Nanohole Plasmons in Optically Thin Gold Films

Abstract

The optical properties of single nanoholes in optically thin (t = 20 nm) gold films on glass have been studied experimentally and theoretically. The measured elastic scattering spectra from the nanoholes exhibit a broad resonance in the red part of the visible spectrum, which is qualitatively similar to localized surface plasmon (LSP) resonances in gold nanodisks. The hole resonance red-shifts with increasing hole diameter (D = 60−107 nm), similar to particle LSP resonances. These features could be well reproduced by electrodynamic simulations based on the boundary element method (BEM). Further, the electric field distribution around the resonant nanoholes, obtained from the BEM simulations, exhibits a clear electric dipole pattern. This confirms the assignment of the hole resonance to a dipolar LSP resonance mode. However, in comparison with Au nanodisks of similar size, the hole LSP resonance exhibits a shorter dephasing time (τ). This observation can be understood in terms of an additional decay channel that is dominated by the short wavelength antisymmetric bound (ab) surface plasmon polariton (SPP) mode of the surrounding Au film. Experimental verification of the LSP-SPP coupling is obtained from near-field scanning optical microscopy images, which exhibit interference fringes due to SPP emission from the hole. The fringe periodicity corresponds to a SPP wavelength of λab ≈ 285 nm, much less than both the free space wavelength λ0 = 633 nm or the wavelength of the more well-known symmetric leaky mode.