Proc. Natl Acad. Sci. USA (2012)

Optical fields can be used to drive transitions between different states in quantum systems, but some transitions are much more likely than others. When visible light interacts with an atom, for example, electric dipole transitions are much more common than other transitions, such as electric quadrupole and magnetic dipole transitions. This happens because an atom is much smaller than the wavelength of visible light so, to a good approximation, it is exposed to a spatially uniform electric field, and such an electric field can only drive electric dipole transitions. However, Prashant Jain and co-workers have shown that these selection rules need to be rewritten when nanostructures are used to manipulate electromagnetic fields on the nanoscale.

Jain and co-workers — who are based at Berkeley, the Hebrew University of Jerusalem and Tel Aviv University — used a combination of theory and numerical modelling to study what happens when a semiconductor nanorod near a metal nanosphere is exposed to a spatially uniform electric field. They find that the strength of transitions allowed by the electric dipole selection rule increase, and that the strength of transitions forbidden by this selection rule increase by an even larger factor. They also predict changes in the wavelength of many transitions. The results have implications for spectroscopy and applications such as light harvesting and optical imaging.