which is the cavity frequency we introduced in Eq. 4. Experimentally, it is observed that only when ω c = ω 0 (or at k ∥ = 0, ω k = ω Q , known as the normal incidence condition) will there be VSC effects . For a red-detuned cavity (ω c < ω Q ), there are still a finite number of modes (with a finite value of k ∥ ) to fulfill the resonance condition ω k = ω 0 and generate polariton states. This is referred to as the oblique incidence, in which one can observe the splitting of the energy levels and the formation of polariton states. Surprisingly, there is no observed VSC effect even though polariton states are formed at a finite k ∥ . Despite recent theoretical progress , there is no widely accepted theoretical explanation for the VSC effect which happens only at the normal incidence. Here, we generalize the single mode expression in Eq. 16 into many modes and explain the normal incidence condition. In Supplementary Note 9, we carry out a detailed theoretical analysis.