PHANTOM · Photophysical Applications of Nano-Optics to Molecules

Photophysical Applications of Nano-Optics to Molecules

When molecules are embedded in specifically–designed optical nanostructures, their interaction with light can be enhanced, reaching conditions at which coherent light-matter coupling prevails over all other processes. In this realm, known and the strong light-matter coupling regime, new quantum states (known as cavity polaritons) are formed and the energetic landscape of the molecules may significantly change. As has been demonstrated several times during the past years the creation of new states and the rearrangement of the molecular energy landscape give rise to exciting opportunities in chemistry and material science, as it provides a new tool to control materials process by coupling to photonics structures. In the PhANToM project, supported by the Marie Curie Career Integration Grant, we explored several avenues in which strong light-matter coupling can be used in order to tailor molecular photophysical processes in organic dyes. We successfully coupled phosphorescent molecules to microcavities and showed that this coupling alters the emission properties of the molecules and their internal dynamics, as was revealed in ultrafast spectroscopic measurements. Moreover, we studied how intermolecular processes are affected by coupling to artificial photonic structures: in normal circumstances, such interactions can occur for extremely short molecular separations of a few nanometers. However, in this project we demonstrated that energy-transfer in a donor-acceptor pair can be enhanced and efficiently occur over distances of ~100 nm when the molecules are embedded in a hybrid photonic/plasmonic structure which acts as mediator for energy transfer. Such enhancement is highly desirable in organic electro-optic devices and may find important applications in organic photovoltaic cells, where energy should be conveyed from the light-absorbing molecules to the active layer which converts the excitation into electrical current. In a different route of the project we constructed a unique time-resolved microscopy system which permits the ultrafast monitoring of spatiotemporal dynamics in optically-excited samples. We used this system to study how the coherent coupling between the molecular excitations (Frenkel excitons) to propagating photons changes the transport properties of organic materials. We succeeded in recording the motion of polaritons in strongly-coupled organic microcavities, over a time-scale of several picoseconds and observed for the first time the gradual expansion of polariton ensembles over distances of several microns, which provides the first direct evidence for long-range transport in organic systems. Our results prove that the poor mobility of organic materials, which significantly limits the performances of organic-electronics devices, can be overcome by using the phenomena of strong coupling, with a potential increase of 3 orders of magnitude in mobility. Finally, we constructed composite molecule-nanoparticle assemblies to study strong light-matter coupling on a single-molecule level. We used DNA-directed self-assembly to construct metallic nano-particle dimers with a predefined geometry and to precisely place the dye molecule exactly at the gap separating the nanoparticles. In such self-assembled composite structures plasmonic effects give rise to an enormous optical field confinement and field enhancement, which amplifies light-matter interaction and allows strong light-matter coupling with individual molecules at room temperature. Such nanometric devices can be used to study quantum-optical effects in molecular system and to study and control molecular processes on a single-molecule level.

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