Non-Hermitian effects on the exciton-polariton dispersion in 2D materials

Non-Hermitian effects are often discussed in the context of exceptional points. However, in this paper we present an example of a non-trivial non-Hermitian effect that is not based on exceptional points. In a two-dimensional system, such as a transition-metal dichalcogenides (TMD) monolayer, in which optically active resonances exist (in this example excitons), the coupling to the electromagnetic radiation field creates 2D polaritons. Their mass can change its sign depending on the exciton’s dephasing. We discuss the difference between longitudinal (L) and transverse (T) polaritons and present proposals for possible experimental observations of the predicted mass-sign reversal.