Design and fabrication of nanorod in nanohole arrays with highly tunable enhanced optical transmission

The complex Ag nanohole structures with an Ag nanorod inside the hole are designed and investigated both numerically and experimentally. Based on finite-difference-time-domain (FDTD) calculations, these structures show strong polarization angle-dependent optical properties. Additionally, a new extraordinary optical transmission (EOT) mode with tunable resonance wavelength is observed at the near-IR region due to the enhanced local radiation of the nanorods as well as electromagnetic coupling to the nanoholes. The resonance wavelength of the new EOT mode can be red-shifted continuously from the near-IR region to the mid-IR region (~ 20μm ) by varying array periodicity and nanorod length. By combining nanosphere lithography with oblique angle deposition, large-area Ag nanorod in nanohole structures can be fabricated on glass substrates. This fabrication technique enables the production of the complex nanohole arrays with controlled hole diameter, thickness, and rod structure inside the hole. The observed transmission at the new EOT mode can increase from ~41% of the nanohole to ~78% of the compound structure. The tunable EOT wavelength and strong polarization-dependent optical properties make the structure ideal for applications such as ultrathin optical filters, polarizers, surface-enhanced spectroscopies, etc.