Rare-earth doped nanocrystals possess optical transitions with significant either electric or magnetic dipole characters. They are of considerable interest for understanding and engineering light-matter interactions at the nanoscale with numerous applications in nanophotonics. Here, we study the ${}^{5}{D}_{0}{\ensuremath{\rightarrow}}^{7}{F}_{1}$ transition dipole vector in individual
NaYF4:Eu3+ nanorod crystals by Fourier and confocal microscopies. A single-crystal host matrix leads to narrow emission lines at room temperature that permit separation of the Stark sublevels resulting from the crystal-field splitting. We observe a fully magnetic transition and low variability of the transition dipole orientation over several single nanorods. We estimate the proportion of the dipole transitions for the Stark sublevels. We also determine an effective altitude of the rod with respect to the substrate. The narrow emission lines characteristic of
NaYF4:Eu3+ ensure well-defined electric or magnetic transitions, and are thus instrumental for probing locally their electromagnetic environment by standard confocal microscopy.
The Purcell effect is commonly used to increase light emission by enhancing the radiative decay of electric dipole transitions. In this Letter, we demonstrate that the opposite effect, namely, the inhibition of electric dipole transitions, can be used to strongly enhance light emission via magnetic dipole transitions. Specifically, by exploiting the differing symmetries of competitive electric and magnetic dipole transitions in trivalent europium, we demonstrate a fourfold enhancement of the far-field emission from the (5)D(0)→(7)F(1) magnetic dipole transition in trivalent europium. We show that this strong enhancement is well predicted by a three-level model that couples the individual Purcell enhancement factors of competitive transitions from the same excited state.