Credit: © APS

In a metal, the excitation of the collective motion of electrons — plasmons — produces an intense electric field. Sergei Kuhn and co-workers1 at ETH in Zurich, Switzerland, have now used this electron excitation in a 100-nm gold nanoparticle to create a ‘nano-antenna’ capable of enhancing the fluorescence of a single molecule.

Kuhn and co-workers studied high quantum efficiency terrylene molecules that were vertically embedded in a 20-nm-thick matrix film. These molecules have an emission line within the wavelength range of the gold nanoparticle’s plasmon excitation spectrum, which is critical to observing the fluorescence enhancement. A terrylene molecule experienced a highly inhomogeneous electric field when the nanoparticle was brought within 10 nm of it; a distance much less than the wavelength of the radiation. Such a field has a very different effect on optical properties than excitation from a homogeneous plane wave. By scanning the gold particle in 1-nm steps across a single molecule, a 20-fold increase in the fluorescence intensity and decay rate was observed.

The experimental set-up functions as an 'apertureless' near-field optical microscope with an image resolution of 65 nm. Fundamentally, the high spatial resolution of the measurements yields detailed information about molecular optics that can be compared to three-dimensional calculations.