Unveiling the Invisible: Turning Infrared Light Visible

Up-conversion of light holds significant potential across various fields, particularly in defence and optical communications. A groundbreaking development by the team at the Indian Institute of Science (IISc) introduces a novel approach using a 2D material to create a non-linear optical mirror stack. This innovative design enables up-conversion along with widefield imaging capabilities. The stack comprises multilayered gallium selenide attached to a gold reflective surface, with a silicon dioxide layer in between.

Conventional infrared imaging relies on specialized low-energy bandgap semiconductors or micro-bolometer arrays, primarily detecting heat or absorption signatures from the subject under observation. Infrared imaging plays a crucial role in various domains, ranging from astronomy to chemistry. For instance, passing infrared light through a gas allows scientists to discern specific gas properties based on how the light behaves. Such analysis is often unattainable using visible light.

Despite the utility of existing infrared sensors, they are typically bulky, inefficient, and subject to export restrictions due to their significance in defence applications. Hence, there is a pressing need to develop efficient indigenous devices to address these limitations.

The IISc team's method involves directing an input infrared signal and a pump beam onto the mirror stack. The material's nonlinear optical characteristics cause a frequency mixing effect, resulting in an up-converted output beam with enhanced frequency while retaining other properties. Through this technique, they successfully up-converted infrared light with a wavelength of approximately 1550 nm to visible light at 622 nm, detectable using conventional silicon-based cameras.

"This process maintains coherence, preserving the input beam's properties in the output. Therefore, any pattern imprinted in the input infrared frequency automatically transfers to the new output frequency," explains Varun Raghunathan, the study's corresponding author and Associate Professor in the Department of Electrical Communication Engineering (ECE), as published in Laser & Photonics Reviews.

Lead author Jyothsna KM aligning optical beams for up-conversion experiments (Photo: Harinee Natarajan)

Gallium selenide's high optical nonlinearity is highlighted as a key advantage by Raghunathan, indicating that a single photon of infrared light and a single photon of the pump beam can combine to produce a single photon of light with an up-converted frequency. Remarkably, the team achieved up-conversion using a mere 45 nm-thick layer of gallium selenide, making it more cost-effective compared to traditional devices with larger crystals. The performance was also on par with current state-of-the-art up-conversion imaging systems.

Jyothsna K Manattayil, the study's first author and a PhD student at ECE, elaborates on the use of a particle swarm optimization algorithm to expedite the calculation of the optimal layer thicknesses. The thickness directly influences the wavelengths that gallium selenide can transmit and up-convert, necessitating adjustments based on the intended application. She notes, "Our experiments utilized infrared light at 1550 nm and a pump beam at 1040 nm. However, the system's versatility was evident as performance remained consistent across a broad range of infrared wavelengths, from 1400 nm to 1700 nm."

Looking ahead, the researchers aim to expand their work to up-convert longer wavelength light and enhance device efficiency by exploring alternative stack configurations. Raghunathan emphasizes the global interest in infrared imaging without traditional sensors, suggesting that their innovation could be transformative for such applications.