We have experimentally determined the energies of the ground and first four excited excitonic states of the fundamental optical transition in monolayer WS2, a model system for the growing class of atomically thin two-dimensional semiconductor crystals. From the spectra, we establish a large exciton binding energy of 0.32 eV and a pronounced deviation from the usual hydrogenic Rydberg series of energy levels of the excitonic states. We explain both of these results using a microscopic theory in which the non-local nature of the effective dielectric screening modifies the functional form of the Coulomb interaction. These strong but unconventional electron-hole interactions are expected to be ubiquitous in atomically thin materials.

The ability to control the size of the electronic bandgap is an integral part of solid-state technology. Atomically thin two-dimensional crystals offer a new approach for tuning the energies of the electronic states based on the unusual strength of the Coulomb interaction in these materials and its environmental sensitivity. Here, we show that by engineering the surrounding dielectric environment, one can tune the electronic bandgap and the exciton binding energy in monolayers of WS2 and WSe2 by hundreds of meV. We exploit this behaviour to present an in-plane dielectric heterostructure with a spatially dependent bandgap, as an initial step towards the creation of diverse lateral junctions with nanoscale resolution.

We report a determination of the complex in-plane dielectric function of monolayers of four transition metal dichalcogenides: MoS 2 , MoSe 2 , WS 2 and WSe 2 , for photon energies from 1.5-3 eV. The results were obtained from reflection spectra using a Kramers-Kronig constrained variational analysis. From the dielectric functions, we obtain the absolute absorbance of the monolayers. We also provide a comparison of the dielectric function for the monolayers with the corresponding bulk materials.

Control of the optical properties of matter on ultrashort timescales is of both fundamental interest and central importance for applications in photonics. It is desirable to achieve pronounced changes over a broad spectral range using the least possible amount of material. Here, we demonstrate a dramatic change over a spectral range of hundreds of meV on the femtosecond timescale in the optical response of atomically thin two-dimensional crystals of the transition-metal dichalcogenide WS 2 following excitation by intense optical pump pulses. Our findings reveal the role of extremely strong Coulomb interactions. At the direct gap, we observe a Mott transition from excitonic states to free carriers, accompanied by a giant bandgap renormalization of approximately 500 meV and the development of population inversion. M any-body phenomena arising from the interaction between charge carriers are of central importance in condensedmatter physics. In semiconductors, Coulomb interactions can lead to the formation of stable many-particle electronic states, such as excitons, trions, excitonic molecules and dropletons, as well as macroscopic condensates 1-5 . Excitonic quasi-particles appear as pronounced resonances in the optical response and can dominate absorption and emission spectra 6 . These many-body interactions can be strongly altered in the presence of elevated electron-hole densities 7 , providing a method of modifying the material's response by optical carrier injection via ultrashort, intense laser pulses. From the fundamental point of view, this approach offers an excellent experimental method to probe manybody interactions under controlled conditions. For applications, these phenomena can be exploited in photonic devices ranging from lasers to optical switches, saturable absorbers, and modulators. In this respect, the emerging field of atomically thin two-dimensional materials, such as semiconducting transition-metal dichalcogenides (TMDCs) 8-11 , shows much promise. These materials provide strong light-matter coupling, with optical absorption around 10-20% in layers as thin as 0.6 nm 12-14 and extremely efficient Coulomb interactions, with exciton binding energies on the order of 0.5 eV (refs 15-19). Recent theoretical studies of monolayer TMDCs 20 predict that high carrier densities should produce massive changes in the optical response. However, despite a number of studies addressing the dynamics of excitons 21-35 , the behaviour of these systems under strong photoexcitation in the regime of a dense electron-hole plasma beyond the Mott transition remains unexplored experimentally.Here we access this regime by subjecting WS 2 mono-and bilayers to strong photoexcitation, injecting carriers via ultrashort laser pulses with fluences up to several mJ cm -2 . We show that the optical response of atomically thin TMDC crystals changes dramatically across a spectral range of many hundreds of meV at high electron-hole densities. We observe both a complete disappearance of the main excitonic resonance and the developmen...