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Functional optical metalens made from 2D materials

19 Nov 2018

US-Taiwanese partnership uses materials such as hexagonal boron nitride and molybdenum disulfide.

New types of ultrathin materials that could replace the conventional glass lenses used in cameras and imaging systems. Typically the new sorts of metalenses are not made of glass. Instead, they consist of materials constructed at the nanoscale in arrays of columns or fin-like structures like Fresnel lenses.

But although metalenses are much thinner than glass lenses, they still rely on relatively high aspect ratio structures, in which the column or fin-like structures are much taller than they are wide, making them prone to collapsing and falling over. Furthermore, these structures have always been near the wavelength of light they’re interacting with in thickness — until now.

A team from the University of Washington, MD, USA, (UW) and National Tsing Hua University, Taiwan, have developed functional metalenses that measure between one-tenth to one-half the thickness of the wavelengths of light that they focus. The metalenses, constructed from layered 2D materials, are as thin as 190 nm across. The work has been described in Nano Letters.

“This is the first time that researchers have shown that it is possible to create a metalens out of 2D materials,” commented article author Arka Majumdar, an assistant professor of physics and of electrical and computer engineering at UW. The design principles can be used for the creation of metalenses with more complex, tunable features, added Majumdar, who is also a faculty researcher with the UW’s Molecular Engineering & Sciences Institute and Institute for Nano-Engineered Systems.

Majumdar’s team has been studying the design principles of metalenses for several years, and previously constructed metalenses for full-color imaging.

Overcoming design limitation

The challenge in this project was to overcome an inherent design limitation in metalenses: in order for a metalens material to interact with light and achieve optimal imaging quality, the material had to be roughly the same thickness as the light’s wavelength in that material. This restriction ensures that a full zero to two-pi phase shift range is achievable, which guarantees that any optical element can be designed. For example, a metalens for a 500nm lightwave (green light) would itself need to be about 500nm thick, although this thickness can be decreased as the refractive index of the material increases.

Majumdar and his team have synthesized functional metalenses that are much thinner than this theoretical limit — one-tenth to one-half the wavelength. First, they constructed a metalens from sheets of layered 2D materials; the team employed widely-studied 2D materials such as hexagonal boron nitride and molybdenum disulfide.

A single atomic layer of these materials provides a small phase shift that is unsuitable for efficient lensing. So instead the UW team used multiple layers to increase the thickness, although the thickness remained too small to reach a full two-pi phase shift.

“We had to start by deciding what type of design would yield the best performance given the incomplete phase,” commented co-author Jiajiu Zheng, a doctoral student in electrical and computer engineering.

To make up for the shortfall, the team employed mathematical models that were originally formulated for liquid-crystal optics. These, in conjunction with the metalens structural elements, allowed the researchers to achieve high efficiency even if the whole phase shift is not covered.

Promising work

In addition to achieving a new approach to metalens design at record-thinness levels, the team says that this work shows the promise of making new devices for imaging and optics entirely out of 2D materials.

“These results open up an entirely new platform for studying the properties of 2D materials, as well as constructing fully functional nanophotonic devices made entirely from these materials,” commented Majumdar. “Additionally, these materials can be easily transferred on any substrate, including flexible materials, paving a way towards flexible photonics.”

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