Second-Order Nonlinearity of Excitons in hBN-Encapsulated Monolayer Transition Metal Dichalcogenides

Monolayer transition metal dichalcogenides (1L-TMDs) are expected for applications to next-generation optoelectronic devices, owing to strong coupling with light. Optical properties of 1L-TMDs are dominated by excitons (electron-hole pairs), which are stable even at room temperature. This is because exciton binding energies are as large as a few hundred meV, derived from the atomic thickness. The low-dimensionality also gives arise to the nonhydrogenic energy level structure of 1L-TMDs excitons unlike those in bulk semiconductors [1]. So far, phenomenological Rytova-Keldysh potential [2], [3] is used for the analyses on the s-series exciton level structure observed in linear spectroscopies [4], [5]. In addition, Berry curvature effects are expected to contribute to the splitting of p-series excitons observed in third-order nonlinear optical responses [6]. However, no unified model picture has been established for the 1L-TMDs exciton level structure.