Exciton Up-conversion in Transition Metal Dichalcogenide Monolayers

The optical properties of Transition Metal Dichalcogenide (TMD) monolayers are governed by robust excitons. We have systematically investigated the excited exciton states by photoluminescence up-conversion spectroscopy in high quality MoS2, MoSe2, MoTe2 and WSe2 monolayers (MLs) encapsulated in hBN [1]. The excitation laser is tuned into resonance with the A:1s exciton ground state transition, and we observe for all the investigated MLs clear emission of excited exciton states (A:2s, A:3s…) up to 450 meV above the laser energy. The optical transitions are further investigated by reflectivity, photoluminescence excitation, and resonant Raman scattering, confirming their origin as excited excitonic states. We interpret the efficient up-conversion process as the consequence of exciton-exciton interactions where one of the excitons is annihilated while the second exciton acquires large extra energy. This mechanism is expected to be quite weak because it should satisfy both energy and momentum conservation laws. However our model calculations suggest an efficient exciton-exciton (Auger) scattering mechanism specific to TMD monolayers involving an excited conduction band, thus generating high-energy excitons with small wave vectors.
[1] B. Han et al, PRX 8, 031073 (2018)