Atomic-scale resolution of molecular light emission in a picocavity

Abstract

Light emission from a single emitter in a nanophotonic environment is usually described by means of the local density of photonic states acting on a point dipole, an approach which captures the basic essence of the excitation and emission rates of the emitter. However, the emergence of a new type of plasmonic picocavity, which combines the large enhancement of a metallic nanogap and the atomic-scale enhancement of a few atoms in the cavity has allowed for achieving levels of resolution in the Ångstrom scale, capable to dissect the internal structure of light emitted from a single molecule. Under this conditions, the molecular emitter cannot be considered as a point dipole, and a full description of the emitter's internal dipolar structure is needed. A theoretical approach which accounts both for the spatial distribution of the electronic transition density in the molecule, and for the local density of photonic states produced in the picocavity is presented here. The electronic transition dipole of the molecule is calculated within a quantum chemistry framework of Density Functional Theory (DFT), and the local density of photonic states of the picocavity is addressed by a proper modal expansion following classical electrodynamics. We apply this approach to describe light emission maps from a single tautomer located in a picocavity, as obtained in Scanning Tunneling Microscopy (STM), and reproduce and understand intramolecular features in light emission from a free-base phthalocyanine (H2Pc). The use of plasmonic picocavities thus opens the door for photons to access the atomic scale.