Nonlinear Plasmonics with 2D Excitons*

We investigate the interactions between propagating surface plasmon polaritons (SPP) in a metallic waveguide and excitons in WSe₂ monolayer. The WSe₂ monolayer was encapsulated by few layer thick hBN and transferred onto a lithographically defined gold waveguide using dry transfer technique. SPPs were launched by focusing a tunable laser onto an integrated input grating coupler and scattered out using another grating coupler. Both linear and nonlinear response of SPP-exciton interactions were measured. We report a very large linear absorption of SPP ~ 73% when the SPP is tuned to the exciton energy. For the nonlinear response, we report both optical-pumped and SPP-pumped differential SPP transmission measurements. For the SPP-pumped case, we report a large differential transmission (DT/T) over 4%. These results are highly consistent with a theoretical model of the linear and nonlinear SPP-exciton response. In our model, the SPP-exciton wave is based on an eigenmode analysis in the linear response regime, which is then perturbatively extended to the nonlinear regime. We believe that our device architecture and analysis could pave the road towards all-plasmonic modulators, amplifiers and transistors.