In the 2D spectra of the dimer coupled to the cavity, we further observe a significant increase in polariton coherence time with increased cavity coupling strength, compared to the excitonic coherence time for the system outside the cavity. At room temperature with a cavity loss rate of Γ = 10 meV, the coherence lifetime of a molecular dimer can be significantly prolonged up to 200 fs (from < 50 fs outside the cavity), with a light-matter coupling strength of Ω R ≈ 124 meV. The prolonged offdiagonal oscillations are the direct consequence of the polaron decoupling effect where the light-matter coupling effectively decreases the exciton-phonon coupling of the system (see Eq. 46), leading to a longer polariton coherence time. This is a clear theoretical demonstration of the polaron decoupling effect by simulating the polariton 2DES spectra. Similar experimental conditions for Γ and Ω R have been recently achieved when coupling CdSe nanoplatelets to a dielectric optical cavity, and we envision observing such an effect from direct experimental measurements in the future. Fig. presents the dynamics with the initial condition starting from a non-Hermitian density matrix |g1⟩⟨e0|, which is the necessary component for simulating optical response functions. Because of the asymmetry of the coherence elements in the initial density matrix, we see the rise of imaginary populations. Here, we can see the coherent transfer between the |g1⟩, |e0⟩, and |g0⟩ states and the decay of all magnitudes due to loss.