Asymmetry of Fano resonances in exciton-plasmon interaction

Abstract

The spectral signature of a sharp molecular transition can be enhanced by its coherent coupling with a broad plasmonic resonance supported by a metallic antenna. This interaction is the basis of surface-enhanced spectroscopy where a low amount of molecules can be detected. We focus on the weak -coupling regime, that is found if the coupling strength between the molecular exciton and the plasmonic resonance is weak compared to the losses in the system. The extinction spectrum that arises from this interaction corresponds to the well-known Fano line shape. The main features of the Fano spectra can be understood using a simple model of two coupled harmonic oscillators, where one of them represents the plasmon driven by an external force. However, this approach can miss some experimental features. For example, this model predicts a perfectly symmetrical spectral feature if the plasmonic and the molecular resonances spectrally match. In contrast experimental results show an asymmetrical Fano line shape, which was initially asociated to the effect of plasmonic higher order modes. In this work we use the Mie's theoretical framework to model exactly the interaction of a molecular exciton (described as a two level system) with the plasmonic response of a spherical nanoantenna. This approach allows to study the contribution of higher-order modes and other effects to the exciton-plasmon interaction. The understanding of the fine details of the Fano features can allow for extracting more information from the spectroscopic signal in state-of-the-art experiments.