Ultrafast Switching of Trion Emitters in Monolayer MoSe₂

Excitons in two-dimensional transition metal dichalcogenides emerged as a unique nanoscale platform offering strong light-matter coupling, spin-valley locking and exceptional tunability. Moreover, their properties and optical response change drastically in the presence of free charges, leading to the formation of new quasiparticles known as trions or Fermi polarons. The physics of such Bose-Fermi quasiparticle mixtures have attracted considerable interest in the scientific community. However, there are limitations to how fast the optical response of these states can be manipulated, restricting the majority of applications to a static regime. <br/><br/>Here, we show how to overcome this challenge by using low-energy photons in the THz frequency range. We demonstrate the conversion of trions into excitons in two-dimensional materials on ultrafast timescales of a few picoseconds by applying short THz pulses after the optical excitation. Monitoring the time-resolved photoluminescence dynamics, a strong quenching of the trion population induced by the THz radiation is observed, accompanied by a simultaneous increase of the exciton emission. The process is highly sensitive to the energy of the THz photons, that has to match the trion binding energy of the material. Furthermore, the observed switching is found to be highly reproducible when both the THz power and the time delay between optical and THz pulses are tuned. Our results provide a promising experimental tool for fundamental research of light-emitting excitation mixtures and offer pathways towards technological developments of nanophotonic devices based on atomically thin materials.