Nanotechnology Technology

Meta-Lens Able to Record at Smaller than a Wavelength of Light

Scanning electron microscope micrograph of the fabricated meta-lens. The meta-lens is made of titanium dioxide nanofins on a glass substrate. Scale bar: 2 mm (Image courtesy of the Capasso Lab)

Researchers from the Harvard John A. Paulson School of Engineering and Applied Sciences (or SEAS) have found a way to create the very first planar lens that works very efficiently within the visible spectrum of light. This completely covers the entire range of colors from red all the way to blue. The new lens is able to resolve nanoscale features that are less than a wavelength of light apart from one another, using an ultrathin array of teeny waveguides, called metasurface, that bend light as it passes through. The complete study can be found in Science journal.

Curved lenses are commonly used in cameras and telescopes. In order to minimize distortion and resolve clearer images multiple lenses are stacked on one another, leading to large microscopes and lengthy telephoto lenses. Technology has changed a lot over the years but it is still very difficult to create both compact and thin lenses.

Senior author of the paper, Federico Capasso says this technology holds the potential of being quite revolutionary as it works within the visible spectrum. This means it holds the ability to replace lenses on a wide range of devices. In the not so distant future, metalenses will be produced to the masses at a fraction of the cost that conventional lenses require.

Bernard Kress from Partner Optical Architech at Microsoft says that correcting for chromatic spread over the visible spectrum in an effective way using only a single flat optical element wasn’t something that was possible prior to the research. He says the Capasso group’s metalens will allow integration of broadband imaging systems in a lightweight and compact design. This will allow upcoming generations of optical sub-systems to remove their need for large weight, size, power and costs.

meta lens
Schematic exhibiting the ultra-thin meta-lens. The lens is made of titanium dioxide nanofins on a glass substrate. The meta-lens focuses an incident light (penetrating from bottom and propagating upward) to a spot (yellow space) smaller than the incident wavelength. (Image credit: Peter Allen)

To be able to focus red, blue and green light, the team had to use materials that would not absorb or scatter light. Rob Devlin, co-author of the paper said the material had to be able to confine light to a specific space with a large refractive index. The material also had to be something that was already being put to use within the industry.

Titanium dioxide was used to build the nanoscale array of smooth and high-aspect ratio nanostructures that form the very heart of the metalens. The goal was to build a single planar lens with a very large numerical aperture, allowing it to focus light into a spot that is smaller than a single wavelength. First author of the paper, Mohammadreza Khorasaninejad explains that the more tightly light is focused; the smaller the focal spot is possible to be, giving a far more enhanced resolution of the image.

Light passing through the meta-lens is focused by millions of nano structures (Capasso Lab)

The team designed the array to resolve a structure that was about 400 nanometers across. At this scale, the metalens was able to offer much better focus than any lens on the market today. Wei Ting Chen, coauthor of the study says normal lenses require hand polishing that is completely precise. If there are any changes in the curvature, the performance of the lens will drop very dramatically. The new lens is created in just one step, with a single layer of lithography that holds everything as it must be in order to work at top capacity.

Researchers are excited to be the very first to discover such a large-scale real-life application in the metamaterials field. They have solved the difficult challenge of building a visible-range metalens that accomplishes high numerical aperture as well as high efficiency. Reaching these two goals at the same time is astounding, which will mean big things in wearable optics, virtual reality and augmented reality applications.