Separation of Homogeneous and Inhomogeneous Broadening using Two-Dimensional Coherent Spectroscopy

Separating the contributions of homogeneous dephasing from inhomogeneous broadening in spectral linewidths is essential for connecting optical spectra to microscopic dissipation and disorder mechanisms. Voigt fits to one-dimensional spectra, such as photoluminescence, yield strongly correlated Gaussian and Lorentzian widths, so that neither width can be determined independently with confidence. We quantitatively show that two-dimensional coherent spectroscopy (2DCS) reduces this degeneracy by providing orthogonal spectral slices, diagonal and cross-diagonal, with complementary sensitivity to homogeneous and inhomogeneous broadening processes. As a demonstration, we measure the exciton resonance in hBN-encapsulated MoSe$_2$ at 8 K. A joint uncertainty-weighted fit maps the full $\chi^2(\sigma,\gamma)$ landscape to quantify parameter covariance. Compared with linear Voigt analysis, 2DCS yields more compact confidence regions and markedly reduced parameter correlation, enabling reliable separation of the contributions to the excitonic linewidth.