Kirigami brings new dimensions of cutting-edge nanophotonics to ion beam patterning

Ion beam milling, with its nanometer precision for pattern etching, quickly became an invaluable tool to create novel and highly controllable geometries on optically active surfaces. A myriad of (typically 2-D) photonic metasurfaces now exists with unique properties developed to couple and manipulate all sorts of light.

With a new twist of creativity by using “nano-kirigami,” work from Liu et al. opens up an entirely new category of metasurfaces with abilities typically unavailable to etched-pattern devices. The resulting 3-D structures exhibit uniaxial broadband polarization conversion, as well as handedness-sensitive phase properties.

Their method creates the 3-D nanostructures by etching 2-D patterns in approximately 80-nanometer-thick gold films with a focused ion beam (FIB), and then exploiting the mechanical stresses created in the film by the ion collisions and implantations from the FIB’s parallel mode irradiation. While this works to create local, independent structures, the authors used a close-loop process to create significantly more complex pinwheel geometries. The global features of these larger structures are dependent on the formation of each substructure.

While the close-loop method requires more modeling to properly predict and design the resulting functionality, it can ultimately produce much more advanced 3-D nanogeometries. The pinwheels presented in this work possesses four-fold rotational symmetry (along the z-axis). This leads to the unusual feature of having uniaxial polarization and phase properties for incident light normal to the surface, as opposed to needing a particular incident polarization as is typically the case.

“Our pinwheel structures possess suspended features for reconfiguration potentials, which may open up new possibilities for the exploration of functional and reconfigurable micro-/nano-photonic and electronic devices,” said Jianfang Li, one of the work’s authors.

Source: “Nano-kirigami metasurfaces by focused-ion-beam induced close-loop transformation,” by Zhiguang Liu, Huifeng Du, Zhi-Yuan Li, Nicholas X. Fang, and Jiafang Li, APL Photonics (2018). The article can be accessed at https://doi.org/10.1063/1.5043065 .