Light emitted by molecules embedded within metal nanoparticle clusters is strongly enhanced by interaction with surface plasmons. This allows, for example, the observation of Raman scattering from individual molecules. The symmetry of the metal cluster may affect the Raman-scattered light by generating new polarization states. This article reviews the use of symmetry theory to analyze the plasmonic normal modes of metal nanoparticle trimers. The lowest bright energy modes are degenerate for an equilateral triangle but split when the symmetry is broken. When a single molecule in the gap between two of the particles emits, it excites the plasmon modes, typically off-resonance, and the ensuing interference between the modes rotates the polarization of the emitted light. This so-called Raman optical activity can generate circularly polarized light at the Raman frequency. This curious phenomenon, which was demonstrated experimentally, may prove useful for future plasmonic devices.
