# Enhancement of and interference among higher order multipole transitions in molecules near a plasmonic nanoantenna

## TL;DR

This paper explores how higher order multipole transitions in molecules near plasmonic nanoantennas can be enhanced or suppressed through interference effects, extending beyond the electric dipole approximation.

## Contribution

It introduces a new framework combining quantum-chemical calculations and numerical simulations to analyze interference among multiple multipolar emission channels.

## Key findings

- Strong enhancement of magnetic dipole and electric quadrupole emissions.
- Interference effects can lead to suppression of molecular radiation.
- Placement and orientation control can fully suppress transition probabilities.

## Abstract

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.