Van der Waals semiconductors have provided a new platform to extend the physics and applications of cavity exciton polaritons, thanks to their flexibility of integration, large oscillator strength, and novel moiré physics. However, the inhomogeneous broadening of excitons, originating from strain, defect, or interface fluctuation, has been an obstacle to further exploring coherent quantum phenomena and devices in this community. In this work, we show that the contribution of exciton inhomogeneous broadening to polariton linewidth can be fully suppressed by increasing Rabi splitting. The results can be well reproduced theoretically- by modeling the excitonphoton coupling using a non-Hermitian Hamiltonian. Our findings not only shed light on the development of coherent Van der Waals polariton devices but also demonstrate a general conclusion on the correlation between the inhomogeneous broadening of a solid-state oscillator and the linewidth of its corresponding light-matter hybrids.

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• Exciton polariton
• Excitons
• Transition metal dichalcogenides
• Reflectivity