Nanoscale kirigamis manipulate light
Described in the Science Advances journal in a paper titled “Nano-kirigami with giant optical chirality”, the technique relies solely on the ion beam irradiation of a 80nm-thick free-standing gold film.
Instead of relying on stimuli like temperature changes, volume variations, or capillary forces to exert differential strains in cut-out geometries and bow them out of plane, the researchers entirely used gallium-based Focused Ion Beam (FIB) to not only cut out intricate shapes (at high intensity milling), but also to create controlled and localized tensile stress through lower intensity irradiation.
Upon ion irradiation, some of the gold atoms are sputtered away from the surface and the resulting vacancies cause grain coalescence which induces tensile stress close to the film surface, they explain in the paper.
Simultaneously, gallium ions are also implanted into the film, which induces compressive stress, and it is this stress differential across the first 20nm of the gold’s film that determines the overall film deformation, the researchers report.
They were able to simplify the gold film’s behaviour as a bilayer model for predictive modelling upon selective irradiation.
Hence, with pre-programmed irradiation, the researchers were able to cut-out and 3D shape various kirigami patterns at the nanoscale. They expect this novel manufacturing technique to find applications in the design of functional structures for plasmonics, nanophotonics, optomechanics but also MEMS/NEMS, to name a few.
One example cited and demonstrated in the paper is the out-of-plane twisting through nano-kirigami, to yield unique electromagnetic properties such as 3D optical chirality. They leveraged giant optical chirality effects by designing arrays of 3D microscale pinwheel-like structures with a lattice periodicity of 1.45µm, observing distinct circular dichroism (causing different absorption losses for right-hand or left-hand circularly polarized light-waves.
Optical measurements showed that the circular dichroism spectra of LH and RH 3D pinwheel structures exhibited nearly opposite signs with similar amplitudes.
Massachusetts Institute of Technology – www.mit.edu
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