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Strong Coupling of Organic Molecules and Plasmons

Final Report Summary - STROCOMP (Strong Coupling of Organic Molecules and Plasmons)

The improvement of fabrication technology over the last decades enables the accurate creation of almost arbitrarily shaped nanoscale metal structures. In such systems, quasi-bound surface modes (plasmons) provide strong, sub-wavelength confinement of electromagnetic fields. This confinement leads to strongly increased coupling between light and matter, and increases the possible spatial resolution, making it possible to surpass the diffraction limit of conventional optics. These properties make plasmonics a quickly growing and multidisciplinary subject, with applications in physics, chemistry, biology and engineering. A particularly relevant topic is the coupling of quantum emitters (such as atoms, molecules, quantum dots, or color centers in diamond) to plasmons. By concentrating light with the use of plasmons, the mismatch between the absorption cross section of the emitter and the size of the light beam can be circumvented. It is then even possible to reach the strong coupling regime, where the elementary excitations become hybrid states with mixed light-matter character, so called exciton-polaritons. The major aim of StroCOMP was to develop new insights into the strong coupling between plasmons and organic molecule excitations. Due to their complex molecular structure, organic molecules provide a richer structure and dynamics than many other emitters, which both complicates their understanding and opens new avenues for their exploitation. Over the duration of StroCOMP, we have been able to significantly improve our understanding of such systems.

In particular, we have been able to demonstrate that strong light-matter coupling can be used to significantly increase the propagation length and thus the transport efficiency of molecular excitations. This effect, which exploits the long-range nature of the involved photonic modes as compared to typical exciton transport lengths (limited by disorder and small site-to-site couplings in most organic materials), could lead to important applications in, for example, organic solar cells. In such solar cells, the efficiency of energy transport after absorption of a photon can be an important rate-limiting step for energy conversion, as the exciton has to be transported to a reaction site for splitting into an electron-hole pair.

In addition, we have demonstrated that strong light-matter coupling does not only affect "external" degrees of freedom (such as transport properties), but can also lead to significant changes in internal molecular degrees of freedom, which determine the chemical properties of the molecules. In particular, we developed a general theoretical approach that combines the well-known tools of molecular physics and chemistry with those of cavity quantum electrodynamics. This approach describes the changes in molecular properties based on an extension of the well-known Born-Oppenheimer approximation to the interaction with confined light modes. We were then able to show that strong light-matter coupling can be used to both suppress existing photochemical reactions as well as open new reaction pathways. In particular, we have focused on collective molecular effects due to the fact that in typical experiments, many molecules interact with a single light mode. This collective coupling makes it necessary to treat all molecules coupled to the light mode at the same time, leading to the concept of a "supermolecule". We have shown that this can lead even to reactions involving many molecules after absorption of just a single photon.

Finally, we have worked on our understanding of the condensation and quantum degeneracy of plasmon-exciton-polaritons. Due to the bosonic character of these quasi-particles, exciton-polariton condensation can lead to laser-like emission at much lower threshold powers than in conventional photon lasers. In work published in Optica and performed in collaboration with the group of Jaime Gómez Rivas, we demonstrated for the first time polariton condensation/lasing in a plasmonic system. We observed plasmon-exciton-polariton lasing within a dark mode in an array of metallic nanoparticles with an extremely low threshold power, which interestingly is reduced by increasing the degree of light-matter coupling in spite of the degradation of the quantum efficiency of the active material. In addition, we were able to show that a plasmonic nanocavity coupled to a mesoscopic ensemble of emitters under coherent pumping can generate non-classical light even when the number of emitters is large. This surprising result is in stark contrast to the situation of a bare ensemble of emitters, for which the generated light becomes classical even for a relatively small number of emitters. This makes plasmonic strong coupling a promising route for generating nonclassical light beyond the single emitter level.

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