  Metallic bilayer structures have been shown to emit strong terahertz (THz)
pulses. We present a predictive multiscale mode that simulates optically
induced spin-currents in a Fe/Pt-heterostructure and the emitted electric
field. Electronic effects are treated on the nano scale using the Boltzmann
transport equation for the dynamics of out-of-equilibrium charge carriers,
numerically solved with a particle-in-cell code. The optical effects are
simulated with a formalism that bridges the nanometer scale of the structure to
the micrometer scale of the emitted waves. The approach helps to understand
recent experimental findings on the basis of microscopic scattering effects and
transport phenomena. Our theory's versatility allows it to be readily adapted
to a wide spectrum of spintronic THz emitter designs. As an example, we show
how the THz generation efficiency, defined as output to input power ratio, can
be improved and optimized using serially stacked layers in conjunction with THz
anti-reflection coatings.
