Seeing Meta Lenses

Seeing it sensing it.  Machine seeing, sensing.

FLAT LENSES MADE OF NANOSTRUCTURES TRANSFORM TINY CAMERAS AND PROJECTORS  In Spectrum IEEE

Metalenses are finally moving into consumers’ hands

Inside today’s computers, phones, and other mobile devices, more and more sensors, processors, and other electronics are fighting for space. Taking up a big part of this valuable real estate are the cameras—just about every gadget needs a camera, or two, three, or more. And the most space-consuming part of the camera is the lens.

The lenses in our mobile devices typically collect and direct incoming light by refraction, using a curve in a transparent material, usually plastic, to bend the rays. So these lenses can’t shrink much more than they already have: To make a camera small, the lens must have a short focal length; but the shorter the focal length, the greater the curvature and therefore the thickness at the center. These highly curved lenses also suffer from all sorts of aberrations, so camera-module manufacturers use multiple lenses to compensate, adding to the camera’s bulk.

With today’s lenses, the size of the camera and image quality are pulling in different directions. The only way to make lenses smaller and better is to replace refractive lenses with a different technology.

That technology exists. It’s the metalens, a device developed at Harvard and commercialized at Metalenz, where I am an applications engineer. We create these devices using traditional semiconductor-processing techniques to build nanostructures onto a flat surface. These nanostructures use a phenomenon called metasurface optics to direct and focus light. These lenses can be extremely thin—a few hundred micrometers thick, about twice the thickness of a human hair. And we can combine the functionality of multiple curved lenses into just one of our devices, further addressing the space crunch and opening up the possibility of new uses for cameras in mobile devices.

Before I tell you how the metalens evolved and how it works, consider a few previous efforts to replace the traditional curved lens.

Conceptually, any device that manipulates light does so by altering its three fundamental properties: phase, polarization, and intensity. The idea that any wave or wave field can be deconstructed down to these properties was proposed by Christiaan Huygens in 1678 and is a guiding principle in all of optics.

a hand holding a tweezer, with a blowout showing an array of small pillarsIn this single metalens [between tweezers], the pillars are less than 500 nanometers in diameter. The black box at the bottom left of the enlargement represents 2.5 micrometers. METALENZ

In the early 18th century, the world’s most powerful economies placed great importance on the construction of lighthouses with larger and more powerful projection lenses to help protect their shipping interests. However, as these projection lenses grew larger, so did their weight. As a result, the physical size of a lens that could be raised to the top of a lighthouse and structurally supported placed limitations on the power of the beam that could be produced by the lighthouse.

French physicist Augustin-Jean Fresnel realized that if he cut a lens into facets, much of the central thickness of the lens could be removed but still retain the same optical power. The Fresnel lens represented a major improvement in optical technology and is now used in a host of applications, including automotive headlights and brake lights, overhead projectors, and—still—for lighthouse projection lenses. However, the Fresnel lens has limitations. For one, the flat edges of facets become sources of stray light. For another, faceted surfaces are more difficult to manufacture and polish precisely than continuously curved ones are. It’s a no-go for camera lenses, due to the surface accuracy requirements needed to produce good images.

Another approach, now widely used in 3D sensing and machine vision, traces its roots to one of the most famous experiments in modern physics: Thomas Young’s 1802 demonstration of diffraction. This experiment showed that light behaves like a wave, and when the waves meet, they can amplify or cancel one another depending on how far the waves have traveled. The so-called diffractive optical element (DOE) based on this phenomenon uses the wavelike properties of light to create an interference pattern—that is, alternating regions of dark and light, in the form of an array of dots, a grid, or any number of shapes. Today, many mobile devices use DOEs to convert a laser beam into “structured light.” This light pattern is projected, captured by an image sensor, then used by algorithms to create a 3D map of the scene. These tiny DOEs fit nicely into small gadgets, yet they can’t be used to create detailed images. So, again, applications are limited.   ... '