Nature Commun. 3, 979 (2012)

Magnetic dipole transitions at optical frequencies are such rare events that they are often considered to be forbidden transitions. Now Tim Taminiau and colleagues from the United States, Spain and the Netherlands have quantified the magnetic nature of light emission by using energy- and momentum-resolved spectroscopy of the rare-earth ion trivalent europium (Eu3+). The researchers prepared thin films of europium-doped yttrium oxide (Eu3+:Y2O3) and engineered interference effects to distinguish between the light emission resulting from electric dipoles and magnetic dipoles. Emissions from electric and magnetic dipoles differ in symmetry and polarization. An experiment was designed in which constructive interference would occur between emitted and scattered light for an electric dipole, as the electric field is symmetric with respect to the dipole axis, and, conversely, destructive interference would occur for a magnetic dipole. To analyse the emissions quantitatively, rate equations were used to model the transitions. A transition at 592 nm was found to be predominantly magnetic (90.8 ± 2%) and a transition at 612 nm was found to be electric (99 ± 1%).