Miniaturized fractal optical nanoantennas defined by focused helium ion beam milling

Abstract

It has been shown in the past that fractal geometries are beneficial for radio and communication antenna designs in terms of bandwidth and gain. Recently, this concept was extended to plasmonic nanoantennas. Here, we present a fabrication method based on electron beam lithography and focused helium ion beam milling to further miniaturize dimer nanoantennas of 0th, 1st and 2nd order Sierpiński fractals. With this state-of-the-art approach, it becomes feasible to experimentally move their resonance conditions into the sub-micron wavelength regime, while maintaining excellent pattern definition and achieving sub-10 nm gap sizes for high near-field enhancement. These highly sophisticated nanostructures are numerically simulated and analyzed by dark-field scattering spectroscopy to monitor the effects of the fractal structuring on the scattering spectra and near-field enhancement.

Bio-Inspired Nanomembranes as Building Blocks for Nanophotonics, Plasmonics and Metamaterials

“…Figure 12 shows some selected types of nanoantennas. It presents (a) conductive cylindrical scatterer arrays [ 180 ]; (b) nanohole array in conductive surface (optical aperture nanoantennas) [ 181 ], (c) metallic nanodimers [ 182 ]; (d) nanodot array (nanoparticle chain) [ 183 ]; (e) bowtie nanoantennas [ 184 ]; (f) diabolo array [ 185 ]; (g) square spiral nanoantennas [ 186 ]; (h) round spiral nanoantennas [ 187 ]; (i) end-to-end two-wire array [ 188 ]; (j) hexagonal lattice array [ 189 ]; (k) Sierpinski fractal nanoantenna [ 190 ]; (l) Yagi-Uda array [ 191 ]. Besides the geometries shown in Figure 12 , there are many other nanoantenna designs (e.g., bull’s eye, triangular lattice, V-shaped, logarithmic, Archimedean and Euler spirals, oligomer nanoantennas, crossed bowties, multiparticle common-gap antennas, etc.…”

A review of defect engineering, ion implantation, and nanofabrication using the helium ion microscope

“…(j) Gold bow-tie plasmonic antenna with internal fractal structure on glass fabricated by helium FIB milling of an EBL bow-tie pre-structure. Adapted from [ 141 ], Seitl et al, Miniaturized fractal optical nanoantennas defined by focused helium ion beam milling, Nanotechnology, Vol. 31(7), 075301, first published 14 November 2019, Copyright © 2019 IOP Publishing Ltd. Reproduced with permission.…”

“…In the latter, Laible et al also investigated the fabrication of gold pre-stuctures by EBL and argon ion milling (rectangles or connected bow-tie shapes) as another means to reduce the milling time for large assemblies, whereby the final nanostructures themselves were defined using optimized helium FIB milling strategies based on multidirectional scans. In follow-up work by the same group, bow-tie nanoantennae with fractal internal geometries were fabricated, using bow-tie pre-structures prepared by EBL that were then milled with the helium FIB to accurately define the nanogaps and the internal fractal patterns [ 141 ] ( Figure 6j ).…”

Helium-ion-beam nanofabrication: extreme processes and applications

“…Then, plasmonic nanoantennas could be patterned with the maximum precision in a reasonable time on the polycrystalline gold film or single-crystalline gold flakes [106]. As shown in figure 15(C), these strategies are used to further miniaturize zero-, first-, and second-order Sierpiński fractal dimer nanoantennas and to investigate the scattering spectrum and high near-field enhancement [107]. Plasmonic dipole nanoantennas with 5 nm-wide gaps were also fabricated by focused HIB direct writing.…”