  The interaction between the electromagnetic field and plasmonic
nanostructures leads to both the strong linear response and inherent nonlinear
behavior. In this paper, a time-domain hydrodynamic model for describing the
motion of electrons in plasmonic nanostructures is presented, in which both
surface and bulk contributions of nonlinearity are considered. A coupled
Maxwell-hydrodynamic system capturing full-wave physics and free electron
dynamics is numerically solved with the parallel finite-difference time-domain
(FDTD) method. The validation of the proposed method is presented to simulate
linear and nonlinear responses from a plasmonic metasurface. The linear
response is compared with the Drude dispersion model and the nonlinear
terahertz emission from a difference-frequency generation process is validated
with theoretical analyses. The proposed scheme is fundamentally important to
design nonlinear plasmonic nanodevices, especially for efficient and broadband
THz emitters.
