The Indian Institute of Science team used a 2D material to design what they call a non-linear optical mirror stack to increase or up-convert the frequency of short infrared light to the visible range, combined with widefield imaging capability.
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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. This up-conversion of light has diverse applications, especially in defence and optical communications, said IISc.

“The human eye can only see light at certain frequencies (called the visible spectrum), the lowest of which constitutes red light. Infrared light, which we can’t see, has an even lower frequency than red light. IISc researchers have now fabricated a device to increase or up-convert the frequency of short infrared light to the visible range,” IISc said.

2D material used

The institute added that 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.

It said that 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.

However, existing infrared sensors are bulky and not very efficient. They are also export-restricted because of their utility in defence. There is, therefore, a critical need to develop indigenous and efficient devices.

The method used by the IISc team involves feeding an input infrared signal along with a pump beam onto the mirror stack. The nonlinear 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 a wavelength of around 1,550 nm to 622 nm visible light. The output light wave can be detected using traditional silicon-based cameras.

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.

Worldwide interest

“There is a lot of interest worldwide in doing infrared imaging without using infrared sensors. Our work could be a game-changer for those applications,” said Varun Raghunathan, associate professor, Department of Electrical Communication Engineering.