Novel effects in strong light-matter coupling

In the last decade a host of seminal experimental results have demonstrated that properties and dynamics of molecules and solids can be modified and controlled by coupling strongly to the electromagentic field of a photonic environment, e.g. an optical cavity. For a detailed understanding of such changes it becomes necessary to use first-principles approaches to strong light-matter interactions. In this talk I will discuss the fundamental setting for such ab-initio methods, the Pauli-Fierz quantum field theory in Coulomb gauge, introduce quantum-electrodynamical density-functional theory as an efficient and accurate simulation technique and highlight novel effects that become accessible. Among others I demonstrate how conduction and absorption properties are modified, how collective strong coupling can induce strong local modifications and how bound states in the continuum appear.