Nanoplasmonic Enhancement of Molecular Fluorescence: Theory and Numerical Modeling

Significant emission enhancement of fluorescent molecules placed in close proximity to metallic nanoparticles has been observed. Recent advances in nanotechnology have enabled the introduction of plasmon-enhanced molecular fluorescence in various applications. Comprehensive theory of the physics behind this enhancement mechanism has also been developed. However, most of the existing analytical tools are applicable mainly for particular nanoparticles in either spherical or ellipsoidal shapes. Since the plasmonic enhancement of molecular fluorescence is dependent on various parameters such as shape, size, and distribution of nearby nanoparticles, it is crucial to have more powerful analysis tools to be able to handle any arbitrary nanoparticles. For this purpose, the 3D finite element method, which is a commonly used technique for arbitrary structures, is implemented and reported in this paper. The emitting molecule is assumed to be an electric dipole point source. The fluorescence enhancement factor is described in term of a local electric field-enhancement factor and the quantum yield of the system. The model is validated by comparison to the approximate quasistatic model and the exact Mie theory. It provides more accurate results than those of the quasistatic model, which makes it become the powerful numerical approach for investigation of arbitrary nanostructure influence on molecular fluorescence. It is then applied for investigating the emission characteristics of the fluorescent molecule when it is placed in the vicinity of more complicated structures including dimers and chains of coupled nanoparticles. It is found that these coupled nanoparticle configurations provide stronger fluorescence enhancement than the single nanoparticle of the same particle size when the inter-particle gap is small. It is attributed to the higher electric-field enhancement in the inter-particle gap region via strong surface plasmon coupling effects of two neighboring nanoparticles.