Improving mid-infrared imaging and sensing

American and Chinese researchers are developing a way of taking images in the mid-infrared (mid-IR) part of the spectrum for applications including thermal imaging, biomedical sensing, and free-space communication.

The mid-IR band of electromagnetic radiation is a particularly useful part of the spectrum; it can provide imaging in the dark, trace heat signatures, and provide sensitive detection of many biomolecular and chemical signals. But optical systems for this band of frequencies have been hard to make, and devices using them are highly specialised and expensive. Now, the researchers say they have found a highly efficient and mass-manufacturable approach to controlling and detecting these waves.

The approach – developed by teams from MIT, the University of Massachusetts at Lowell, University of Electronic Science and Technology of China, and the East China Normal University – uses a flat, artificial material composed of nanostructured optical elements, instead of the usual thick, curved-glass lenses used in conventional optics. These elements are said to provide on-demand electromagnetic responses and are made using techniques like those used for computer chips, meaning that manufacturing is scalable.

“There have been remarkable demonstrations of metasurface optics in visible light and near-infrared, but in the mid-infrared it’s moving slowly,” said MIT’s Tian Gu. As they began this research, he added, the question was, since they could make these devices extremely thin, could they also make them efficient and low-cost? The team members now say they have achieved this.

The device uses an array of precisely shaped thin-film optical elements called ‘meta-atoms’ made of chalcogenide alloy, which has a high refractive index. These meta-atoms have thicknesses that are a fraction of the wavelengths of the light being observed, and collectively they can perform like a lens. They provide nearly arbitrary wavefront manipulation that isn’t possible with natural materials at larger scales, but they have a tiny fraction of the thickness, and thus only a tiny amount of material is needed.

The devices are said to transmit 80% of the mid-IR light with optical efficiencies up to 75%, representing a significant improvement over existing mid-IR metaoptics. They can also be made lighter and thinner than conventional IR optics. Using the same method, by varying the pattern of the array the researchers can arbitrarily produce different types of optical devices, including a simple beam deflector, a cylindrical or spherical lens, and complex aspheric lenses. The lenses have been demonstrated to focus mid-IR light with the maximum theoretically possible sharpness, known as the diffraction limit.

The team says these techniques allow the creation of metaoptical devices, which can manipulate light in more complex ways than can be achieved using conventional bulk transparent materials. The devices can also control polarisation and other properties.