Next, we explore the effect of detuning on polariton lineshape. Near the resonance condition (ω c = ω 0 + λ), the system enters the polaron decoupling regime which leads to linewidth narrowing, and the polariton lineshape is described by Eq. 30 under the fast limit (or Eq. 29 under the slow limit). On the other hand, in the large detuning limit (Θ N → 0 or π/2), the system behaves like an uncoupled system where the light and matter components are well-separated. The polariton linewidths remain Γ ex for the matter end and Γ c for the photon end. Fig. presents the spectral lineshapes under different detunings near the resonance condition (ω c = ω 0 + λ = 2.03 eV). In the numerical simulations, we control the parameters to always stay in the strong coupling regime (see Eq. 23 as well as Appendix B) to make the polariton peaks distinguishable from each other. Here, we fix N = 4 and g c = 68.1 meV. The cavity frequency varies from ω c = 1.83 eV to ω c = 2.23 eV. Fig. shows the results with a cavity loss rate Γ c = 44.15 meV and Γ ex = 76.5 meV. One can see that for the red-detuned cases (ω c < ω 0 + λ), the LP is dominated by the photon component, with a width Γ-∼ Γ c , while the UP is dominated by the exciton component so that Γ+ ∼ Γ ex . Moving towards the resonance condition (ω c = ω 0 + λ = 2.03 meV), the polariton linewidths are narrower due to the polaron decoupling effect. Further, for the bluedetuned cases (ω c > ω 0 +λ), Γ-∼ Γ ex and Γ+ ∼ Γ c . The FWHM of all the polariton peaks are directly obtained from the numerically simulated A ± N (ω).