Fig. presents the collective coupling effect of polariton spectra by varying the number of molecules N under both the fast and the slow limits under the zero detuning case. Panels (a)-(c) are the results under the fast limit with ω f = 24.8 meV. Fig. presents the polariton spectra by fixing single molecule coupling strength g c = 68.1 meV and varying N . The total Rabi splitting is Ω R = 2 √ N g c . One can see that as N increases, the total Rabi splitting increases. The polariton lineshape are Lorentzian-like, and the linewidth becomes narrower due to the polaron decoupling effect. Fig. presents the polariton spectra by fixing the total Rabi splitting Ω R = 385.2 meV and varying N . The single molecule coupling strength is