Optical Lenses out of 2D Materials

Prototypes of ultrathin metalenses made of 2D materials such as hexagonal boron nitride and molybdenum disulfide. (Source: Liu et al., Nano Lett.)

In recent years, physicists and engineers have been designing, construc­ting and testing different types of ultrathin materials that could replace the thick glass lenses used today in cameras and imaging systems. Criti­cally, these meta­lenses are not made of glass. Instead, they consist of materials constructed at the nanoscale into arrays of columns or fin-like structures. These formations can interact with incoming light, directing it toward a single focal point for imaging purposes.

But even though metalenses are much thinner than glass lenses, they still rely on 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 and the National Tsing Hua Univer­sity in Taiwan constructed func­tional metalenses that are one-tenth to one-half the thickness of the wave­lengths of light that they focus. Their metalenses, which were constructed out of layered 2D materials, were as thin as 190 nano­meters.

“This is the first time that someone has shown that it is possible to create a metalens out of 2D materials,” said Arka Majumdar, a UW assistant pro­fessor of physics and of electrical and computer engi­neering. Their design principles can be used for the creation of meta­lenses with more complex, tunable features, added Majumdar.

Majumdar’s team has been studying the design principles of meta­lenses for years, and previously constructed meta­lenses for full-color imaging. But 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. In mathe­matical terms, 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 500-nano­meter lightwave would need to be about 500 nano­meters in thickness, though this thickness can decrease as the refrac­tive index of the material increases.

Majumdar and his team were able to synthesize functional meta­lenses that were much thinner than this theoretical limit – one-tenth to one-half the wavelength. First, they con­structed the metalens out of sheets of layered 2D materials. The team used widely studied 2D materials such as hexagonal boron nitride and molyb­denum disul­fide. A single atomic layer of these materials provides a very small phase shift, unsuitable for efficient lensing. So the 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 figuring out what type of design would yield the best perfor­mance given the incom­plete phase,” said 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. They tested the metalens’ efficacy by using it to capture different test images, including of the Mona Lisa and a block letter W. The team also demon­strated how stretching the metalens could tune the focal length of the lens.

In addition to achieving a wholly new approach to metalens design at record-thin levels, the team believes that its experi­ments show 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,” said Majumdar. Addi­tionally, these materials can be easily trans­ferred on any substrate, including flexible materials, paving a way towards flexible photonics. (Source: Univ. Washington)

Reference: C.-H. Liu et al.: Ultrathin van der Waals Metalenses, Nano Lett. 186961 (2018); DOI: 10.1021/acs.nanolett.8b02875

Link: Nano Optoelectronic Integrated System Engineering, Dept. of Physics, University of Washington, Seattle, USA

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