are the Hopfield coefficients. In Eq. 30, we have explicitly ignored the 1/T * 2 contribution (c.f. Eq. 23 and Eq. 25). With a larger light-matter detuning ∆, the first term in Eq. 31 decreases due to a reduced J ν (∆E + ) originated from a larger energy gap between |+⟩ state and the dark states manifold. On the other hand, the second term in Eq. 31 increases because of increased J ν (∆E -) with a smaller energy gap between |-⟩ state and the dark states manifold. As such, there will be a turnover of 1/T 2 as one increases the light-matter detuning ∆ = ω c -(ω x +λ) from zero value to positive values. The impact of cavity loss, which affects both |±⟩ states, is to cause additional decoherence from population transfer to the |G, 1⟩ state. Fig. verify such a turnover of T 2 as a function of the detuning obtained from HEOM simulations, at various cavity quality factors from Q = 167 to Q = 1500. Further analysis of the turnover using the formalism of Eq. 30 is provided in Sec. VIII of the Supporting Information.