  A plasmonic modulator is a device that controls the amplitude or phase of
propagating plasmons. In a pure plasmonic modulator, the presence or absence of
a pump plasmonic wave controls the amplitude of a probe plasmonic wave through
a channel. This control has to be mediated by an interaction between disparate
plasmonic waves, typically requiring the integration of a nonlinear material.
In this work, we demonstrate the first 2D semiconductor nonlinear plasmonic
modulator based on a WSe2 monolayer integrated on top of a lithographically
defined metallic waveguide. We utilize the strong coupling between the surface
plasmon polaritons, SPPs, and excitons in the WSe2 to give a 73 percent change
in transmission through the device. We demonstrate control of the propagating
SPPs using both optical and SPP pumps, realizing the first demonstration of a
2D semiconductor nonlinear plasmonic modulator, with a modulation depth of 4.1
percent, and an ultralow switching energy estimated to be 40 aJ.
