Fano resonances in plasmonic heptamer nano-hole arrays

Abstract

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.

Helium focused ion beam direct milling of plasmonic heptamer-arranged nanohole arrays

“…As discussed in our previous work [24], an HNH array can support several excitations, including propagating SPPs due to the periodicity of the array, LSPPs resonating on HNHs, and WA waves. Unlike our previous work [24], where the HNH array had G = 10 nm and was covered by a dielectric material of refractive index n s = 1.33, here we have G = 15 nm and n s = 1. Increasing G and decreasing the superstrate refractive index n s result in a blue shift of the resonant wavelengths.…”

“…The optical transmittance of an HNH array with the same structural parameters as the fabricated one was calculated for vertical linearly polarized incident light, as shown in Figure 5A. As discussed in our previous work [24], an HNH array can support several excitations, including propagating SPPs due to the periodicity of the array, LSPPs resonating on HNHs, and WA waves. Unlike our previous work [24], where the HNH array had G = 10 nm and was covered by a dielectric material of refractive index n s = 1.33, here we have G = 15 nm and n s = 1.…”

“…The geometrical parameters, including the diameter of each nanohole (D), the interhole separation (G), the periodicity of the array (P), and the thickness of the Au film (T), are chosen according to the desired spectral range of the resonances. Here, we chose D = 100 nm, G = 15 nm, P = 480 nm, and T = 55 nm for application in the near-infrared spectral region [24]. Figure 1B shows the He ion microscope (HIM) images in tilted (54°) and top views of HNHs in a 55-nm-thick Au film taken after direct milling via HeFIB.…”

“…Heptamer-arranged nanoholes (HNHs) are hexagonal close-packed arrangements of seven nanoholes in a metal film, inspired by complementary arrangements of metal nanoparticles [23]. Arrays of HNHs in a metal film are very interesting for their highly complex, rich spectral responses and pronounced Fano resonances involving propagating surface plasmon polaritons (SPPs), localized SPPs (LSPPs), and Wood's anomaly (WA) waves [24]. However, the fabrication of HNH arrays in a metal film is very challenging due to the small distance between the holes required for close-packing (10-15 nm) and the commensurate need for high verticality sidewalls in a 30-to 60-nm-thick metal film [24]; that is, nanoscale features with a 4:1 aspect ratio are required.…”

“…Arrays of HNHs in a metal film are very interesting for their highly complex, rich spectral responses and pronounced Fano resonances involving propagating surface plasmon polaritons (SPPs), localized SPPs (LSPPs), and Wood's anomaly (WA) waves [24]. However, the fabrication of HNH arrays in a metal film is very challenging due to the small distance between the holes required for close-packing (10-15 nm) and the commensurate need for high verticality sidewalls in a 30-to 60-nm-thick metal film [24]; that is, nanoscale features with a 4:1 aspect ratio are required. Here, we report novel techniques developed using HeFIB milling of an Au film to form high-quality arrays of HNHs and demonstrate the operation of the arrays via optical measurements.…”