Researchers at the Indian Institute of Science (IISc) have fabricated a device to increase or ‘up-convert’ the frequency of short infrared light to the visible range.
By enabling infrared imaging without the use of heavy and expensive infrared sensors, the new device could advance defence and optical communications applications.
Existing infrared sensors bulky and limited by restrictions
Traditional infrared imaging uses exotic low-energy bandgap semiconductors or micro-bolometer arrays, which usually pick up heat or absorption signatures from the object being studied. For example, when infrared light is passed through a gas, sensing how the light changes can help scientists tease out specific properties of the gas. Such sensing is not always possible using visible light.
However, existing infrared sensors are bulky and are export-restricted because of their utility in defence.
In a first, the IISc team used a 2D material to design what they call a non-linear optical mirror stack to achieve this up-conversion, combined with widefield imaging capability. The stack consists of multilayered gallium selenide fixed to the top of a gold reflective surface, with a silicon dioxide layer sandwiched in between.
The method involves feeding an input infrared signal along with a pump beam onto the mirror stack. The non-linear optical properties of the material constituting the stack result in a mixing of the frequencies, leading to an output beam of increased (up-converted) frequency, but with the rest of the properties intact. Using this method, they were able to up-convert infrared light of wavelength around 1,550nm to 622nm visible light. The output light wave can be detected using traditional silicon-based cameras.
“This process is coherent – the properties of the input beam are 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,” explained Varun Raghunathan, Associate Professor in the Department of Electrical Communication Engineering (ECE) and corresponding author of a study published in Laser & Photonics Reviews.
Gallium selenide boasts high optical nonlinearity
The advantage of using gallium selenide, he added, is its high optical nonlinearity, which means that a single photon of infrared light and a single photon of the pump beam could combine to give a single photon of light with up-converted frequency. The team was able to achieve the up-conversion even with a thin layer of gallium selenide measuring just 45nm.
The small size makes it more cost-effective than traditional devices that use centimetre-sized crystals. Its performance was also found to be comparable to current state-of-the-art up-conversion imaging systems.
Going forward, the researchers plan to extend their work to up-convert light of longer wavelengths. They are also trying to improve the efficiency of the device by exploring other stack geometries.
“There is a lot of interest worldwide in doing infrared imaging without using infrared sensors. Our work could be a gamechanger for those applications,” said Raghunathan.
