VSC are re-scaled by a factor of 0.5 to be consistent with Fig. . Here, we choose D = 1 µm for the effective mode diameter, the effective cavity size L = 1 mm (probing area) to discretize the 2D cavity dispersion relation when using Eq. 29 (solid lines), with ω m = 5ω c which generates a total number of M ≈ 10 6 modes for the 2D FP cavity. We use the same ω m value to perform integration using Eq. 34 (open circles). The details are provided in Supplementary Note 10, section B. One can observe that the resonance peak is still centered around ω c = ω 0 but slightly red-shifted, demonstrating the normal incidence condition. The approximate analytic expression of k2D VSC in Eq. 35 gives a similar long tail for ω c < ω 0 but a much sharper decay for ω c ≥ ω 0 . Overall, the resonance peak is asymmetric as it tails toward the lower energy regions. Future VSC experiments are required to explore if there is any asymmetry in the rate constant profile. Experimental Connections. The current theory is valid for N = 1 or a few molecules strongly coupled to the cavity, such that the individual light-matter coupling η c is strong. Experimentally, it is now possible to achieve strong (or even ultra-strong ) light-matter couplings between a plasmonic nanocavity and a few vibrational modes , such that Ω R ≫ τ -1 c (for N = 1). In these experimental setups , the current theory ( kVSC in Eq. 17) can be directly applied, and all of the predictions could be verified experimentally, e.g., the τ c behavior in Fig. and various scaling relations in Fig. .