Guided 2D exciton-polaritons, resulting from the strong coupling of excitons in semiconductors with nonradiating waveguide modes, provide an attractive approach toward developing novel on-chip optical devices. These quasiparticles are characterized by long propagation distances and efficient nonlinear interactions 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 of 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 line width, 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.
Keywords: excitons; guided polaritons; motional narrowing.; solid immersion lens; strong coupling; transition metal dichalcogenides.