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A new research field is emerging in which ensembles of molecules are collectively hybridised with light in a process known as strong coupling. This hybridisation leads to the formation of new states that are part light and part matter, states known as polaritons. Here we offer an entry point into the field of molecular strong coupling. We include an overview of the essential phenomena and an introduction to the conceptual framework – considerable use is made of simple classical physics models since they are helpful in developing an intuitive understanding. Open questions are identified and discussed, as well as some of the exciting experimental and theoretical challenges that lie ahead.

We show that strong light–matter coupling can be used to overcome a long-standing problem that has prevented efficient optical emission from carbon nanotubes. The luminescence from the nominally bright exciton state of carbon nanotubes is quenched due to the fast nonradiative scattering to the dark exciton state having a lower energy. We present a theoretical analysis to show that by placing carbon nanotubes in an optical microcavity the bright excitonic state may be split into two hybrid exciton–polariton states, while the dark state remains unaltered. For sufficiently strong coupling between the bright exciton and the cavity, we show that the energy of the lower polariton may be pushed below that of the dark exciton. This overturning of the relative energies of the bright and dark excitons prevents the dark exciton from quenching the emission. Our results pave the way for a new approach to band-engineering the properties of nanoscale optoelectronic devices.