Strong coupling to generate complex birefringence – metasurface in the middle etalons

We have measured the optical signatures of strong coupling between the resonance of etalons and plasmon antenna arrays in transmission and polarization. Planar etalons in the middle of which a plasmon antenna array is placed show anticrossings in transmission between the etalon resonances and plasmon antenna resonance, which we map as function of frequency, etalon opening and oscillator strengh. We argue that the proper interpretation of strong coupling and the magnitude of the Rabi splitting requires a `metasurface-in-the-middle’ cavity model, and is distinct from strong coupling between a cavity and a dispersive material. Furthermore, we quantitatively connect the Rabi splitting to the electrostatic antenna polarizability, i.e., the polarizability in absence of radiative damping corrections. Finally, we demonstrate that the strong coupling brings very strong polarization conversion effects, as the hybrid modes provide for a strong retardance that can be leveraged for linear birefringence and dichroism.