The optical properties of gold heptamer nanohole arrays have been investigated theoretically and numerically. This structure support pronounced Fano resonances with high transmittance (~50%) and narrow bandwidths (down to 12 nm). The Fano features arise from the interference between light directly transmitted through the holes, and light indirectly scattered through the excitation of localized surface plasmon polaritons (LSPPs), propagating surface plasmon polaritons (SPPs), or/and waves related to Wood's anomaly (WA). The mechanisms behind the generation of these resonances are revealed by observing near-field distributions, altering the structural parameters and applying the Bloch wave model. Furthermore, it is shown that Fano resonances associated with LSPPs exhibit high surface (2 nm/nm) and bulk sensitivities (400 nm/RIU). However, the highest figure of merit (~24 RIU) occurs for a Fano resonance involving a WA and SPP mode.

We fabricate plasmonic heptamer-arranged nanohole (HNH) arrays by helium (He) focused ion beam (HeFIB) milling, which is a resist-free, maskless, direct-write method. The small He+ beam spot size and high milling resolution achieved by the gas field-ionization source used in our HeFIB allows the milling of high aspect ratio (4:1) nanoscale features in metal, such as HNHs incorporating 15 nm walls of high verticality between holes in a 55-nm-thick gold film. Drifts encountered during the HeFIB milling of large arrays, due to sample stage vibrations or He beam instability, were compensated by a drift correction technique based on in situ He ion imaging of alignment features. Our drift correction technique yielded 20 nm maximum dislocation of HNHs, with 6.9 and 4.6 nm average dislocations along the horizontal and vertical directions, respectively. The measured optical resonance spectra of the fabricated plasmonic HNH arrays are presented to support the fabrication technique. Defects associated with HeFIB milling are also discussed.

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The optical properties of nanohole oligomers in a gold film have been investigated theoretically and experimentally. Helium focused ion beam milling was used to create arrays of close-packed heptamer-arranged nanoholes (HNH) in a gold film on a fused silica substrate. The holes are slightly elliptical about a nominal diameter of 100 nm and are separated by 15 nm Au regions (“bridges”) to form close-packed heptamer arrangements. Optical responses are reported as transmittance spectra, showing various Fano resonances due to different excitations on the structure: surface plasmon polaritons localized to the HNH, symmetric and asymmetric surface plasmon polaritons propagating along with the film, and surface waves associated with Wood’s anomaly. All resonances observed involve the coupling (bonding and anti-bonding) of magnetic dipoles within the holes of HNH and in some cases of magnetic hexapoles within the central hole. The sensing performance was investigated by measuring the bulk sensitivity of the resonances, anticipating application to biosensing, yielding 380 nm RIU−1 in the best case. Surface-enhanced Raman scattering measurements were also performed on HNH, where adventitious carbon was detected directly through the identification of D and G bands (spectra taken from nearby un-milled Au areas were essentially featureless).

Fano interference is predicted in a structure consisting of a periodic array of uncoupled gold nano-disks (AuNDs) supported by a Bragg stack. The phenomenon occurs due to resonant interference between the reflection spectrum of the Bragg stack and localized surface plasmon resonances (LSPRs) on the AuNDs. Fitting the computational results to the Fano lineshape formula yields the Fano parameters of the resonances including their linewidth. One of the most important advantages of the structure is that the linewidth of the Fano resonances (∼9 nm) is much narrower than the bandwidth of the LSPR nano-disks (∼80 nm) and the reflection bandwidth of the Bragg stack on which the disks are supported (∼100 nm). The narrow Fano linewidth and the ease with which they can be interrogated using perpendicularly incident light makes the structure attractive for biosensing applications. The bulk sensitivities and figure of merit of Fano resonances in a proposed design are 44 nm/RIU (redshift) and −52 nm/RIU (blueshift), and ∼4 RIU−1, respectively.