In situ observation of plasmon tuning in a single gold nanoparticle during controlled melting.

In this Letter we study the relations among shape, symmetry, and plasmon resonance shift in a single gold nanoparticle during laser melting. A beam of an argon ion laser is focused on a selected particle, while its optical and shape properties can be observed with the help of a combined dark-field/photoluminescence microscope and an atomic force microscope, respectively. Starting from a spherical shape, radiation pressure forms the melting gold particle into an upright standing rod on a glass substrate, showing a characteristic dipole scattering pattern. A red-shift of the photoluminescence signal and the scattering spectrum is observed. The melting process can be controlled allowing the formation of different particle heights and plasmon resonance shifts. In situ tuning of the plasmon resonance of individual particles is possible with this reversible melting process.