Surface plasmon enhanced electric field versus Förster resonance energy transfer near core-shell nanoparticle

The process of electric field enhancement and molecular energy transfer near core-shell nanoparticles is compared using multipole spectral expansion based closed form expressions. It is a general notion that localized surface plasmon modes of plasmonic structures greatly enhance the molecular energy transfer between a donor-acceptor pair and the electric field intensity. Most of such studies are conducted for donor-acceptor pairs placed far away so that the bare interaction energy is very small due to its r⁻⁶ character. Relative to this tiny bare interaction energy, surface plasmon induced interaction energy may be very large, for example, in the core-shell nanoparticle, which is a hybrid plasmonic structure of a sphere and a cavity. However, when molecules are positioned very close to each other and, therefore, the Coulumbic interaction between them is very strong, the enhancement factor is found to remain quite low. Moreover, the integrated analysis and comparison of electric field enhancement and molecular energy transfer near core-shell nanoparticles has not been done. Since core-shell nanoparticles exhibit rich plasmonic features in their optical spectra, they can be used for spectral fine tuning and have found applications in cancer diagnostics. Importantly, both processes, namely, electric field enhancement and molecular energy transfer, are treated using closed form expressions derived using multipole spectral expansion. Higher order plasmonic modes are shown to play a crucial role. The present work can be directly used as a formula, e.g., for designing surface plasmon based biosensors and estimating energy exchange between proteins and excitonic interactions in quantum dots.