Photon Upconversion\nof Defect-Bound Excitons in hBN-Encapsulated\nMoS<sub>2</sub> Monolayer

Atomic defects associated\nwith vacancies in two-dimensional\ntransition\nmetal dichalcogenide monolayers efficiently trap charged carriers\nand strongly localize excitons. Defects in semiconducting monolayers\nare seldomly utilized for enhancing optical phenomena, although they\nmay provide resonant intermediate states within the energy band gap\nfor applications with multiphoton excitations, like highly efficient\nand thermally robust photon upconversion. In an MoS<sub>2</sub> monolayer\nencapsulated by hBN with high defect and resident electron densities,\nwe observe an upconversion of localized exciton (X<sub>L</sub>) emission\nwith a huge energy gain of up to 290 meV. The upconverted X<sub>L</sub> emission is robust up to temperatures of about 120 K and exhibits\na sublinear or a nearly linear laser power dependence for the energy\ngain of about 100 meV and above 200 meV, respectively. The upconversion\nmechanism is explained by a cooperative energy transfer process between\nthe photocreated and resident electrons, in which hybridized pairs\nof single sulfur vacancies likely act as real intermediate states.\nAdditionally, we find a weak upconversion of the neutral exciton photoluminescence\nwith an energy gain of about 350 meV for quasi-resonant excitation\nof the X<sub>L</sub> exciton. It is attributed to a two-step, two-photon\nabsorption.