AsianScientist (Nov. 23, 2018) – Researchers have constructed functional metalenses that are as thin as one-tenth of the wavelength of light they focus. These findings, published in Nano Letters, could advance the field of nanophotonics.
The thick glass lenses used in today’s cameras and imaging systems might one day be replaced by metalenses, ultrathin lenses that focus light using nanoscale arrays of columns or fin-like structures rather than glass. Even though metalenses are much thinner than glass lenses, they still rely on what are known as high aspect ratio structures which make them prone to collapsing or falling over.
Furthermore, metalenses are typically as thick as the wavelength of light that they interact with. For example, a metalens for a 500-nanometer lightwave—which in the visual spectrum is green light—would need to be about 500 nanometers in thickness, though this thickness can decrease as the refractive index of the material increases.
A team of researchers from National Tsing Hua University, Taiwan, and the University of Washington, US, has now successfully constructed functional metalenses that are one-tenth to one-half the thickness of the wavelengths of light that they focus. Their metalenses were as thin as 190 nanometers—less than 1/100,000ths of an inch thick.
“This is the first time that someone has shown that it is possible to create a metalens out of 2D materials,” said senior and co-corresponding author Assistant Professor Arka Majumda of the University of Washington.
To overcome the theoretical limit imposed by the wavelength of light, the researchers used sheets of layered 2D materials. Although a single atomic layer of materials such as hexagonal boron nitride or molybdenum disulfide only produces a very small phase shift, stacking multiple layers together increased the lens’ ability to focus light.
“We had to start by figuring out what type of design would yield the best performance given the incomplete phase,” said co-author Mr. Zheng Jiajiu.
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.
The article can be found at: Liu et al. (2018) Ultrathin van der Waals Metalenses.
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Source: University of Washington.
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