It is pointed out that inter-branch scattering (between UP and dark states) is crucial to explain the difference between the UP and LP linewidths . To be specific, the UP linewidth is usually much broader than the linewidth of the LP state, mainly due to the UP → LP and UP → dark states (DS) decay channels, which causes additional broadening to UP compared to LP. By assuming that the additional broadening effect is additive, one can express the UP linewidth by further incorporating inter-branch transitions, i.e.,