  Guided 2D exciton-polaritons, resulting from the strong coupling of excitons
in semiconductors with non-radiating waveguide modes, provide an attractive
approach towards developing novel on-chip optical devices. These quasiparticles
are characterized by long propagation distances and efficient nonlinear
interaction but cannot be directly accessed from the free space. Here we
demonstrate a powerful approach for probing and manipulating guided polaritons
in a Ta2O5 slab integrated with a WS2 monolayer using evanescent coupling
through a high-index solid immersion lens. Tuning the nanoscale lens-sample gap
allows for extracting all the intrinsic parameters of the system. We also
demonstrate the transition from weak to strong coupling accompanied by the
onset of the motional narrowing effect: with the increase of exciton-photon
coupling strength, the inhomogeneous contribution to polariton linewidth,
inherited from the exciton resonance, becomes fully lifted. Our results enable
the development of integrated optics employing room-temperature
exciton-polaritons in 2D semiconductor-based structures.
