Fig. 1: Coherent population oscillations of excitons in 2D plasmonic structures.

a Depiction of the 2D material plasmonic structure. Surface plasmon polaritons (SPPs) are launched at the input of the device by focusing a free space laser onto either/both input couplers. The SPPs propagate through the waveguide where they interact with excitons in the WSe₂ monolayer, encapsulated in hBN. The SPPs are coupled back to free space photons by the output gratings. The upper inset depicts the hBN-WSe₂-hBN heterostructure on top of the waveguide. The axes used for the theory and simulations are shown. b Optical image of the main device used in the experiments. Scale bar 5 µm. The red dotted lines show the active region of the WSe₂ and blue dashed and dotted lines show the outlines of the top and bottom hBN, respectively. c Broad normalized differential transmission spectra (DT/T) for co-propagating SPP excitation. The green line denotes where the WSe₂ is pumped for d. The inset shows the schematic for co-propagating SPPs. d High-resolution DT/T spectra for co-propagating SPPs being pumped at 1.731 eV (716 nm). The inset depicts the pump and probe coupling to the X⁰ neutral exciton state where ω_pr is the probe frequency, ω_pp is the pump frequency, and δ is the detuning between the pump and probe frequencies.