Bengaluru: In a significant upgrade to efforts in making infrared light accessible to the human eye, researchers at the Indian Institute of Science (IISc) have developed a device that increases, or “up-converts”, the frequency of short infrared light to the visible range.
The work is a step forward in developing efficient, home-grown devices in this domain since utilities in defence make infrared sensors export-restricted. It could also help overcome limitations of existing infrared sensors that are bulky and not very efficient, IISc said on Thursday. The researchers pitch the device as potentially impactful in infrared imaging applications that do not use sensors.
Infrared light, which humans cannot see, has a frequency lower than red light which itself comes with the lowest frequency in the visible spectrum. The study has been published in Laser & Photonics Reviews.
To achieve the up-conversion, the researchers worked with a non-linear optical mirror stack that consists of multilayered gallium selenide fixed to a gold reflective surface, with a silicon dioxide layer sandwiched in between.
The team fed an input infrared signal along with a pump beam onto the mirror stack. The nonlinear optical properties of the material in the stack caused a mixing of the frequencies, resulting in an output beam of increased (up-converted) frequency, keeping the rest of the properties intact. An up-conversion of infrared light of wavelength around 1550 nm to 622 nm visible light was recorded. IISc said the output light wave could be detected using traditional silicon-based cameras.
Varun Raghunathan, Associate Professor in the Department of Electrical Communication Engineering (ECE) and corresponding author of the study, said properties of the input beam were preserved at the output. “This means that if one imprints a particular pattern in the input infrared frequency, it automatically gets transferred to the new output frequency,” he said.
Infrared imaging and sensing come with wide-ranging applications, especially in defence and optical communications. Another case in point is the passing of infrared light through a gas that helps scientists analyse changes in the light, and derive specific properties of the gas.
The thickness of the material in the mirror stack needs to be tweaked to suit the application. “In our experiments, we have used infrared light of 1550 nm and a pump beam of 1040 nm. But that doesn’t mean that it won’t work for other wavelengths,” Jyothsna K Manattayil, PhD student at ECE and first author, said.
The team was able to achieve the up-conversion even with a thin layer of gallium selenide measuring just 45 nm. The small size of the device makes it more cost-effective than traditional devices that use centimetre-sized crystals, IISc said.