Improving Infrared Imaging

Scanning Electron Microscope image shows a few of the carefully designed shaped of the chalcogenide glass deposited on a clear substrate. The shapes, which the researchers call meta-atoms, determine how mid-infrared light is bent when passing through the material. (Source: L. Zhang et al., MIT)

A new way of taking images in the mid-infrared part of the spectrum, developed by researchers at MIT and elsewhere, could enable a wide variety of applications, including thermal imaging, biomedical sensing, and free-space communi­cation. The mid-infrared 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 specia­lized and expensive. Now, the researchers say they have found a highly efficient and mass-manu­facturable approach to controlling and detecting these waves.

The new approach uses a flat, artificial material composed of nano­structured optical elements, instead of the usual thick, curved-glass lenses used in conven­tional optics. These elements provide on-demand electro­magnetic responses and are made using techniques similar to those used for computer chips. “This kind of meta­surface can be made using standard micro­fabri­cation techniques,” Tian Gu says. “The manu­facturing is scalable.” He adds that “there have been remarkable demons­trations of meta­surface optics in visible light and near-infrared, but in the mid-infrared it’s moving slowly.”

The new device uses an array of precisely shaped thin-film optical elements made of chalco­genide alloy, which has a high refractive index that can form high-perfor­mance, ultrathin structures. These meta-atoms, with shapes resembling block letters like I or H, are deposited and patterned on an IR-trans­parent substrate of fluoride. The tiny shapes 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’s not 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. “It’s funda­mentally different from conventional optics,” Gu says.

The process “allows us to use very simple fabri­cation techniques,” Gu explains, by thermally eva­porating the material onto the substrate. They have demons­trated the technique on 6-inch wafers with high throughput, a standard in micro­fabrication, and “we’re looking at even larger-scale manu­facturing.” The devices transmit 80 percent of the mid-IR light with optical efficiencies up to 75 percent, represen­ting signi­ficant improve­ment over existing mid-IR meta­optics. They can also be made far 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.

These techniques allow the creation of meta­optical devices, which can mani­pulate light in more complex ways than what can be achieved using conven­tional bulk transparent materials, Gu says. The devices can also control polari­zation and other properties. Mid-IR light is impor­tant in many fields. It contains the charac­teristic spectral bands of most types of molecules, and penetrates the atmo­sphere effec­tively, so it is key to detecting a wide range of substances such as in environ­mental monitoring, as well as for military and industrial appli­cations, the researchers say. Since most ordinary optical materials used in the visible or near-infrared bands are totally opaque to these wave­lengths, mid-IR sensors have been complex and expen­sive to make. So the new approach could open up entirely new potential appli­cations, including in consumer sensing or imaging products, Gu says. (Source: MIT)

Reference: L. Zhang et al.: Ultra-thin high-efficiency mid-infrared transmissive Huygens meta-optics, Nat. Commun. 9, 1481 (2018); DOI: 10.1038/s41467-018-03831-7

Link: Photonic Materials, Dept. of Materials Science & Engineering, Massachusetts Institute of Technology, Cambridge, USA

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