From defense to astronomy to environmental science, multiple sectors rely on IR imaging and sensing to uncover critical information that cannot be detected by light in the visible range. Existing IR sensors are often bulky and inefficient. In addition, many countries restrict exports of IR sensors, making the development of indigenous IR imaging and sensing devices a necessity. In response to this need, researchers at the Indian Institute of Science (IISc) developed a device to upconvert the frequency of short IR light to the visible range. According to a release from the IISc, the researchers designed a nonlinear optical mirror stack and used it to achieve widefield upconversion imaging from the NIR to the visible wavelengths. The mirror stack comprises gallium selenide (GaSe) layers attached to the top of a gold reflective surface. A silicon dioxide layer is sandwiched between the GaSe layer and gold reflector to provide a dielectric spacer. GaSe is a 2D material that is known for its high optical nonlinearity. (From left): Schematic of the nonlinear optical mirror used for upconversion imaging. Energy diagram showing the sum frequency generation process used for upconversion. Representative upconverted images of IISc logo and spokes where the object pattern at 1550 nm is upconverted to a 622-nm wavelength. Courtesy of Jyothsna K. Manattayil. The team used a sum frequency generation (SFG) process to upconvert 1550-nm NIR input to 622-nm visible output in the presence of a pump beam radiating at 1040 nm. The researchers fed the input IR signal and pump beam onto the mirror stack. The nonlinear optical properties of the GaSe material provided upconverted frequency by enabling a single photon of IR light to combine with a single photon of the pump beam. The GaSe material allowed for a blending of frequencies, which resulted in an output beam that exhibited an upconverted frequency, while keeping the rest of its properties intact. The team was able to achieve the upconversion from IR to visible with just a thin, 45-nm layer of GaSe. The researchers detected the output light wave using traditional silicon-based cameras. “This process is coherent — the properties of the input beam are preserved at the output,” professor Varun Raghunathan said. “This means that if one imprints a particular pattern in the input infrared frequency, it automatically gets transferred to the new output frequency.” Researcher Jyothsna K. Manattayil aligns optical beams for upconversion experiments. Courtesy of Harinee Natarajan. The researchers optimized the nonlinear optical mirror stack by using a particle swarm optimization algorithm to maximize the detected SFG signal. The algorithm allows the team to quickly calculate the optimal thickness of the GaSe layers that are needed to achieve output in the desired visible range. The wavelengths that can pass through GaSe for upconversion vary, the team said, depending on the thickness of the GaSe layer. The thickness of the material can be finetuned depending on the wavelength requirements for a given application. “In our experiments, we have used infrared light of 1550 nm and a pump beam of 1040 nm. But that doesn’t mean that it won’t work for other wavelengths,” researcher Jyothsna K. Manattayil said. “We saw that the performance didn’t drop for a wide range of infrared wavelengths, from 1400 nm to 1700 nm.” The team performed image upconversion experiments that demonstrated real-time Fourier-plane imaging and real-time Fourier-domain processing with good imaging fidelity and efficiency. The researchers found that the performance of the nonlinear optical mirror stack was comparable to current state-of-the-art upconversion imaging systems. The small size of the mirror stack makes the device more cost-effective than traditional upconversion imaging devices that use centimeter-sized crystals and typically require long lengths, high pump power, and careful phase matching between interacting waves. IR imaging and sensing are used in many different industries. For example, by sending IR light through gas and then sensing how the light changes, scientists can discern specific properties in the gas. Sensing capabilities like these are not always possible using visible light. The IISc team’s approach to upconverting from NIR to visible wavelengths can be used for defense and optical communications, among other applications. Going forward, the team plans to extend its work to include the upconversion of longer wavelengths of light. The researchers are also exploring other stack geometries that might improve the efficiency of the device. “There is a lot of interest worldwide in doing infrared imaging without using infrared sensors,” Raghunathan said. “Our work could be a gamechanger for those applications.” The research was published in Laser & Photonics Reviews (www.doi.org/10.1002/lpor.202400374).