Fig. further shows the polariton linewidths plotted against 1/N , which exhibit a linear trend. The blue and red open circles are the FWHM of the LP and the UP obtained from the LA spectra computed from our HEOM simulations, respectively. The blue and red dashed lines are the corresponding linear fitting results, with the coefficient of determination R 2 ≈ 0.999 for both LP and UP. Moreover, the slope of the fitting curve for the UP is c (eV) smaller than the LP, meaning the UP is less susceptible to the polaron decoupling effect than the LP. These results are in perfect accordance with the 1/N scaling relation predicted in Eq. 30b under the fast limit. As a corollary, under the large N limit and with the total Rabi splitting Ω R ≫ Γ ex , the exciton broadening gradually vanishes at the polariton frequencies and only the cavity broadening Γ c /2 remains (if non-zero). As we can see from Fig. that the blue dashed line extends roughly through the origin.