Light based control of Exciton Polaritons in Van der Waals Semiconductors

In this work we focus on exciton polaritons in the Van der Waals (vdW) Semiconductor, WS₂, under intense femto-second (fs) photo-excitation. Photo-excitation initiates a blueshift of the exciton polariton. This corresponds to an extremely large photo-induced change of the dielectric function. Detailed investigation of the dispersion and the fluence dependence indicate the observed effect arises from light-based control of excitons. The change to the dielectric function evolves over two distinct timescales. On the femtosecond timescale a large coherent contribution is observed. On the picosecond (ps) timescale part of the effect is found to persist. The equilibrium state is fully recovered approximately one hundred ps after the photo-excitation event. The information-rich data-sets available from interrogating the exciton-polariton allow us to explore the pathways toward active control of excitons. Additionally, our experimental approach allows us to access key quantities, such as the group velocity and real-space confinement of the exciton polariton, in the non-equilibrium state. Finally, our findings indicate that strong-light matter interactions in vdW semiconductors may be used to achieve all-optical control of significant magnitude.