The far-field linear and nonlinear optical response of excitons in two-dimensional (2D) semiconducting transitional metaldichalcogenides (TMDs), such as MoS_2 and WSe_2, have been the subject of intense investigation over the past decade.Here, we report on our experimental results measuring the linear and nonlinear response of surface plasmon polaritons(SPPs) propagating on metallic waveguides interacting with excitons in a single WSe_2 monolayer. The WSe_2 monolayerwas encapsulated in hexagonal boron nitride and transferred on top of a metallic waveguide. The measurements werecarried out at low temperature (below 11 K). We measured the linear absorption of SPPs by excitons, resulting in a 73 %attenuation of the transmitted probe. To determine the nonlinear response, we performed both optical pump-SPP probeand SPP pump-SPP probe experiments. For the SPP pump case, a differential transmission response exceeding 4 % wasachieved. Time-resolved pump-probe measurements reveal a fast component of the nonlinear response of 290 fs with aslower 13.7 ps component, consistent with previous optical measurements. These plasmonic structures could open up newopportunities to probe fundamental light-matter interactions in 2D material-plasmonic heterostructures.
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