Neutral and Charged Inter-Valley Biexcitons in Monolayer Transition Metal Dichalcogenides

In monolayer transition metal dichalcogenides, tightly bounded exciton and trion dominate the optical properties. Higher-order correlated states, such as biexciton, are possible but are difficult to unambiguously identified with linear optical spectroscopy alone. With polarization resolved two dimensional coherent spectroscopy, unambiguously signatures of neutral and charged inter-valley biexcitons are observed in monolayer MoSe2 with \textasciitilde 20 meV and \textasciitilde 3meV binding energies, which are consistent with variational and Monte Carlo calculations. These higher--order correlated states consist of quasiparticles formed at opposite valleys with large crystal momentum difference, making them a unique type of bound states with no direct analog in conventional semiconductors. Our findings offer new opportunities for developing ultrathin biexciton lasers andpolarization-entangled photon sources and for creating exotic exciton-polariton condensates.