Delivering VR in Perfect Focus With Nanostructure Meta-lenses

If wearing a virtual reality or augmented reality headset is ever to become commonplace, hardware manufacturers will need to figure out how to make the devices small and lightweight while ensuring their images are sharp and clear. Unfortunately, this task faces a key limitation in optics: Conventional lenses are curved glass objects that focus different wavelengths of light in different locations, which would show viewers blurry images. As a result, pretty much anything with a lens—from tiny smartphone cameras to large-scale projectors—uses multiple lenses, which add weight, thickness, and complexity, increasing cost.

We’ve figured out a new way to manufacture fully transparent, ultracompact lenses capable of properly focusing every color in the spectrum to the same point. Because our lens comprises specially designed nanostructures, which do not exist in nature, to focus light, we call it a “meta-lens.” It has the advantages of being ultracompact while capable of delivering higher-quality imaging across a wider spectrum of light than most traditional lenses, without requiring multiple lenses.

Bending Light

For centuries, most lenses for telescopes, glasses, and other optical equipment have been manufactured by grinding glass into a rough curved shape and then polishing it to cleanly and clearly bend light. However, these lenses can’t focus light of every color on the same point.

It is a basic property of light that different colors—or frequencies—travel at different speeds in a lens. They cannot reach the same point at the same time, resulting in blurred images.

Different frequencies of light bend and travel differently in a lens. Image Credit: Lucas V. Barbosa

To reduce this effect, commercial lens manufacturers construct complicated optical devices with many separate lenses, each precisely ground into curves and aligned to focus its range of wavelengths in just the right place. However, they end up with large, heavy and complex lenses—nothing that would be easy to wear comfortably as part of a VR experience.

The Power of Nanostructures

To replace these enormous and expensive precision-engineered products, we start with a millimeter-thick sheet of regular flat glass. On it, we place a layer of carefully designed rectangular nanostructures, a million times thinner than the glass layer, made of titanium dioxide, which is totally transparent to visible light.

The nanonstructures as viewed by a scanning electron microscope. Image Credit: Capasso Group, Harvard University, CC BY-ND

The nanostructures are designed to bend incoming light rays by increasingly greater angles the farther they hit the meta-lens from its center so that all rays are focused in the same spot. To secure the nanostructures onto the glass substrate, we use lithography, a technique widely used to mass-produce computer chips.

In 2016, we showed that using flat glass with nanostructures could focus light of one specific color just as well as a traditional curved lens. But in that research, what we made suffered from the same age-old problem as curved glass: Each color focused on a different location. To have our flat lenses form high-quality images, all the light—regardless of its color—must focus on the same point.