  Room temperature strong coupling of WS_2 monolayer exciton transitions to
metallic Fabry-Perot and plasmonic optical cavities is demonstrated. A Rabi
splitting of 101 meV is observed for the Fabry-Perot cavity, more than double
those reported to date in other 2D materials. The enhanced magnitude and
visibility of WS_2 monolayer strong coupling is attributed to the larger
absorption coefficient, the narrower linewidth of the A exciton transition, and
greater spin-orbit coupling. For WS_2 coupled to plasmonic arrays, the Rabi
splitting still reaches 60 meV despite the less favorable coupling conditions,
and displays interesting photoluminescence features. The unambiguous signature
of WS_2 monolayer strong coupling in easily fabricated metallic resonators at
room temperature suggests many possibilities for combining light-matter
hybridization with spin and valleytronics.
