A new publication from Opto-Electronic Sciences; DOI 10.29026/oes.2024.240017 , discusses all chalcogenide glass metalens.
Visual perception plays a crucial role in human understanding of the world: the eyes receive scattered light from object surfaces, which is focused onto the retina through the eye lens, allowing us to see vibrant scenes. However, this process of visible light imaging depends on external illumination and is easily affected by environmental factors like smoke. According to Planck's law, objects with a temperature (above -273.15 ℃) emit light spontaneously, with the radiation spectrum of room temperature objects mainly concentrated in the long-wave infrared range (wavelengths ranging from 8 to 12 μm). Long-wave infrared imaging technology can provide high-quality imaging in low-light conditions, haze, smoke, and other complex environments, and is sensitive to changes in temperature. Therefore, it has wide-ranging applications in military, security, medical, and other fields. Currently, long-wave infrared cameras are bulky and heavy. Developing lightweight long-wave infrared imaging systems can effectively support the development of emerging fields such as autonomous driving and UAV vision. Achieving this goal hinges on the development of lightweight, thin, and mass-producible long-wave infrared metalenses, offering an effective implementation solution.
Recently, a research team from Ningbo University led by Yixiao Gao and Xiang Shen, in collaboration with teams from Zhejiang University and Nottingham Trent University in the UK, have proposed a novel type of long-wave infrared metalens. This metalens is directly etched with high aspect ratio microstructures on the surface of chalcogenide glass, enabling efficient focusing and imaging of long-wave infrared light fields. It offers advantages such as simple structure and ease of mass production.
In recent years, the development of all-dielectric metalens technology has leveraged artificially structured subwavelength arrays to flexibly control the phase, amplitude, and polarization of incident light fields. These metalenses are characterized by their flat, ultra-thin, compact, and easily integrable properties, making them highly suitable for integrated optical systems. However, all-dielectric metalenses face two main challenges in the long-wave infrared spectrum: (1) common optical materials such as silica glass are opaque, and (2) fabricating micron-thick dielectric microstructures on heterogeneous substrates leads to reliability issues in complex environments, such as thermal mismatch-induced microstructure collapse.
To address these challenges, researchers propose directly fabricating metalenses on surfaces of materials transparent to long-wave infrared radiation, such as silicon and germanium. However, silicon absorbs in the long-wave infrared range, while germanium has a high thermal expansion coefficient and increased absorption at elevated temperatures. Chalcogenide glass, composed of chalcogen elements (sulfur, selenium, or tellurium), is an amorphous material with a high refractive index, extremely low absorption losses, and excellent optical and thermal stability in the long-wave infrared range. It has demonstrated promising applications in optical fibers and integrated photonics devices. Directly patterning superstructured lenses on chalcogenide glass surfaces thus presents a new approach to realizing long-wave infrared metalenses.
Keywords: chalcogenide glasses / long wave infrared / metalens
