In Fig. , panels (a)-(c) provides the 2D spectra of monomers coupled to a cavity with no cavity loss. We observe the Rabi splitting of the lower and upper polariton diagonal peaks at t 2 = 0 fs. These peaks are also stretched along the diagonal, indicating the inhomogeneous distribution caused by the low-frequency bath modes (see Eq. 32) coupled to the exciton (Eq. 33), and also both upper and lower polaritons (Eq. 46). The characteristic off-diagonal coherence peaks in the 2D spectra are direct indications of the coupling between the exciton and the cavity, described in Eq. 37. With an increase in waiting time t 2 , the off-diagonal coherence peaks oscillate and the diagonals slowly become more homogeneously broadened (along the anti-diagonal direction) due to spectral diffusion, which arises mainly due to the decay of bath correlation and the loss of the initial memory of the system (commonly referred to as the circularisation), making the excitation axis ω 1 more independent of the emission axis ω 3 .