Infrared images reach the visible range of light without sensors

June 21. () –

Researchers of the Indian Institute of Sciences (IISc) have made a device to increase the frequency of shortwave infrared light to the visible range.

The human eye can only see light at certain frequencies (called the visible spectrum), the lowest of which is red light. Infrared light, which we cannot see, It has an even lower frequency than red light.

Light upconversion has various applications, especially in defense and optical communications. For the first time, the IISc team used a 2D material to design what they call a stack of non-linear optical mirrors to achieve this upconversion, combined with wide-field imaging capability. The stack consists of multilayer gallium selenide fixed to the top of a reflective gold surface, with a layer of silicon dioxide sandwiched between them.

Traditional infrared imaging uses exotic low-energy bandgap semiconductors or microbolometer arrays, which typically collect heat or absorption signatures of the object being studied.

Infrared imaging and detection are useful in a variety of areas, from astronomy to chemistry. For example, when infrared light passes through a gas, detecting how the light changes can help scientists discover specific properties of the gas. This detection is not always possible using visible light.

However, the Existing infrared sensors are bulky and not very efficient. In addition, its export is restricted due to its usefulness in defense.

The method used by the IISc team involves introducing an input infrared signal along with a pump beam into the mirror stack. The non-linear optical properties of the stack material result in a mixing of the frequencies, resulting in an output beam of increased (upconverted) frequency, but with all other properties intact. Using this method, they were able to upconvert infrared light of wavelength around 1550nm to 622nm visible light. The output light wave can be detected using traditional silicon-based cameras.

“This process is consistent: the properties of the input beam are preserved at the output. This means that if a particular pattern is printed at the input infrared frequency, it is automatically transferred to the new output frequency,” he explains. it’s a statement Varun Raghunathan, associate professor in the Department of Electrical Communications Engineering (ECE) and corresponding author of the study, published in Laser & Photonics Reviews.

The advantage of using gallium selenide, he adds, is its high optical nonlinearity, meaning that a single photon of infrared light and a single photon of the pump beam they could combine to give a single photon of up-converted frequency light.

The team was able to achieve upconversion 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 centimeter-sized crystals. Its performance was also found to be comparable to current state-of-the-art upconversion imaging systems.

Jyothsna K Manattayil, ECE student and first author, explains that they used a particle swarm optimization algorithm to speed up the calculation of the correct thickness of the necessary layers. Depending on the thickness, the wavelengths that can pass through the gallium selenide and become thicker layers will vary. This means that the thickness of the material must be adjusted according to the application.

“In our experiments, we have used 1,550 nm infrared light and a 1,040 nm pump beam. But that doesn’t mean it doesn’t work for other wavelengths,” he says. “We saw that performance did not decrease for a wide range of infrared wavelengths, from 1,400 nm to 1,700 nm.”

In the future, the researchers plan to expand their work to convert longer wavelength light into thicker layers. They are also trying to improve the efficiency of the device by exploring other stack geometries.

“There is great interest throughout the world in obtain infrared images without using infrared sensors. Our work could be a game-changer for those applications,” says Raghunathan.