New infrared imager converts infrared light into clear images

Imagine mapping an individual’s blood vessels while simultaneously monitoring heart rate without coming into contact with the person’s skin, seeing through fog and smog, and seeing through silicon wafers to verify composition and the quality of the electronic cards.

The new infrared imager is slim and compact with a large area display. Image credit: Ning Li.

The examples above are just some of the capabilities of a new infrared imager designed by a research team led by electrical engineers from the University of California, San Diego.

This imager detects a part of the infrared spectrum, called short wave infrared light (wavelengths between 1000 and 1400 nm), which is just beyond the visible spectrum (between 400 and 700 nm). But shortwave infrared imaging should not be confused with thermal imaging, which detects relatively longer infrared wavelengths emitted from the body.

The new infrared imager works by shining short-wave infrared light onto a target area or object, then changing the low-energy infrared light – which is reflected back to the device – to shorter wavelengths and higher energy that can be seen by the human eye.

It makes invisible light visible.

Tina Ng, Professor of Electrical and Computer Engineering, UC San Diego Jacobs School of Engineering

Although infrared imaging technology has been around for many years, the majority of systems are complex, bulky and expensive, typically requiring a separate display and camera. These systems are usually made from inorganic semiconductors, which are rigid, expensive and contain dangerous elements, such as lead and arsenic.

The new infrared imager designed by Ng’s research team solves these problems. The Imager integrates the display and sensors into a single slim device, making it simple and compact.

The infrared imager was designed using organic semiconductors. It is therefore safe, flexible and economical to use in biomedical applications. This technology also offers improved image resolution over some of its inorganic counterparts.

Described recently in the journal Advanced Functional Materials, the new imager offers more advantages. It visualizes more of the shortwave infrared spectrum, between 1000 and 1400 nm, compared to current analog systems which typically see just below 1200 nm.

To date, the new imager has one of the largest display sizes of infrared imagers, i.e. 2 cm2 in the zone. And since the imager was created using thin-film processes, it’s economical and easy to scale up to create even larger screens.

Convert infrared photons into visible photons

The new imager is made up of many semiconductor layers, each measuring hundreds of nanometers thick and placed on top of each other.

Three of these layers, each composed of a different organic polymer, are the main players in the imager: an organic light-emitting diode (OLED) display layer, a photodetector layer, and an electron blocking layer between the of them.

The photodetector layer generates an electric current by absorbing short wave infrared light or low energy photons.

This electrical current travels to the OLED display layer where it is transformed into a visible image, i.e. high energy photons. The electron blocking layer – an intermediate layer – prevents the OLED display layer from losing current. It is this mechanism that allows the device to create a clearer image.

This process of transforming low energy photons into higher energy photos is called upconversion. This upconversion process is electronic, which is a unique feature.

The advantage of this is that it allows direct conversion from infrared to visible in a thin and compact system. In a typical IR imaging system where the upconversion is not electronic, you need an array of detectors to collect the data, a computer to process that data, and a separate screen to display that data. . This is why most of the existing systems are bulky and expensive.

Ning Li, Study First Author and Postdoctoral Researcher, UC San Diego Jacobs School of Engineering

Ning Li works in Ng’s lab.

Another unique feature of the imager is that it can effectively offer electronic and optical readouts.

This makes it multifunctional.

Ning Li, Study First Author and Postdoctoral Researcher, UC San Diego Jacobs School of Engineering

For example, when the team shone infrared light on the back of a person’s hand, the new imager offered a clear image of the person’s blood vessels while simultaneously capturing the person’s heart rate.

Investigators also used their new infrared imager to see through a silicon wafer and smog. In a demonstration, the team placed an “EXIT” patterned photomask in a compact smog-filled chamber. In another demonstration, they positioned a “UCSD” patterned photomask on the back of a silicon wafer.

Infrared light penetrates both silicon and smog, allowing the imager to observe the letters in these two demonstrations. This would come in handy for applications such as inspecting silicon chips for faults and helping self-driving cars see in poor weather conditions.

The team is currently investigating ways to improve the efficiency of the imager

The study was funded by the National Science Foundation (ECCS-1839361) and the Samsung Advanced Institute of Technology. It was carried out in part at the San Diego Nanotechnology Infrastructure (SDNI) at the University of California, San Diego, a member of the National Nanotechnology Coordinated Infrastructure, which is financially supported by the National Science Foundation (grant ECCS-1542148).

Infrared imager (Ng lab)

Video credit: Ning Li.

Journal reference:

Li, N. et al. (2021) Organic upconversion imager with dual electronic and optical readouts for detection of shortwave infrared light. Advanced functional materials.


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