Seeing the invisible: IISc researchers develop device that makes infrared light visible

Seeing the invisible: IISc researchers develop device that makes infrared light visible

Bengaluru: Researchers at the Indian Institute of Science (IISc) have developed a unique device that can have wide-ranging applications in defence, communications and scientific imaging. infrared light In visible light,
“The human eye can only see light at certain frequencies (called the visible spectrum), the lowest frequency of which is red light.Infrared light, which we cannot see, has a frequency even lower than red light. Our researchers have now created a device that can increase or “up-convert” the frequency of short infrared light to the visible range,” IISc said.
The IISc team has designed a “nonlinear optical mirror stack” using a 2D material, gallium selenide, that amplifies or “up-converts” the frequency of short infrared light into the visible range.
Pointing out that conventional infrared imaging relies on bulky and inefficient sensors whose export is restricted even for defence uses, IISc said its device offers an indigenous and efficient alternative by combining 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 an input produces a particular pattern in the infrared frequency, it is automatically 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 Lasers and Photonics Review.


Left to right: Schematic diagram 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 the IISc logo and spoke where the object pattern at 1550 nm is up-converted to 622 nm wavelength (Credit: IISc/Jyotsna KM)

The advantage of using gallium selenide, Raghunathan says, is its high optical nonlinearity, which means that a photon of infrared light and a photon from the pump beam can combine to give a photon of light with a highly-converted frequency.
“The team was able to achieve up-conversion even with a thin layer of gallium selenide measuring just 45 nm. Due to its small size it is more cost-effective than conventional devices using centimetre-sized crystals. Its performance was also found to be comparable to current state-of-the-art up-conversion imaging systems,” he said.
The performance of their device was found to be comparable to current state-of-the-art up-conversion imaging systems.
Jyotsna K Mantayil, PhD student in ECE and first author, said they used particle swarm optimization algorithms to speed up calculation of the correct thickness of the layers. Depending on the thickness, the wavelengths that pass through and are up-converted by the gallium selenide will vary. This means the thickness of the material needs to be adjusted depending on the application.
“In our experiments, we used infrared light of 1,550 nm and a pump beam of 1,040 nm. But this does not mean that it will not work for other wavelengths. We observed that the performance did not degrade for a wide range of infrared wavelengths from 1,400 nm to 1,700 nm,” he said.
Moving forward, the researchers plan to expand their work further. Convert Up light of longer wavelength. They are also trying to improve the efficiency of the device by exploring other stack geometries.
“There is a lot of interest around the world in doing infrared imaging without using infrared sensors. Our work could make a big difference for those applications,” says Raghunathan.

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