Strong coupling between surface plasmons and molecular excitons may lead to the formation of new hybrid states—polaritons—that are part light and part matter in character. A key signature of this strong coupling is an anti-crossing of the exciton and surface plasmon modes on a dispersion diagram. In a recent report on strong coupling between the plasmon modes of a small silver nano-rod and a molecular dye, it was shown that when the oscillator strength of the exciton is large enough, an additional anti-crossing feature may arise in the spectral region where the real part of the permittivity of the excitonic material is zero. However, the physics behind this double anti-crossing feature is still unclear. Here, we make use of extensive transfer matrix simulations to explore this phenomenon. We show that for low oscillator strengths of the excitonic resonance, there is a single anti-crossing arising from strong coupling between the surface plasmon and the excitonic resonance, which is associated with the formation of upper and lower plasmon–exciton polaritons. As the oscillator strength is increased, we find that a new mode emerges between these upper and lower polariton states and show that this new mode is an excitonic surface mode. Our study also features an exploration of the role played by the orientation of the excitonic dipole moment and the relationship between the modes we observe and the transverse and longitudinal resonances associated with the excitonic response. We also investigate why this type of double splitting is rarely observed in experiments.
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14 January 2021
Research Article|
January 11 2021
Single vs double anti-crossing in the strong coupling between surface plasmons and molecular excitons
Wai Jue Tan
;
Wai Jue Tan
a)
1
Department of Physics and Astronomy, University of Exeter
, Exeter EX4 4QL, United Kingdom
a)Author to whom correspondence should be addressed: wjt206@exeter.ac.uk
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Philip A. Thomas
;
Philip A. Thomas
b)
1
Department of Physics and Astronomy, University of Exeter
, Exeter EX4 4QL, United Kingdom
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Isaac J. Luxmoore
;
Isaac J. Luxmoore
c)
2
College of Engineering, Mathematics and Physical Sciences, University of Exeter
, Exeter EX4 4QF, United Kingdom
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William L. Barnes
William L. Barnes
d)
1
Department of Physics and Astronomy, University of Exeter
, Exeter EX4 4QL, United Kingdom
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a)Author to whom correspondence should be addressed: wjt206@exeter.ac.uk
b)
Electronic mail: p.thomas2@exeter.ac.uk
c)
Electronic mail: i.j.luxmoore@exeter.ac.uk
d)
Electronic mail: w.l.barnes@exeter.ac.uk
Note: This paper is part of the JCP Special Topic on Polariton Chemistry: Molecules in Cavities and Plasmonic Media.
J. Chem. Phys. 154, 024704 (2021)
Article history
Received:
November 16 2020
Accepted:
December 20 2020
Citation
Wai Jue Tan, Philip A. Thomas, Isaac J. Luxmoore, William L. Barnes; Single vs double anti-crossing in the strong coupling between surface plasmons and molecular excitons. J. Chem. Phys. 14 January 2021; 154 (2): 024704. https://doi.org/10.1063/5.0037864
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