Nanokirigami to Manipulate Light at the Nanoscale

Different patterns of slices through a thin metal foil are made by a focused ion beam. These patterns cause the metal to fold up into predetermined shapes, which can be used for such purposes as modifying a beam of light. (Source: S. Liu et al., MIT)

Nano­kirigami has taken off as a field of research in the last few years; the approach is based on the ancient arts of origami and kiri­gami, which allows cutting as well as folding, but applied to flat materials at the nano­scale, measured in billionths of a meter. Now, researchers at MIT and in China have for the first time applied this approach to the creation of nano­devices to mani­pulate light, poten­tially opening up new possi­bilities for research and, ultimately, the creation of new light-based communi­cations, detection, or compu­tational devices.

Using methods based on standard microchip manu­facturing tech­nology, Nicholas Fang and his team used a focused ion beam to make a precise pattern of slits in a metal foil just a few tens of nano­meters thick. The process causes the foil to bend and twist itself into a complex three-dimen­sional shape capable of selec­tively filtering out light with a particular polari­zation. Previous attempts to create func­tional kirigami devices have used more compli­cated fabri­cation methods that require a series of folding steps and have been primarily aimed at mecha­nical rather than optical functions, Fang says. The new nano­devices, by contrast, can be formed in a single folding step and could be used to perform a number of dif­ferent optical functions.

For these initial proof-of-concept devices, the team produced a nano­mechanical equivalent of specia­lized dichroic filters that can filter out circu­larly polarized light that is either “right-handed” or “left-handed.” To do so, they created a pattern just a few hundred nano­meters across in the thin metal foil; the result resembles pinwheel blades, with a twist in one direction that selects the corres­ponding twist of light. The twisting and bending of the foil happens because of stresses intro­duced by the same ion beam that slices through the metal. When using ion beams with low dosages, many vacancies are created, and some of the ions end up lodged in the crystal lattice of the metal, pushing the lattice out of shape and creating strong stresses that induce the bending.
“We cut the material with an ion beam instead of scissors, by writing the focused ion beam across this metal sheet with a prescribed pattern,” Fang says. “So you end up with this metal ribbon that is wrink­ling up” in the precisely planned pattern.

“It’s a very nice connec­tion of the two fields, mechanics and optics,” Fang says. The team used helical patterns to separate out the clock­wise and counter­clockwise polarized portions of a light beam, which may represent “a brand new direction” for nano­kirigami research, he says. The technique is straight­forward enough that, with the equations the team developed, researchers should now be able to calcu­late backward from a desired set of optical charac­teristics and produce the needed pattern of slits and folds to produce just that effect, Fang says.

“It allows a predic­tion based on optical func­tionalities” to create patterns that achieve the desired result, he adds. “Previously, people were always trying to cut by intuition” to create kirigami patterns for a parti­cular desired outcome. The research is still at an early stage, Fang points out, so more research will be needed on possible appli­cations. But these devices are orders of magnitude smaller than conven­tional counter­parts that perform the same optical functions, so these advances could lead to more complex optical chips for sensing, compu­tation, or communications systems or bio­medical devices, the team says.

For example, Fang says, devices to measure glucose levels often use measure­ments of light polarity, because glucose molecules exist in both right- and left-handed forms which interact dif­ferently with light. “When you pass light through the solution, you can see the concen­tration of one version of the molecule, as opposed to the mixture of both,” Fang explains, and this method could allow for much smaller, more efficient detectors. Circular polari­zation is also a method used to allow multiple laser beams to travel through a fiber-optic cable without inter­fering with each other. “People have been looking for such a system for laser optical communi­cations systems” to separate the beams in devices called optical isolaters, Fang says. “We have shown that it’s possible to make them in nano­meter sizes.” (Source: MIT)

Reference: Z. Liu et al.: Nano-kirigami with giant optical chirality, Sci. Adv. 4, eaat4436 (2018); DOI: 10.1126/sciadv.aat4436

Link: Nanophotonics and 3D Nanomanufacturing Laboratory, Dept. of Mechanical Engineering, Massachusetts Institute of Technology, Cambridge, USA

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