  Recent optical studies of monolayer transition metal dechalcogenides have
demonstrated that their excitonic absorption feature splits into two widely
separated peaks at finite carrier densities. The additional peak is usually
attributed to the presence of trions, bound states of two electrons and a hole
or an electron and two holes. Here we argue that in the density range over
which the trion peak is well resolved, it cannot be interpreted in terms of
weakly coupled three-body systems, and that the appropriate picture is instead
one in which excitons are dressed by interactions with a Fermi sea of excess
carriers. This coupling splits the exciton spectrum into a lower energy
attractive exciton-polaron branch, normally identified as a trion branch, and a
higher energy repulsive exciton-polaron branch, normally identified as an
exciton branch. We have calculated the frequency and doping dependence of the
optical conductivity and found that: (i) the splitting varies linearly with the
Fermi energy of the excess quasiparticles; (ii) the trion peak is dominant at
high carrier densities; (iii) and the trion peak width is considerably smaller
than that of the excitonic peak. Our results are in good agreement with recent
experiments.
