Probing the Electromagnetic Local Density of States with a Strongly Mixed Electric and Magnetic Dipole Emitter

Abstract: We identify a solid-state quantum emitter whose room-temperature radiative decay is mediated by a nearly equal mixture of isotropic electric dipole (ED) and magnetic dipole (MD) transitions. Using energy-momentum spectroscopy, we experimentally show that the near-infrared $^3$T$_2{\rightarrow}^3$A$_2$ emission from divalent-nickel-doped magnesium oxide (Ni$^{2+}$:MgO) is composed of $\sim$50% MD and $\sim$50% ED transitions. We then demonstrate that the spontaneous emission rate of these ions near planar interfaces is determined by the combined electric and magnetic local density of optical states (LDOS). This electromagnetic LDOS probes the total mode density, and thus similar to thermal emission, these unique electronic emitters effectively excite all polarizations and orientations of the electromagnetic field.