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-...

Moiré superlattices can induce correlated‐electronic phases in twisted van der Waals materials: strongly correlated quantum phenomena emerge, such as superconductivity and the Mott‐insulating state. However, moiré superlattices produced through artificial stacking can be quite inhomogeneous, which hampers the development of a clear correlation between the moiré period and the emerging electrical and optical properties. Here, it is demonstrated in twisted‐bilayer transition‐metal dichalcogenides that low‐frequency Raman scattering can be utilized not only to detect atomic reconstruction, but also to map out the inhomogeneity of the moiré lattice over large areas. The method is established based on the finding that both the interlayer‐breathing mode and moiré phonons are highly susceptible to the moiré period and provide characteristic fingerprints. Hyperspectral Raman imaging visualizes microscopic domains of a 5° twisted‐bilayer sample with an effective twist‐angle resolution of about 0.1°. This ambient methodology can be conveniently implemented to characterize and preselect high‐quality areas of samples for subsequent device fabrication, and for transport and optical experiments.

We investigate WSe 2 -MoSe 2 heterobilayers with different twist angles h 6 d between the two layers by low-frequency Raman scattering. In sufficiently aligned samples with h ¼ 0 or h ¼ 60 and d Շ 3 , we observe an interlayer shear mode (ISM), which is a clear sign of a commensurate bilayer structure, i.e., the layers must undergo an atomic reconstruction to form R-type or H-type stacking orders. We find slightly different ISM energies of about 18 cm -1 and 17 cm -1 for H-type and R-type reconstructions, respectively, independent of the exact value of h 6 d. Our findings are corroborated by the fact that the ISM is not observed in samples with twist angles, which deviate by d > 3 from 0 or 60 . This is expected since, in such incommensurate structures, with the possibility of Moir e-lattice formation, there is no restoring force for an ISM. Furthermore, we observe the ISM even in sufficiently aligned heterobilayers, which are encapsulated in hexagonal Boron nitride. This is particularly relevant for the characterization of high-quality heterostructure devices.

We experimentally study the radii of excitons in hBN-encapsulated WS2 monolayers by means of magneto-optical reflectance spectroscopy at cryogenic temperatures in magnetic fields up to 29 T. We observe field-induced energy shifts of the exciton ground and excited states due to valley Zeeman and diamagnetic effects. We find the g factor of the first excited state of −4.2 ± 0.1 to be essentially equal to that of the ground state of −4.35 ± 0.1. From diamagnetic shifts we determine the root mean square radii of the excitons. The radius of the first excited state is found to be 5 -8 nm and that of the ground state around 2 nm. Our results further confirm the Wannier-Mott nature of the exciton quasiparticles in monolayer semiconductors and the assignment of the optical resonances in absorption-type measurements. They also provide additional support for the applicability of the effective mass hydrogenlike models in these systems.

We report about the investigation of twisted MoSe 2 homo-and MoSe 2 -WSe 2 heterobilayers by means of low-frequency Raman spectroscopy (LFRS) and low-temperature micro photoluminescence (µPL). In room-temperature LFRS experiments on both, twisted MoSe 2 homobilayers and twisted MoSe 2 -WSe 2 heterobilayers, we observe moiré phonons, i.e. folded acoustic phonon modes due to the moiré superlattice. In the heterobilayers, we can identify moiré phonons of both materials, MoSe 2 and WSe 2 . While the twist angles for the homobilayers are relatively precisely known from the applied tear-and-stack preparation method, the twist angles of the heterobilayers have to be determined via second-harmonic-generation microscopy on monolayer regions of the samples, which has significant uncertainties. We show that by the moiré phonons of the heterobilayers, the relative twist angles can be determined on a local scale with much higher precision. We apply our technique for the investigation of a large area H-type (twist angle θ = 60 • + δ) MoSe 2 -WSe 2 heterobilayer. These investigations show that spatial regions, which can be identified to be atomically reconstructed (i.e. δ = 0 • ) by the observation of an interlayer shear mode in LFRS experiments, exhibit a strong, momentum-allowed interlayer-exciton signal in low-temperature µPL. On the contrary, regions, where moiré phonons are observed, i.e. which can be identified to be rigidly twisted by a misalignment angle in the range of 5 • ≲ |δ| ≲ 6 • , exhibit no significant interlayer-exciton signals.