In atomically thin transition metal dichalcogenides (TMDs), reduced dielectric screening of the Coulomb interaction leads to strongly correlated many-body states, including excitons and trions, that dominate the optical properties. Higher-order states, such as bound biexcitons, are possible but are difficult to identify unambiguously using linear optical spectroscopy methods alone. Here, we implement polarization-resolved two-dimensional coherent spectroscopy to unravel the complex optical response of monolayer MoSe2 and identify multiple higher-order correlated states. Decisive signatures of neutral and charged inter-valley biexcitons appear in cross-polarized two-dimensional spectra as distinct resonances with respective ~20 meV and ~5 meV binding energies--similar to recent calculations using variational and Monte Carlo methods. A theoretical model taking into account the valley-dependent optical selection rules reveals the specific quantum pathways that give rise to these states. Inter-valley biexcitons identified here, comprised of neutral and charged excitons from different valleys, offer new opportunities for creating exotic exciton-polariton condensates and for developing ultrathin biexciton lasers and polarization-entangled photon sources.
@article{arxiv.1609.02008,
title = {Neutral and Charged Inter-Valley Biexcitons in Monolayer MoSe$_2$},
author = {Kai Hao and Lixiang Xu and Judith F. Specht and Philipp Nagler and Kha Tran and Akshay Singh and Chandriker Kavir Dass and Christian Schüller and Tobias Korn and Marten Richter and Andreas Knorr and Xiaoqin Li and Galan Moody},
journal= {arXiv preprint arXiv:1609.02008},
year = {2017}
}