Ultrastrong Coupling of a Single Molecule to a Plasmonic Nanocavity: A First-Principles Study

Abstract

Ultrastrong coupling (USC) is a distinct regime of light-matter interaction in which the coupling strength is comparable to the resonance energy of the cavity or emitter. In the USC regime, common approximations to quantum optical Hamiltonians, such as the rotating wave approximation, break down as the ground state of the coupled system gains photonic character due to admixing of vacuum states with higher excited states, leading to ground-state energy changes. USC is usually achieved by collective coherent coupling of many quantum emitters to a single mode cavity, whereas USC with a single molecule remains challenging. Here, we show by time-dependent density functional theory (TDDFT) calculations that a single organic molecule can reach USC with a plasmonic dimer, consisting of a few hundred atoms. In this context, we discuss the capacity of TDDFT to represent strong coupling and its connection to the quantum optical Hamiltonian. We find that USC leads to appreciable ground-state energy modifications accounting for a non-negligible part of the total interaction energy, comparable to k B T at room temperature.

Ab Initio Molecular Cavity Quantum Electrodynamics Simulations Using Machine Learning Models

“…All of the parameters we used in our simulations are consistent with these state-of-the-art experiments and should be able to be directly tested with the single molecule in the plasmonic cavity experiment. We should also note that, for the case of a plasmonic cavity coupled to a single molecule, the single-mode PF Hamiltonian might not be an appropriate model. − A more desired approach is to use ab initio theory , to describe both molecules and the plasmonic nanoparticles, due to the strong coupling between them and also because the “cavity mirror” is getting close enough to the molecule and its ab initio description becomes necessary …”

Picocavities: a Primer

“…While desirable to construct larger atomic-scale structures in nanocavities, these are constrained by extremely strong surface forces (see below), and only single (or few) atom picocavities are observed. Recent papers proposing to develop (ultra)­strong coupling with single emitter electronic transitions using picocavity fields − are intriguing but must be treated with caution currently (see SI).…”

Self-Dissociation of Polar Molecules in a Confined Infrared Vacuum

“…This was first demonstrated with semiconductors [8] and superconducting circuits [9], and more recently using organic microcavities [10][11][12]. Recent breakthroughs in nanophotonics have implemented strong coupling between an individual molecule and a plasmonic field, by stabilizing the confined field of a localized plasmon resonance in optical regime to just a few cubic nanometers [13][14][15][16]. Emerging materials with tunable plasmonic resonances can be used to produce strong field confinements in the near and mid-infrared [17][18][19], which would enable studies of quantum light-matter interaction with high-frequency intramolecular vibrations.…”