Excitons in monolayer transition metal dichalcogenides (TMDs) dominate their optical response due to exceptionally large binding energies arising from their two-dimensional nature. Several theoretical models have been proposed to describe this excitonic behavior, however, it remains unclear which model most accurately captures the underlying physical properties of the response. In this Letter, we experimentally measure the optical response of different high-quality monolayer TMD heterostructures and compare the results to the different theoretical models to address this uncertainty. Our results univocally show that in high-quality TMD heterostructures, quantum mechanical interactions in the form of a quantum pure dephasing plays a dominant role compared to other physical constituents. This finding is especially significant owing to the inability of other approaches to experimentally isolate and measure the quantum pure dephasing contribution to the excitonic response. These findings establish a robust framework for understanding and predicting the optical properties of excitons in monolayer TMD-based heterostructures, which are important for both fundamental research and optoelectronic device applications.

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• Excitons
• 2-dimensional systems
• Transition metal dichalcogenides
• Optical spectroscopy