  Recently, we predicted theoretically that in cavities that support several
longitudinal modes, strong coupling can occur in very different manners,
depending on the system parameters. Distinct longitudinal cavity modes are
either entangled with each other via the material or independently coupled to
the exciton mode. Here we experimentally demonstrate the transition between
those two regimes as the cavity thickness is gradually increased while
maintaining fixed coupling strength. We study the properties of the system
using reflection and emission spectroscopy and show that even though the
coupling strength is constant, different behavior in the spectral response is
observed along the coupling-decoupling transition. In addition, we find that in
such multimode cavities pronounced upper-polariton emission is observed, in
contrast to the usual case of a single-mode cavity. Furthermore, we address the
ultrafast dynamics of the multimode cavities by pump-probe spectroscopic
measurements and observe that the transient spectra significantly change
through the transition.
