  Band engineering stands as an efficient route to induce strongly correlated
quantum many-body phenomena. Besides inspiring analogies among diverse physical
fields, tuning on demand the group velocity is highly attractive in photonics
because it allows unconventional flows of light. $\Lambda$-schemes offer a
route to control the propagation of light in a lattice-free configurations,
enabling exotic phases such as slow-light and allowing for highly optical
non-linear systems. Here, we realize room-temperature intercavity Frenkel
polaritons excited across two strongly coupled cavities. We demonstrate the
formation of a tuneable heavy-polariton, akin to slow light, appearing in the
absence of a periodic in-plane potential. Our photonic architecture based on a
simple three-level scheme enables the unique spatial segregation of photons and
excitons in different cavities and maintains a balanced degree of mixing
between them. This unveils a dynamical competition between many-body scattering
processes and the underlying polariton nature which leads to an increased
fluorescence lifetime. The intercavity polariton features are further revealed
under appropriate resonant pumping, where we observe suppression of the
polariton fluorescence intensity.
