The spontaneous emission of a single molecule is substantially modified close to a metallic nanostructure. We study the spectral behavior of the radiative and nonradiative decay rates and of the local-field factor in the vicinity of a plasmon resonance. We show that the highest fluorescence enhancement is obtained for an emission wavelength redshifted from the plasmon resonance, and that quenching always dominates at plasmon resonance. These results may have experimental implications in spectroscopy and monitoring of elementary light sources.

A detailed study of time-averaged electromagnetic forces on subwavelength-sized particles is presented. An analytical decomposition of the force into gradient and scattering-plus-absorption components is carried out, on the basis of which the attractive or repulsive behavior of the force is explained. Small metallic particles are shown to experience both kinds of forces; which kind also depends on the excitation of surface plasmons. Resonances give rise to enhancements of both the scattering and the absorption forces, but the gradient force can become negligible. Also, close to resonant wavelengths, the gradient force can be maximum, while both the scattering and the absorption forces remain large. Comparisons of analytic results with rigorous calculations allow the establishment of ranges of validity of the dipolar approximation for these forces.

Duplication of double-stranded DNA (dsDNA) requires a fine-tuned coordination between the DNA replication and unwinding reactions. Using optical tweezers, we probed the coupling dynamics between these two activities when they are simultaneously carried out by individual Phi29 DNA polymerase molecules replicating a dsDNA hairpin. We used the wild-type and an unwinding deficient polymerase variant and found that mechanical tension applied on the DNA and the DNA sequence modulate in different ways the replication, unwinding rates, and pause kinetics of each polymerase. However, incorporation of pause kinetics in a model to quantify the unwinding reaction reveals that both polymerases destabilize the fork with the same active mechanism and offers insights into the topological strategies that could be used by the Phi29 DNA polymerase and other DNA replication systems to couple unwinding and replication reactions.