  Organic microcavities are photonic nanostructures that strongly confine the
electromagnetic field, allowing exotic quantum regimes of light-matter
interaction with disordered organic semiconductors. The unambiguous
interpretation of the spectra of organic microcavities has been a long-standing
challenge due to several competing effects involving electrons, vibrations and
cavity photons. Here we present a theoretical framework that is able to
describe the main spectroscopic features of organic microcavities consistently.
We introduce a class of light-matter excitations called dark vibronic
polaritons, which strongly emit but only weakly absorb light in the same
frequency region of the bare electronic transition. Successful comparison with
experimental data demonstrates the applicability of our theory. The proposed
microscopic understanding of organic microcavities paves the way for the
development of optoelectronic devices enhanced by quantum optics.
