For the GSB (panels a-c) and SE pathways (d-f), we only observe peaks at upper and lower polariton frequencies and also at the coherence position between upper and lower polaritons. This can be directly understood by looking at the dipole matrix elements of this particular system (see Supporting Information, Sec. I), where there are large transition dipoles from the ground state to |±⟩ polaritons. The middle polariton (dark state |D⟩) is not visible in the 2D signal, because the transition dipole from the ground state to the middle polariton is very small. In fact, µ GD = ⟨D|μ|G⟩ is ≈ 35 times smaller than the corresponding transition dipoles to the upper polariton µ +G or the lower polariton µ -G . Since the GSB and SE pathways are only involved in transitions between the ground and first excitation manifold, we do not observe the signatures of the dark state due to its very low transition amplitude (|µ GD | 2 ∝ (1/35) 2 = 0.0008). In Fig. of the Supporting Information, we further present the contribution of different polariton states to the GSB and SE peaks, based on the magnitude of |µ αG | 2 (where |α⟩ ∈ {|+⟩, |D⟩, |-⟩}. The full results of t 2 > 0 spectra are presented in Fig. of the Supporting Information. With a finite cavity loss (Γ = 10 meV), one can expect a similar trend as shown in Fig. . The 2DES peaks will be more homogeneously broadened and the peak intensity will be lost at a faster rate, with an increased loss rate, as shown in Fig. of the Supporting Information.