Trion-to-exciton upconversion dynamics in monolayer WSe₂

We investigate the trion-to-exciton upconversion dynamics in a large-scale mechanically exfoliated tungsten diselenide (WSe2) monolayer covered by poly(methyl methacrylate) using two-color ultrafast pump-probe spectroscopy. With resonantly pumping the trion and probing the exciton transitions, we find a fast trion-to-exciton upconversion process with the upconversion time about ∼2 ps. Combined with the temperature-dependent transient reflection and helicity-resolved Raman measurements, we further demonstrate that the upconversion is indeed a phonon-mediated process in our sample.