Encapsulation Narrows and Preserves the Excitonic Homogeneous Linewidth of Exfoliated Monolayer MoSe2

Measurements of the homogeneous linewidth of exfoliated MoSe2 monolayers reveal significant mate-rial improvement with encapsulation, but also higher than expected material inhomogeneity. Multidimen-sional coherent spectroscopy also directly demonstrates that exfoliated monolayer MoSe2 encapsulated in hexagonal boron nitride has an enhanced coherence time and would be useful in high-optical-power applications, where unencapsulated samples would not. We directly measure an excitonic homogeneous linewidth of exfoliated MoSe2 that is 0.20 +/- 0.02 meV, corresponding to a dephasing time T-2 approximate to 3.3 ps. Encapsulation reduces the sample inhomogeneity. The inhomogeneous linewidth is roughly 5 times larger than the homogeneous linewidth in even the highest-quality encapsulated materials. Encapsulated MoSe2 samples are unaffected by conditions that entirely degrade unencapsulated samples.