which is 4N times smaller than outside the cavity case. Intuitively, this is because when the collective Rabi oscillation period is shorter than the time scales for vibrational motion, the excitons can exchange their energy with the cavity mode many times before the nuclei have time to reorganize their configuration to the excited state potential. For large N , the reorganization energy will approach to zero, and the equilibrium positions of the polaritonic states potential and the ground state potential are aligned . The width of the optical lineshape (such as polariton absorption) that corresponds to |G, 0⟩ → |±⟩ opti-cal transition is contributed by both exciton and cavity broadenings. Here we consider the linear absorption (LA) spectra. A concise review for the LA spectra lineshape theory is provided in Appendix A, in which a simple twostate model is discussed.