Visible infrared light conversion: The new device uses 2D material to upconvert infrared light

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 cannot see, has an even lower frequency than red light. Researchers at the Indian Institute of Science (IISc) have now produced a device to boost or «convert» the frequency of short infrared light into visible light.

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

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

Infrared imaging and sensing is 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 sensation is not always possible using visible light.

However, existing infrared sensors are cumbersome and not very efficient. They are also restricted from export because of their defensive utility. Therefore, there is a critical need to develop indigenous and efficient devices.

The method used by the IISc team involves feeding an infrared signal along with a pump beam into the mirror stack. The nonlinear optical properties of the material making up the stack result in a mixing of frequencies, leading to an output beam with an increased (up-converted) frequency, but with the rest of the properties 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 outgoing light wave can be detected using traditional silicon-based cameras.

«This process is coherent – the properties of the input beam are preserved in the output. This means that if one embeds a particular pattern in the input infrared frequency, it is automatically transferred to the new output frequency,» explains Varun Raghunathan, Professor of Associate. in the Department of Electrical Communication 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, which means that a single photon of infrared light and a single photon of the pump beam can combine to give a single photon. of up-converted frequency light alone.

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 modern upconversion imaging systems.

Jyothsna K Manattayil, Ph.D. student in ECE and first author, explains that they used a particle stack 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 the gallium selenide and be up-converted will vary. This means that the thickness of the material must be changed depending on the application.

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

Next, the researchers plan to expand their work to 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 around the world in making infrared images without using infrared sensors. Our work could be a game changer for those applications,» says Raghunathan.