AsianScientist (Dec. 7, 2017) – Scientists in South Korea and the United Kingdom have developed optical devices made of graphene and gold which can control the intensity and polarity of light. They published their findings in Advanced Optical Materials.
Metasurfaces are two-dimensional materials that can effectively control the electric and magnetic components of light and other electromagnetic waves, bending them in specific directions. Controlling the beam’s direction can bring out interesting phenomena, the most incredible being the ‘invisibility cloak effect,’ where light waves bypass an object, recreating the image beyond the object, as flowing water in a river would bypass a stone.
In this study, scientists at the Center for Integrated Nanostructure Physics within South Korea’s Institute for Basic Science, in collaboration with researchers from the University of Birmingham and the Korea Advanced Institute of Science and Technology, developed flat lenses with tunable features.
Specifically, the researchers pierced micrometer-sized, U-shaped holes in a gold sheet and coated it with graphene, producing what they called a metalens. As the shape of common convex lenses allows light to be concentrated (or focused) on a spot, the pattern of the tiny apertures of the metalens focuses the incoming beam on a specific point.
In addition, the micrometer-sized holes can change light polarization. The team used circularly polarized waves, that is, a light beam where the direction of the electric field is corkscrew spiraling, and demonstrated that their metalenses could convert the left-circular polarization wave (going counterclockwise if viewed from the front) into a right-circular polarization (clockwise).
The researchers managed to obtain a conversion rate of 35 percent. Converting circular polarization could be useful in various scientific domains, for example, biosensing and telecommunications.
To control even more properties, the scientists took advantage of graphene’s unique electronic features and used them to tune the output beam’s intensity or amplitude. Here, graphene plays the role of the exposure of a camera.
In the case of the camera, the amount of light entering the instrument is varied by controlling the shutter’s opening time and size. These metalenses instead regulate the exposure via an electric tension applied to the graphene sheet, without the need for bulky components. When voltage is applied to the graphene layer, the output beam becomes weaker.
“Using metalenses, you can make microscopes, cameras and tools used in very sensitive optical measurements much more compact,” said study first author Dr. Kim Teun-Teun of the University of Birmingham.
The article can be found at: Kim et al. (2017) Amplitude Modulation of Anomalously Refracted Terahertz Waves with Gated-Graphene Metasurfaces.
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Source: Institute for Basic Science.
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