Metalenses, or flat lenses, are collections of tiny nanostructures arranged on a flat surface that focus incident light in a similar manner to conventional curved refractive lenses. Although metalenses for visible light can be extremely light and thin compared to their refractive counterparts, a big challenge is that their nanostructures have to be smaller than the wavelength of the light they are manipulating. For visible light, these nanostructures have to be 200 times smaller than a strand of hair. Their tiny geometries also mean that a large diameter lens at the scale of those used in backyard telescopes or professional cameras will require many billions of these nanostructures, a challenge even for modern chip-making tools.

In this DARPA-funded collaborative study between the Capasso group at Harvard University, the Large Optics Fabrication and Testing group at the University of Arizona, and the Air Force Research Laboratory, the authors create and comprehensively characterize the world’s largest metalens designed for visible light. This is a 10 cm diameter metalens that is just 0.5 mm thin and weighs 15 grams, 42 times thinner and 16.5 times lighter than the equivalent refractive lens with the same focusing power. This metalens is fabricated using deep ultraviolet lithography and techniques that are compatible with commercial semiconductor processing tools, even bypassing certain exposure area limitations of these tools. The resultant metalens, with its 18.7 billion glass nanopillars, was evaluated through a wide-range of experimental and numerical techniques to quantify its tolerance to various sources of imperfections and harsh operating environments. In particular, extreme temperature swings over 400 degrees Celsius and ultrasonic vibrations did not degrade optical performance, demonstrating the robustness of the glass metalens platform. 

As a vivid demonstration of its optical performance, the 10 cm metalens was used to image a variety of celestial objects, ranging from dark sunspots against a bright Sun, to the terminator on the Moon (shadowed boundary of the Moon), and even the North American nebula, a faint cloud of ionized gas that is too dim to be seen with the naked eye. Ultralight and ultra-compact metalenses have the potential to revolutionize space-based and airborne imaging and remote sensing - and it all begins with the tiniest of nanostructures working in tandem with billions of others. 

Read the article in ACS Nano here and the Harvard SEAS press release here.