FIB creates shapes to control light


Rebecca Pool

Wednesday, July 11, 2018 - 10:30
Image: FIB carves different patterns of slices through a gold nanofilm that cause the foil to fold up into predetermined shapes [MIT]
Using a focused ion beam, MIT researchers and colleagues have carved precise patterns into metal foil to create 3D shapes that manipulate light.
These new nanodevices open up new possibilities for research and, ultimately, the creation of new light-based communications, detection, or computational devices.
The so-called research field of nanokirigami is based on the ancient art of kirigami - making 3D shapes by folding and cutting paper - but is applied to flat materials at the nanoscale.
Professor Nicholas X Fang from Mechanical Engineering, and colleagues, used a dual-beam FIB/SEM system - FEI Helios 600i - to make a precise pattern of slits in a gold foil just a few tens of nanometres thick. 
The process  could be programmed into one step and in situ monitored by the SEM, and prompted the foil to bend and twist itself into a complex 3D shape, capable of selectively filtering out light with a particular polarization.
Crucially, the one-step  nano-kirigami method also avoided the prescribed multistep procedures in traditional kirigami or origami techniques.
For these initial proof-of-concept devices, Fang used the nanomechanical equivalent of dichroic filters that filter out circularly polarized light.
To do so, they created a pattern just a few hundred nanometres across in the thin metal foil resembling pinwheel blades, with a twist in one direction that selects the corresponding twist of light.
According to Fang, when using ion beams with low dosages, vacancy defects are created within the foil, pushing the metal crystal lattice out of shape and creating strong stresses that induce the bending.
The researchers used helical patterns to separate out the clockwise and counterclockwise polarized portions of a light beam, which they reckon could represent 'a brand new direction' for nanokirigami research.
"By using the topography-guided stress equilibrium, rich 3D shape transformation such as buckling, rotation, and twisting of nanostructures is precisely achieved, which can be predicted by our mechanical modeling," highlights Wang. "Benefiting from the nanoscale 3D twisting features, giant optical chirality is achieved in an intuitively designed 3D pinwheel-like structure,"
“Previously, people were always trying to cut by intuition and create kirigami patterns for a particular desired outcome," he adds.
The latest 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.
Research is published in Science Advances.
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