Fig. shows the rephasing signal for N = 1, 2, 5 and 25 respectively. In panel (a), we observe four diagonally elongated peaks with one molecule coupled to the cavity mode. The lower diagonal peak at (ℏω 1 ≈ -50 meV, ℏω 3 ≈ -50 meV) corresponds to the lower polariton excitation, and the upper diagonal peak at (ℏω 1 ≈ 50 meV, ℏω 3 ≈ 50 meV) corresponds to the upper polariton excitation. The cross peaks at (ℏω 1 ≈ -50 meV, ℏω 3 ≈ 50 meV), and at (ℏω 1 ≈ 50 meV, ℏω 3 ≈ -50 meV) correspond to coherent energy transfer between the upper and lower polaritonic states. In panel (b), we present the rephasing 2DES for 2 molecules coupled to the cavity mode. All of the peaks are diagonally elongated and appear at a location similar to that in panel (a). With increasing N , we can observe a significant reduction in the linewidth of various peaks, due to the polaron decoupling effect 1, (that the reorganization energy λ ∝ 1/N ), even though √ N ℏg c is fixed. This linewidth narrowing with an increasing N is also evident in the linear spectra. 1, The size of all the peaks is reduced, which is equivalent to the reduction of polariton linewidths in the linear spectra with increasing N as shown in Fig. and Fig of Paper I 1 . In addition, we also see the appearance of an asymmetrical negative signal (derivative lineshape) in both the lower and upper diagonal peaks. This is observed because of the excited-state absorption transitions (ESA) from the upper and lower polaritons to higher double exciton states in the double excitation manifold , which is only possible for N > 1. As such, the appearance of this ESA peak is a signature of the collective coupling of molecules to the cavity mode. As we increase the number of molecules to N = 5 in panel (c), the peaks appear at locations similar to those in panels molecules coupled to the cavity respectively. The peak size decreases with an increasing number of molecules because of the linewidth reduction as we observed in linear spectra in Fig. of Paper-I 1 . The off-diagonal coherence peaks also appear in the same locations as the rephasing signals but they become more intense with an increasing number of molecules. This can be attributed to the polaron decoupling effect which effectively reduces the strength of coupling between the bath modes and polariton states, creating a homogeneous environment for the polaritons and thus intensifying the relative coherence transfer through non-rephasing pathways. Similar enhancement of polariton coherence has been observed in recent exact quantum dynamics simulations as well as analytic theory. Fig. presents the total pure-absorptive spectra for different N , which is obtained by direct addition of the rephasing (Fig. ) and non-rephasing signals (Fig. ) and then normalizing the signal according to the maximum peak intensity. Panel (a) presents the pure-absorptive 2DES of one molecule coupled to the cavity mode. The upper and lower diagonal peaks correspond to the upper and lower polariton eigenstates. These peaks are overall diagonally elongated, indicating that the polaritonic states are coupled to an inhomogenous bath environment. Panel (b) represents the pure absorptive 2DES of N = 2 molecules coupled to the cavity. The overall size of the peaks has reduced, and the diagonal peaks appear to have relatively reduced in the diagonal direction (the homogeneous broadening has become very similar to the inhomogeneous molecule). We also see the appearance of ESA signatures in the diagonal peaks which is due to the formation of double-exciton states which allow for the transitions between single excitons in the first excitation manifold with the double excitons in the second excitation manifold. Comparing all the panels, we can observe that with an increase in N , the overall linewidth becomes homogenous and the 2DES is mostly dominated by the non-rephasing pathways. This is due to the effective reduction of coupling strength between the polariton states and the bath modes which makes the non-rephasing pathways more favorable. Fig. presents different Liouville pathways for the different 2DES signals used in pure-absorptive 2D electronic spectra. The vertical direction represents the increasing time of propagation. The light-red and sky-blue arrows represent the system perturbation with the external laser field. The light-red arrow pointing in the right direction represents an electric field with a negative phase factor (e -iωt , where ω is the frequency of the radiation field), and the sky-blue arrow, pointing in the left direction represents the electric field with a positive phase factor (e iωt ). The vertical yellow, dark-red, and dark-blue arrows represent the evolution of the system after the first, second, and third laser perturbations, respectively. Within the ladders, we represent the state of the system as either coherence or population states. For example, |G 0 ⟩ corresponds to the collective ground state, |P 1 ⟩ (and |P ′ 1 ⟩) represent the polariton eigenstates in the first excitation manifold (like the upper polariton, |+⟩ and the lower polariton |-⟩) and |P 2 ⟩ correspond to the polariton eigenstates in the second excitation manifold. Each of the diagrams is read from bottom to up in the vertical direction of increasing time. The detailed definitions of these states are expressed in Eqs. 23 Rephasing, respectively, at t 2 = 0 fs. In panel (a), we observe diagonally elongated peaks at (ℏω 1 ≈ -50 meV, ℏω 3 ≈ -50 meV) and at (ℏω 1 ≈ 50 meV, ℏω 3 ≈ 50 meV) which correspond to the energetic locations of the lower and upper polariton eigenstates respectively. We also observe cross-diagonal peaks at (ℏω 1 ≈ -50 meV, ℏω 3 ≈ 50 meV) and (ℏω 1 ≈ 50 meV, ℏω 3 ≈ -50 meV) which indicate the coherent energy transfer between the brightpolariton states.