Heterostructures of atomically thin van der Waals bonded monolayers have opened a unique platform to engineer Coulomb correlations, shaping excitonic 1-3 , Mott insulating 4 , or superconducting phases 5,6 . In transition metal dichalcogenide heterostructures 7 , electrons and holes residing in different monolayers can bind into spatially indirect excitons 1,3,8-11 with a strong potential for optoelectronics 11,12 , valleytronics 1,3,13 , Bose condensation 14 , superfluidity 14,15 , and moiré-induced nanodot lattices 16 . Yet these ideas require a microscopic understanding of the formation, dissociation, and thermalization dynamics of correlations including ultrafast phase transitions. Here we introduce a direct ultrafast access to Coulomb correlations between monolayers; phase-locked mid-infrared pulses allow us to measure the binding energy of interlayer excitons in WSe2/WS2 hetero-bilayers by revealing a novel 1s-2p resonance, explained by a fully quantum mechanical model. Furthermore, we trace, with subcycle time resolution, the transformation of an exciton gas photogenerated in the WSe2 layer directly into interlayer excitons. Depending on the stacking angle, intra-and interlayer species coexist on picosecond scales and the 1s-2p resonance becomes renormalized. Our work provides a direct measurement of the binding energy of interlayer excitons and opens the possibility to trace and control correlations in novel artificial materials.In monolayers of transition metal dichalcogenides (TMDs), the confinement of electronic motion into two dimensions and the suppression of dielectric screening facilitate unusually strong Coulomb interaction 17-20 . This gives rise to excitons with giant binding energies of several hundred meV 17 , small Bohr radii 18 and ultrashort radiative lifetimes 19 . When two monolayers are contacted with type-II band alignment, the conduction band minimum and the valence band maximum are located in two different layers 7 . Owing to their proximity, electron-hole (e-h) pairs in adjacent monolayers are still subject to strong mutual Coulomb attraction. Interband photoluminescence combined with theory has, indeed, provided evidence of interlayer excitons 8,21-23 . Because the composite electron and hole wavefunctions overlap only weakly in space, these excitons are long lived -a key asset for future applications 1,3,14-16,24 .Yet, the weak coupling to light renders these quasiparticles inaccessible to interband absorption spectroscopy. Hence the binding energies of interlayer excitons, which depend sensitively on the delocalization of the electronic wavefunctions over the heterostructure 23 , have not been measured.Signatures of the ultrafast interlayer charge transfer have been studied by interband spectroscopy 8,21 .These techniques, however, cannot measure Coulomb correlations or the formation of interlayer excitons, on the intrinsic ultrashort timescales.Meanwhile, phase-locked electromagnetic pulses in the terahertz (THz) and mid-infrared (MIR) range have directly accessed ultrafast low-...

Exciton binding energies of hundreds of meV and strong light absorption in the optical frequency range make transition metal dichalcogenides (TMDs) promising for novel optoelectronic nanodevices. In particular, atomically thin TMDs can be stacked to heterostructures enabling the design of new materials with tailored properties. The strong Coulomb interaction gives rise to interlayer excitons, where electrons and holes are spatially separated in different layers. In this work, we reveal the microscopic processes behind the formation, thermalization and decay of these fundamentally interesting and technologically relevant interlayer excitonic states. In particular, we present for the exemplary MoSe2-WSe2 heterostructure the interlayer exciton binding energies and wave functions as well as their time-and energy-resolved dynamics. Finally, we predict the dominant contribution of interlayer excitons to the photoluminescence of these materials. arXiv:1804.08412v1 [cond-mat.mes-hall]

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