Atomic quadrupolar fluorescence in the presence of a spherical nanoantenna: cesium 6 S 1/2 − 6 D 5/2

We examine the fluorescence of a quadrupolar transition occurring in a three-level atom near a spherical nanoantenna with plane-wave excitation; the atom can be only excited by means of a quadrupolar transition, and it can decay either by a direct quadrupolar transition or a ladder decay composed of two dipolar transitions. Since dipolar transitions are unavoidable, fluorescence from them is also studied. Specifically, we analyze the fluorescence rates of the radiative transitions of the cesium atom (6 S 1/2 −6 D 5/2 ), as well as the involving competing processes, namely, the excitation rate and quantum yield. In relation to the case without a nanoantenna: (1) the quadrupolar quantum yield can be enhanced by about 2.55 orders of magnitude; (2) the excitation rate of the quadrupolar transition reaches an enhancement between 2 and 3.7 orders of magnitude; (3) because of the simultaneous enhancement of the excitation rate and quantum yield, the quadrupolar fluorescence rate can be enhanced by 4.7 orders of magnitude, enabling the practical detection of the fluorescence from the quadrupolar transition, whereas the dipolar fluorescence rates are augmented about 2.18 orders of magnitude. These findings occur when the atom is placed in an interval less than 10 nm from the surface of the nanoantenna. In addition, even in the presence of the nanoantenna, the quadrupolar fluorescence rate is smaller than the dipolar one; the ratio of these quantities with respect to its free-space value can be improved at most 2.3 orders of magnitude. Our setup is simple for practical implementation; hence, our work might impact applications related to atomic spectroscopy and sensing as well as control and manipulation of light emission and quantum states.