Abstract
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. Here, we implement polarization-resolved two-dimensional coherent spectroscopy (2DCS) 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 and ∼5 meV binding energies—similar to recent calculations using variational and Monte Carlo methods. A theoretical model considering the valley-dependent optical selection rules reveals the quantum pathways that give rise to these states. Inter-valley biexcitons identified here, comprising of neutral and charged excitons from different valleys, offer new opportunities for developing ultrathin biexciton lasers and polarization-entangled photon sources.
Highlights
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
While a few experimental studies have reported the bound biexciton in monolayer TMDs, significant discrepancies between the measured[23,24,25,26,27] and calculated[28,29,30,31,32] binding energy remain, which may arise for several reasons
In photoluminescence (PL) spectroscopy of WSe2 and WS2 TMD monolayers, the emergence of a new peak whose intensity grows super-linearly with pump fluence has been assigned to the neutral biexciton, but resonances associated with the charged biexciton or electron plasma excitations may appear at a similar energy[33]
Summary
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. Experimental studies that can clearly identify possible bound states between excitons and trions and the associated internal valley indices are critical in guiding the development of a microscopic theory describing these higher-order correlated states in atomically thin semiconductors. We shed new light on higher-order bound states by identifying and distinguishing neutral and charged biexcitons in monolayer MoSe2 using resonant two-dimensional coherent spectroscopy (2DCS)[37]. The polarization dependence of the 2D spectrum is reproduced with density matrix calculations taking into account the valley-dependent optical selection rules These results suggest that both types of biexcitons are inter-valley in nature and consist of two excitons with large difference in crystal momentum, making them unique types of higher-order bound states with no direct analogue in conventional semiconductors
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