Metasurfaces: Light-Bending, Wafer-Thin Lenses with a Bright Future 

RESEARCH BRIEF

By Pawel Slabiak • March 31, 2023

Center for Advanced Imaging Innovation and Research

Metasurfaces are tiny: this sample, smaller than a penny, holds thirteen of them with room to spare.

A metasurface is an arrangement of nanoparticles on a substrate.

The individual meta-atoms that make up a metasurface are just a few hundred nanometers in diameter.

This interaction can be exploited to direct incident light, giving metalenses the potential to enable smaller, flatter lenses than traditional optics allow.

The meta-atoms, similar in size to the wavelength of visible light, enter into resonance as light passes through them.

Increasingly fine manipulation of light with nanoengineered surfaces is possible thanks to nanomanufacturing technologies and computer modeling tools.

"You can achieve the function you want," said Haogang Cai, PhD, assistant professor of radiology at NYU Langone Health, whose lab investigates metasurfaces.

Here's a metalens pattern created by an optimization algorithm developed by Dr. Cai and colleagues.

The variations in size and distance between the nanodiscs control the resulting resonance and optical function.

A computer simulation shows where light is expected to focus after passing through the metalens.

Gold line marks the location of the metasurface.

And the manufactured metalens creates a focal point similar to the one predicted by the model.

Gold line marks the location of the metasurface.

Advanced designs enable a single metalens to aim different wavelengths at the same spot in order to concentrate white light.

Conventional optics require several lenses to produce such achromatic focus.

"I am particularly interested in using this technique in biomedical applications, such as miniaturized bioimaging systems like endoscopes," said Dr. Cai.

Dr. Cai's team has created a flat metasurface that replicates the focusing of a conventional cylindrical lens used in light sheet microscopy.

The corresponding metalens is 2 by 2 millimeters (about the size of a sesame seed) and hair-thin.

A cylindrical lens in a light sheet microscope is approximately  6 millimeters long.

Dr. Cai has also led the development of metaholograms that direct light differently depending on the surrounding medium.

In water, the metalens projects a logo of Argonne National Laboratory.

In air, light passing through the metalens projects an image of the NYU torch.

"Metasurfaces may present an opportunity for dynamic tuning of their optical function," said Dr. Cai.

Control mechanisms could include changes in the local dielectric environment, such as in liquid crystal.

One application Dr. Cai's team is working on is the detection of biomolecules, like glucose particles or viruses.

Medium-specific behavior can be useful in biological sensing.

Beyond medical imaging, optical metasurfaces hold promise for holographic displays and augmented-reality wearables.

"We started with very basic research ... but nowadays I try to think more about applications."

"The freedom of this technology is that you can generate new hybrid devices," said Dr. Cai.

Credits: Research images courtesy of Haogang Cai. Cylinder rendering by Volodimir Zozulinskyi/Shutterstock. Photographs, text, and production by Pawel Slabiak.

Related Publications

cai2r.net

Cai H, Srinivasan S, et al. Inverse design of metasurfaces with non-local interactions. npj Comput Mater 6, 116 (2020). doi: 10.1038/s41524-020-00369-5

Cai H, et al. Polarization-Insensitive Medium-Switchable Holographic Metasurfaces. ACS Photonics 2021, 8, 9, 2581-2589. doi: 10.1021/acsphotonics.1c00836

Zhang X, Cai H, et al. A universal metasurface transfer technique for heterogenous integration. Nanophotonics, 2023.  doi: 10.1515/nanoph-2022-0627