  Spontaneous emission of quantum emitters can be modified by engineering their
optical environment. This allows a resonant nanoantenna to significantly modify
the radiative properties of a quantum emitter. In this article, we go beyond
the common electric dipole approximation for the molecular electronic
transition and take light-matter coupling through higher order multipoles into
account. We investigate, by means of theory and numerical simulations, a strong
enhancement of the magnetic dipole and electric quadrupole emission channels of
a molecule adjacent to a plasmonic patch nanoantenna. While this on its own had
been considered, the assumption in prior work usually has been that each
molecular transition is dominated only by one of those multipolar emission
channels. This leads naturally to the notion of discussing the modified
emission in terms of a modified local density of states defined for each
specific multipolar transition. In reality, this restricts the applicability of
the approach, since specific molecular transitions occur via multiple
multipolar pathways that have to be considered all at once. Here, we introduce
a framework to study interference effects between higher order transitions in
molecules by (a) a rigorous quantum-chemical calculation of their multipolar
moments and (b) by a consecutive investigation of the transition rate upon
coupling to an arbitrarily shaped nanoantenna. Based on that formalism we
predict interference effects between these transition channels. This allows for
a strong suppression of radiation by exploiting destructive interference. Our
work suggests that placing a suitably chosen molecule at a well defined
position and at a well defined orientation relative to a nanoantenna can fully
suppress the transition probability.
