Metalenses offer integrated focusing for infrared imaging devices

Metasurfaces, which manipulate phase differences to bend and shape waves, can serve as lenses to focus light in unique ways. By carefully designing subwavelength patterns, it’s possible to create ultrathin metalenses that can sit in front of photodetectors and aid in light collection. Metalenses and other microscopic optical elements, paired with imaging devices, are currently limited in their integration because they are made from different materials than the detectors.

Zhang et al. have now fashioned metalenses out of a semiconductor commonly used as a substrate for infrared focal plane arrays, potentially allowing for the creation of integrated devices for infrared imaging. It’s a technique that authors say could scale to arrays with a large number of pixels.

The team designed broadband and narrowband infrared metalenses, each about 30 microns in diameter and 2 microns thick. They patterned both lens designs in a 10 by 10 grid on a gallium antimonide substrate. They used a microscope to scan and image different planes inside the substrate and observed a tripling of intensity in the focal plane as a result.

The authors suspect there is still room for improvement in the design, as the measured focusing efficiency is about 20 percent less than the average they found in simulations.

Work is ongoing to integrate the metalenses with a real array, with each pixel in the array sitting behind its own metalens. Having the lens and detector integrated into a single assembly could provide improved sensitivity and also increase the required operating temperature for focal plane arrays.

Source: “Solid-immersion metalenses for infrared focal plane arrays,” by Shuyan Zhang, Alexander Soibel, Sam A. Keo, Daniel Wilson, Sir. B. Rafol, David Z. Ting, Alan She, Sarath D. Gunapala, and Federico Capasso, Applied Physics Letters (2018). The article can be accessed at https://doi.org/10.1063/1.5040395 .