Strong light-matter coupling: parametric interactions in a cavity and   free-space
  We consider parametric interactions of laser pulses in a coherent macroscopic ensemble of resonant atoms, which are possible in the strong coupling regime of light-matter interaction. The spectrum condensation (lasing at collective vacuum Rabi sidebands) was studied in an active cavity configuration. Parametric interactions under the strong light-matter coupling were proved even in free space. In contrast to bichromatic beats in a cavity, they were shown to appear due to interference between polaritonic wave packets of different group velocities. 
I. B. Mekhov, V. S. Egorov, V. N. Lebedev, P. V. Moroshkin, I. A.   Chekhonin, S. N. Bagayev