Nanoscale kirigamis manipulate light

July 12, 2018 //By Julien Happich
Nanoscale kirigamis manipulate light
Getting their inspiration from traditional Japanese paper-cutting and folding techniques known as kirigami, researchers from the Massachusetts Institute of Technology have demonstrated a simple one-step fabrication method that could prove really versatile to design complex 3D structures at the nanoscale.

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.

Showing macro-kirigami and nano-kirigami side by side. (A) Camera images of the paper kirigami process of an expandable dome. (B) SEM images of an 80-nm-thick gold film, a 2D concentric arc pattern and a 3D microdome. The high-dose FIB milling corresponds to the “cutting” process, and the global low-dose FIB irradiation of the sample area (enclosed by the dashed ellipse) corresponds to the “buckling” process in nano-kirigami. (C to F) A 12-blade propeller and (G to J) a four-arm pinwheel formed in a macroscopic paper and a gold nanofilm, respectively. Scale bars in SEM images is 1um. Credit: Liu et al., Sci. Adv. 2018;4: eaat4436.

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