In a first for optical lenses, Columbia Engineering researchers have created a flat lens capable of full-color imaging.

Ordinary simple lenses are limited by an inability to focus light of different colors to a single spot due to dispersion. As a result, different colors are never in focus at the same time, and an image formed by a simple lens is inevitably blurred.

The conventional way to solve this problem is to stack multiple lenses; this routes the light through different paths and allows the whole light wave to arrive at the focal point at the same time. Conventional lenses are designed to add an increasing amount of delay to light as it moves toward the lens center, which is why they are thinner at the edges. This comes, however, at the cost of increased complexity and weight.

The new lens is flat, much thinner than a sheet of paper (just one micron thick) and needs no additional elements to offer performance comparable to top-of-the-line compound lens systems.

Led by applied physics professor Nanfang Yu, the Columbia team used their expertise in optical "metasurfaces,” or engineered two-dimensional structures, and built their lens from “meta-atoms” in order to control light propagation. Each meta-atom has a size that is a fraction of the wavelength of light. By patterning a very thin, flat layer of nanostructures on a substrate as thin as a human hair, the researchers’ meta-lenses were able to achieve the same function as a much thicker and heavier conventional lens system.

Moreover, the lenses were made by standard 2D planar fabrication techniques similar to those used for fabricating computer chips. The potential for mass-manufacturing is promising: modern computer chips need numerous nanostructure layers to be defined – some as many as 100 – whereas the lenses need just one. They also do not require the costly and time-consuming process of grinding and polishing.

The meta-lenses can focus a large range of colors of any arbitrary polarization state, meaning it will work not only in a controlled lab setting, but also in real-world ambient-light applications. It also works with transmitted light, as might be found in an optical system.

"Because they are so thin, they have the potential to drastically reduce the size and weight of any optical instrument or device used for imaging, such as cameras, microscopes, telescopes and even our eyeglasses," said Yu. "Think of a pair of eyeglasses with a thickness thinner than a sheet of paper, smartphone cameras that do not bulge out, thin patches of imaging and sensing systems for driverless cars and drones, and miniaturized tools for medical imaging applications."

The team anticipates that their meta-lenses could replace bulky lens systems, comparable to the way flat-screen TVs have replaced CRTs. Nader Engheta, a University of Pennsylvania professor who was not involved in this study but is an expert in nanophotonics and metamaterials, agrees that the potential exists for a diverse set of applications involving imaging, sensing and compact camera technology.

The research is published in the Oct. 3, 2018, edition of the journal Light: Science & Applications.