Coherent Electronic Coupling in Monolayer MoSe$_{2}$

Monolayer transition metal dichalcogenides (TMDs) have emerged as an interesting class of two-dimensional materials due to their unique optical properties, such as a crossover from an indirect-to-direct bandgap as well as valley-specific optical selection rules. A striking feature in the linear optical spectra of TMDs is pronounced neutral and charged excitons (trions), with significantly larger binding energies than conventional semiconductors due to reduced screening. Using ultrafast two-color pump-probe spectroscopy, we demonstrate that Coulomb interactions responsible for the large binding energies in monolayer MoSe$_{2}$ also lead to strong coherent coupling between excitons and trions. Signatures for coherent coupling appear as isolated cross-peaks in a 2D spectrum obtained by tuning the pump and probe wavelengths through the resonances. While incoherent population transfer may partially contribute to one of the peaks, density matrix calculations reveal that the unique peak lineshapes arise from coherent exciton-trion many-body interactions, whose strength is significantly larger compared to conventional semiconductor quantum wells. Strong exciton-trion coherent coupling demonstrated here makes TMDs an excellent platform for future coherent opto-electronic devices.