Twisting and bending nanoscale materials creates novel optical devices

Nanokirigami has taken off as a field of research over the past few years. The approach is based on the ancient arts of origami (making three-dimensional shapes by folding paper) and kirigami (which allows cutting as well as folding), but applied to flat materials at the nanoscale.

Now, researchers at Massachusetts Institute of Technology (MIT) and in China have for the first time applied this approach to the creation of nanodevices for manipulating light. This potentially opens up new possibilities for research and, ultimately, the creation of new light-based communications, detection or computational devices.

The findings are reported in a paper in Science Advances by MIT professor of mechanical engineering Nicholas Fang and five others. Using methods based on standard microchip manufacturing technology, 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. Using ion beams with low dosages creates many vacancies in the crystal lattice of the metal and some of the ions end up lodged in these vacancies, 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 counter-clockwise 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, devices to measure glucose levels often use measurements of light polarity, because glucose molecules exist in both right- and left-handed forms that 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."

This story is adapted from material from MIT, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.