Signature of 2p exciton in hBN-encapsulated monolayer MoSe2 revealed by sum frequency generation spectroscopy

Monolayer (1L) MoSe 2 is a two-dimensional direct-gap semiconductor. Due to the low dimensionality and reduced dielectric screening, careers experience stronger Coulombic interaction than that in conventional bulk materials, which results in excitonic binding energy as large as hundreds of meV [1] . Recent hBN encapsulation technique enabled one to observe s-series excitons with the principal quantum number of up to n=4 [2] . In that report, their energy positions are analyzed using the Keldysh potential model [3] , which describes Coulombic interaction in thin materials. However, nonnegligible additional factors such as Berry curvature [4] also affect excitonic structure. For example, p-series excitons are sensitive to Berry curvature contribution due to their finite angular momenta. To disentangle these effects, it is important to access optically dark 2p excitons. So far, no direct observation of the 2p exciton has been reported for hBN-encapsulated sample and only the indirect estimation of the energy separation between 2s and 2p excitons was made by the optical Stark effect [5] .