Strongly coupled, high-quality plasmonic dimer antennas fabricated using a sketch-and-peel technique

Abstract

A combination of helium- and gallium-ion beam milling together with a fast and reliable sketch-and-peel technique is used to fabricate gold nanorod dimer antennas with an excellent quality factor and with gap distances of less than 6 nm. The high fabrication quality of the sketch-and-peel technique compared to a conventional ion beam milling technique is proven by polarisation-resolved linear dark-field spectromicroscopy of isolated dimer antennas. We demonstrate a strong coupling of the two antenna arms for both fabrication techniques, with a quality factor of more than 14, close to the theoretical limit, for the sketch-and-peel–produced antennas compared to only 6 for the conventional fabrication process. The obtained results on the strong coupling of the plasmonic dimer antennas are supported by finite-difference time-domain simulations of the light-dimer antenna interaction. The presented fabrication technique enables the rapid fabrication of large-scale plasmonic or dielectric nanostructures arrays and metasurfaces with single-digit nanometer scale milling accuracy.

The patterning toolbox FIB-o-mat: Exploiting the full potential of focused helium ions for nanofabrication

“…Hence, a nanopatterning approach that is able to realize sub-10 nm gaps in a reproducible manner is highly desired. He ion beam milling already demonstrated these capabilities in the fabrication of strongly coupled dimers with gap distances of less than 6 nm [ 62 – 64 ]. In all previous cases, the antenna shapes were pre-fabricated with Ga ion beam milling or lithographic approaches and only single-line cuts were performed with He ions to create dimer antennas.…”

Scalable Fabrication of Metallic Nanogaps at the Sub‐10 nm Level

“…[143] A number of variations to the SPL procedure have been reported. [144][145][146][147][148] For instance, instead of using a negative resist, it is possible to directly create nano-trenches in a deposited metal film by FIB milling before selectively removing the metal outside of the trenches by peeling, see Figure 14. [142] In the case of a silicon substrate, it has been shown that slight over-milling of the unwanted metal causes a thin layer of silicon atoms to be sputtered onto the side walls of the retained metal, which serves to protect the retained metal from damage during the subsequent peeling step.…”

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

“…In comparison, gallium FIB milling produced gap widths larger than 20 nm, V-shaped side walls, rounded edges and a perturbed top surface. By optimizing the gold film itself to enable more uniform milling rates, for example, by increasing the grain sizes in polycrystalline films by annealing [ 179 – 180 ], or by using single-crystal gold [ 181 ], further improvements in both gap size and reproducibility can be achieved. Additional plasmonic structures that have been fabricated by helium FIB milling include trimeric assemblies of resonant nanoapertures [ 182 ], arrays of nanostructured radial resonant apertures [ 183 ], and plasmonic heptamer nanohole arrays [ 139 ] ( Figure 6h ), all in gold films, as well as multiresonant plasmonic nanoslit cavities [ 184 ] and plasmonic tetramer nanoantennae [ 51 ] in crystalline gold platelets.…”

“…Compared with the gallium FIB, sketch-and-peel using the helium FIB enables finer cuts and, in the first demonstrations, the fabrication of arrays of plasmonic dimer structures with a gap size of 15 nm was achieved [ 186 ]. Subsequent iterations of the method routinely report gap sizes of 5 nm [ 180 – 181 ], including the fabrication of heart-shaped nanodimers defined using a shared boundary in the center to create the nanogap as opposed to milling the dimer outline and gap separately [ 140 ] ( Figure 6i ). To further increase the efficiency of the process, a combination of gallium and helium FIB milling can be applied, the former for the coarser outlines and the latter for the fine cuts forming the nanogaps [ 180 , 187 ].…”

“…Subsequent iterations of the method routinely report gap sizes of 5 nm [ 180 – 181 ], including the fabrication of heart-shaped nanodimers defined using a shared boundary in the center to create the nanogap as opposed to milling the dimer outline and gap separately [ 140 ] ( Figure 6i ). To further increase the efficiency of the process, a combination of gallium and helium FIB milling can be applied, the former for the coarser outlines and the latter for the fine cuts forming the nanogaps [ 180 , 187 ]. The advantage of using single-crystal gold to ensure uniform sputter rates, and thus ultimately enable milling of narrower cuts with enhanced reproducibility, has also been demonstrated for the sketch-and-peel method [ 181 ].…”