditions.
“All-organic upconverters could lead to applications that can’t be realised with current technology. For example, invisible night-vision devices can be directly integrated into car windscreens without affecting the visual field,” said Hany.
The team is now working on shifting the dye’s absorption further into the SWIR range. They are also using machine-learning techniques to find new dye molecules capable of sensing SWIR waves, but also improve the device’s stability and sensitivity.