A team of researchers at the University of California, San Diego has developed a new infrared imager that can see through smog and fog, map out a person’s blood vessel, and at the same time monitor the heart rate, without actually touching the person’s skin.
The research supported by the National Science Foundation (ECCS-1839361) and Samsung Advanced Institute of Technology, was performed in part at the San Diego Nanotechnology Infrastructure (SDNI) at UC San Diego, a member of the National Nanotechnology Coordinated Infrastructure, which is supported by the National Science Foundation (grant ECCS-1542148).
The infrared imager can also see through silicon wafers to inspect the composition and quality of electronic boards, while detecting a part of the infrared spectrum with wavelengths from 1000 to 1400 nanometers dubbed ‘short wave infrared light’ right outside the 400 to 700 nanometers visible spectrum.
It should be noted that the shortwave infrared imaging is not the same with thermal imaging that detects much longer infrared wavelengths given off by the body.
The imager functions when it shines shortwave infrared light on an area of interest, converts the low energy infrared light reflecting back to the device into shorter, higher-energy wavelengths visible to the human eye.
Tina Ng, a Professor of Electrical and Computer Engineering at the University of California, San Diego Jacobs School of Engineering affirmed that the process ‘makes the invisible light visible’.
Even thought the infrared imaging technology has been in function for decades, most of the systems are expensive, bulky and complex, most times requiring a separate camera and display. Aside this, they are produced with the use of inorganic semiconductors, that are hitherto costly, rigid, while also having toxic elements like arsenic and lead.
The developed infrared imager seeks to solve and overcome these challenges as it combines the sensors and the display into a thin device, thereby making it compact and simple. It is created with organic semiconductors, saving cost, while at the same time flexible and safe to use in biomedical applications. The imager provides better image resolution than many of its organic counterparts.
Aside the enumerated, the new infrared imager has other advantages. One of them is that is sees more of the shortwave infrared spectrum, from 1000 to 1400 nanometers, where existing similar systems had most times seen below 1200 nanometers.
The Imager also has the largest display sizes of infrared images , with 2 square centimeters in area. It is easy and not expensive to scale up to have larger displays as the imager is tuned and fabricated using thin film processes.
Invigorating Infrared photons to visible photons
Made up of multiple semiconducting layers, with hundreds of thin, stacked nanometers on top of each other, the imager has three of these layers that are made of different organic polymer standing as the image key players: a photo detector layer, an organic light-emitting diode (OLED) display layer, and an in between electron-blocking layer.
Shortwave infrared light with low energy protons are absorbed by the photodetector layer, and then generates electric current, which then flows to the OLED display layer, with an onward conversion to a visible image with low energy protons. There is an in between layer called the ‘electron-blocking layer’ that prevents the OLED display layer from losing current, in the process enabling the device to produce a clearer, more readable image.
The conversion of low energy photons to high energy photons highlighted above known as ‘upconversion’ is an electronic process, with the first author of the research, Ning Li, a postdoctoral researcher in Ng’s lab adding that:
“The advantage of this is it allows direct infrared-to-visible conversion in one thin and compact system. 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.”
According to Li, the imager is effective in providing optical and electronic readouts, a feature that makes it multifunctional. For instance, when infrared light is shined at the back of a subject’s hand, a clearer picture of the subject’s blood vessels is provided by the imager while recording the subject’s heart rate.
The team furthermore used the infrared imager to see through smog and silicon wafer in a demonstration where a photomask was placed and patterned in a small chamber filled with smog with the ‘EXIT’ tag. They also placed a photo mask patterned with “UCSD” behind a silicon wafer. The imager sees the letters in these demonstrations after the infrared light had penetrated through smog and silicon. This process would be useful in everyday applications for self-driven cars to see clearly in bad weather. It would also be useful for the inspection of silicon chips, while looking out for defects.
It is believed that the team of researchers will focus on improving the imager’s efficiency to achieve clearer results.
Reference: “Organic Upconversion Imager with Dual Electronic and Optical Readouts for Shortwave Infrared Light Detection” by Ning Li, Naresh Eedugurala, Dong-Seok Leem, Jason D. Azoulay and Tse Nga Ng, 19 February 2021, Advanced Functional Materials.