Abstract
Exciton-polaritons (EPs) are quasiparticles formed by the hybridization of excitons with light modes. As organic semiconductors sustain stable excitons at room-temperature, these materials are being actively studied for room temperature polaritonic devices1–3. This is typically in the form of cavity-based systems, where molecules are confined between metallic or dielectric mirrors 4–6 or in a plasmonic gap 7,8. In such systems strong light-matter coupling gives rise to polariton splittings on the order of 200 to 300 meV 6. A wide range of phenomena have been demonstrated in cavity-polariton systems including super-fluidity9, precisely controlled chemical reactions10 and long-range energy propagation11. Here, using a range of chemically diverse model organic systems we show that interactions between excitons and moderately confined photonic states within the (thin) film can lead to the formation of EPs, with a defined lifetime, even in the absence of external cavities. We demonstrate the presence of EPs via angular dependent splittings in reflectivity spectra on the order of 30 meV and collective emission from ~5 ×107 coupled molecules. Additionally, we show that at room temperature these EPs can transport energy up to ~270 nm at velocities of ~5 ×106 m s-1. This propagation velocity and distance is sensitive to, and can be tuned by, the refractive index of the external environment. However, although sensitive to the nanoscale morphology the formation of the exciton-polariton states is a general phenomenon, independent of underlying materials chemistry, with the principal material requirements being a high oscillator strength per unit volume and low disorder. These results and design rules will enable the harnessing of EP effects for a new application in optoelectronics, light harvesting 9,12,13 and cavity controlled chemistry without the limiting requirement of an external cavity.
Shaping quantum photonic states using free electrons