Strong coupling between a dielectric nanocavity and a monolayer transition metal dichalcogenide

We demonstrate strong coupling between light in a dielectric nanocavity with deep sub-wavelength confinement and excitons in a monolayer of molybdenum ditelluride. Avoided crossing is demonstrated by both photoluminescence and reflection measurements, from which we extract a light-matter interaction strength of $g_{\mathrm{PL}} =\SI{5.3\pm0.3}{\milli\eV}$ and $g_{\mathrm{R}} =\SI{4.7\pm0.7}{\milli\eV}$, respectively. The associated Rabi splitting is twice as large as the system's losses. These values are in good agreement with values obtained by a novel exciton reaction coordinate formalism, yielding $g_{\mathrm{theory}} = \SI{5.2\pm0.7}{\milli\eV}$. The strong light-matter interaction, combined with low losses and sub-wavelength confinement of light, black demonstrates a new regime of light-matter interactions where strong nonlinearities at the single-photon level are expected.