  Heterostructures of atomically thin van der Waals bonded monolayers have
opened a unique platform to engineer Coulomb correlations, shaping excitonic,
Mott insulating, or superconducting phases. In transition metal dichalcogenide
heterostructures, electrons and holes residing in different monolayers can bind
into spatially indirect excitons with a strong potential for optoelectronics,
valleytronics, Bose condensation, superfluidity, and moir\'e-induced nanodot
lattices. 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.
