Excited exciton states in monolayer MoS2 encapsulated in hexagonal Boron nitride

The optical properties of transition metal dichalcogenide monolayers such as MoS2 and WSe2 are dominated by excitons, Coulomb bound electron hole pairs, with binding energies of several hundred meV. This gives rise to a series of resonances in linear and non-linear optical spectra in analogy to the Rydberg states of the hydrogen atom. Due to the broad and often defect related optical transitions in MoS2 monolayers exfoliated directly onto SiO2 substrates detailed information on excited exciton states was inaccessible.
Here we reveal in reflectivity and in photoluminescence excitation spectroscopy (PLE) the presence of the first two excited states of the A-exciton in MoS2 monolayers encapsulated with hexagonal boron nitride (hBN) [1]. We see a clear increase both in intensity and in the degree of valley polarization of the neutral exciton emission when the laser excitation energy coincides with these excited Rydberg states. We also demonstrate efficient PL up-conversion when in resonance with the A exciton, resulting in emission 200 meV above the laser energy. These results have allowed us to estimate the exciton binding energy to be around 220 meV in h-BN encapsulated MoS2 monolayer samples.
[1] Fabian Cadiz et al, PRX 7, 021026 (2017)