16 December 2021

Color-Sorting Metalenses Boost Imaging Sensitivity

Technology advance enables filter-free imaging sensors that are poised to improve low-light and fast imaging for smartphones and autonomous vehicles

WASHINGTON — Researchers have shown that newly designed pixel-scale metasurface lenses — flat surfaces that use nanostructures to manipulate light — can be used to make imaging sensors that are roughly three times more sensitive than those used today. The new sensor architecture could enable digital cameras that can image faster or in conditions with less light.

Caption: Researchers have developed a metalens that acts as both a color splitter and a lens. It can be directly integrated on sensor pixels to create a filter-free sensor with increased sensitivity. 

Credit: Masashi Miyata, NTT Device Technology Labs

“Traditional imaging sensors such as the ones used in smartphones, wearable devices and autonomous vehicles have a limited sensitivity because they rely on color filters placed over each pixel,” said research team leader Masashi Miyata from NTT Device Technology Labs in Japan. “Our new metalenses are made from a highly-engineered surface that can collect light while simultaneously separating primary colors without any color filters, opening a pathway to dramatically improve sensitivity.”

In Optica, Optica Publishing Group’s journal for high-impact research, Miyata and colleagues report that filter-free color sensors made with the new metalenses significantly enhanced signal levels without sacrificing color image quality or spatial resolution. And because the new metalenses are made using a CMOS-compatible process, they could easily be integrated onto current sensors to create filter-free imaging devices.

“We envision our metalenses playing an important role in the development of filter-free color image sensors that exceed current sensitivity limits,” said Miyata. “These new sensors could one day let people more easily capture night views with smartphones or enable new cameras that accurately capture high-speed objects, which will be useful in security and autonomous driving.”

Caption: This scanning electron microscopy image shows the 1250-nm-tall nanoposts used to create the metalens array.

Credit: Masashi Miyata, NTT Device Technology Labs

Eliminating filters

In a conventional sensor, color information is acquired by using color filters that absorb a portion of the light. A red filter, for example, lets through only red wavelengths while absorbing all the other wavelengths. This means that only about 30% of the light is actually detected.

To boost sensitivity, the NTT researchers designed a metalens array that acquires color information without optical loss through a process known as color sorting. This involves splitting the light into red, green and blue and then focusing each color onto different pixels. The pixel-scale metalens array was created by etching nanoposts into a 1250-nm-thick layer of silicon nitride.

Although other pixel-scale color splitters have been experimentally demonstrated, they haven’t been practical for consumer devices because they were either inefficient, affected by the light’s polarization or sensitive to light that might hit the sensor from an oblique angle. The new metalenses, however, are based on a dispersion-enriched metasurface platform that makes them polarization-insensitive and suppresses spectral crosstalk for all the color pixels. Because the metalenses are so efficient at focusing light, their color sorting performance isn’t affected by oblique light.

Caption: This color microscopy image shows the measured light profile on the sensor pixels under normal illumination. The efficiency of color sorting and focusing into each target pixel can be observed.

Credit: Masashi Miyata, NTT Device Technology Labs

Evaluating sensor performance

The researchers used an optical microscope to mimic the way that light would travel through a metalens array before reaching a sensor. This experiment showed that, compared with a filter-based sensor, the metalens-based sensor generates color images with 2.83-fold enhanced signal levels without sacrificing color quality.

Optical simulation studies also showed that the metalens-based sensor architecture exhibited less image degradation due to sensor noise, which is often the limiting factor in dark-scene or ultra-fast imaging. Now that they have demonstrated the new sensor concept, the researchers plan to create and test an integrated device by directly mounting a metalens array onto an image sensor.

“We hope our work will further boost the development of practical optical devices and systems based on metasurfaces,” said Miyata. “With their ability to flatten and shrink optical components while drastically enhancing performance, we believe that optical metasurfaces can be applied not only to image sensors but also to various optoelectronics devices such as those used in displays, projectors and augmented or virtual reality devices.”

Paper: M. Miyata, N. Nemoto, K. Shikama, F. Kobayashi, T. Hashimoto, “Full-color-sorting metalenses for high-sensitivity image sensors,” Optica, 8(12), 1596-1604 (2021) https://doi.org/10.1364/OPTICA.444255.

About Optica

Optica is an open-access journal dedicated to the rapid dissemination of high-impact peer-reviewed research across the entire spectrum of optics and photonics. Published monthly by Optica Publishing Group, the Journal provides a forum for pioneering research to be swiftly accessed by the international community, whether that research is theoretical or experimental, fundamental or applied. Optica maintains a distinguished editorial board of more than 60 associate editors from around the world and is overseen by Editor-in-Chief Prem Kumar, Northwestern University, USA. For more information, visit Optica.

About Optica Publishing Group (formerly OSA)

Optica Publishing Group is a division of the society, Optica (formerly OSA), Advancing Optics and Photonics Worldwide. It publishes the largest collection of peer-reviewed and most-cited content in optics and photonics, including 18 prestigious journals, the society’s flagship member magazine, and papers and videos from more than 835 conferences. With over 400,000 journal articles, conference papers and videos to search, discover and access, our publications portfolio represents the full range of research in the field from around the globe.

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