  Nanosphere-on-mirror plasmonic antennas, each having a monolayer graphene or
MoS2 sheet in the gap, were pumped with a femtosecond laser. Abrupt turnings in
the scattering linewidth and peak intensity trends as the laser power changed
were experimentally observed. Theoretical modelling of dynamic plasmon
evolvement attributes the turning to transitioning between two plasmon states,
with a universal switching threshold of four-wave mixing efficiency around
0.14%. This bistability is rendered by a strong feedback from the nonlinear gap
current to the gap plasmons, and involvement of both four and high-order wave
mixing. This work reveals a pathway to making energy efficient nonlinear
plasmonic elements.
