Manipulating valley-sensitive light-matter states in monolayer transition metal dichalcogenides

Control over the interactions between light and matter underlies many classical and quantum applications. In recent years, 2D layered semiconductors have gained prominence for optoelectronics because of their strong excitonic effects and capacity for van der Waals assembly. One of the unique features of these monolayer materials, the valley pseudospin, can be manipulated by controlling the local properties of optical fields. Here, we discuss two manifestations of this optical control across different regimes of coupling. In a strongly coupled regime, we discuss the dynamics of valley-polarized hybrid light-matter states, or exciton-polaritons, in a monolayer MoS2 embedded in a microcavity. Different dynamics of valley-polarized exciton-polaritons can be accessed with microcavity engineering by tuning system parameters such as cavity decay rate and exciton-photon coupling strength. Comparison of predictions and measurements demonstrate the ability to intentionally modify exciton-polariton valley characteristics, illustrating the microcavity as a tool for manipulating and engineering valley dynamics in 2D materials. In the weak coupling regime optical selection rules give rise to the valley-selective optical Stark shift. We discuss recent advances in probing this effect with improved sensitivity. Both of these complementary approaches show how the valley structure of monolayer materials yield interesting light-matter phenomena that allow tuning of optical properties.