Evaluation of surface roughness of metal films using plasmonic Fano resonance in attenuated total reflection

Abstract

Attenuated total reflection (ATR) by surface plasmon polariton (SPP) is a method for evaluating the dispersion relation of SPP from the position of a dip in the reflection spectrum. However, recent studies have shown that the dips are displaced from SPP resonance because they are produced by a type of Fano resonance, i.e., the interference between the resonant reflection process accompanied by resonant excitation of SPP and the direct reflection process without resonant excitation. This result suggests that the system properties difficult to be achieved in the dispersion relation of SPP can be characterized using the ATR method. In this study, we investigate the effect of surface roughness due to nanosized dimples created in the initial stage of pitting corrosion on the ATR spectrum, from the viewpoint of Fano resonance. Using the temporal coupled-mode method, it is shown that the Fano resonance in ATR is caused by the phase change of direct reflection because of the absorption on the metal surface, and the spectral shape is determined by this phase, along with the ratio of the external (radiative) decay rate to the total decay rate of the resonant mode. Moreover, it is clarified that the internal and external decay rates extracted from the ATR spectrum provide information on corrosion, such as the effective thickness of the metal film and the randomness in dimple distribution.

Detection of Initial Stage of Aluminum Corrosion in NaCl Solution Utilizing Surface Plasmon Resonance

“…The reflectance at the dip is determined by the balance between the material loss and the radiation loss. Here, the radiation from the surface plasmon on the NaCl-solution side of the Al film to the substrate increases exponentially, while the material loss tends to decrease, as the Al thickness decreases [30]. Zero reflectance corresponds to the balanced (critical) point.…”

“…The main reason for this broadening is the increased radiation. The resonant width becomes large due to the increase of radiation loss of the surface plasmon, and the asymmetry of the Fano line shape is enhanced due to the interference between the increased radiation from the surface plasmon and the directly reflected wave [30].…”

Analysis of Effects of Surface Roughness on Sensing Performance of Surface Plasmon Resonance Detection for Refractive Index Sensing Application

“…Theoretical papers on SPR sensors [ 35 , 42 , 43 ] have reported the theoretical sensitivity for SPR calculated using ideally smooth metal surfaces. In addition, the effects of roughness on SPR characteristics, resonant conditions [ 44 ] and reflectance [ 45 ], transmittance, and absorption spectra [ 46 ] have been reported and investigated. For example, research groups have proposed a dispersion-relation model [ 45 , 47 ] and measured a rough-surface plasmon sensor’s propagation length [ 48 ].…”

“…In addition, the effects of roughness on SPR characteristics, resonant conditions [ 44 ] and reflectance [ 45 ], transmittance, and absorption spectra [ 46 ] have been reported and investigated. For example, research groups have proposed a dispersion-relation model [ 45 , 47 ] and measured a rough-surface plasmon sensor’s propagation length [ 48 ]. In addition, Yang et al [ 49 ] experimentally validated that the plasmonic angle depended on the roughness of the metal film using the surface-annealing method.…”