An Ultrathin Flat Lens with a Broad Bandwidth

Schematic of the fishnet metalens and a closeup of its unit cell. The metalens is made of titanium dioxide. (Source: Kante Lab, UCB)

Researchers at UC Berkeley have created an ultrathin, flat optical lens whose performance can finally rival that of bulkier, tradi­tional lenses on the market. The development could lead to game-changing advances in solar energy, virtual reality technology, medical imaging and other appli­cations reliant upon optics. The lens measures 350-nanometers thick with an efficiency of 70% over a frequency spanning visible to infrared light. “This is the first photonic system with such a broad bandwidth and such high effi­ciencies in an ultrathin flat lens,” said Boubacar Kanté, associate professor of electrical engin­eering and computer sciences and faculty scientist at Lawrence Berkeley National Laboratory. “This is, simply, the thinnest, most efficient, broadest band flat lens in the world.”

The size and weight of optical lenses have presented signi­ficant obstacles in the effort to miniaturize opto­electronics and other devices. Tradi­tional lenses, one of the most basic components in optics, are usually made with bulk materials and curved surfaces to bend and focus incoming light waves. Such lenses are able to capture 65–75% of the incident light, but they are relatively heavy and susceptible to chromatic aberrations, the distortions that result due to the dependence of the refractive index on the color of light. Flat lenses use engineered designs and charac­teristics of different metamaterials to bend light. They are appealing to researchers because such metalenses are capable of mani­pulating light at subwave­length scales, and they can overcome chromatic aberra­tions, but only in a very narrow band of light. Until now.

Going flat has often meant sacrificing performance in lenses. Previous metalenses delivered lower effi­­­­ciencies of 20–40% in lenses measuring 600–800 nanometers thick. The researchers came up with an innovative solution to improve the light-capturing capabilities of their flat metalens. They used electron-beam lithography to shape a fishnet pattern onto a titanium dioxide wafer. To mimic the curvature of a traditional lens, they used a gradient in which smaller holes were formed in the center and larger ones were positioned around the edges.

They demons­trated the ability of their fishnet-achromatic-metalens to capture 70% of incoming light in frequencies ranging from 640 nanometers to 1200 nanometers. Light entering the fishnet metalens within that broad octave band of wavelengths would be focused at a single point on the other side of the lens. “We are very excited by these results because many appli­cations required the simul­taneous processing of multiple wavelengths in a broad spectrum,” said Kanté. “This is the case for solar energy appli­cations where we need to focus all color of light for efficient solar cells or solar concen­trators.”

The lighter-weight metalens also reduces the power consumption for a variety of appli­cations, said Kanté. As an example, satellites, where every ounce counts, would require less fuel to launch and operate if metalenses replaced tradi­tional ones. A good next step, Kanté said, would be to develop processes that could scale up for larger scale production. (Source: UCB)

Reference: A. Ndao et al.: Octave bandwidth photonic fishnet-achromatic-metalens, Nat. Commun. 11, 3205 (2020); DOI: 10.1038/s41467-020-17015-9

Link: Kanté Lab, Dept. of Electrical Engineering and Computer Sciences, University of California, Berkeley, USA

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