Exciton dynamics of cavity-coupled monolayer semiconductors.

The space-time symmetry breaking has significant effects in the electronic structures of 2D crystals, typically in the monolayer transition metal dichalcogenides (TMDs). Violation of the parity symmetry allows the valley states with certain polarizations, i.e., K+ and K- which present distinct optical responses. The time-reversal symmetry protects the energy degeneracy of the two valley states. However, the electron-hole exchange and many-exciton interactions may have a trade-off with the valley polarization properties, as they open new channels for electronic-state relaxation and couplings. Using chiral optical cavities, we develop a strong-coupling scheme with the TMDs valley excitons so as to control the polarization degree. We found the exciton polaritons that can incredibly control the exciton dynamics and the many-exciton interactions, so for the optical emission. An in-depth study of these effects was carried out by developing a 2D chiral pump-probe spectroscopy. We developed a time-, frequency- and chirality-resolved signal for a real-time monitoring of the valley exciton dynamics. An effective many-exciton Hamiltonian was developed for a microscopic theory for the 2D spectroscopic signal; the biexciton modes are readily accessed by the excited-state absorption component. Our work provides a comprehensive theory for the TMD monolayers at ultrafast timescales, which would be insightful for the study of low-dimensional materials and polariton physics.