Strong coupling is achieved when the coherent energy exchange between a confined electromagnetic field mode (e.g., a cavity mode or a surface plasmon) and material excitations becomes faster than the decay and decoherence of either constituent. This creates a paradigmatic hybrid quantum system with eigenstates that have mixed light-matter character (polaritons). Organic molecules are a particularly suitable system to achieve strong coupling due to their large transition dipole moments and binding energies.
In the last years, it has been realized that strong coupling can be used to modify a wide range of material properties and to achieve new functionalities. We will first discuss how strong coupling can lead to an extraordinary increase of energy transport efficiency in disordered organic systems, even for subradiant states that are not themselves coupled to the light modes. We will then show how strong coupling between mid-IR frequency light modes and vibrational transitions in molecules affects the dynamics and nonlinear response, such as the Raman scattering signal, of organic materials. Exploiting the hybrid light-matter nature of polaritons, this can be used to convert a Raman laser into a new type of optical parametric oscillator with two mutually coherent output beams. Finally, we will discuss the influence of strong coupling on internal molecular structure and chemical reactions, which cannot be explained using standard two-level models of the molecules. We introduce a first-principles model that fully takes into account photonic, electronic and nuclear degrees of freedom, and explains how the Born-Oppenheimer potential energy surfaces that determine molecular dynamics are modified under strong coupling. This can be used to, e.g., almost completely suppress photochemical reactions such as photoisomerization. Surprisingly, this suppression works more efficiently when many molecules are coupled to a single light mode due to a “collective protection” effect in the delocalized polaritonic state.