Simulation of two-dimensional electronic and double-quantum coherence spectra of molecular polaritons

We simulate two-dimensional electronic spectroscopy (2DES) and double-quantum coherence (DQC) signals for a dissipative Tavis-Cummings model using the third-order optical response function formalism. Cavity photon loss, excited state population relaxation, and fast dephasing are included through Lindblad terms, while static energetic disorder is treated using the density matrix equations of motion. The 2DES spectra contain lower polariton (LP) and upper polariton (UP) diagonal peaks together with LP-UP cross peaks, thereby resolving excitation-detection correlations that are absent in pump-probe spectra. Decomposition into ground state bleaching, stimulated emission, and excited state absorption pathways shows that the 2DES are the residual of cancellations among pathways of opposite sign, with the incomplete cancellation originating from the two-level structure of the emitters. The waiting time dependent amplitudes of the 2DES peaks exhibit damped Rabi oscillations. As the bright state coherence and population decay, the diagonal-cross peak contrast is reduced. The remaining long time signal is determined by dark state relaxation and is no longer associated with either polariton branch. DQC spectra provide a complementary view of the same model. The double-quantum frequency axis resolves the structure of the double-excitation manifold, while the T1 dependence monitors the phase evolution and decay of coherence between the ground and single-excitation states. The DQC signal amplitude decays rapidly and oscillates with a period approximately twice that of the 2DES oscillations.