Magnetic Purcell Enhancement by Magnetic Quadrupole Resonance of Dielectric Nanosphere Antenna

Dielectric nanoantennas have been suggested for manipulating the spontaneous emission of a magnetic dipole through the magnetic Purcell effect. However, the experimentally observed enhancement of the magnetic dipole emission has been marginal and much smaller than the theoretical prediction. Here, we develop a composite system, that is, a Si nanosphere decorated with Eu³⁺ complexes, in which magnetic dipole emission of Eu³⁺ is efficiently coupled to the magnetic Mie modes of the nanosphere. By means of single particle spectroscopy, we systematically investigate the light scattering and photoluminescence spectra of the coupled system with diameters of Si nanospheres from 130 to 245 nm. We demonstrate that, by tuning the magnetic quadrupole Mie resonance to the ⁵D₀–⁷F₁ magnetic dipole transition of Eu³⁺, the branching ratio between the magnetic and electric dipole transitions is enhanced up to 7×. We also show the magnetic Purcell enhancement in an ensemble of Si nanospheres with size distribution.