Non-Hermitian dispersion sign reversal of radiative resonances in two dimensions

In a recent publication [Wurdack et al., Nat. Commun. 14, 1026 (2023)], it was shown that in microcavities containing atomically thin semiconductors, non-Hermitian quantum mechanics can lead to negative exciton polariton masses. Here, we study the mass-sign reversal in two-dimensional systems and show that it is not contingent on the cavity. We show that it can occur generally in radiative resonances in two dimensions and derive conditions for it (critical dephasing threshold, etc.). In monolayer transition-metal dichalcogenides, this phenomenon is not invalidated by the strong electron-hole exchange interaction, which is known to make the exciton massless.