  Dark modes represent a class of forbidden transitions or transitions with
weak dipole moments between energy states. Due to their low transition
probability, it is difficult to realize their interaction with light, let alone
achieve the strong interaction of the modes with the photons in a cavity.
However, by mutual coupling with a bright mode, the strong interaction of dark
modes with photons is possible. This type of mediated interaction is widely
investigated in the metamaterials community and is known under the term
electromagnetically induced transparency (EIT). Here, we report strong coupling
between a plasmonic dark mode of an EIT-like metamaterial with the photons of a
1D photonic crystal cavity in the terahertz frequency range. The coupling
between the dark mode and the cavity photons is mediated by a plasmonic bright
mode, which is proven by the observation of a frequency splitting which depends
on the strength of the inductive interaction between the plasmon bright and
dark modes of the EIT-like metamaterial. In addition, since the plasmonic dark
mode strongly couples with the cavity dark mode, we observes four polariton
modes. The frequency splitting by interaction of the four modes (plasmonic
bright and dark mode and the two eigenmodes of the photonic cavity) can be
reproduced in the framework of a model of four coupled harmonic oscillators.
