The study of light-matter interaction has led to many fundamental discoveries as well as to the development of new technology. In this thesis, we investigate the interaction between light and matter in different mesoscopic systems such us Fabry-Perot cavities with fixed and/or moving mirrors (optomechanical cavities) and superconducting circuits. In the context of optomechanical cavities, we isolate genuine quantum contributions of the interaction between an optical field and a mechanical mirror and study how to probe nonlinearities of the mechanical motion. We also investigate dynamical corrections, arising from an initial non-equilibrium configuration of the system, to the Casimir energy induced by the interaction between a quantum multimode field and the quantum fluctuations of the movable mirror. In a cavity scenario, we further consider such kind of dynamical corrections for the Casimir-Polder force between an excited atom and a perfectly conducting mirror, finding new features that can allow for an easier way to single-out the dynamical Casimir-Polder effect. In the context of superconducting circuits, we explore the light-matter interaction between microwave fields and artificial atoms in the ultrastrong coupling regime, where the system displays a high degree of entanglement. We show how to extract these (otherwise inaccessible) quantum correlations, and how such correlations can potentially be exploited as a resource for entanglement-based applications. In all these investigations we provide feasible experimental scenarios where such new effects can be probed.