  Strong Coulomb interaction in 2D materials provides unprecedented
opportunities for studying many key issues of condensed matter physics, such as
co-existence and mutual conversions of excitonic complexes, fundamental optical
processes associated with their conversions, and their roles in the celebrated
Mott transition. Recent lasing demonstrations in 2D materials raise important
questions about the existence and origin of optical gain and possible roles of
excitonic complexes. While lasing occurred at extremely low densities dominated
by various excitonic complexes, optical gain was observed in the only
experiment at densities several orders of magnitude higher, exceeding the Mott
density. Here, we report a new gain mechanism involving charged excitons or
trions well below the Mott density in 2D molybdenum ditelluride. Our combined
experimental and modeling study not only reveals the complex interplays of
excitonic complexes well below the Mott transition, but also provides
foundation for lasing at extremely low excitation levels, important for future
energy efficient photonic devices.
