Giant enhancement of dipole-forbidden transitions via lattices of plasmonic nanoparticles

We demonstrate theoretically that electric dipole-forbidden atomic transitions can be excited by the interaction of the quadrupole moment of the transition with the electric near-field in the vicinity of an illuminated periodic array of core–shell metallic nanoparticles via surface-plasmon excitation. The rate of the quadrupole transition is enhanced by nine orders of magnitude relative to the illumination of the atom by a plane wave rendering the transition experimentally observable. At the same time, the enhancement of the quadrupole transition rate means an enhancement of the quadrupole force at the same level, enabling the subwavelength trapping of the atom at the optical near-field landscape. The calculations are based on rigorous electrodynamic calculations based on the layer-multiple-scattering theory for periodic nanostructures of spherical scatterers. The quadrupole transition is studied by means of electromagnetic multipole angular momentum theory which provides a closed formula for the corresponding atomic transition rate.