Concentric rings of microscopic structures enable a flat lens to focus light at wavelengths across the visible range [Image: Menon Lab, University of Utah]
Researchers at the University of Utah, USA, have engineered a flat lens that delivers the optical performance needed to capture full-color pictures of distant astronomical objects (Appl. Phys. Lett., doi: 10.1063/5.0242208). Combining a large aperture with the ability to focus light across a broad range of visible wavelengths, the lens offers a small and lightweight solution for imaging systems that are destined to fly onboard satellites or space telescopes.
Overcoming chromatic dispersion
The lens is based on the idea of a Fresnel zone plate, in which a series of concentric rings etched on a flat surface diffracts the incoming light so that it converges at the desired focus. While some large-aperture versions of these flat lenses have been designed for space applications, strong chromatic dispersion has restricted their use to single-wavelength imaging. The researchers believe that a modified design, called a multilevel diffractive lens, could overcome that limitation. In these lenses, the concentric rings are formed from micrometer-scale surface structures, which enable more precise and efficient manipulation of the incoming light.
By using computer simulations to analyze the optical performance produced by microstructures with different geometries, the Utah team designed a pattern of rings that could effectively focus light at wavelengths ranging from 400 to 800 nm. The optimized configuration, which requires 10,000 concentric rings with a fixed width of 5 µm and heights varying between 0 and 2.4 µm, yields a lens with a diameter of 100 mm, a focal length of 200 mm and a numerical aperture of 0.24.
When used to image the sun, the camera was able to reveal the position of individual sunspots on the solar surface.
To fabricate the design with high precision, the researchers exploited a process called grayscale lithography, in which semiconductor tools are used to pattern the microstructures in a photoresist layer on top of a thin glass wafer. The finished lens weighs just 25 g, a factor of eight lighter than a conventional curved lens of the same size.
Full-color images
Tests showed that the fabricated lens could produce focused spots at 17 separate wavelengths within the target range. The researchers then incorporated the lens into a telephoto camera, producing a color image of the moon that resolved various geological features, such as titanium-rich volcanic flows, that cannot be seen with standard systems. When used to image the sun, the camera was able to reveal the position of individual sunspots on the solar surface.
The team also assembled a hybrid telescope that combines the microstructured lens as the primary focusing element with a refractive lens as the eyepiece. This hybrid instrument was used to image terrestrial objects as well as the lunar surface, while measurements confirmed that the telescope could accurately focus light across the visible range. Further simulations suggest that such imaging systems could be eight times thinner than those based on a Fresnel zone plate, while still collecting light with the same efficiency and offering the benefit of full-color imaging.