  Excitons in monolayer transition-metal-dichalcogenides (TMDs) dominate their
optical response and exhibit strong light-matter interactions with
lifetime-limited emission. While various approaches have been applied to
enhance light-exciton interactions in TMDs, the achieved strength have been far
below unity, and a complete picture of its underlying physical mechanisms and
fundamental limits has not been provided. Here, we introduce a TMD-based van
der Waals heterostructure cavity that provides near-unity excitonic absorption,
and emission of excitonic complexes that are observed at ultra-low excitation
powers. Our results are in full agreement with a quantum theoretical framework
introduced to describe the light-exciton-cavity interaction. We find that the
subtle interplay between the radiative, non-radiative and dephasing decay rates
plays a crucial role, and unveil a universal absorption law for excitons in 2D
systems. This enhanced light-exciton interaction provides a platform for
studying excitonic phase-transitions and quantum nonlinearities and enables new
possibilities for 2D semiconductor-based optoelectronic devices.
