The new device uses 2D material to upconvert infrared light

The human eye only sees light at certain frequencies (called the visible spectrum), the lowest of which is red light. Infrared light, which we cannot see, has an even lower frequency than red light. Researchers at the Indian Institute of Science (IISc) have now created a device that can enhance or “upconvert” the frequency of short-wave infrared light into the visible range.

Upconversion of light has a variety of applications, especially in defense and optical communications. First, the IISc team used the 2D material to design a so-called nonlinear optical mirror stack to achieve upconversion combined with wide-angle imaging capabilities. The stack consists of a multilayer of gallium selenide attached to the top of a gold reflective surface, with a layer of silicon dioxide in between.

Traditional infrared imaging uses exotic, low-energy bandgap semiconductors or microbolometer arrays that typically capture heat or absorption signatures from the object under examination.

Infrared imaging and detection is useful in a variety of fields, from astronomy to chemistry. For example, when infrared light is passed through a gas, detecting changes in the light can help scientists learn the specific properties of the gas. Such detection is not always possible using visible light.

However, existing infrared sensors are bulky and inefficient. They are also restricted from export due to their utility in defense. There is, therefore, an urgent need to develop indigenous and efficient devices.

The method used by the IISc team involves feeding the input infrared signal along with the pump beam onto a stack of mirrors. The nonlinear optical properties of the material making up the stack cause frequency mixing, leading to an output beam with an increased (up-converted) frequency, but with the other properties intact. Using this method, they managed to convert infrared light with a wavelength of approximately 1,550 nm into visible light with a wavelength of 622 nm. The output light wave can be detected using traditional silicon cameras.

“The process is consistent – the properties of the input beam are retained at the output. This means that if you imprint a specific pattern on the input infrared frequency, it will automatically be transferred to the new output frequency,” explains Varun Raghunathan, associate professor at the Department of Electrical Communications Engineering (ECE) and co-author of the study published in Laser and Photonics Reviews.

The advantage of using gallium selenide, he adds, is its high optical nonlinearity, which means that a single photon of infrared light and a single photon of the pump beam can combine to create a single photon of increased frequency light.

The team managed to achieve upconversion even with a thin gallium selenide layer as short as 45 nm. Its small size makes it more cost effective than traditional devices using centimeter crystals. Its performance was also found to be comparable to current state-of-the-art upconversion imaging systems.

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

“In our experiments, we used infrared light with a wavelength of 1550 nm and a pump beam with a wavelength of 1040 nm. But that doesn’t mean it won’t work at other wavelengths,” he says. “We observed that performance did not decline over a wide range of infrared wavelengths, from 1,400 nm to 1,700 nm.”

In the future, the scientists plan to expand their work to include upward conversion of light with longer wavelengths. They are also trying to improve the device’s performance by exploring other stack geometries.

“There is great interest around the world in performing infrared imaging without the use of infrared sensors. Our work could be a game changer for these applications,” says Raghunathan.