With an increase in cavity loss to Γ = 10 meV in panels (d)-(f) or to Γ = 20 meV in panels (g)-(i), one observes two prominent features in the 2D spectra. (1) Both the diagonal and the off-diagonal peaks become more homogeneous (further broadened along the anti-diagonal direction), even at t 2 = 0 fs. This is purely due to the effect of the Lindblad treatment for the cavity loss, which adds a homogeneous dephasing channel for the decay of the coherences in the reduced system density matrix, thus homogenizing the peaks. The homogeneous nature of the photonic bath is a direct result of Markovian treatment via Lindbladian, which is equivalent to a bath described by a very high cutoff frequency. (2) The second laser pulse puts most of the ensemble in the population state (see Fig. in the Supplemental Materials), and so a decay in signal intensity along t 2 can be mostly attributed to excited population decay. Now, the addition of an extra loss channel from the cavity causes the populations to decay proportional to e -Γdt (Lindblad-type decay). Therefore, with an increase in the cavity loss rate Γ, one observes a faster decay of the signal intensity at a given t 2 . This is obvious when comparing panel (e) for Γ = 10 meV or panel (h) for Γ = 20 meV to panel (b) for Γ = 0. Fig. presents the polariton 2DES spectra when varying the light-matter coupling strength g c at a fixed cavity loss rate Γ = 10 meV. With an increase in g c , the separation between the lower and upper polariton peaks increases, just like those in the linear spectra as presented in Fig. . As a result of Rabi splitting, the overlap between the diagonal and cross peaks decreases, making it easier to track the behavior of individual peaks.