Anapole due to a tangentially polarized dipole near a subwavelength sphere: Inhibition and enhancement of spontaneous emission

Abstract

There exist oscillating current distributions that cannot generate radiation; these configurations are known as anapoles. Our goal is to theoretically demonstrate that an oscillating electric dipole, located in the vicinity of a subwavelength sphere and polarized tangentially, can yield an anapole. This paper complements our recent study [J. R. Zurita-S\'anchez, Phys. Rev. Research 1, 033064 (2019)], devoted to an anapole arising from an electric dipole with radial polarization. Our methodology relies on the multipolar expansion of the excitation and scattered electromagnetic fields with respect to the center of the spherical particle. We found that an anapole arises when the effective electric and magnetic dipoles, induced by the dipolar source, vanish. The aforementioned anapole condition considers that the lowest multipolar order contribution is responsible for the radiative response of the emitter. However, high-order multipoles yield an unavoidable residual radiated power which is much smaller still than the radiated power of the dipole in the absence of the scatterer. Particularly, for a nonabsorbing sphere, the geometrical and dielectromagnetic parameters that lead to an anapole state are determined. As a practical scenario, we exploit the anapole excitation for controlling the spontaneous emission decay rate of an excited atom (or a molecule). In comparison to the decay rate of an excited atom without the scatterer, the radiative decay in the presence of the particle can be inhibited by a factor of about three orders of magnitude; this factor is quite large for an open system like ours. Furthermore, we show that by varying the orientation of the atomic transition dipole moment from a tangential direction, where the anapole condition is satisfied, to a radial direction, the atom can change from a state in which the spontaneous emission rate is inhibited to one in which this rate is enhanced. Hence, the anapole setup enables the versatile control of the radiative properties of an atom, and it provides the possibility for implementing applications related to the sensing and localization of atoms.