Universität zu Köln

Schäfer, Oliver Roland (2025). Influence of Molecular Order on Strong Light-Matter Interaction. PhD thesis, Universität zu Köln.

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Abstract

In this work, we investigate the influence of molecular order on (strong) light-matter interaction. In the strong-coupling regime, the material properties are modified, e.g., chemical reactions, energy transport in transistors, and long-range energy propagation, due to newly formed hybrid states, called polaritons. We fabricate planar microcavities with the merocyanine dye HB238 that forms aggregates with both J- and H-like transitions. We achieve simultaneous strong light-matter coupling with both transitions and two different cavity modes, resulting in the formation of four polaritons. The transition dipole moment of the H-transition is perpendicular to the substrate plane, resulting in a highly polarization-dependent response of the strong-coupling behavior. This is the first report on both a strongly-coupled H-transition and a transition dipole moment located out-of-plane. We study the system with polarization- and angle-resolved reflection and emission spectra. We further fabricate microcavity samples with different numbers of coherently coupled molecules, which can be interpreted as varying dynamic disorder. We find that the H-transition requires more order than the J-transition to reach the strong coupling regime. Next, we introduce aligned seven-atom wide armchair-edge graphene nanoribbons as a template for HB238 for light-matter studies, which yields uniaxially aligned transition dipole moments of the J-transition. In a planar microcavity we observe similar behavior as previously reported for single crystals or aligned polymers: the light-matter coupling strength is increased and the J-polaritons are highly polarization dependent due to the alignment, making templating an intriguing method to orient molecules, because it can be used for various molecules with controlled film thicknesses. We further probe the orientational order of the templated film inside the microcavity by polarization-resolved confocal microscopy and emission spectroscopy. We find that the molecules are well oriented by the seven-atom wide armchair-edge graphene nanoribbons. A multi-oscillator model is applied to both the strongly-coupled amorphous and the templated HB238 film. Mainly one polariton contributes to the coupling behavior of the templated film, indicating that the templated film consists of the aligned and a less aggregated component. The aligned component mainly contributes to the light-matter coupling. The light-matter interaction can be tailored by other systems that confine light. We fabricate square lattices of gold nano-cylinders and deposit templated HB238 on-top to control the light-matter coupling strength by polarization. We find that the coupling strength to plasmonic lattices is largely influenced by dynamic disorder and a weaker coupling strength with templated HB238 than with amorphous layers, which contrasts the findings in planar microcavities.

Item Type:	Thesis (PhD thesis)
Creators:	Creators Email ORCID ORCID Put Code Schäfer, Oliver Roland oliver.roland.schaefer@rwth-aachen.de UNSPECIFIED UNSPECIFIED
URN:	urn:nbn:de:hbz:38-796076
Date:	2025
Language:	English
Faculty:	Faculty of Mathematics and Natural Sciences
Divisions:	Faculty of Mathematics and Natural Sciences > Department of Chemistry > Institute of Physical Chemistry
Subjects:	Natural sciences and mathematics Physics Chemistry and allied sciences
Uncontrolled Keywords:	Keywords Language strong light-matter coupling English molecular aggregates English molecular templating English
Date of oral exam:	13 August 2025
Referee:	Name Academic Title Lindfors, Klas Prof. Dr Sokolowski, Moritz Prof. Dr
Funders:	RTG-2591 'TIDE - Template-designed Organic Electronics' (Deutsche Forschungsgemeinschaft), DFG project 426882575, Instrument funding for nanofabrication by the Deutsche Forschungsgemeinschaft in cooperation with the Ministerium für Kunst und Wissenschaft of North Rhine-Westphalia (project 448775637), German Excellence Initiative (QM2)
Refereed:	Yes
URI:	http://kups.ub.uni-koeln.de/id/eprint/79607