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Two-dimensional excitons formed in quantum materials such as monolayertransition-metal dichalcogenides and their strong light-matter interaction haveattracted unrivalled attention by the research community due to theirextraordinarily large oscillator strength as well as binding energy, and theinherent spin-valley locking. Semiconducting few-layer and monolayer materialswith their sharp optical resonances such as WSe2 have been extensively studiedand envisioned for applications in the weak as well as strong light-mattercoupling regimes, for effective nano-laser operation with various differentstructures, and particularly for valleytronic nanophotonics motivated by thecircular dichroism. Many of these applications, which may benefit heavily fromthe two-dimensional electronic quasiparticle's properties in such films,require controlling, manipulating and first of all understanding the nature ofthe optical resonances that are attributed to exciton modes. While theory andprevious experiments have provided unique methods to the characterization andclassification efforts regarding the band structure and optical modes in 2Dmaterials, here, we directly measure the quasiparticle energy-momentumdispersion for the first time. Our results for single-layer WSe2 clearlyindicate an emission regime predominantly governed by free excitons, i.e.Coulomb-bound electron-hole pairs with centre-of-mass momentum andcorresponding effective mass. Besides uniquely evidencing the existence of freeexcitons at cryogenic temperatures optically, the fading of the dispersivecharacter for increased temperatures or excitation densities reveals atransition to a regime with profound role of charge-carrier plasma or localizedexcitons regarding its emission, debunking the myth of free-exciton emission atelevated temperatures.

Shedding light on the monolayer-WSe2 exciton's nature by optical effective-mass measurements