When molecular vibrational excitations are coupled to the optical cavity, one generates the vibrational polaritons, as illustrated in Fig. , where a vibrationally excited state with 0 photons in the cavity |v 1 , 0⟩ (black energy levels) hybridizes with the ground vibrational state with 1 photon in the cavity |v 0 , 1⟩ (red energy levels) that is in resonance to |v 1 , 0⟩. The resulting hybridized states |±⟩ (green and blue energy levels) that are energetically separated by the Rabi-splitting Ω R (with details of the Rabi splitting provided in Section 5.1, Eq. 194). The Rabi-splitting is spectroscopically visible if it is larger than the rates of other competing dissipative processes (typically estimated from the spectral line-widths), such as solvent dissipation or cavity loss, and consequently, the light-matter coupling is said to be in the vibrational strong coupling regime. Note that this is only a schematic based on the JC type of model, where we only considered a single vibrational DOF coupled to a single cavity mode. In actual experiments, an estimated N = 10 6 ∼ 10 10 molecules are collectively coupled to the Fabry-Pérot cavity for each cavity mode. Thus, one needs to use the Tavis-Cummings (TC) model (see Sec. been achieved and the Rabi-splitting has been experimentally observed. As expected from the Tavis-Cummings (TC) model (see Sec. 1.2), the Rabi-splitting linearly increases with √ N . This collective effect has been verified experimentally Fig. , where the Rabi splitting measured from the transmission spectra linearly depends on the N/V (see Eq. 17 This suppression of the rate constant will only happen under the "resonant condition" when the cavity frequency is close (in resonance) with a molecular vibrational frequency. Specifically, in Fig. , the resonant vibrational frequency refers to the Si-C vibrational stretching frequency. 108 Further, Fig. demonstrates the key features of this type of VSC experiment, with the width and shape of the transmission spectra (black solid line) being similar to the cavity-modified rate constant (red dashed line). Similar studies that have observed cavity suppression include Ref. 366 that studied Prins cyclization and Ref. 367 that very recently studied the urethane addition reaction.