Make shortwave infrared light visible with a single component


The infrared (IR) photodetector looks like a sandwich of several layers. IR light is absorbed into the organic photodetector (OPD), creating electrical charges. Credit: Empa

Infrared (IR) light is invisible to humans. However, some animals, such as rattlesnakes or blood-sucking bats, can perceive infrared radiation and use it to find food. But even for humans, the ability to see in the shortwave IR (SWIR) range would sometimes be useful. With the help of starlight alone, you could see quite clearly at night. Mechanics would be able to see the heat of a soldering tip at a glance. And fruit dealers could spot damaged produce even before the rotting process begins.

But IR light has a “problem”: it is weaker than visible light and UV light on the other side of the light spectrum. So while UV light makes white shirts and dancers’ teeth shine in a club – all it takes is a fluorescent dye in laundry detergent – infrared light is difficult to make visible to the human eye. This is because dyes can convert high energy light directly to low energy light, but not the other way around.

A complete IR camera on a chip

IR cameras therefore require sophisticated electronics to capture IR light, an electronic amplifier and finally a screen to display the artificially generated image. It’s expensive. Today’s standard SWIR cameras for industrial use cost around 7,000 Swiss francs.

Empa researchers Roland Hany, Karen Strassel, Wei-Hsu and Michael Bauer have now successfully captured SWIR light – and made it visible – with just one component. The device developed at Empa is essentially an OLED display with three additional layers (see graphic). IR light passes through an electrically conductive window over a dye layer in a photodetector. Inside, electrons begin to migrate, their movement being amplified by an electrical voltage. The electrical charges then migrate into the OLED layer, where they produce a green light spot. Electronic signal processing by a computer is not necessary: ​​The incoming (invisible) SWIR light is amplified in an “analog” way, so to speak, and displayed directly on the screen. The color of the visible light emitted (blue, green, yellow or red) can be adjusted by selecting the dye in the OLED.

Useful for night vision and for sorting beans

SWIR light is useful for many applications in the food industry, logistics or crafts. For example, you can view the temperature of soldering tips or monitor the cooling of newly manufactured jars and bottles. SWIR light makes wet objects darker which is useful for sorting coffee beans or black olives: stones and metal objects as impurities glow among all dark (wet) fruits on a conveyor belt.

The key to Roland Hany’s SWIR screen lies in special dyes that he and his colleagues have been studying for some time, called squaraines. The name comes from the basic structure of the chemical molecule, squaric acid. This class of dyes was first discovered in the 1960s and is characterized by deep colors and high temperature stability. The researchers chemically modified squaric acid so that it absorbs in the range of SWIR light. “At the moment, we are working with dyes that absorb at just under 1,000 nanometers,” Hany explains. “But we are already working to shift absorption to longer wavelengths, further in the IR range. If we are successful, our sensor will be able to detect water and humidity much better than it can. currently doing. “

Looking for an industrial partner

Hany likes to call the module he developed with his group OUC, or organic upscaling device. This is because it converts weak IR light into stronger visible light (“upconversion”) and works by using thin layers of dye made from carbon-based (“organic”) chemistry. One problem is that the know-how for manufacturing organic optoelectronic devices on an industrial scale is mainly located in Asia. Hany is, however, confident that his discovery will materialize soon: “At the moment, we are working to increase the sensitivity of the module and improve its long-term stability. ”

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More information:
Karen Strassel et al, Shortwave infrared absorbing squaraine dyes for all-organic optical conversion devices, Advanced Materials Science and Technology (2021). DOI: 10.1080 / 14686996.2021.1891842

Provided by the Swiss Federal Laboratories for Materials Science and Technology

Quote: Make Shortwave Infrared Light Visible with Single Component (2021, October 26) Retrieved October 26, 2021 from .html

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