“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. Our researchers have now fabricated a device to increase or “up-convert” the frequency of short infrared light to the visible range,” IISc said.
The IISc team has designed what they call a “nonlinear optical mirror stack” using a 2D material, gallium selenide, to increase or “up-convert” the frequency of short infrared light to the visible range.
Pointing out that traditional infrared imaging relies on bulky and inefficient sensors that are also export-restricted for defence uses, IISc said its device offers an indigenous and efficient alternative by mixing an infrared input signal with a pump beam to produce an output visible light beam while preserving the properties of the original infrared.
“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,” explains Varun Raghunathan, associate professor in the Department of Electrical Communication Engineering (ECE) and corresponding author of the study published in Laser & Photonics Reviews.
From Left to Right: Schematic of the nonlinear optical mirror used for up-conversion imaging. Energy diagram showing the sum frequency generation process used for up-conversion. Representative up-converted images of IISc logo and spokes where the object pattern at 1550 nm is upconverted to 622 nm wavelength (Credit: IISc/Jyothsna KM)
The advantage of using gallium selenide, Raghunathan adds, 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 45 nm. 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,” he said.
Their device’s performance was found to be on par with current state-of-the-art up-conversion imaging systems.
Jyothsna K Manattayil, PhD student at ECE and first author, said that they used a particle swarm optimisation algorithm to speed up the calculation of the right thickness of layers needed. Depending on the thickness, the wavelengths that can pass through gallium selenide and get up-converted will vary. This means that the material thickness needs to be tweaked depending on the application.
“In our experiments, we have used infrared light of 1,550 nm and a pump beam of 1,040 nm. But that doesn’t mean that it won’t work for other wavelengths. We saw that the performance didn’t drop for a wide range of infrared wavelengths, from 1,400 nm to 1,700 nm,” she said.
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,” says Raghunathan.
