Photoinduced ultrafast quasiparticle dynamics and exciton-phonon coupling in layered semiconductors.

In van der Waals layered materials, the dielectric screening is reduced due to two-dimensional space confinement, resulting in the enhancement of the excitonic binding energy up to hundreds of meV and of excitonic stability at room temperature. The scientific community focuses its attention on understanding and manipulating the sub-picosecond exciton dynamics driven by light absorption. Typically, the dynamic of an exciton is affected by the interaction with other system's degrees of freedom. Recently, it was proposed that exciton-phonon coupling plays a crucial role in the incoherent relaxation dynamics as well as in the coherence and diffusion of excitons. However, how exciton-phonon coupling manifests in the energy and time domain is still an open debate between theory and experiment. To find a unified picture, we investigate the time-resolved broadband reflectivity of the layered semiconductor bismuth tri-iodide due to its high binding energy and the predicted presence of exciton-phonon interaction. Our joint effort between experiment and theory enables the isolation of the spectral fingerprints for the optical detection of exciton-phonon coupling in a layered semiconductor.