  The interaction of organic molecules and molecular aggregates with
electromagnetic fields that are strongly confined inside optical cavities
within nanoscale volumes, has allowed the observation of exotic quantum regimes
of light-matter interaction at room temperature, for a wide variety of cavity
materials and geometries. Understanding the universal features of such organic
cavities represents a significant challenge for theoretical modelling, as
experiments show that these systems are characterized by an intricate
competition between coherent and dissipative processes involving entangled
nuclear, electronic and photonic degrees of freedom. In this review, we discuss
a new theoretical framework that can successfully describe organic cavities
under strong light-matter coupling. The theory combines standard concepts in
chemical physics and quantum optics to provide a microscopic description of
vibronic organic polaritons that is fully consistent with available
experiments, and yet is profoundly different from the common view of organic
polaritons. We show that by introducing a new class of vibronic polariton wave
functions with a photonic component that is dressed by intramolecular
vibrations, the new theory can offer a consistent solution to some of the
long-standing puzzles in the interpretation of organic cavity
photoluminescence. Throughout this review, we confront the predictions of the
model with spectroscopic observations, and describe the conditions under which
the theory reduces to previous approaches. We finally discuss possible
extensions of the theory to account for realistic complexities of organic
cavities such spatial inhomogeneities and the multi-mode nature of confined
electromagnetic fields.
