Incorporating QM/MM molecular dynamics into the few-mode quantization approach for light-matter interactions in nanophotonic structures

TL;DR

This work addresses the challenge of describing strongly coupled light–matter dynamics for organic emitters in highly multimodal, lossy nanophotonic environments. It introduces a framework that couples ab initio QM/MM molecular dynamics with a few-mode quantized electromagnetic field, deriving the photonic Hamiltonian from the environmental spectral density $\mathbf{J}_{ij}(\omega)$ via Maxwell simulations and Green's functions, and solving the dynamics within the single-excitation subspace. Key contributions include validation against a Lindblad master equation for TLSs, demonstration that molecular degrees of freedom and disorder do not destroy strong coupling but lift degeneracies, and demonstration of cavity-mediated energy transfer as well as photochemistry-driven transfer enabled by pseudomodes. The approach enables in silico design of molecule–nanocavity architectures and offers a path to integrating organic emitters into photonic circuits with high fidelity and geometric flexibility, paving the way for optimized nanoscale light–matter devices.

Abstract

In the context of light-matter interactions between organic chromophores and confined photons of (plasmonic) nano-resonators, we introduce a general framework that couples ab initio QM/MM molecular dynamics with few-mode field quantization to simulate light-matter interactions of molecular emitters at the nanoscale. Arbitrary, lossy, and spatially inhomogeneous photonic environments are represented by a minimal set of interacting modes fitted to their spectral density, while geometry-dependent molecular properties are computed on the fly. Applications to few-molecule strong coupling show that strong coupling persists when molecular degrees of freedom and disorder are included for the chosen system consisting of a nanoparticle dimer coupled to multiple emitters. At the same time, symmetry-protected degeneracies of two-level models are lifted. The framework further reveals how spatial field inhomogeneity and molecular disorder shape cavity-mediated energy transfer, illustrated for an HBQ-Methylene Blue donor-acceptor combination in a five-emitter system.

Figures (5)

• Figure 1: (a) Schematic representation of five point-like emitters within the gap of a silver nano-particle dimer oriented along the $z-$axis. When accounting for the molecular degrees of freedom, a Rhodamine model depicted in the inset is placed at each spot. (b) Physical (plain purple line) and fitted (yellow pointed-dashed line) spectral densities of the central emitter with its dipole moment oriented along the $z$-axis. The absorption of the emitters corresponds to a delta function (black vertical line) when treated as ideal two-level systems (TLSs). (c) Time-evolution of the population of the five TLSs resonantly coupled to the dipolar mode of the optical resonator. (d) Same population dynamics but when the confined electromagnetic field within the nano-cavity is described by a single mode.
• Figure 2: (a) Fitted spectral densities of the hybrid light-matter system shown alongside the absorption of the molecular emitters in gas phase at various temperatures and in solution at 300 K. (b) Zoom into the dipolar mode of the spectral density and its overlap with the molecular absorption under different conditions. (c) Time evolution of the populations of the Rhodamines, the dipolar mode of the silver nanoparticle dimer, and the overall ground state (system losses) for gas-phase emitters at 0 K. (d) Same populations at 4 K, (e) 10 K, (f) 20 K, (g) 100 K, and (h) in solution (water, QM/MM model) at 300 K.
• Figure 3: (a) Time evolution of the excited-state population of the central Rhodamine molecule within the setup of \ref{['fig:TLS']} across 101 few-mode QM/MM MD simulations. The average over these 101 realizations is shown with black diamonds. (b), (c), (d), and (e) Time evolution of the populations of the four peripheral molecular emitters. (f) Population dynamics of the dipolar mode of the silver nanoparticle dimer. (g) Population dynamics of the modes forming the broad pseudo-mode (peak at $\sim$5.35 eV in \ref{['fig:TLS']}). The average over the 101 realizations is shown with yellow diamonds. (h) Ground-state occupation reflecting losses of the silver nanoparticle dimer. Note the linear scale in this panel.
• Figure 4: (a) Time evolution of the excited-state population of the central Rhodamine in the setup of \ref{['fig:TLS']} for 100 few-mode QM/MM MD simulations with initial excitation on the central molecule. The average over these 100 realizations is shown with black diamonds. (b), (c), (d), and (e) Time evolution of the populations of the four peripheral molecular emitters. (f) Population dynamics of the dipolar mode of the silver nanoparticle dimer. (g) Population dynamics of the modes forming the broad pseudo-mode (peak at $\sim$5.35 eV in \ref{['fig:TLS']}). The average over the 100 realizations is shown with yellow diamonds. (h) Ground-state occupation due to losses of the silver nanoparticle dimer. Note the linear scale in this panel.
• Figure 5: (a) Schematic of five molecular emitters within the gap of a silver nanoparticle dimer oriented along the $z$-axis. HBQ (left inset) occupies the central site, and Methylene Blue molecules (right inset) occupy the peripheral positions. (b) Effective spectral density perceived by HBQ shown with the absorption of Methylene Blue (light blue, orange, dark blue, and red lines). Black dots indicate the fitted-mode frequencies $\tilde{\omega}_k$ forming the pseudo-mode in our model. (c) Time evolution of the excited-state populations of the five emitters (HBQ: green; MeB1: light blue; MeB2: orange; MeB3: dark blue; MeB4: red), together with that of the bright dipolar mode (purple) and the pseudo-mode (black; broad peak at $\sim$4 eV). The ground-state population (yellow) increases steadily due to system losses. (d) O-H and H-N distances in HBQ (inset). An H-N distance of $\sim$0.105 nm corresponds to a bonded state.