Exciton-Polaritons in Two-Dimensional Materials

Polaritons are hybrid states of matter and light that form under strong and ultrastrong light-matter coupling. While the original research on polaritons focused on matter in external optical cavities, the concept of self-hybridization was introduced for polariton formation in thin flakes of layered materials. Here we discuss polaritons in two-dimensional (2D) monolayers as the ultimate limit of light-matter hybridization. Free standing monolayers of transition metal dichalcogenides show propagating exciton-polaritons that form between a photonic mode waveguided by the monolayer and the excitonic states of the material. We measured the dispersion of this exciton-polariton and found that it shows the characteristic back-bending observed for strong-light matter coupling. We discuss the consequences of these excitations, e.g., the prospect of combining polaritons in van-der-Waals type materials and the question of what limits the signature of matter contribution to polaritons in ultrathin 2D crystals.