Turning on visible infrared light: The new device uses two-dimensional materials to convert infrared light

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, has an even lower frequency than red light. Researchers at the Indian Institute of Science (IISc) have now developed a device to amplify or “convert” the frequency of short-infrared light into the visible range.

Light up-conversion has a variety of applications, especially in defense and optical communications. Initially, the IISc team used a two-dimensional material to design what is called a nonlinear optical mirror stack to achieve this high conversion, along with extended imaging capability. The stack consists of a multilayer gallium selenide fixed on top of a gold reflective surface with a layer of silicon dioxide in between.

Traditional infrared imaging uses low-energy bandgap semiconductors or microbolometer arrays, which typically pick up heat or absorption signatures from the object being studied.

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

However, existing infrared sensors are bulky and inefficient. They also have export restrictions due to their use in defense. Therefore, there is a fundamental need to develop native and efficient devices.

The method used by the IISc team involves feeding the incoming infrared signal along with the pump beam onto the mirror stack. The nonlinear optical properties of the stack constituents result in a mixing of frequencies, resulting in an output beam with an increased frequency (up-conversion), but the rest of the properties are intact. Using this method, they were able to convert infrared light with a wavelength of about 1550 nm into visible light of 622 nm. The output light wave can be detected using traditional silicon-based cameras.

Associate Professor Varun Raghunathan explains: “The process is coherent – the characteristics of the input beam are preserved in 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. be.” in the Department of Electrical Communications Engineering (ECE) and corresponding author of this study published in Laser and photonic investigations.

He adds that the advantage of using gallium selenide is its high optical nonlinearity, meaning that a single photon of infrared light and a single photon of the pump beam can combine to form a high-frequency photon of light.

The team was able to achieve upconversion even with a thin layer of gallium selenide only 45 nm in size. 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 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 appropriate thickness of the required layers. Depending on the thickness, the wavelengths that can pass through gallium selenide and be converted to high will vary. This means that the thickness of the material must be changed depending on the application.

“In our experiments, we used 1550 nm infrared light and 1040 nm pump beam. But that doesn’t mean it won’t work for other wavelengths,” he says. We found that performance did not degrade over a wide range of infrared wavelengths, from 1400 nm to 1700 nm.

In the future, the researchers plan to expand their work to convert light with longer wavelengths. They are also trying to improve device performance 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 be a game changer for these applications,” says Raghunathan.