Novel 3D Au nanohole arrays with outstanding optical properties

In this paper, a novel nanohole array with three-dimensional structure was generated by colloidal lithography existing of a free-standing nanohole array that experiences the same environment on both sides in 3D space. In a practicable way, the nanoholes were lifted up from the substrates, resulting in undulate morphology. Therefore we name this a 3D nanohole array (3D-NHA), and the performance in optical applications was much improved compared to conventional 2D nanohole arrays (2D-NHAs) due to a nearly perfect surface plasmon energy matching on both surfaces of the arrays. There was a 1.5-fold improvement in extraordinary optical transmission and 1.7-fold enhancement in response sensitivity towards a change of refractive index compared to 2D-NHAs. Furthermore, a (1, 1) Au/air transmission peak appeared in the transmission spectra and showed high relative sensitivity and an excellent linear response over a wide spectral range. Besides improved optical properties, good transferability of nanohole arrays without substrate dependence was designed and realized experimentally for the first time due to the unique protuberant holes of 3D-NHAs.