under the single mode limit (or under the 1D cavity case, see Eq. 23). As expected, the 𝑘 VSC expression in Eq. 30 should contain several characteristic physical constants, including the speed of light 𝑐 in 𝜔 c (see Eq. 3) as it is related to light-matter interaction, Planck's constant ℏ in 𝑔 c (see Eq. 9) as it should be a quantum theory, and Boltzmann's constant 𝑘 B in 𝑛(𝜔 0 ) as it is a thermally activated theory. We adopt a model system used in Ref. to demonstrate the basic trend of 𝑘 VSC predicted by the current theory. The schematic of the model is provided in Fig. , whereas the details are provided in Supplementary Material, Sec II.