Flat Lens with Achromatic Focus Captures Detailed Images of Sun and Moon

A multilevel diffractive lens from the University of Utah could serve as a lightweight alternative to conventional refractive systems for long-range imaging. The large-aperture flat lens focuses light as effectively as traditional curved lenses and captures color accurately. Astrophotography imaging systems could use the lens to acquire images in applications where space is at a premium, such as on aircraft, satellites, and space-based telescopes.

The researchers used a computational framework to design and test the lens. “Our computational techniques suggested we could design multilevel diffractive flat lenses with large apertures that could focus light across the visible spectrum,” professor Rajesh Menon said.

Using an inverse-design approach and grayscale lithography, the researchers created a flat lens that is 100 mm in diameter and 2.4 μm thick. The lens has a 200-mm focal length and is optimized for the 400- to 800-nm wavelength range.

For centuries, lenses have worked the same way — that is, by using curved glass or plastic to bend the light and focus the image. The thicker and heavier the lens, the more it bends the light, and the stronger the magnification.

The weight and bulk required for curved lenses to be powerful enough to support large telescopes makes them impractical for ground- and space-based telescopes. These telescopes instead rely on massive, curved mirrors, which are thinner and lighter than curved lenses, to bend light and bring the image into focus. However, mirrors can also blot or distort portions of the image.

Researchers at the University of Utah developed a large-diameter diffractive lens that enables detailed, color-accurate imaging in air and space. Courtesy of Scilight: 2025, 2025 (6). doi: 10.1063/10.0035853.

Diffractive lenses can be just as lightweight as mirrors without the drawbacks. However, the diffractive lenses that are currently in use tend to be limited in their capacity and challenging and expensive to make.

A Fresnel zone plate (FZP) is a type of flat lens that uses concentric ridges instead of a thick, curved surface to focus light. This approach makes the lens lightweight and compact, but prevents it from producing true colors. Rather than bending all the wavelengths of visible light at the same angle, the ridges of an FZP lens diffract the wavelengths at different angles, resulting in an image with chromatic aberrations.

To create a flat lens that could depict colors accurately, the researchers created a pattern of microscopic, concentric rings on the substrate of their lens. They optimized the concentric rings of microscopic indentations on their flat lens to bring all wavelengths of light into focus at the same time. Unlike the ridges of FZPs, which are optimized for a single wavelength, the size and spacing of the indentations on the new lens keep the diffracted wavelengths of light close enough together to produce a full-color, in-focus image.

In experiments resolving spatial frequencies up to 181 line-pairs per millimeter, the researchers demonstrated the capability of the flat lens to capture high-quality, full-color images of the moon, sun, and distant terrestrial scenes. The lens acquired color-enhanced lunar images that revealed geological features and solar images that identified sunspots.

The team used hyperspectral point-spread function characterization to confirm that the flat lens could achieve achromatic focusing. It also integrated the multilevel diffractive lens with a refractive achromatic lens to form a hybrid telescope, which could significantly reduce the weight of the lens for airborne and space-based imaging applications.

“Simulating the performance of these lenses over a very large bandwidth, from visible to near-infrared, involved solving complex computational problems involving very large datasets,” professor Apratim Majumder, who led the research, said. “Once we optimized the design of the lens’ microstructures, the manufacturing process involved required very stringent process control and environmental stability.”

The simulations and experimental results demonstrate the exciting potential of large-area, achromatic flat lenses for astrophotography as well as other fields.

“Our demonstration is a stepping stone towards creating very large aperture, lightweight flat lenses with the capability of capturing full-color images for use in air-and-space-based telescopes,” Majumder said.

The research was published in Applied Physics Letters (www.doi.org/10.1063/5.0242208).