  Two-dimensional excitons formed in quantum materials such as monolayer
transition-metal dichalcogenides and their strong light-matter interaction have
attracted unrivalled attention by the research community due to their
extraordinarily large oscillator strength as well as binding energy, and the
inherent spin-valley locking. Semiconducting few-layer and monolayer materials
with their sharp optical resonances such as WSe2 have been extensively studied
and envisioned for applications in the weak as well as strong light-matter
coupling regimes, for effective nano-laser operation with various different
structures, and particularly for valleytronic nanophotonics motivated by the
circular dichroism. Many of these applications, which may benefit heavily from
the two-dimensional electronic quasiparticle's properties in such films,
require controlling, manipulating and first of all understanding the nature of
the optical resonances that are attributed to exciton modes. While theory and
previous experiments have provided unique methods to the characterization and
classification efforts regarding the band structure and optical modes in 2D
materials, here, we directly measure the quasiparticle energy-momentum
dispersion for the first time. Our results for single-layer WSe2 clearly
indicate an emission regime predominantly governed by free excitons, i.e.
Coulomb-bound electron-hole pairs with centre-of-mass momentum and
corresponding effective mass. Besides uniquely evidencing the existence of free
excitons at cryogenic temperatures optically, the fading of the dispersive
character for increased temperatures or excitation densities reveals a
transition to a regime with profound role of charge-carrier plasma or localized
excitons regarding its emission, debunking the myth of free-exciton emission at
elevated temperatures.
