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Shortwave-IR imager promises new multifunctional devices

11 May 2021

University of California San Diego combines sensor and display into one thin device.

A project at the University of California, San Diego (UCSD) has developed a novel shortwave infrared (SWIR) imaging platform that could be valuable in several diverse applications.

SWIR radiation, from around 1000 to 1400 nanometers, typically consists of photons reflected back from an object, rather than emitted by it in the manner of longer-wavelength IR thermal radiation. This can potentially provide a greater degree of contrast in an imaging operation.

In addition, SWIR wavelengths pass through a number of potentially troublesome barriers, such as water vapor and fog, as well as through certain solid materials including silicon, for potential industrial inspection uses.

Military applications are also available, with Intevac Photonics awarded $1.8-million in March 2021 to develop a gated SWIR sensor for high-energy laser tracking systems, building on technology used in the US Airborne Laser (ABL) program.

The hurdle for industrial SWIR imaging has been the need for necessary upconversion of SWIR data into usable visible imaging, typically involving complicated or expensive equipment, built around rigid inorganic semiconductors.

"There remains a critical need for large‐area imaging technologies that operate in the shortwave infrared spectral region," noted the project in its paper, published in Advanced Functional Materials.

"Upconversion imagers that combine photo‐sensing and display in a compact structure are attractive, since they avoid the costly and complex process of pixilation. However, upconversion device research is primarily focused on the optical output, while electronic signals from the imager remain underutilized."

UCSD aimed to tackled the problem by using organic semiconductors to create a low cost and flexible imaging platform, with the sensors and display combined into one thin device which could provide both optical and electronic signals.

According to the project, the imager is made up of multiple stacked semiconducting layers, including layers of three different organic polymers able to act as a photodetector layer and an OLED display layer, with an electron-blocking layer in between.

"The photodetector layer absorbs SWIR photons and generates an electric current, which flows to the OLED display layer, where it gets converted into a visible image," commented UCSD. "An electron-blocking layer keeps the OLED display layer from losing any current and enables the device to produce a clearer image."

Monitoring both blood vessels and heart rate

Designing the imager around established thin-film fabrication processes should make the imager cheaper to manufacture and readily scaled-up to display sizes larger than the 2-square-centimeters of the UCSD's proof-of-concept device - already one of the largest display sizes of such IR imagers to date according to the project.

In trials, the device was used to image a photomask patterned with the word EXIT in a small chamber filled with smog, and a second photomask patterned with UCSD behind a silicon wafer, indicating the potential value of the new platform for applications such as guidance systems in poor visibility or the inspection of silicon chips for defects.

"The advantage of the upconversion process being electronic is that it allows direct infrared-to-visible conversion in one thin and compact system,” said Ning Li of UCSD. "In a typical IR imaging system where upconversion is not electronic, you need a detector array to collect data, a computer to process that data, and a separate screen to display that data. This is why most existing systems are bulky and expensive."

In practice this also means that the device can be multifunctional, providing both optical and electronic readouts, as when the researchers directed infrared light onto the back of a subject's hand. The imager provided a clear picture of the subject's blood vessels, while also recording the subject’s heart rate, according to Ning Li.

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