Today's glass-based lenses are bulky and resist miniaturization. To address the problem, two different imaging methods — a type of lens designed for nanoscale interaction with lightwaves, and robust computational processing — were combined successfully to create full-color images.
The ultrathin lens is part of a class of engineered objects known as metasur-faces — 2D analogs of metamaterials that are manufactured materials with physical and chemical properties not normally found in nature. A metasur-face-based lens (metalens) consists of flat, microscopically patterned material surfaces designed to interact with lightwaves. To date, images taken with meta-lenses yield clear images for only small slices of the visual spectrum. The new metalens — in conjunction with computational filtering — yields full-color images with very low levels of aberrations across the visual spectrum.
Instead of manufactured glass or silicone, metalenses consist of repeated arrays of nanometer-scale structures, such as columns or fins. If properly laid out at these miniscule scales, the structures can interact with individual lightwaves with precision that traditional lenses cannot. Since metalenses are also so small and thin, they take up much less room than the bulky lenses of cameras and high-resolution microscopes. Meta-lenses are manufactured by the same type of semiconductor fabrication process that is used to make computer chips.
In experiments producing images with metalenses, the optimal wavelength range so far has been very narrow — at best around 60 nanometers wide with high efficiency. But the visual spectrum is 300 nanometers wide.
Today's metalenses typically produce accurate images within their narrow optimal range, such as an all-green image or an all-red image. For scenes that include colors outside of that optimal range, the images appear blurry, with poor resolution and other defects known as chromatic aberrations. For a rose in a blue vase, for example, a red-optimized metalens might pick up the rose's red petals with few aberrations, but the green stem and blue vase would be unresolved blotches with high levels of chromatic aberrations.
If a single metalens could produce a consistent type of visual aberration in an image across all visible wavelengths, the researchers thought to resolve the aberrations for all wavelengths afterward using computational filtering algorithms. For the rose in the blue vase, this type of meta-lens would capture an image of the red rose, blue vase, and green stem — all with similar types of chromatic aberrations that could be tackled later using computational filtering.
A metalens was constructed whose surface was covered by tiny, nanometers-wide columns of silicon nitride. These columns were small enough to diffract light across the entire visual spectrum, which encompasses wavelengths ranging from 400 to 700 nanometers. The arrangement and size of the silicon nitride columns in the metalens were designed so that it would exhibit a spectrally invariant point spread function. Essentially, this feature ensures that, for the entire visual spectrum, the image would contain aberrations that can be described by the same type of mathematical formula. Since this formula would be the same regardless of the wavelength of light, the researchers could apply the same type of computational processing to correct the aberrations.
Unlike many other metasurface-based imaging systems, the new approach isn't affected by the polarization state of light, which refers to the orientation of the electric field in the 3D space in which lightwaves are traveling.