We first examine the effect of collective coupling to polariton linewidths due to the polaron decoupling effect. By doing so, we set the overall Rabi splitting to be much larger than the bath characteristic frequency ω f , and for simplicity, we do not include the cavity loss (Γ c = 0). The cavity frequency is set to be in resonance with the exciton energy (ω c = ω 0 + λ). On the other hand, we keep the overall Rabi splitting Ω R < 400 meV so that η ≡ Ω R /(2ω c ) < 0.1 which does not enter into the ultrastrong coupling regime , ensuring that the RWA is valid and the dipole self-energy (DSE) term is negligible . There are two schemes to explore the polaron decoupling effect to polariton linewidths, one is fixing the single molecule coupling strength g c and the other is fixing the overall Rabi splitting Ω R , which does not influence the scaling relation of the polariton linewidth with N .