Abstract
The coupling plasmon of a hybrid nanostructure, silver island (SI) associated with silver nanoparticle (SNP), on metal-enhanced fluorescence (MEF) was studied theoretically. We used the multiple multipole method to analyze the plasmon-mediated enhancement factor on the fluorescence of a molecule immobilized on SNP and located in the gap zone between SI and SNP; herein, the SI was modeled as an oblate spheroid. Numerical results show that the enhancement factor of the hybrid nanostructure is higher than that of a SNP or a SI alone due to the coupled gap mode. This finding is in agreement with the previous experimental results. In addition, the plasmon band of the structure is broadband and tunable, which can be red-shifted and broadened by flattening or enlarging SI. Based on this property, the hybrid nanostructure can be tailored to obtain the optimal enhancement factor on a specific molecule according to its excitation spectrum. Moreover, we found that there is an induced optical force allowing SNP be attracted by SI. Consequently, the gap is reduced gradually to perform a stronger MEF effect.
Highlights
In the past decade, using metallic nanostructures to perform the metal-enhanced fluorescence (MEF; or called surface-enhanced fluorescence) has attracted a lot of attentions [1,2,3,4,5,6,7,8,9,10,11]
The wavelength-dependent MEF of a hybrid nanostructure, a silver island (SI) associated with a silver nanoparticle (SNP), upon a molecule located in the gap zone was studied theoretically by multiple multipole (MMP) method, where the SI was modeled as an equivalent oblate spheroid
Numerical results illustrate that the excitation rate for a molecule, immobilized on SNP and located in the gap zone between SI and SNP, at the excitation stage very depends on the gap size and the distance from SNP; the smaller the gap and distance, the larger the excitation rate
Summary
In the past decade, using metallic nanostructures to perform the metal-enhanced fluorescence (MEF; or called surface-enhanced fluorescence) has attracted a lot of attentions [1,2,3,4,5,6,7,8,9,10,11]. Due to the localized surface plasmon resonance (SPR) of gold or silver nanostructures, the local electric field in their vicinity can be enhanced significantly to raise the excitation rate on a nearby molecule [11,12,13,14,15,16,17]. The plasmon-mediated Forster resonance energy transfer (FRET) between the excited molecule and the nanostructure facilitates the emission of the fluorescence, so as to raise the quantum yield and reduce the lifetime of the fluorescence dramatically [6,7,8, 15,16,17,18,19,20,21]. Purcell effect has elucidated that the environment can modify the spontaneous emission of
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