Strong coupling effects in plasmonic-molecular systems

Maria Bancereks Faculty of Physics, University of Warsaw

Maria Bancerek (a), Katarzyna Kluczych-Korch (a), Tomasz J. Antosiewicz (a)

Faculty of Physics, University of Warsaw

Strong light-matter interaction in the system offers the possibility of modifying its properties through the emergence of new, hybridized cavity-exciton eigenmodes, called polaritons [1]. Among various systems used to study strong coupling plasmonic-molecular systems gain attention as one of the simplest yet capable of reaching the strong coupling regime, owing to the small mode volumes of the plasmonic nanostructures and plasmonic resonance tunability [2, 3]. Given the spectral overlap of the plasmonic resonance and the molecular transition and their spatial proximity, one can observe Rabi splitting in the absorption spectrum of the system, indicating the formation of so-called lower and upper plexcitonic branches. Employing TDDFT calculations, we numerically study metallic nanoparticles and aromatic molecules assemblies in order to underpin the atomic-scale changes induced by the strong coupling. We investigate how the coupling changes with the nanoparticle-molecule separation, leading to changes in the absorption spectrum of the system. We show how intensity of molecular transitions increases when placing a molecule closer to the nanoparticle up to a certain distance (~5 Å) at which molecular transitions couple to the variety of nanoparticle states, resulting in mixed metallic-molecular transitions between hybridized states. We investigate how studied effects influence hot carrier generation and show carrier transfer between two subparts of the system, i.e., direct hot electron transfer from the molecule to the nanoparticle and the modified energy landscape of generated holes and electrons.

Figure 1. Photoabsorption spectra for a Mg nanoparticle – tetracene coupled system for various gaps and induced electron density for gap = 5 Å at frequencies corresponding to lower and upper polaritons (LP, UP) (a). Molecular/mixed transitions contributions to photoabsorption for LP (b); inset shows wavefunctions of states constituting an exemplary mixed transition at gap = 3 Å.

[1] Baranov, D.G. et al. Novel Nanostructures and Materials for Strong Light–Matter Interactions. ACS Photonics 5, 24-42 (2018)

[2] Törmä, P. & Barnes, W. L. Strong coupling between surface plasmon polaritons and emitters: a review. Rep. Prog. Phys. 78, 013901 (2015)

[3] Kuisma, M. et al. Ultrastrong Coupling of a Single Molecule to a Plasmonic Nanocavity: A First-Principles Study. ACS Photonics 9, 3, 1065–1077 (2022)